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Open AccessArticle

Dynamical Filtering Highlights the Seasonality of Surface-Balanced Motions at Diurnal Scales in the Eastern Boundary Currents

by

Antonio Quintana

,

Hector S. Torres

and

Jose Gomez-Valdes

Fluids 2022, 7(8), 271; https://doi.org/10.3390/fluids7080271 - 08 Aug 2022

Viewed by 1027

Abstract

Balanced motions (BM) and internal gravity waves (IGW) account for most of the kinetic energy budget and capture most of the vertical velocity in the ocean. However, estimating the contribution of BM to both issues at time scales of less than a day [...] Read more.

Balanced motions (BM) and internal gravity waves (IGW) account for most of the kinetic energy budget and capture most of the vertical velocity in the ocean. However, estimating the contribution of BM to both issues at time scales of less than a day is a challenge because BM are obscured by IGW. To study the BM regime, we outlined the implementation of a dynamical filter that separates both classes of motion. This study used a high-resolution global simulation to analyze the Eastern Boundary Currents during the winter and summer months. Our results confirm the feasibility of recovering BM dynamics at short time scales, emphasizing the diurnal cycle in winter and its dampening in summer due to local stratification that prevents large vertical excursion of the surface boundary layer. Our filter opens up new possibilities for more accurate estimation of the vertical exchanges of any tracers at any vertical level in the water column. Moreover, it could be a valuable tool for studies focused on wave–turbulence interactions in ocean simulations. Full article

(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)

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Open AccessArticle

Influence of Turbulence Effects on the Runup of Tsunami Waves on the Shore within the Framework of the Navier–Stokes Equations

by

,

,

and

Fluids 2022, 7(3), 117; https://doi.org/10.3390/fluids7030117 - 20 Mar 2022

Cited by 1 | Viewed by 1273

Abstract

This paper considers turbulence effects on tsunami runup on the shore in tsunami simulations using the system of three-dimensional Navier–Stokes equations. The turbulence effects in tsunami propagation and runup are studied by solving the problem of a wave propagating in a nonuniform-bottom pool [...] Read more.

This paper considers turbulence effects on tsunami runup on the shore in tsunami simulations using the system of three-dimensional Navier–Stokes equations. The turbulence effects in tsunami propagation and runup are studied by solving the problem of a wave propagating in a nonuniform-bottom pool and collapsing with a barrier. To solve this problem, we used the turbulence model, RANS SST (Reynolds-averaged Navier–Stokes shear stress transport). We compared the wave profiles at different times during wave propagation, runup, and collapse. To quantify the turbulence effects, we also compared the forces acting on the basin bottom. We demonstrated that the turbulence had almost no effect on the shape of the wave and the way of its propagation (except collapse). However, turbulence effects during the runup and collapse became noticeable and could boost the flow (increasing the pressure force and the total force) by up to 25 percent. Full article

(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)

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Open AccessArticle

Interannual Bottom-Intensified Current Thickening Observed on the Continental Slope Off the Southeastern Coast of Hokkaido, Japan

by

,

,

and

Fluids 2022, 7(2), 84; https://doi.org/10.3390/fluids7020084 - 19 Feb 2022

Cited by 2 | Viewed by 1462

Abstract

By rotary empirical orthogonal function and coastal-trapped wave mode analyses, we analyzed current velocity data, collected from 2001 to 2016. The data were obtained by an acoustic Doppler current profiler, deployed upward at a location of 41°39.909′ N, 144°20.695′ E, on a 2630-m [...] Read more.

By rotary empirical orthogonal function and coastal-trapped wave mode analyses, we analyzed current velocity data, collected from 2001 to 2016. The data were obtained by an acoustic Doppler current profiler, deployed upward at a location of 41°39.909′ N, 144°20.695′ E, on a 2630-m deep continental slope seabed off the southeastern coast of Hokkaido, Japan. The results indicate that the current intensifies toward the bottom and is directed nearly toward the shore, reaching an average speed of ~2.5 cm s⁻¹ just above the bottom. The thickness of the along-slope northward component of the bottom-intensified current varied within the range of 50–350 m. We found that the current thickness change was caused by oceanic barotropic disturbances, produced by the intensification of the Aleutian Low, largely related to the El Niño–Southern Oscillation and modified through the excitation of bottom-trapped modes of coastal-trapped waves. This finding improves the prediction accuracy of the the bottom-intensified current change, being beneficial for suspended sediment studies, construction and maintenance of marine structures, planning of deep drilling, and so on. Full article

(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)

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Open AccessArticle

Note on the Bulk Estimate of the Energy Dissipation Rate in the Oceanic Bottom Boundary Layer

by

Xiaozhou Ruan

Fluids 2022, 7(2), 82; https://doi.org/10.3390/fluids7020082 - 18 Feb 2022

Viewed by 1333

Abstract

The dissipation of the kinetic energy (KE) associated with oceanic flows is believed to occur primarily in the oceanic bottom boundary layer (BBL), where bottom drag converts the KE from mean flows to heat loss through irreversible mixing at molecular scales. Due to [...] Read more.

The dissipation of the kinetic energy (KE) associated with oceanic flows is believed to occur primarily in the oceanic bottom boundary layer (BBL), where bottom drag converts the KE from mean flows to heat loss through irreversible mixing at molecular scales. Due to the practical difficulties associated with direct observations on small-scale turbulence close to the seafloor, most up-to-date estimates on bottom drag rely on a simple bulk formula (

C_{d} U^{3}

) proposed by G.I. Taylor that relates the integrated BBL dissipation rate to a drag coefficient (

C_{d}

) as well as a flow magnitude outside of the BBL (U). Using output from several turbulence-resolving direct numerical simulations, it is shown that the true BBL-integrated dissipation rate is approximately 90% of that estimated using the classic bulk formula, applied here to the simplest scenario where a mean flow is present over a flat and hydrodynamically smooth bottom. It is further argued that Taylor’s formula only provides an upper bound estimate and should be applied with caution in the future quantification of BBL dissipation; the performance of the bulk formula depends on the distribution of velocity and shear stress near the bottom, which, in the real ocean, could be disrupted by bottom roughness. Full article

(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)

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Open AccessArticle

Effect of the Coastline Geometry on the Boundary Currents Intruding through the Gap

by

Joseph Kuehl

and

Vitalii A. Sheremet

Fluids 2022, 7(2), 71; https://doi.org/10.3390/fluids7020071 - 08 Feb 2022

Cited by 3 | Viewed by 1168

Abstract

The problem of a geophysical western boundary current negotiating a gap in its supporting boundary is considered. For traditional straight, parallel gaps, such systems are known to exhibit two dominant states, gap penetrating and leaping, with the transitional dynamics between states displaying hysteresis. [...] Read more.

The problem of a geophysical western boundary current negotiating a gap in its supporting boundary is considered. For traditional straight, parallel gaps, such systems are known to exhibit two dominant states, gap penetrating and leaping, with the transitional dynamics between states displaying hysteresis. However, for more complex geometries, such as angled or offset gap configurations, the question of multiple states and hysteresis is unresolved. In such cases, the inertia of the western boundary current is oriented into the gap, hence the assumption that increased inertia promotes gap penetrating loop current states. Here we address the problem numerically in an idealized setting. It is found that despite the inertia of the current being directed into the gap, for large western boundary current transport values, leaping states will be present. That is, we show here that the presence of multiple states with hysteresis for gap-leaping western boundary current systems is robust to both angled and offset gap geometries. Full article

(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)

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Open AccessArticle

The Impacts of Gustiness on Air–Sea Momentum Flux

by

Meng Lyu

,

Henry Potter

and

Clarence O. Collins

Fluids 2021, 6(10), 336; https://doi.org/10.3390/fluids6100336 - 22 Sep 2021

Cited by 1 | Viewed by 1017

Abstract

The exchange of momentum across the air–sea boundary is an integral component of the earth system and its parametrization is essential for climate and weather models. This study focuses on the impact of gustiness on the momentum flux using three months of direct [...] Read more.

The exchange of momentum across the air–sea boundary is an integral component of the earth system and its parametrization is essential for climate and weather models. This study focuses on the impact of gustiness on the momentum flux using three months of direct flux observations from a moored surface buoy. Gustiness, which quantifies the fluctuations of wind speed and direction, is shown to impact air–sea momentum fluxes. First, we put forward a new gustiness formula that simultaneously evaluates the impact of fluctuations in wind direction and speed. A critical threshold is established using a cumulative density function to classify runs as either gusty or non-gusty. We find that, during runs classified as gusty, the aerodynamic drag coefficient is increased up to 57% when compared to their non-gusty counterparts. This is caused by a correlated increase in vertical fluctuations during gusty conditions and explains variability in the drag coefficient for wind speeds up to 20 m/s. This increase in energy is connected with horizontal fluctuations through turbulent interactions between peaks in the turbulent spectra coincident with peaks in the wave spectra. We discus two potential mechanistic explanations. The results of this study will help improve the representation of gustiness in momentum flux parameterizations leading to more accurate ocean models. Full article

(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)

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Open AccessFeature PaperArticle

Moored Flux and Dissipation Estimates from the Northern Deepwater Gulf of Mexico

by

,

,

,

and

Albert J. Williams III

Fluids 2021, 6(7), 237; https://doi.org/10.3390/fluids6070237 - 30 Jun 2021

Cited by 3 | Viewed by 1343

Abstract

Results from a pilot program to assess boundary mixing processes along the northern continental slope of the Gulf of Mexico are presented. We report a novel attempt to utilize a turbulence flux sensor on a conventional mooring. These data document many of the [...] Read more.

Results from a pilot program to assess boundary mixing processes along the northern continental slope of the Gulf of Mexico are presented. We report a novel attempt to utilize a turbulence flux sensor on a conventional mooring. These data document many of the features expected of a stratified Ekman layer: a buoyancy anomaly over a height less than that of the unstratified Ekman layer and an enhanced turning of the velocity vector with depth. Turbulent stress estimates have an appropriate magnitude and are aligned with the near-bottom velocity vector. However, the Ekman layer is time dependent on inertial-diurnal time scales. Cross slope momentum and temperature fluxes have significant contributions from this frequency band. Collocated turbulent kinetic energy dissipation and temperature variance dissipation estimates imply a dissipation ratio of 0.14 that is not sensibly different from canonical values for shear instability (0.2). This mixing signature is associated with production in the internal wave band rather than frequencies associated with turbulent shear production. Our results reveal that the expectation of a quasi-stationary response to quasi-stationary forcing in the guise of eddy variability is naive and a boundary layer structure that does not support recent theoretical assumptions concerning one-dimensional models of boundary mixing. Full article

(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)

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Open AccessArticle

Direct Numerical Simulation of Sediment Transport in Turbulent Open Channel Flow Using the Lattice Boltzmann Method

by

,

,

and

Fluids 2021, 6(6), 217; https://doi.org/10.3390/fluids6060217 - 09 Jun 2021

Cited by 3 | Viewed by 2039

Abstract

The lattice Boltzmann method is employed to conduct direct numerical simulations of turbulent open channel flows with the presence of finite-size spherical sediment particles. The uniform particles have a diameter of approximately 18 wall units and a density of [...] Read more.

The lattice Boltzmann method is employed to conduct direct numerical simulations of turbulent open channel flows with the presence of finite-size spherical sediment particles. The uniform particles have a diameter of approximately 18 wall units and a density of

ρ_{p} = 2.65 ρ_{f}

, where

ρ_{p}

and

ρ_{f}

are the particle and fluid densities, respectively. Three low particle volume fractions

ϕ = 0.11 %

, 0.22%, and 0.44% are used to investigate the particle-turbulence interactions. Simulation results indicate that particles are found to result in a more isotropic distribution of fluid turbulent kinetic energy (TKE) among different velocity components, and a more homogeneous distribution of the fluid TKE in the wall-normal direction. Particles tend to accumulate in the near-wall region due to the settling effect and they preferentially reside in low-speed streaks. The vertical particle volume fraction profiles are self-similar when normalized by the total particle volume fractions. Moreover, several typical transport modes of the sediment particles, such as resuspension, saltation, and rolling, are captured by tracking the trajectories of particles. Finally, the vertical profiles of particle concentration are shown to be consistent with a kinetic model. Full article

(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)

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Graphical abstract

Open AccessArticle

Flow Past Mound-Bearing Impact Craters: An Experimental Study

by

Diego Gundersen

,

Gianluca Blois

and

Kenneth T. Christensen

Fluids 2021, 6(6), 216; https://doi.org/10.3390/fluids6060216 - 09 Jun 2021

Cited by 1 | Viewed by 1364

Abstract

An experimental investigation into the flow produced by mound-bearing impact craters is reported herein. Both an idealized crater and a scaled model of a real martian crater are examined. Measurements were performed using high-resolution planar particle image velocimetry (PIV) in a refractive-index matching [...] Read more.

An experimental investigation into the flow produced by mound-bearing impact craters is reported herein. Both an idealized crater and a scaled model of a real martian crater are examined. Measurements were performed using high-resolution planar particle image velocimetry (PIV) in a refractive-index matching (RIM) flow environment. Rendering the crater models optically invisible with this RIM approach provided unimpeded access to the flow around and within each crater model. Results showed that the mean flow within the idealized crater exhibits more structural complexity compared to its moundless counterpart. Second-order statistics highlighted regions of minimal and elevated turbulent stresses, the latter of which revealed a complex interaction between shear layers that are present at the upstream and downstream parts of the rim and the central mound. Periodic vortex shedding of quasi-spanwise vortices from the upstream rim was revealed by POD-filtered instantaneous flow fields. Vertical flapping of this shear layer resulted in vortices occasionally impinging on the inner wall of the downstream rim. Further, conditional averaging analysis suggested periodic lateral oscillations of wall-normal vortices within the crater rim region reminiscent of those observed for flow inside spherical dimples. These results have implications for intra- to extra-crater mass and momentum exchange, and for sediment transport processes. Lastly, experiments with the Gale Crater model showed both similarities with and differences from the primary flow features found for the idealized model. Full article

(This article belongs to the Special Issue Boundary Layer Processes in Geophysical/Environmental Flows)

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