Effects of Nonlinearity on Velocity, Acceleration and Pressure Gradient in Free-Stream Zone of Solitary Wave over Horizontal Bed—An Experimental Study

: For solitary waves on a horizontal bed, the study deals experimentally with the high ratio of wave height ( H 0 ) to still water depth ( h 0 ) that ampliﬁes the wave nonlinearity. The value of H 0 /h 0 tested in a wave ﬂume ranges from 0.050 to 0.550, indicating the shift from a quasi-linear solitary wave to a highly nonlinear one. A high-speed particle image velocimetry (HSPIV) and a ﬂow visualization technique of particle-trajectory tracking method are utilized to measure velocity ﬁelds and identify near-bed ﬂow structures. The unsteady free-stream velocities with equal magnitude take place in a free-stream zone, FSZ). The FSZ underlies the internal ﬂow zone, over which the external free surface of solitary wave exists and is situated beyond the boundary layer. The spatio-temporal variation of free-stream velocity, moving in phase with the free surface elevation, characterizes the pattern of pressure gradient in the FSZ and thus dominates the behavior of boundary layer ﬂow. Accordingly, nonlinear effects on the time series as well as the maximum values of horizontal velocity, particle acceleration, and pressure gradient in the FSZs of solitary waves are presented. Before, at, and after the wave crest’s intersection with a given measurement location, favorable, zero, and adverse pressure gradients occur in the FSZ, respectively. For H 0 / h 0 = 0.179, 0.363, and 0.550, the values of the dimensionless maximum free-stream velocity are about 3.10, 5.32, and 6.20 times that (= 0.0473) for H 0 /h 0 = 0.050; and the corresponding values of the dimensionless maximum adverse pressure gradient are about 5.74, 14.54 and 19.84 times that (= 0.0061) for H 0 / h 0 = 0.050. This evidence highlights the nonlinear effect on the kinematic and hydrodynamic features of solitary waves. Finally, the effect of nonlinearity on the relationship between the dimensionless time for the maximum adverse pressure gradient in the FSZ and that for the incipient ﬂow reversal in the bottom boundary layer is explored for the ﬁrst time. It is found that the incipient ﬂow reversal takes place immediately after the maximum adverse pressure gradient, together with a decrease in the dimensionless time for ﬂow reversal if H 0 / h 0 increases. The fact accentuates the nonlinear effect on the incipient ﬂow reversal right above the bed.


Introduction
The movement of a solitary wave was first observed in situ by Russell [1] with quite a stable waveform traveling over a long distance.His experimental results indicated that the wave propagated at a nearly constant wavelength (or celerity) yet a small attenuation in wave height [2], thus recognized as one type of permanent wave.A solitary wave is also regarded as one type of long-wave because the wave propagates persistently over shallow water.As reported by Grue et al. [3] and Madsen et al. [4], a combination of multiple solitary waves with distinct wave heights and different separation times was observed in the field during tsunami events.Further, El et al. [5] and Grilli et al. [6] demonstrated both numerically and experimentally that a series of isolated leading solitary waves emerged while an undular bore traveled at a decreasing water depth.Based on the illustrations mentioned above, a single solitary wave or multiple solitary waves take place in the navigation (or irrigation) channel, estuary, coastal zone, or ocean strait.The fact highlights the importance of associated studies on solitary waves.
Limited experimental investigations focus on the free surface elevations and particle velocities of solitary waves over a horizontal bed (Lee et al. [17] and Lin et al. [18][19][20]).Employing a resistance wave gauge and laser Doppler velocimetry, Lee et al. [17] measured the time series of free surface elevations as well as the counterparts of horizontal and vertical velocities for solitary waves with a ratio H 0 /h 0 = 0.11-0.29.Comparisons of the experimental results with those predicted by Boussinesq, McCowan, Munk, and Grimshaw solutions were then made, aiming to examine the validity of these analytic solutions.Comparisons of the measured free surface elevations with the predicted ones by the four solutions all showed good agreement.However, comparisons between the measured horizontal and vertical velocities and the predicted ones indicated that the Boussinesq solution matched satisfactorily with the experimental data trend.Note that the nonlinear effect of solitary waves was not discussed in their study.
Using the HSPIV, Lin et al. [18] demonstrated the "similarity profiles" for the dimensionless horizontal velocity; and for the dimensionless time lead of horizontal velocities in the boundary layer flows induced by solitary waves traveling over a horizontal bed with H 0 /h 0 = 0.096-0.386.Their study lacked in exploring the nonlinear effect on the kinematic and hydrodynamic features.As reported by Lin et al. [19,20], the features of (local and convective) accelerations and pressure gradient all over the internal flow as well as the relevant "similarities and Froude number similitudes" were elucidated only for a solitary wave (with H 0 /h 0 = 0.363) propagating on a horizontal bed.Further, as indicated by Lin et al. [20], fairly good predictions of horizontal and vertical velocities could be obtained by the Boussinesq solution if the linear wave celerity is employed in the computation.A series of studies on kinematic and hydrodynamic features or flow similarity and Froude number similitude was reported in Lin et al. [21][22][23][24][25][26] for the run-up and run-down motions of solitary waves traveling over 1:15-1:3 sloping beaches.Recently, Lin et al. [27] studied the characteristics of flow velocity and pressure gradient of an undular bore propagating on a horizontal bed.
Based on the above literature survey, it is still rudimentary concerning the role that the nonlinear effects play in the relevant kinematic and hydrodynamic features of solitary waves over a horizontal bed.In other words, as influenced by the nonlinearity, the corresponding features of flow velocities, particle accelerations, and pressure gradients of solitary waves are not made clear.Further, the relationship between the time instants for the occurrence of the maximum adverse pressure gradient in the FSZ and the incipient flow reversal in the boundary layer has not been investigated.Thus, using an HSPIV and a flow visualization technique, this study focuses on these unknown kinematic and hydrodynamic features, aiming to enhance the understanding of long-wave mechanics.
The outline of this paper is as follows.Section 2 illustrates the experimental set-up and instrumentation, followed by Section 3 with the validation tests.The results and discussions are presented in Section 4. Finally, the findings are concluded in Section 5.

Wave Flume and Coordinate System
Experiments were performed in a wave flume (14.00 m long, 0.25 m wide, and 0.50 m high) at the Department of Civil Engineering, National Chung Hsing University, Taiwan.A piston-type wave maker (WM-PT-S, Chen Hseng Machine Co., Ltd., Taichung, Taiwan, ROC), placed at one end of the flume and had a maximum stroke of 49.0 cm, was actuated by a servomotor to follow the wave-plate trajectory (Goring [28]) and produce a solitary wave.As reported by Lin et al. [15,[18][19][20][21][22][23][24][25][26], the generation of a satisfactory solitary wave by the wave maker was highly repeatable, together with the dispersive tail-wave train almost indiscernible.
A Cartesian coordinate system, (x, y), is set up with its origin (0, 0) on the horizontal bed surface (Figure 1).Axis x is along the flume and axis y is upward from the surface of the bed.The specified measuring section (SMS), x = 0, is positioned 800.0 cm from the wave maker at rest.Let t denote time, t s = (h 0 /g) 1/2 a representative time scale with g being the gravity acceleration, thus leading to the dimensionless time, T [= t/t s = t × (g/h 0 ) 1/2 ].It should be mentioned that t = 0 specifies the time instant when the wave crest passes the SMS.The velocity components, (u, v) = (u[x, y, t], v[x, y, t]), represent the horizontal and vertical velocities of water particles along the (x, y) directions, respectively.
The outline of this paper is as follows.Section 2 illustrates the experimental set-up and instrumentation, followed by Section 3 with the validation tests.The results and discussions are presented in Section 4. Finally, the findings are concluded in Section 5.

Wave Flume and Coordinate System
Experiments were performed in a wave flume (14.00 m long, 0.25 m wide, and 0.50 m high) at the Department of Civil Engineering, National Chung Hsing University, Taiwan.A piston-type wave maker (WM-PT-S, Chen Hseng Machine Co., Ltd., Taichung, Taiwan, ROC), placed at one end of the flume and had a maximum stroke of 49.0 cm, was actuated by a servomotor to follow the wave-plate trajectory (Goring [28]) and produce a solitary wave.As reported by Lin et al. [15,[18][19][20][21][22][23][24][25][26], the generation of a satisfactory solitary wave by the wave maker was highly repeatable, together with the dispersive tail-wave train almost indiscernible.
A Cartesian coordinate system, (x, y), is set up with its origin (0, 0) on the horizontal bed surface (Figure 1).Axis x is along the flume and axis y is upward from the surface of the bed.The specified measuring section (SMS), x = 0, is positioned 800.0 cm from the wave maker at rest.Let t denote time, ts = (h0/g) 1/2 a representative time scale with g being the gravity acceleration, thus leading to the dimensionless time, T [= t/ts = t × (g/h0) 1/2 ].It should be mentioned that t = 0 specifies the time instant when the wave crest passes the SMS.The velocity components, (u, v) = (u[x, y, t], v[x, y, t]), represent the horizontal and vertical velocities of water particles along the (x, y) directions, respectively.

Wave Gauge, HSPIV, and Flow Observation
As shown in Figure 1, two ultrasonic wave gauges (Banner U-Gage S18U, Banner Engineering Corp., Tenth Avenue North, MN, USA) were placed at x = −150.0cm and the SMS measured the free surface elevations, η(x, t).The former was utilized as a reference signal to trigger the start of data sampling when it reached the preset threshold (i.e., a specified voltage somewhat smaller than the one corresponding to the wave crest).The latter obtained the temporal variation of free surface elevation, η(0, t), and then determined H0 at the SMS.
Instantaneous velocity fields were measured by an HSPIV system, comprising a high-speed digital camera and a laser head.A 5 W argon-ion laser (Coherent Innova-90, Santa Clara, CA, USA) was the light source.A cylindrical lens expanded the laser beam from the laser head to form a 1.5 mm thick fan-shaped light sheet, which was then directed

Wave Gauge, HSPIV, and Flow Observation
As shown in Figure 1, two ultrasonic wave gauges (Banner U-Gage S18U, Banner Engineering Corp., Tenth Avenue North, MN, USA) were placed at x = −150.0cm and the SMS measured the free surface elevations, η(x, t).The former was utilized as a reference signal to trigger the start of data sampling when it reached the preset threshold (i.e., a specified voltage somewhat smaller than the one corresponding to the wave crest).The latter obtained the temporal variation of free surface elevation, η(0, t), and then determined H 0 at the SMS.
Instantaneous velocity fields were measured by an HSPIV system, comprising a highspeed digital camera and a laser head.A 5 W argon-ion laser (Coherent Innova-90, Santa Clara, CA, USA) was the light source.A cylindrical lens expanded the laser beam from the laser head to form a 1.5 mm thick fan-shaped light sheet, which was then directed upwards via the bottom glass along the half span of the wave flume.Titanium dioxide particles (mean diameter 1.8 µm) were uniformly seeded into the water volume in the flume.The digital camera (Phantom VEO640 or Phantom V5.1, Vision Research Inc., Wayne, NJ, USA), with a maximum framing rate of 500/1000/1600-2500 Hz at a resolution of (1152 × 1152)/(1024 × 512)/(2048 × 1152) pixel, captured the instantaneous particle-laden images.Before commencing the cross-correlation computation for the instantaneous velocity fields, the Laplacian edge-enhancement technique (Adrain and Westerweel [29]) and contrast enhancement method (Cowen and Monismith [30]) were utilized to brighten the traced particles and sharpen the particle edges in the images.The HSPIV algorithm allowed the instantaneous velocity field to be computed from a pair of images, starting with an interrogation window from 64 × 64 pixels and ending at 8 × 8 pixels.
This study aims to reveal the effect of the nonlinearity of solitary waves on horizontal velocities, particle accelerations, and pressure gradients in the FSZs, located in the upper portions of near-bottom zones.Therefore, the fields of view of the camera in velocity measurements (and also for instantaneous flow observation) by the HSPIV were set with distinct sizes as listed in Table 1, and their centers were located at the SMS.The framing rate of the camera was set at 1600-2500 or 1000 or 500 Hz, depending on the experimental conditions.Note that the images captured by the HSPIV also provide the information of instantaneous flow field with the "particle-dotted" pattern, which is beneficial to precisely identify the time for the occurrence of the incipient flow reversal (i.e., water particles starting to move opposite to the wave propagation direction) exactly above the bed.A total of 20 repeated runs for the velocity measurements were performed for each case.The particle-trajectory tracking method helped visualize the flow structure in the nearbottom zone, with the purpose to observe the commencement of flow reversal right beyond the surface of the horizontal bed.The camera, laser light sheet, and seeding particles used for this technique were all the same as those for velocity measurements by the HSPIV.The fields of view for flow observations had a size of (2.00-2.11)cm in length and (1.00-1.18)cm in height, centered at the SMS.For each case, a total of three repeated runs were carried out to capture the flow fields characterized by the "pathlined" pattern.The sampling rates of the camera were set at 30-100 Hz, depending on the experimental conditions (see Table 1).

Experimental Conditions
Nine experimental cases were tested with H 0 in the range 0.4-4.4cm but at the same h 0 = 8.0 cm, thus H 0 /h 0 = 0.05-0.55(Cases A-I).The ratio reflects a solitary wave's transition from a quasi-linear state to a highly nonlinear one [11,15,17,18,[23][24][25].To acquire the time series of instantaneous horizontal velocity in the free-stream zone at the SMS, a symmetric 11-point smoothing scheme with distinct weightings was used to eliminate noises in the velocity data.The ensemble-average method was then employed for the 20 repeated runs to obtain the time series of ensemble-averaged horizontal velocity.Table 1 summarizes the nine experimental conditions.For all cases, the values of the linear and nonlinear wave celerity, C [= (gh 0 ) 1/2 ] and C 0 (= [g(h 0 + H 0 )] 1/2 ), as well as the counterparts of C 0 /C are listed in Table 1.The relationship between C 0 /C and H 0 /h 0 is regressed to be C 0 /C = (0.4546H 0 /h 0 + 1.0).For Case E, only the flow observation using particle-trajectory Water 2022, 14, 3609 5 of 24 tracking was conducted without performing velocity measurement by the HSPIV.These un-done counterparts were previously made throughout the full water depth and partially reported in Lin et al. [18][19][20].In this study, elucidation of the kinematic and hydrodynamic features for Case E was achieved by mining the data banks that were set up during the experimental stage of the three studies.

Validation Tests
Figure 2 presents a comparison of the original time series of free surface elevation η o (t) for five runs, which are randomly selected from a total of 20 runs for Case E. Their profiles of the time series almost collapse onto each other without prominent discernible distinction.The fact still holds true for the other eight experimental cases.The evidence demonstrates the high repeatability of the solitary waves generated by the wave maker.A comparison of the time series of ensemble-averaged free surface elevation η(t) (i.e., obtained by phaseaveraging over 20 repeated runs) with those predicted by solitary wave theory [31] is illustrated in Figure 3 for Case E. The trend of measured data in η(t) is in good accordance with that predicted by solitary wave theory.
11-point smoothing scheme with distinct weightings was used to eliminate noises in the velocity data.The ensemble-average method was then employed for the 20 repeated runs to obtain the time series of ensemble-averaged horizontal velocity.Table 1 summarizes the nine experimental conditions.For all cases, the values of the linear and nonlinear wave celerity, C [= (gh0) 1/2 ] and C0 (= [g(h0 + H0)] 1/2 ), as well as the counterparts of C0/C are listed in Table 1.The relationship between C0/C and H0/h0 is regressed to be C0/C = (0.4546H0/h0 + 1.0).For Case E, only the flow observation using particle-trajectory tracking was conducted without performing velocity measurement by the HSPIV.These un-done counterparts were previously made throughout the full water depth and partially reported in Lin et al. [18][19][20].In this study, elucidation of the kinematic and hydrodynamic features for Case E was achieved by mining the data banks that were set up during the experimental stage of the three studies.

Validation Tests
Figure 2 presents a comparison of the original time series of free surface elevation ηo(t) for five runs, which are randomly selected from a total of 20 runs for Case E. Their profiles of the time series almost collapse onto each other without prominent discernible distinction.The fact still holds true for the other eight experimental cases.The evidence demonstrates the high repeatability of the solitary waves generated by the wave maker.A comparison of the time series of ensemble-averaged free surface elevation η(t) (i.e., obtained by phase-averaging over 20 repeated runs) with those predicted by solitary wave theory [31] is illustrated in Figure 3 for Case E. The trend of measured data in η(t) is in good accordance with that predicted by solitary wave theory.velocity data.The ensemble-average method was then employed for the 20 repeated runs to obtain the time series of ensemble-averaged horizontal velocity.Table 1 summarizes the nine experimental conditions.For all cases, the values of the linear and nonlinear wave celerity, C [= (gh0) 1/2 ] and C0 (= [g(h0 + H0)] 1/2 ), as well as the counterparts of C0/C are listed in Table 1.The relationship between C0/C and H0/h0 is regressed to be C0/C = (0.4546H0/h0 + 1.0).For Case E, only the flow observation using particle-trajectory tracking was conducted without performing velocity measurement by the HSPIV.These un-done counterparts were previously made throughout the full water depth and partially reported in Lin et al. [18][19][20].In this study, elucidation of the kinematic and hydrodynamic features for Case E was achieved by mining the data banks that were set up during the experimental stage of the three studies.

Validation Tests
Figure 2 presents a comparison of the original time series of free surface elevation ηo(t) for five runs, which are randomly selected from a total of 20 runs for Case E. Their profiles of the time series almost collapse onto each other without prominent discernible distinction.The fact still holds true for the other eight experimental cases.The evidence demonstrates the high repeatability of the solitary waves generated by the wave maker.A comparison of the time series of ensemble-averaged free surface elevation η(t) (i.e., obtained by phase-averaging over 20 repeated runs) with those predicted by solitary wave theory [31] is illustrated in Figure 3 for Case E. The trend of measured data in η(t) is in good accordance with that predicted by solitary wave theory.Further, for Case E, Figure 4a,b demonstrates two ensemble-averaged velocity fields for 1.0 cm ≤ y ≤ 8.0 cm at t = −0.045and 0.045 s, respectively.The measurement errors of these velocities, obtained by using HSPIV, are estimated herein by the mass flux method (Chang and Liu [32]).Based on the two-dimensional flow field, a check is first done by calculating the mass flux of each measuring grid, M f = |∂u/∂x + ∂v/∂y| × dA.Note that, due to mass conservation and flow continuity, M f should be equal to zero theoretically.For the velocity field for Case E, the grid lengths are ∆x = ∆y = 0.1167 cm, and the corresponding area is dA = ∆x × ∆y = 0.0136 cm 2 .A representative mass flux, M fr [= (u wc ) max × ∆y], is designated with (u wc ) max [= 36.94 cm/s] being the maximum horizontal velocity at wave crest for T = 0.The relative error is then defined by M f /M fr .The values of M f /M fr are found to be below 2.0%.Moreover, for a typical horizontal velocity of 20.0 or 40.0 cm/s in the flow field, the measurement error of horizontal velocity is about 0.3 or 0.46 cm/s (Lin et al. [18,22,24]), rendering the relative error to be 1.5% or 1.2%.These two pieces of evidence do indicate high precision in the velocity measurements utilizing the HSPIV system.
Further, for Case E, Figure 4a,b demonstrates two ensemble-averaged velocity fields for 1.0 cm ≤ y ≤ 8.0 cm at t = −0.045and 0.045 s, respectively.The measurement errors of these velocities, obtained by using HSPIV, are estimated herein by the mass flux method (Chang and Liu [32]).Based on the two-dimensional flow field, a check is first done by calculating the mass flux of each measuring grid, Mf = |∂u/∂x + ∂v/∂y| × dA.Note that, due to mass conservation and flow continuity, Mf should be equal to zero theoretically.For the velocity field for Case E, the grid lengths are Δx = Δy = 0.1167 cm, and the corresponding area is dA = Δx × Δy = 0.0136 cm 2 .A representative mass flux, Mfr [= (uwc)max × Δy], is designated with (uwc)max [= 36.94 cm/s] being the maximum horizontal velocity at wave crest for T = 0.The relative error is then defined by Mf/Mfr.The values of Mf/Mfr are found to be below 2.0%.Moreover, for a typical horizontal velocity of 20.0 or 40.0 cm/s in the flow field, the measurement error of horizontal velocity is about 0.3 or 0.46 cm/s (Lin et al. [18,22,24]), rendering the relative error to be 1.5% or 1.2%.These two pieces of evidence do indicate high precision in the velocity measurements utilizing the HSPIV system.As pointed out by Lee et al. [17] and Lin et al. [20], the Boussinesq solution incorporated with C in the calculation provides a good prediction of flow velocity.For Case E, Figure 5a-c show comparisons of the ensemble-averaged horizontal velocities u(t) obtained at y = 7.04, 4.40, and 1.44 cm, respectively, with those predicted by the Boussinesq solution.Good accordance between the experimental data and the theoretic predictions is evidenced, reconfirming the satisfactorily precise velocity measurements by the HSPIV system.Moreover, Figure 6a-c presents the comparisons between the time series of local acceleration Al(t) (calculated from the time series of the ensemble-averaged horizontal velocity) and those predicted by wave theory at y = 7.04, 4.40, and 1.44 cm.Note that the details of the calculation procedure for the former are addressed later in Sec.4.3, and the latter is directly computed by taking the time derivative of the horizontal velocity predicted by the Boussinesq solution.Not only for the entire trend but also for the individual magnitudes in these time series, fairly good agreement is thus confirmed.As pointed out by Lee et al. [17] and Lin et al. [20], the Boussinesq solution incorporated with C in the calculation provides a good prediction of flow velocity.For Case E, Figure 5a-c show comparisons of the ensemble-averaged horizontal velocities u(t) obtained at y = 7.04, 4.40, and 1.44 cm, respectively, with those predicted by the Boussinesq solution.Good accordance between the experimental data and the theoretic predictions is evidenced, reconfirming the satisfactorily precise velocity measurements by the HSPIV system.Moreover, Figure 6a-c

presents the comparisons between the time series of local acceleration
A l (t) (calculated from the time series of the ensemble-averaged horizontal velocity) and those predicted by wave theory at y = 7.04, 4.40, and 1.44 cm.Note that the details of the calculation procedure for the former are addressed later in Sec.4.3, and the latter is directly computed by taking the time derivative of the horizontal velocity predicted by the Boussinesq solution.Not only for the entire trend but also for the individual magnitudes in these time series, fairly good agreement is thus confirmed.Water 2022, 14, 3609 8 of 24

Results and Discussion
A series of experimental results of geometric, kinematic, and hydrodynamic features with dimensionless forms are presented in the following.The representative length and time scales considered are h 0 or [h 0 + H 0 ] (or simply H 0 ) and (h 0 /g) 1/2 or [(h 0 + H 0 )/g] 1/2 .The two scales result in the representative velocity scale being C = (gh 0 ) 1/2 or C 0 (= [g(h 0 + H 0 )] 1/2 ), which is equivalent to the linear or nonlinear wave celerity.The length and time scales selected herein are h 0 and (h 0 /g) 1/2 , together with the use of H 0 to normalize the time series of free surface elevation, η(T).To highlight the nonlinear effect, the nonlinear wave celerity, C 0 (= [g(h 0 + H 0 )] 1/2 ), is particularly used as the velocity scale.Note that C 0 /C, =; (04546H 0 /h 0 , +; 10); as evidenced in Table 1, thus allowing prompt transformation between C 0 and C. Further, the acceleration (or pressure gradient) scale amounts to the ratio of the velocity scale to the time scale, i.e., equal to g.

Elucidation of FSZs from Velocity Profiles
To show the presence of the FSZ beyond the boundary layer and underlying the internal zone, it is appropriate to first use the velocity profiles all over the full water depth.
For Case E at the SMS (x/h 0 = 0), Figure 7a,b-h present the time series of dimensionless free surface elevation, η(T)/H 0 , as well as the profiles of dimensionless horizontal and vertical velocities, u(y/h 0 , T)/C 0 and v(y/h 0 , T)/C 0 , at T = −2.50,−1.39, −0.50, 0, 0.5, 1.39, and 2.50 (as marked by dashed lines with 1 − 7 in Figure 7a), respectively.The times before, at, and after the intersection of the wave crest with the SMS correspond to −6.00 ≤ T < 0, T = 0, and 0 < T ≤ 6.00, respectively, with an ascending, constant, and descending free surface.With reference to T = 0, the free surface elevations for −6.00 ≤ T < 0 are symmetric to those for 0 < T ≤ 6.00, exhibiting an even-function form with η(T)/H 0 = η(−T)/H 0 .In addition, as evidenced in Figure 7b-h, the horizontal velocities are positive with u(y/h 0 , T)/C 0 = u(y/h 0 , −T)/C 0 throughout the full water depth at the SMS.However, the vertical velocities are negative, zero, and positive with v(y/h 0 , T)/C 0 = −v(y/h 0 , −T)/C 0 , except for those very close to the bed.
As shown in Figure 7b or Figure 7h for T = −2.50 or 2.50, u(y/h 0 )/C 0 increases from zero at y/h 0 = 0 to 0.082 at y/h 0 = 0.022 or 0.031, characterizing the feature of the boundary layer.It then remains almost constant for y/h 0 = 0.031-0.35,as indicated by the two horizontal dotted lines within which the uniform horizontal velocities exist.This layer is herein termed the FSZ within which the horizontal velocities are denoted as the free-stream velocities.For y/h 0 > 0.35, u(y/h 0 )/C 0 decreases linearly to 0.066 at y/h 0 = y fs /h 0 = 1.08 (i.e., the instantaneous free surface).It is worth mentioning that, for −6.00 ≤ T < −1.39 or 1.39 < T ≤ 6.00 and y/h 0 > 0.35, all the dimensionless horizontal velocity profiles exhibit similar shapes to that at T = −2.50 or 2.50.In particular, the upper portions have a nonuniform, linearly decreasing trend.This differs from the known recognition of long waves having a uniform horizontal velocity profile all over the full water depth.As seen in Figure 7c or Figure 7g at T = −1.39 or 1.39 (i.e., at phase corresponding to the "inflection point" in Figure 7a), u(y/h 0 )/C 0 ranges from zero at the bed to 0.163 at y/h 0 = 0.024 or 0.029, then remains nearly unchanged from the lower limit of FSZ (at y/h 0 = 0.03) to the free surface.
Further, for −1.39 < T ≤ 0 or 0 ≤ T < 1.39 (Figure 7d,e or Figure 7e,f), the free-stream velocity prevails at y/h 0 = 0.026-0.35or 0.027-0.35.Especially, for y/h 0 > 0.35, a distribution with a nonlinear increase in the horizontal velocity is evidenced.The fact is again contrary to the uniform horizontal profile along the full water depth, assumed frequently in the theoretical prediction or numerical simulation of long-wave propagation.The maximum values of dimensionless horizontal velocity, [u(y/h 0 , T)] max /C 0 , occur at T = 0 with the FSZ located at y/h 0 = 0.027-0.35.Note that the extreme value of [u(y/h 0 , 0)] max /C 0 , taking place right at the wave crest (i.e., y/h 0 = y crest /h 0 = 1.363), is equal to 0.357, greater than those (= 0.251) in the FSZ.
As also shown in Figure 7b-d for −6.00 ≤ T < 0 or Figure 7f-h for 0 < T ≤ 6.00, the dimensionless vertical velocity in each profile, v(y/h 0 , T)/C 0 , linearly varies from zero at the bed to a certain positive or negative maximum, [v(y fs /h 0 , T)] max+ /C 0 or [v(y fs /h 0 , T)] max− /C 0 , at the instantaneous free surface.The profile with all positive or negative maxima, [v(y/h 0 )] max+ /C 0 or [v(y/h 0 )] max− /C 0 (= −[v(y/h 0 )] max+ /C 0 < 0), occurs at T = −1.39 or 1.39 (i.e., at the phase of the inflection point in Figure 7a).Further, at a given y/h 0 and −6.00 ≤ T ≤ −1.39 or 1.39 ≤ T ≤ 6.00, v(T)/C 0 increases from nearly zero to [v(y/h 0 )] max+ /C 0 or decreasing from [v(y/h 0 )] max− /C 0 to almost zero.As a contrast, for −1.39 ≤ T ≤ 1.39, it does decrease from the maximum, [v(y/h 0 )] max+ /C 0 , via zero at T = 0, to [v(y/h 0 )] max− /C 0 .It should be emphasized that, at the SMS and with increasing y/h 0 values, the "magnitudes" of vertical velocity remain increasing in the FSZ, in which the unsteady free-stream velocities are almost identical.Figure 8a-g show close-ups for the temporal variation of horizontal and vertical v locity profiles which are obtained in both the boundary layer and the FSZ at T = −2.5 −1.39, −0.50, 0, 0.50, 1.39, and 2.50 for Case E. As similarly indicated in Figure 7b-h, th uniform free-stream velocities uo(T) exist in the FSZ situated inside the range within th two horizontal dotted lines.Further, the boundary layer is located inside the zone betwee the lower horizontal dotted line and the bed (i.e., y/h0 = 0).The boundary layer thickne is defined herewith as a specified height measured upwards from the bed to the heigh where the horizontal velocity u(T) at the edge of the boundary layer is equal to 0.99uo(T as shown at the lower horizontal dotted lines in Figure 8a-g.Note that, as primarily ev denced in Figure 8a to Figure 8g, the boundary layer thickness does increase slightly as varies from −6.00 to 6.00.More details relevant to the kinematic features and unique sim ilarity profiles in the boundary layer flows can be referred to in Lin et al. [18].7b-h, the uniform free-stream velocities u o (T) exist in the FSZ situated inside the range within the two horizontal dotted lines.Further, the boundary layer is located inside the zone between the lower horizontal dotted line and the bed (i.e., y/h 0 = 0).The boundary layer thickness is defined herewith as a specified height measured upwards from the bed to the height where the horizontal velocity u(T) at the edge of the boundary layer is equal to 0.99u o (T), as shown at the lower horizontal dotted lines in Figure 8a-g.Note that, as primarily evidenced in Figure 8a to Figure 8g, the boundary layer thickness does increase slightly as T varies from −6.00 to 6.00.More details relevant to the kinematic features and unique similarity profiles in the boundary layer flows can be referred to in Lin et al. [18].For Case E at the SMS, Figure 9 presents the time series of dimensionless horizontal velocity u(T)/C0 obtained at eight heights for y/h0 = 0.03-0.55.For any one of the time series, horizontal velocity increases from near zero to a maximum of −6.00 ≤ T < 0, indicative of the temporal acceleration in the horizontal direction.For 0 < T ≤ 6.00, u(T)/C0 decreases from its maximum to near zero, suggestive of the temporal deceleration in the horizontal direction.The two features highlight the temporal acceleration equal to zero for T = 0, corresponding to which the maximum horizontal velocity occurs.For y/h0 = 0.05-0.35and T = 0, the maximum u/C0 values are almost the same (about 0.251), smaller than those at y/h0 = 0.39-0.55 in the internal zone and larger than those at y/h0 = 0.03-0.04 in the bottom boundary layer.Further, the temporal variation in u(T)/C0 at y/h0 = 0.05 or 0.35 collapses completely onto those at y/h0 = 0.06-0.31.These evidences strongly indicate that the FSZ exists for y/h0 = 0.05-0.35, in which the magnitudes of free-stream velocities are nearly identical and the horizontal velocity profile is uniform.For the cases examined excluding Case E, the FSZs are located between y/h0 = (0.035~0.055) and (0.335~0.366), which are almost equivalent to y/h0 = 0.05-0.350for Case E. For ease of data analysis, the flow velocities that represent kinematic features of FSZs are mined from the velocity fields only between y/h0 = 0.06 and 0.330.For Case E at the SMS, Figure 9 presents the time series of dimensionless horizontal velocity u(T)/C 0 obtained at eight heights for y/h 0 = 0.03-0.55.For any one of the time series, horizontal velocity increases from near zero to a maximum of −6.00 ≤ T < 0, indicative of the temporal acceleration in the horizontal direction.For 0 < T ≤ 6.00, u(T)/C 0 decreases from its maximum to near zero, suggestive of the temporal deceleration in the horizontal direction.The two features highlight the temporal acceleration equal to zero for T = 0, corresponding to which the maximum horizontal velocity occurs.For y/h 0 = 0.05-0.35and T = 0, the maximum u/C 0 values are almost the same (about 0.251), smaller than those at y/h 0 = 0.39-0.55 in the internal zone and larger than those at y/h 0 = 0.03-0.04 in the bottom boundary layer.Further, the temporal variation in u(T)/C 0 at y/h 0 = 0.05 or 0.35 collapses completely onto those at y/h 0 = 0.06-0.31.These evidences strongly indicate that the FSZ exists for y/h 0 = 0.05-0.35, in which the magnitudes of free-stream velocities are nearly identical and the horizontal velocity profile is uniform.For the cases examined excluding Case E, the FSZs are located between y/h 0 = (0.035~0.055) and (0.335~0.366), which are almost equivalent to y/h 0 = 0.05-0.350for Case E. For ease of data analysis, the flow velocities that represent kinematic features of FSZs are mined from the velocity fields only between y/h 0 = 0.06 and 0.330.In the following, the data of the nine cases (H0/h0 = 0.050-0.550,Table 1) are analyzed to explore the effect of nonlinearity (or H0/h0) not only on the dimensionless free surface elevations; but also on the dimensionless free-stream velocities, horizontal accelerations and pressure gradients in the FSZs of solitary waves.

Nonlinear Effect on Free Surface Elevation and Free-stream Velocity
Figure 10 shows a systematic comparison of the relationships between η(T)/H0 and T for Cases A-I (H0/h0 = 0.050-0.550),along with a comparison of the measured data with the solitary wave theory [31] for each case.It is found that the experimental data are in good agreement with the analytic results for all cases.With an increase in H0/h0, the dimensionless free surface elevation becomes more focused around T = 0 with a narrower bell shape, exhibiting the H0/h0 (or nonlinearity) effect on the free surface profile.In other words, for a solitary wave with a larger H0/h0, a shorter time is taken to generate a complete wave motion.For example, for η(T)/H0 = 5%, the corresponding dimensionless times are examined to be |T| = 11.47,5.85, 4.12, and 2.85 for Case A (H0/h0 = 0.050), Case C (H0/h0 = 0.179), Case E (H0/h0 = 0.363), and Case I (H0/h0 = 0.550).The fact demonstrates the larger the H0/h0, the narrower the temporal range of wave shape.From the physical point of view, this trend shows that the change of ascending or descending free surface elevation per unit time becomes greater in magnitude.Namely, the free-surface slope ∂[η(T)/H0]/∂T gets larger at a specified T at a greater H0/h0, except those at η(T) = 0 and η(0) = H0.It is noted that, as shown in Figure 11 for −14.0 ≤ T ≤ 14.0, the averaged free surface elevation [η(T)/H0]mean decreases with an increase in H0/h0, rendering the relationship expressed by [η(T)/H0]mean = 0.0737(H0/h0) −0.545 .
(1) In the following, the data of the nine cases (H 0 /h 0 = 0.050-0.550,Table 1) are analyzed to explore the effect of nonlinearity (or H 0 /h 0 ) not only on the dimensionless free surface elevations; but also on the dimensionless free-stream velocities, horizontal accelerations and pressure gradients in the FSZs of solitary waves.

Nonlinear Effect on Free Surface Elevation and Free-Stream Velocity
Figure 10 shows a systematic comparison of the relationships between η(T)/H 0 and T for Cases A-I (H 0 /h 0 = 0.050-0.550),along with a comparison of the measured data with the solitary wave theory [31] for each case.It is found that the experimental data are in good agreement with the analytic results for all cases.With an increase in H 0 /h 0 , the dimensionless free surface elevation becomes more focused around T = 0 with a narrower bell shape, exhibiting the H 0 /h 0 (or nonlinearity) effect on the free surface profile.In other words, for a solitary wave with a larger H 0 /h 0 , a shorter time is taken to generate a complete wave motion.For example, for η(T)/H 0 = 5%, the corresponding dimensionless times are examined to be |T| = 11.47,5.85, 4.12, and 2.85 for Case A (H 0 /h 0 = 0.050), Case C (H 0 /h 0 = 0.179), Case E (H 0 /h 0 = 0.363), and Case I (H 0 /h 0 = 0.550).The fact demonstrates the larger the H 0 /h 0 , the narrower the temporal range of wave shape.From the physical point of view, this trend shows that the change of ascending or descending free surface elevation per unit time becomes greater in magnitude.Namely, the free-surface slope ∂[η(T)/H 0 ]/∂T gets larger at a specified T at a greater H 0 /h 0 , except those at η(T) = 0 and η(0) = H 0 .It is noted that, as shown in Figure 11 for −14.0 ≤ T ≤ 14.0, the averaged free surface elevation [η(T)/H 0 ] mean decreases with an increase in H 0 /h 0 , rendering the relationship expressed by [η(T)/H 0 ] mean = 0.0737(H 0 /h 0 ) −0.545 . ( The result is attributable to [η(T)/H0]max = 1.0 for all experimental cases and the larger value of H0 involved in the greater magnitude (i.e., higher nonlinearity) of H0/h0 at the same h0 (= 8.0 cm).  Figure 12 presents a thorough comparison of the temporal variations of dimensionless free-stream velocities in the FSZs, uo(T)/C0, for Cases A-I (H0/h0 = 0.050-0.550).The nonlinear wave celerity C0 (= [g(H0 + h0)] 1/2 ) is used herewith as the velocity scale.It is found that, at a larger H0/h0, the time series of uo(T)/C0 becomes more concentrated around T = 0 with a sharper bell shape.Less time is thus taken to achieve a complete variation of the free-stream velocities in the FSZ.The fact exhibits the H0/h0 (or nonlinearity) effect on the temporal distribution of free-stream velocity.Taking uo(T)/C0 = 0.5% for instance, the dimensionless times are identified to be about |T| = 9.56, 6.77, 5.58, and 4.65 for Case A (H0/h0 = 0.050), Case C (H0/h0 = 0.179), Case E (H0/h0 = 0.363), and Case I (H0/h0 = 0.550), respectively.These data do testify to the relatively narrower temporal range of free-stream velocity induced by a solitary wave with a greater value of H0/h0.At a specified dimensionless time, say |T| < 3.95, this trend does show that the change rate of free-stream velocity, ∂[uo(T)/C0]/∂T (equivalent to the dimensionless local acceleration in the FSZ), The result is attributable to [η(T)/H0]max = 1.0 for all experimental cases and the larger value of H0 involved in the greater magnitude (i.e., higher nonlinearity) of H0/h0 at the same h0 (= 8.0 cm).  Figure 12 presents a thorough comparison of the temporal variations of dimensionless free-stream velocities in the FSZs, uo(T)/C0, for Cases A-I (H0/h0 = 0.050-0.550).The nonlinear wave celerity C0 (= [g(H0 + h0)] 1/2 ) is used herewith as the velocity scale.It is found that, at a larger H0/h0, the time series of uo(T)/C0 becomes more concentrated around T = 0 with a sharper bell shape.Less time is thus taken to achieve a complete variation of the free-stream velocities in the FSZ.The fact exhibits the H0/h0 (or nonlinearity) effect on the temporal distribution of free-stream velocity.Taking uo(T)/C0 = 0.5% for instance, the dimensionless times are identified to be about |T| = 9.56, 6.77, 5.58, and 4.65 for Case A (H0/h0 = 0.050), Case C (H0/h0 = 0.179), Case E (H0/h0 = 0.363), and Case I (H0/h0 = 0.550), respectively.These data do testify to the relatively narrower temporal range of free-stream velocity induced by a solitary wave with a greater value of H0/h0.At a specified dimensionless time, say |T| < 3.95, this trend does show that the change rate of free-stream velocity, ∂[uo(T)/C0]/∂T (equivalent to the dimensionless local acceleration in the FSZ), The result is attributable to [η(T)/H 0 ] max = 1.0 for all experimental cases and the larger value of H 0 involved in the greater magnitude (i.e., higher nonlinearity) of H 0 /h 0 at the same h 0 (= 8.0 cm).

Nonlinear Effect on Local and Convective Accelerations in FSZ
For a solitary wave traveling over a horizontal bed, the local and convective accelerations in the horizontal direction [33,34] are defined as: And Within the FSZ, the unsteady free-stream velocities uo(x, y, t) [= uo(x, t)] are uniform in the vertical direction, thus leading to ∂uo(x, t)/∂y = 0. Therefore, at the SMS and in the FSZ, the local and convective accelerations in the horizontal direction are simplified as: Alo(0, t) = ∂uo(0, t)/∂t ≈ Δuo(0, t)/Δt = Alo(t) (6) And Aco(0, t) = uo(0, t) × ∂uo(0, t)/∂x ≈ uo(0, t) × Δuo(0, t)/Δx = Aco(t).(7) Accordingly, the values of Alo(t) and Aco(t) can be computed, using a central difference scheme, from the instantaneous free-stream velocities obtained at the SMS and in its close neighborhood.From a theoretical point of view, the time and spatial interval, Δt and Δx, should be as small as possible (i.e., approaching zero).However, as reported by Lin et al. [19,20,25,27], even the use of the smallest time or spatial interval in the differential computation would result in failure to find a convergent Alo(t) or Aco(t) value.The reason is attributable to the rapid temporal or spatial fluctuations in the "image-based (or pixelbased)" PIV/HSPIV measurements.
Following the method proposed by Lin et al. [25,27] a range of the promising time or spatial intervals, in which the relative deviation of each computed outcome of Alo(t) or Aco(t) would only vary less than 4.0% of its average, is selected.Detailed illustrations for the trial-and-error calculation of Alo(t) and Aco(t) with different Δt or Δx values, together with the determination of promising time or spatial interval, can be referred to Lin et al. [25,27].In this study, the promising time or interval used is equal to (Δt)promising = 0.01-0.0156s, (i.e., 25 times (Δt)framing [= 1/2500-1/1600 s = 0.0004-0.000625s]) or (Δx)promising = 0.10712-0.248cm (i.e., 13-31 times the grid size (Δx)gs [= (Δy)gs = 0.00824 or 0.008 cm] used in the HSPIV measurements).It should be emphasized that previous investigations [25,27,35] did indicate (Δt)promising or (Δx)promising much larger than Δtframing or (Δx)gs, but not as small as (Δt)framing or (Δx)gs.Like the data processing used in Jensen et al. [35], a symmetric 7-point smoothing scheme with distinct weightings was utilized to smoothen the time series of Alo(t) or Aco(t).
At the SMS, Figure 15 displays a complete comparison of the temporal variations of dimensionless local acceleration in the FSZs, Alo/g, for Cases A-I (H0/h0 = 0.050-0.550).Note

Nonlinear Effect on Local and Convective Accelerations in FSZ
For a solitary wave traveling over a horizontal bed, the local and convective accelerations in the horizontal direction [33,34] are defined as: And Within the FSZ, the unsteady free-stream velocities u o (x, y, t) [= u o (x, t)] are uniform in the vertical direction, thus leading to ∂u o (x, t)/∂y = 0. Therefore, at the SMS and in the FSZ, the local and convective accelerations in the horizontal direction are simplified as: And Accordingly, the values of A lo (t) and A co (t) can be computed, using a central difference scheme, from the instantaneous free-stream velocities obtained at the SMS and in its close neighborhood.From a theoretical point of view, the time and spatial interval, ∆t and ∆x, should be as small as possible (i.e., approaching zero).However, as reported by Lin et al. [19,20,25,27], even the use of the smallest time or spatial interval in the differential computation would result in failure to find a convergent A lo (t) or A co (t) value.The reason is attributable to the rapid temporal or spatial fluctuations in the "image-based (or pixelbased)" PIV/HSPIV measurements.
Following the method proposed by Lin et al. [25,27] a range of the promising time or spatial intervals, in which the relative deviation of each computed outcome of A lo (t) or A co (t) would only vary less than 4.0% of its average, is selected.Detailed illustrations for the trialand-error calculation of A lo (t) and A co (t) with different ∆t or ∆x values, together with the determination of promising time or spatial interval, can be referred to Lin et al. [25,27].In this study, the promising time or interval used is equal to (∆t) promising = 0.01-0.0156s, (i.e., 25 times (∆t) framing [= 1/2500-1/1600 s = 0.0004-0.000625s]) or (∆x) promising = 0.10712-0.248cm (i.e., 13-31 times the grid size (∆x) gs [= (∆y) gs = 0.00824 or 0.008 cm] used in the HSPIV measurements).It should be emphasized that previous investigations [25,27,35] did indicate (∆t) promising or (∆x) promising much larger than ∆t framing or (∆x) gs , but not as small as (∆t) framing or (∆x) gs .Like the data processing used in Jensen et al. [35], a symmetric 7-point smoothing scheme with distinct weightings was utilized to smoothen the time series of A lo (t) or A co (t).
At the SMS, Figure 15 displays a complete comparison of the temporal variations of dimensionless local acceleration in the FSZs, A lo /g, for Cases A-I (H 0 /h 0 = 0.050-0.550).Note that A lo (T)/g ≈ −A lo (−T)/g, exhibits the odd-function feature.As addressed in Figure 8, the free-stream velocity increases from near zero to a maximum for −6.00 ≤ T < 0 or decreases from the maximum to about zero for 0 < T ≤ 6.00, underscoring the temporal acceleration or deceleration in the FSZ.Accordingly, for all cases at the SMS, A lo /g is positive for −6.00 ≤ T < 0 and negative for 0 < T ≤ 6.00.Note that A lo /g = 0 for T = 0, at which the wave crest moves right over the SMS and the free-stream velocity arrives at its maximum.It is interesting to mention that the averaged value of the dimensionless local acceleration, [A lo (T)/g] mean , is equal to almost zero for −6.00 ≤ T ≤ 6.00, which is attributable to the time series of A lo (T)/g featured with an odd-function form.
Water 2022, 14, x FOR PEER REVIEW 16 that Alo(T)/g ≈ −Alo(−T)/g, exhibits the odd-function feature.As addressed in Figure 8, free-stream velocity increases from near zero to a maximum for −6.00 ≤ T < 0 or decre from the maximum to about zero for 0 T ≤ 6.00, underscoring the temporal accelera or deceleration in the FSZ.Accordingly, for all cases at the SMS, Alo/g is positive for − ≤ T < 0 and negative for 0 < T ≤ 6.00.Note that Alo/g = 0 for T = 0, at which the wave c moves right over the SMS and the free-stream velocity arrives at its maximum.It is in esting to mention that the averaged value of the dimensionless local accelerat [Alo(T)/g]mean, is equal to almost zero for −6.00 ≤ T ≤ 6.00, which is attributable to the t series of Alo(T)/g featured with an odd-function form.As also seen in Figure 15, the positive and negative maxima of Alo/g, Alo+/g, and A (≈ −Alo+/g < 0), highlight almost equal magnitude at the dimensionless characteristic t T = TAlo+ (< 0) and T = TAlo− (> 0), respectively.As illustrated in Figures 7 and 8, the tran tion of free-stream velocity is in phase with the free-surface motion of a solitary w Therefore, TAlo+ and TAlo− correspond, in fact, to the inflection points in the time serie both η(T)/H0 and uo(T)/C0.Namely, the slopes of η(T)/H0 and uo(T)/C0 (i.e., ∂[η(T)/H0 and ∂[uo(T)/C0]/∂T) exhibit the local maxima apparently at TAlo+ and TAlo−.As evidence Figure 16, Alo+/g and Alo _ /g increase linearly, and TAlo+ and TAlo− decrease with an increa H0/h0 (or nonlinearity).For example, Alo+/g (= −Alo _ /g = 0.0434, 0.1075, and 0.1523) for C C, E and I are about 6.11, 15.14, and 21.45 times that (=0.0071) for Case A, demonstra the obvious nonlinear effect on Alo+/g and Alo _ /g.The reason is assignable to η(T)/H0 uo(T)/C0 both becoming more concentrated around T = 0 with narrower symmetric shapes for a greater H0/h0.Namely, if H0/h0 gets larger, a shorter time is taken to achie complete motion with a greater value of the maximum free-stream velocity (i.e., tran ing with relatively larger |Δu| under the same Δt), thus resulting in a larger ∂[uo(T)/C0] Figure 17 shows a systematic comparison of the time series of dimensionless con tive acceleration in the FSZs, Aco/g, for Cases A-I (H0/h0 = 0.050-0.550).As illustrate Figure 3, t < 0, t = 0, and t > 0 correspond to the times before, at, and after the wave c intersection with the SMS.This leads to the maximum free-stream velocity, [uo(x)]max, pearing at x < 0, x = 0 or x > 0; and subsequently, ∂uo(t)/∂x < 0, ∂uo(0)/∂x = 0 or ∂uo(t)/∂x at the SMS.Accordingly, Aco/g (= [uo(t) × ∂uo(0, t)/∂x]/g) takes negative, zero, or posi values for T < 0, T = 0, or T > 0, as observed in Figure 17 almost with the odd-function fea [i.e., Aco(T)/g ≈ −Aco(−T)/g].For all cases, the magnitudes of negative and positive max As also seen in Figure 15, the positive and negative maxima of A /g, A lo+ /g, and A lo _ /g (≈ −A lo+ /g < 0), highlight almost equal magnitude at the dimensionless characteristic time T = T Alo+ (< 0) and T = T Alo− (> 0), respectively.As illustrated in Figures 7 and 8, the translation of free-stream velocity is in phase with the free-surface motion of a solitary wave.Therefore, T Alo+ and T Alo− correspond, in fact, to the inflection points in the time series of both η(T)/H 0 and u o (T)/C 0 .Namely, the slopes of η(T)/H 0 and u o (T)/C 0 (i.e., ∂[η(T)/H 0 ]/∂T and ∂[u o (T)/C 0 ]/∂T) exhibit the local maxima apparently at T Alo+ and T Alo− .As evidenced in Figure 16, A lo+ /g and A lo _ /g increase linearly, and T Alo+ and T Alo− decrease with an increasing H 0 /h 0 (or nonlinearity).For example, A lo+ /g (= −A lo _ /g = 0.0434, 0.1075, and 0.1523) for Cases C, E and I are about 6.11, 15.14, and 21.45 times that (= 0.0071) for Case A, demonstrating the obvious nonlinear effect on A lo+ /g and A lo _ /g.The reason is assignable to η(T)/H 0 and u o (T)/C 0 both becoming more concentrated around T = 0 with narrower symmetric bell shapes for a greater H 0 /h 0 .Namely, if H 0 /h 0 gets larger, a shorter time is taken to achieve a complete motion with a greater value of the maximum free-stream velocity (i.e., translating with relatively larger |∆u| under the same ∆t), thus resulting in a larger ∂[u o (T)/C 0 ]/∂T.= TAco+ ≈ 1.090 < 1.390 ≈ −TAlo+ = TAlo−.The fact points out that the dimensionless times the occurrence of Aco−/g and Aco+/g are closer to T = 0 than the counterparts of Alo+/g Alo _ /g.Further, Aco−/g and Aco+/g are about 1/22.0-1/4.3times Alo+/g and Alo _ /g, justify much more contribution from the local acceleration than from the convection accelera in the pressure gradient.Further, the averaged value of the dimensionless convective celeration, [Aco(T)/g]mean, is equal to almost zero for −6.00 ≤ T ≤ 6.00 because the time se of Aco(T)/g is characterized by an odd-function form.

Nonlinear Effect on Pressure Gradient in FSZ
Based on the incompressible Navier-Stokes equations [33,34] for a solitary wave tr eling over a horizontal bed, the gravity term is equal to zero and the contribution of viscous term is negligibly small [19,20,25] and thus omitted.Accordingly, the horizo pressure gradient in the FSZ at the SMS is expressed as: (1/ρ)∂po(0, t)/∂x = −[∂uo(0, t)/∂t + uo(0, t) × ∂uo(0, t)/∂x] Figure 17 shows a systematic comparison of the time series of dimensionless convective acceleration in the FSZs, A co /g, for Cases A-I (H 0 /h 0 = 0.050-0.550).As illustrated in Figure 3, t < 0, t = 0, and t > 0 correspond to the times before, at, and after the wave crest intersection with the SMS.This leads to the maximum free-stream velocity, [u o (x)] max , appearing at x < 0, x = 0 or x > 0; and subsequently, ∂u o (t)/∂x < 0, ∂u o (0)/∂x = 0 or ∂u o (t)/∂x > 0 at the SMS.Accordingly, A co /g (= [u o (t) × ∂u o (0, t)/∂x]/g) takes negative, zero, or positive values for T < 0, T = 0, or T > 0, as observed in Figure 17    Based on the incompressible Navier-Stokes equations [33,34] for a solitary wave tr eling over a horizontal bed, the gravity term is equal to zero and the contribution of viscous term is negligibly small [19,20,25] and thus omitted.Accordingly, the horizon pressure gradient in the FSZ at the SMS is expressed as:

Nonlinear Effect on Pressure Gradient in FSZ
Based on the incompressible Navier-Stokes equations [33,34] for a solitary wave traveling over a horizontal bed, the gravity term is equal to zero and the contribution of the viscous term is negligibly small [19,20,25] and thus omitted.Accordingly, the horizontal pressure gradient in the FSZ at the SMS is expressed as: Thus, P o (t) takes a minus value of [A lo (t) + A co (t)] (i.e., the force per unit mass in the horizontal direction).With the magnitude of A lo (t) being much larger than A co (t), A lo (t) dominates the contribution to P o (t).
Figure 18 shows a thorough comparison of the temporal variations in dimensionless pressure gradient P o (T)/g for all cases.The characteristic dimensionless times for the occurrence of a negative maximum [P o (T)/g] max− (= P o _ /g) and a positive maximum [P o (T)/g] max+ (= P o+ /g) are defined as T Po− (< 0) and T Po+ (> 0), respectively.Similar to the variation feature of A lo (T)/g (see Figure 15), an odd-function form of P o (T)/g is examined by P o+ /g ≈ −P o _ /g and T Po+ = T Alo− ≈ −T Alo+ = −T Po− .P o (T)/g decreases from near zero to P o _ /g for −6.00 ≤ T ≤ T Po− or from P o+ /g to about zero for T Po+ ≤ T ≤ 6.00, indicating an increase in the favorable pressure gradient or a decrease in the adverse pressure gradient in the FSZ.Further, it increases from P o _ /g (< 0), via 0, to P o+ /g (> 0) for T Po− ≤ T < 0, T = 0, and 0 < T ≤ T Po+ , respectively, demonstrating that P o (T)/g varies correspondingly from favorable, via zero, to the adverse pressure gradient.Note that, for −6.00 ≤ T ≤ 6.00, the averaged value of the dimensionless pressure gradient, [P o (T)/g] mean , is equal to almost zero because the time series of P o (T)/g is featured with an odd-function form.
Water 2022, 14, x FOR PEER REVIEW 18 o Thus, Po(t) takes a minus value of [Alo(t) + Aco(t)] (i.e., the force per unit mass in horizontal direction).With the magnitude of Alo(t) being much larger than Aco(t), Alo(t) do inates the contribution to Po(t).
Figure 18 shows a thorough comparison of the temporal variations in dimension pressure gradient Po(T)/g for all cases.The characteristic dimensionless times for the currence of a negative maximum [Po(T)/g]max− (= Po _ /g) and a positive maxim [Po(T)/g]max+ (= Po+/g) are defined as TPo− (< 0) and TPo+ (> 0), respectively.Similar to variation feature of Alo(T)/g (see Figure 15), an odd-function form of Po(T)/g is examined Po+/g ≈ −Po _ /g and TPo+ = TAlo− ≈ −TAlo+ = −TPo−.Po(T)/g decreases from near zero to Po _ /g −6.00 ≤ T ≤ TPo− or from Po+/g to about zero for TPo+ ≤ T ≤ 6.00, indicating an increase in favorable pressure gradient or a decrease in the adverse pressure gradient in the F Further, it increases from Po _ /g (< 0), via 0, to Po+/g (> 0) for TPo− ≤ T < 0, T = 0, and 0 < TPo+, respectively, demonstrating that Po(T)/g varies correspondingly from favorable, zero, to the adverse pressure gradient.Note that, for −6.00 ≤ T ≤ 6.00, the averaged va of the dimensionless pressure gradient, [Po(T)/g]mean, is equal to almost zero because time series of Po(T)/g is featured with an odd-function form.Here an interesting issue arises relevant to whether the occurrence of incipient f reversal precedent to, simultaneous with, or later than the appearance of a maximum verse pressure gradient.Namely, to examine TPo+ > Tifr, TPo+ = Tifr, or TPo+ < Tifr.In this stu Tifr is determined from the flow-visualized images.To well observe the incipient flow versals with "particle-dotted" and "path-lined" images for all cases, the ranges of fram rate of the camera were set at 1600-2500 for the HSPIV measurements and 30-100 Hz flow visualization tests.The former enhanced the temporal resolution (i.e., 1/2500-1/1 s) in precisely identifying Tifr from the continuous display (with frame-by-frame operat of the recorded images.The latter provided visual evidence related to the tempo Figure 19 outlines the relationships of P o+ /g (≈ −P o _ /g) and T Po+ (≈ −T Po− ) versus H 0 /h 0 .It is found that P o+ /g increases and T Po+ , however, decreases with an increasing H 0 /h 0 .Namely, at a larger H 0 /h 0 value, P o+ /g becomes greater and takes place closer to T = 0 (i.e., the "phase" with wave crest right passing through the SMS).For example, for Cases C, E, and I, P o+ /g = 0.0350, 0.0887, and 0.1210 appear at T Po+ = 0.1674, 0.1049, and 0.0943, respectively.The values of P o+ /g are 5.74, 14.54, and 19.84 times that of Case A (= 0.0061, with T Po+ = 0.2463), underlining the distinct effect of nonlinearity on P o (T)/g and P o+ /g.variation of flow structure in the near-bottom zone.Together with a concise description of other cases, the observation results are here detailed only for Case E. For ease of understanding, an original motion picture displaying Case E's flow structure in Supplementary Material, in which eight images of the instantaneous flow structure are drawn and then elucidated as follows.At the SMS and for 0 ≤ T ≤ 6.09, Figure 20a-h presents a series of the instantaneous path-lined images of the flow field (left panels) and the schematic diagrams of the velocity profile (right panels) within −0.10 ≤ x/h0 ≤ 0.10 and 0 ≤ y/h0 ≤ 0.10 for Case E. The water particles in the FSZ all move from the left to right side in each image, as evidenced by the temporal variation of free-stream velocities [i.e., uo(T) > 0] at y/h0 = 0.05-0.35(Figure 8).The longer the length of each pathline, the larger the magnitude of each particle velocity.As observed in Figure 20a, the free-stream velocity reaches its maximum and Po/g = 0 at T = 0. Immediately after the passing of wave crest through the SMS, say 0 < T ≤ 1.67, the lengths and the corresponding free-stream velocities in the FSZ decrease with an increasing T (see Figure 20b,c for T = 0.89 and 1.76).It is thus indicated that the flow in the nearbottom zone, subjected to adverse pressure gradients, does decelerate temporally.
Based on detailed observation from a continuous display (with forward and backward motions) of both path-lined and instantaneously particle-dotted images at the SMS, the local horizontal velocities are found to be almost zero for y/h0 < 0.004 at T = Tifr = 1.67 (Figure 20c).After T > Tifr, flow reversal occurs right beyond the bed.Namely, the pathlines located very near the bed start translating from right to left, indicating that the particle velocities are negative, see Figure 20d-h at T = 2.55-6.09.The counterparts observed in the FSZ move, however, from left to right.This reveals that the corresponding particle velocities are positive, but keep decreasing with T. The thickness of the flow reversal layer increases with an increasing T if T > Tifr.This feature shows that the particle velocities right on the bed and exactly at the edge of flow reversal are both zero with negative velocities in between.Interestingly note that, for Case E, the incipient flow reversal occurs at T = Tifr = 1.67 (or t = tifr = 0.1508 s), soon after but not simultaneously with the occurrence of Po+/g at T = TPo+ = TAlo+ = 1.39 (or t = tPo+ = tAlo+ = 0.1255 s).The reason for such a slight temporal delay with (ΔT)delay = 0.28 [or (Δt)delay = 0.0253 s] is attributable to the viscous damping effect in the bottom boundary layer.Here an interesting issue arises relevant to whether the occurrence of incipient flow reversal precedent to, simultaneous with, or later than the appearance of a maximum adverse pressure gradient.Namely, to examine T Po+ > T ifr , T Po+ = T ifr , or T Po+ < T ifr .In this study, T ifr is determined from the flow-visualized images.To well observe the incipient flow reversals with "particle-dotted" and "path-lined" images for all cases, the ranges of framing rate of the camera were set at 1600-2500 for the HSPIV measurements and 30-100 Hz for flow visualization tests.The former enhanced the temporal resolution (i.e., 1/2500-1/1600 s) in precisely identifying T ifr from the continuous display (with frameby-frame operation) of the recorded images.The latter provided visual evidence related to the temporal variation of flow structure in the near-bottom zone.Together with a concise description of other cases, the observation results are here detailed only for Case E. For ease of understanding, an original motion picture displaying Case E's flow structure in Supplementary Material, in which eight images of the instantaneous flow structure are drawn and then elucidated as follows.
At the SMS and for 0 ≤ T ≤ 6.09, Figure 20a-h presents a series of the instantaneous path-lined images of the flow field (left panels) and the schematic diagrams of the velocity profile (right panels) within −0.10 ≤ x/h 0 ≤ 0.10 and 0 ≤ y/h 0 ≤ 0.10 for Case E. The water particles in the FSZ all move from the left to right side in each image, as evidenced by the temporal variation of free-stream velocities [i.e., u o (T) > 0] at y/h 0 = 0.05-0.35(Figure 8).The longer the length of each pathline, the larger the magnitude of each particle velocity.As observed in Figure 20a, the free-stream velocity reaches its maximum and P o /g = 0 at T = 0. Immediately after the passing of wave crest through the SMS, say 0 < T ≤ 1.67, the lengths and the corresponding free-stream velocities in the FSZ decrease with an increasing T (see Figure 20b,c for T = 0.89 and 1.76).It is thus indicated that the flow in the near-bottom zone, subjected to adverse pressure gradients, does decelerate temporally.
Based on detailed observation from a continuous display (with forward and backward motions) of both path-lined and instantaneously particle-dotted images at the SMS, the local horizontal velocities are found to be almost zero for y/h 0 < 0.004 at T = T ifr = 1.67 (Figure 20c).After T > T ifr , flow reversal occurs right beyond the bed.Namely, the pathlines located very near the bed start translating from right to left, indicating that the particle velocities are negative, see Figure 20d-h at T = 2.55-6.09.The counterparts observed in the FSZ move, however, from left to right.This reveals that the corresponding particle velocities are positive, but keep decreasing with T. The thickness of the flow reversal layer increases with an increasing T if T > T ifr .This feature shows that the particle velocities right on the bed and exactly at the edge of flow reversal are both zero with negative velocities in between.Interestingly note that, for Case E, the incipient flow reversal occurs at T = T ifr = 1.67 (or t = t ifr = 0.1508 s), soon after but not simultaneously with the occurrence  Till now, both the path-lined images (Figure 20a-h) and the temporal variations of [P o (T)]/g (Figure 18) have been demonstrated for the first time to shed light on the relationship between T ifr and T Po+ (= T Alo+ ).For each visualized case other than Case E, the incipient flow reversal does take place immediately after the maximum adverse pressure gradient.
In other words, T ifr is slightly larger than T Po+ .Finally, the relationship between T ifr and H 0 /h 0 is also shown in Figure 19, indicating T ifr decreasing prominently with an increase in H 0 /h 0 .This demonstrates the effect of nonlinearity on the incipient flow reversal in the near-bottom zone.

Conclusions
In this study, with respect to the nonlinear effect on the kinematic and hydrodynamic features in the FSZs of solitary waves for H 0 /h 0 = 0.050-0.550,a series of experimental results at the SMS are presented, systematically showing the results of the relevant flow parameters.The findings are summarized as follows.

1.
For all the cases exclusive of Case E, the FSZs are positioned between y/h 0 = (0.035-0.055) and (0.335-0.366), nearly identical to those between y/h 0 = 0.05 and 0.350 in Case E.

2.
If H 0 /h 0 increases, the dimensionless free surface elevation, η(T)/H 0 , and the dimensionless free stream velocity, u o (T)/C 0 , become more concentrated around T = 0 with a narrower symmetric bell-shape, exhibiting shorter time taken to generate a complete wave motion.This trend indicates that the change of ascending or descending free surface elevation per unit (dimensionless) time becomes greater in magnitude.

3.
For −6.00 ≤ T < 0 and 0 < T ≤ 6.00, the dimensionless free-stream velocity, [u o (T)/C 0 ], increases from near zero to a maximum and decreases from the maximum to about zero, highlighting the temporal acceleration and deceleration in the FSZ. 4.
The relationship between [u o /C 0 ] max and H 0 /h 0 is uniquely expressed in Equation ( 3), stating that the former gets large with an increasing H 0 /h 0 .For H 0 /h 0 = 0.179, 0.363 and 0.550, the values of [u o /C 0 ] max are about 3.10, 5.32, and 6.20 times that (= 0.0473) for H 0 /h 0 = 0.050.This trend demonstrates the nonlinear effect on [u o /C 0 ] max . 5.
The dimensionless local acceleration, A lo /g, is positive for −6.00 ≤ T < 0 and negative for 0 < T ≤ 6.00.At T = 0 with wave crest intersecting the SMS, A lo /g is equal to zero and the free-stream velocity reaches its maximum.6.
The magnitudes of positive and negative maxima in the dimensionless local acceleration, A lo+ /g and A lo− /g (≈ −A lo+ /g), increase linearly, and the counterparts of the dimensionless characteristic time T Alo+ (< 0)and T Alo− (> 0) decrease with an increase in H 0 /h 0 .For H 0 /h 0 = 0.179, 0.363 and 0.550, the values of A lo+ /g are about 6.11, 15.14 and 21.45 times that (= 0.0071) for H 0 /h 0 = 0.050, indicating the nonlinear effect on A lo+ /g and A lo− /g. 7.
The magnitudes of negative and positive maxima in the dimensionless convective acceleration, A co− /g, and A co+ /g, increase when H 0 /h 0 increases.However, their magnitudes are about 1/22.0-1/4.3times those of A lo+ /g and A lo− /g.With the magnitude of A co (T)/g being much smaller than that of A lo (T)/g, the contribution to the dimensionless pressure gradient, P o (T)/g (= −[A lo (T) + A co (T)]/g), is thus governed mainly by A lo (T)/g.8.
P o (T)/g decreases from near zero to P o _ /g (< 0) for −6.00 ≤ T ≤ T Po− or from P o+ /g (≈ −P o _ /g > 0) to about zero for T Po+ ≤ T ≤ 6.00, exhibiting an increase in the favorable pressure gradient or decrease in the adverse pressure gradient in the FSZ.Moreover, it increases from P o _ /g, via 0, to P o+ /g for T Po− ≤ T < 0, T = 0, and 0 < T ≤ T Po+ , indicating the change from favorable, via zero, to an adverse pressure gradient.9.
With an increase in H 0 /h 0 , P o+ /g increases but T Po+ decreases.For H 0 /h 0 = 0.179, 0.363 and 0.550, the values of P o+ /g are about 5.74, 14.54 and 19.84 times that (= 0.0061) for H 0 /h 0 = 0.050, showing the strong nonlinear effect on P o (T)/g and P o+ /g.

Figure 1 .
Figure 1.Schematic diagram showing a solitary wave propagating over a horizontal bed, two wave gauges, the (x, y) coordinate system, and (u, v) velocity components.

Figure 1 .
Figure 1.Schematic diagram showing a solitary wave propagating over a horizontal bed, two wave gauges, the (x, y) coordinate system, and (u, v) velocity components.

Figure 2 .
Figure 2. A comparison among five repeated runs for the original time series of free surface elevation ηo(t) (Case E).

Figure 3 .
Figure 3.A comparison of time series between ensemble-averaged free surface elevation η(t) and predicted one (Case E).

Figure 2 .
Figure 2. A comparison among five repeated runs for the original time series of free surface elevation η o (t) (Case E).

Figure 2 .
Figure 2. A comparison among five repeated runs for the original time series of free surface elevation ηo(t) (Case E).

Figure 3 .
Figure 3.A comparison of time series between ensemble-averaged free surface elevation η(t) and predicted one (Case E).

Figure 3 .
Figure 3.A comparison of time series between ensemble-averaged free surface elevation η(t) and predicted one (Case E).

Figure 5 .
Figure 5.Time series comparisons of horizontal velocity between measured and predicted using Boussinesq solution (Case E) at y = (a) 7.04 cm; (b) 4.40 cm; and (c) 1.44 cm.

Figure 6 .
Figure 6.Time series comparisons between calculated local accelerations (which are based on time series of measured horizontal velocities) and predicted ones using Boussinesq solution (Case E) at y = (a) 7.04 cm; (b) 4.40 cm; and (c) 1.44 cm.

Figure 6 .
Figure 6.Time series comparisons between calculated local accelerations (which are based on time series of measured horizontal velocities) and predicted ones using Boussinesq solution (Case E) at y = (a) 7.04 cm; (b) 4.40 cm; and (c) 1.44 cm.

Figure 6 .
Figure 6.Time series comparisons between calculated local accelerations (which are based on time series of measured horizontal velocities) and predicted ones using Boussinesq solution (Case E) at y = (a) 7.04 cm; (b) 4.40 cm; and (c) 1.44 cm.

Figure
Figure8a-g show close-ups for the temporal variation of horizontal and vertical velocity profiles which are obtained in both the boundary layer and the FSZ at T = −2.50,−1.39, −0.50, 0, 0.50, 1.39, and 2.50 for Case E. As similarly indicated in Figure7b-h, the uniform free-stream velocities u o (T) exist in the FSZ situated inside the range within the two horizontal dotted lines.Further, the boundary layer is located inside the zone between the lower horizontal dotted line and the bed (i.e., y/h 0 = 0).The boundary layer thickness is defined herewith as a specified height measured upwards from the bed to the height where the horizontal velocity u(T) at the edge of the boundary layer is equal to 0.99u o (T), as shown at the lower horizontal dotted lines in Figure8a-g.Note that, as primarily evidenced in Figure8ato Figure8g, the boundary layer thickness does increase slightly as T varies from −6.00 to 6.00.More details relevant to the kinematic features and unique similarity profiles in the boundary layer flows can be referred to in Lin et al.[18].

Figure 8 .
Figure 8. Temporal variation of the horizontal velocity profile obtained in the boundary layer and the FSZ at T = (a) −2.50;(b) −1.39; (c) −0.50;(d) 0; (e) 0.50; (f) 1.39; and (g) 2.50 (CaseE).Note that the FSZ is located within the region as marked by the two horizontal dashed lines, and that the internal zone or boundary layer is situated beyond the upper or beneath the lower horizontal dashed line.

Figure 8 .
Figure 8. Temporal variation of the horizontal velocity profile obtained in the boundary layer and the FSZ at T = (a) −2.50;(b) −1.39; (c) −0.50;(d) 0; (e) 0.50; (f) 1.39; and (g) 2.50 (CaseE).Note that the FSZ is located within the region as marked by the two horizontal dashed lines, and that the internal zone or boundary layer is situated beyond the upper or beneath the lower horizontal dashed line.

Figure 10 .
Figure 10.A comparison of temporal variations in the non-dimensional free surface elevation for H0/h0 = 0.050-0.550.

Figure 10 .
Figure 10.A comparison of temporal variations in the non-dimensional free surface elevation for H 0 /h 0 = 0.050-0.550.

Figure 10 .
Figure 10.A comparison of temporal variations in the non-dimensional free surface elevation for H0/h0 = 0.050-0.550.
free-stream velocity, ∂[u o (T)/C 0 ]/∂T (equivalent to the dimensionless local acceleration in the FSZ), becomes larger in magnitude.For T < 0 or T > 0, ∂[u o (T)/C 0 ]/∂T is positive or negative, indicative of flow acceleration or deceleration in the FSZ for each case.

Figure 12 .
Figure 12.A comparison of temporal variations in the dimensionless free-stream velocity for H 0 /h 0 = 0.050-0.550.

Figure 12 .
Figure 12.A comparison of temporal variations in the dimensionless free-stream velocity fo = 0.050-0.550.

Figure 13 .Figure 13 .
Figure 13.Relationship between the dimensionless maximum free-stream velocity and H0/h0 Figure 13.Relationship between the dimensionless maximum free-stream velocity and H 0 /h 0 .

Figure 15 .
Figure 15.A comparison of temporal variations in the non-dimensional local acceleration for H = 0.050-0.550.

Figure 15 .
Figure 15.A comparison of temporal variations in the non-dimensional local acceleration for H 0 /h 0 = 0.050-0.550.

Figure 17 .
Figure 17.A comparison of temporal variations in the non-dimensional convective acceleration H0/h0 = 0.050-0.550.Note that the vertical scale used herein is distinct from that shown in Figure

Figure 16 .
Figure 16.The relationships of T Alo+ and T Alo− as well as A lo+ /g and A lo _ /g versus H 0 /h 0 .
Figure17shows a systematic comparison of the time series of dimensionless convective acceleration in the FSZs, A co /g, for Cases A-I (H 0 /h 0 = 0.050-0.550).As illustrated in Figure3, t < 0, t = 0, and t > 0 correspond to the times before, at, and after the wave crest intersection with the SMS.This leads to the maximum free-stream velocity, [u o (x)] max , appearing at x < 0, x = 0 or x > 0; and subsequently, ∂u o (t)/∂x < 0, ∂u o (0)/∂x = 0 or ∂u o (t)/∂x > 0 at the SMS.Accordingly, A co /g (= [u o (t) × ∂u o (0, t)/∂x]/g) takes negative, zero, or positive values for T < 0, T = 0, or T > 0, as observed in Figure17almost with the odd-function feature [i.e., A co (T)/g ≈ −A co (−T)/g].For all cases, the magnitudes of negative and positive maxima in the dimensionless convective acceleration, A co− /g and A co+ /g, occur at the dimensionless characteristic time T = T Aco− (< 0) and T = T Aco+ (> 0), and increase with increasing H 0 /h 0 .Note that |T Aco− | and T Aco+ are smaller than |T Alo+ | and T Alo− .For example, for Case E, −T Aco− = T Aco+ ≈ 1.090 < 1.390 ≈ −T Alo+ = T Alo−.The fact points out that the dimensionless times for the occurrence of A co− /g and A co+ /g are closer to T = 0 than the counterparts of A lo+ /g and A lo _ /g.Further, A co− /g and A co+ /g are about 1/22.0-1/4.3times A lo+ /g and A lo _ /g, justifying much more contribution from the local acceleration than from the convection acceleration in the pressure gradient.Further, the averaged value of the dimensionless convective acceleration, [A co (T)/g] mean , is equal to almost zero for −6.00 ≤ T ≤ 6.00 because the time series of A co (T)/g is characterized by an odd-function form.

Figure 17 .
Figure 17.A comparison of temporal variations in the non-dimensional convective acceleration H0/h0 = 0.050-0.550.Note that the vertical scale used herein is distinct from that shown in Figure 4.4.Nonlinear Effect on Pressure Gradient in FSZ

Figure 17 .
Figure 17.A comparison of temporal variations in the non-dimensional convective acceleration for H 0 0 = 0.050-0.550.Note that the vertical scale used herein is distinct from that shown in Figure 15.

Figure 18 .
Figure 18.A comparison of temporal variations in the non-dimensional pressure gradient of the stream for H0/h0 = 0.050-0.550.

Figure 19
Figure 19 outlines the relationships of Po+/g (≈ −Po _ /g) and TPo+ (≈ −TPo−) versus H It is found that Po+/g increases and TPo+, however, decreases with an increasing H Namely, at a larger H0/h0 value, Po+/g becomes greater and takes place closer to T = 0 ( the "phase" with wave crest right passing through the SMS).For example, for Cases C and I, Po+/g = 0.0350, 0.0887, and 0.1210 appear at TPo+ = 0.1674, 0.1049, and 0.0943, resp tively.The values of Po+/g are 5.74, 14.54, and 19.84 times that of Case A (= 0.0061, with = 0.2463), underlining the distinct effect of nonlinearity on Po(T)/g and Po+/g.Here an interesting issue arises relevant to whether the occurrence of incipient f reversal precedent to, simultaneous with, or later than the appearance of a maximum verse pressure gradient.Namely, to examine TPo+ > Tifr, TPo+ = Tifr, or TPo+ < Tifr.In this stu Tifr is determined from the flow-visualized images.To well observe the incipient flow versals with "particle-dotted" and "path-lined" images for all cases, the ranges of fram rate of the camera were set at 1600-2500 for the HSPIV measurements and 30-100 Hz flow visualization tests.The former enhanced the temporal resolution (i.e., 1/2500-1/1 s) in precisely identifying Tifr from the continuous display (with frame-by-frame operat of the recorded images.The latter provided visual evidence related to the tempo

Figure 18 .
Figure 18.A comparison of temporal variations in the non-dimensional pressure gradient of the free stream for H 0 /h 0 = 0.050-0.550.

Figure 19 .
Figure 19.Variation trends of the non-dimensional times for occurrence of flow reversal and maximum adverse pressure gradient, and of the values of maximum adverse pressure gradient for H0/h0 = 0.050-0.550.

Figure 19 .
Figure 19.Variation trends of the non-dimensional times for occurrence of flow reversal and maximum adverse pressure gradient, and of the values of maximum adverse pressure gradient for H 0 /h 0 = 0.050-0.550.

Table 1 .
A list of experimental conditions.
* Note that h 0 = 8.0 cm for all cases.