Application of Large-Scale Rotating Platforms in the Study of Complex Oceanic Dynamic Processes

Abstract

As the core components of geophysical dynamic system, oceans and atmospheres are dominated by the Coriolis force, which governs complex dynamic phenomena such as internal waves, gravity currents, vortices, and others involving multi-scale spatiotemporal coupling. Due to the limitations of in situ observations, large-scale rotating tanks have emerged as critical experimental platforms for simulating Earth’s rotational effects. This review summarizes recent advancements in rotating tank applications for studying oceanic flow phenomena, including mesoscale eddies, internal waves, Ekman flows, Rossby waves, gravity currents, and bottom boundary layer dynamics. Advanced measurement techniques, such as particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), have enabled quantitative analyses of internal wave breaking-induced mixing and refined investigations of vortex merging dynamics. The findings demonstrate that large-scale rotating tanks provide a controllable experimental framework for unraveling the physical essence of geophysical fluid motions. Such laboratory experimental endeavors in a rotating tank can be applied to more extensive scientific topics, in which the rotation and stratification play important roles, offering crucial support for climate model parameterization and coupled ocean–land–atmosphere mechanisms.

1. Introduction

Covering approximately 71% of Earth’s surface, the ocean plays an irreplaceable role in global climate regulation, biogeochemical cycling, and biodiversity conservation. Complex oceanic flow phenomena—including internal waves, gravity currents, Ekman flows, vortices, Rossby waves, and bottom boundary layer dynamics—profoundly influence physical, chemical, and biological processes in marine systems [1,2]. A precise understanding of these phenomena is critical for advancing climate prediction, safeguarding marine ecosystems, optimizing resource exploitation, and ensuring maritime safety.

Geophysical fluid dynamics, the discipline governing large-scale oceanic and atmospheric motions, is fundamentally governed by the Coriolis force, involving intricate dynamics across multiple spatiotemporal scales. Yet the intrinsic complexity of marine environments, the vast disparities in spatiotemporal scales, and the prohibitive costs and technical challenges associated with in situ observations pose substantial obstacles to conducting systematic investigations in natural oceanic systems. Traditional methodologies for ocean observation face inherent limitations. Numerical simulations are constrained by computational resources and parameterization uncertainties, hindering their resolution of subgrid-scale processes [3,4]. In situ observations, on the other hand, often encounter problems with environmental interference, depth accessibility, and insufficient spatial coverage [5]. These challenges highlight the critical need for innovative experimental frameworks that enable the isolation and controlled analysis of individual forcing mechanisms driving oceanic currents.

As core facilities in experimental fluid mechanics, large-scale rotating platforms (>10 m diameter) overcome these limitations by enabling the precise control of rotational speed, water depth, topography, and stratification. These platforms replicate rotation-dominated oceanic environments with exceptional fidelity, providing a unique opportunity to investigate complex flow phenomena under controlled laboratory conditions [6]. By simulating Earth’s rotational effects, rotating platforms empower researchers to conduct detailed observations, measurements, and analyses of multi-scale hydrodynamic processes. This capability has significantly expanded the scope and depth of marine research, establishing rotating platforms as indispensable tools for advancing oceanographic theories and practical applications. Their integration into modern marine science not only provides new avenues for fundamental discoveries but also holds immense potential for addressing real-world challenges in climate resilience and sustainable ocean management.

2. Large-Scale Rotating Platform Experiments for Complex Oceanic Dynamics

2.1. Mesoscale Eddies

Oceanic eddies, as critical dynamic phenomena in the ocean, play a pivotal role in energy transport, material mixing, and marine ecosystems [7,8,9,10]. Given the limitations of in situ observational resolution and uncertainties in numerical parameterizations, laboratory experiments utilizing rotating tanks have emerged as a vital tool for elucidating eddy dynamics [11,12,13]. By precisely controlling rotation rates, density stratification, and external forcing mechanisms, these experiments capture mesoscale to submesoscale eddy behaviors, reconstruct their generation, stability, and evolution mechanisms, and bridge critical knowledge gaps between theory and field observations.

Rotating tank experiments employ mechanical driving (e.g., towed cylinders, grid turbulence) or thermal forcing to simulate eddy phenomena triggered by natural processes such as topographic effects and shear instabilities. Longhetto et al. (1996, 1997) reproduced lee cyclogenesis in a stratified rotating basin by establishing a three-layer baroclinic system [14,15]. At the interface of two distinct density layers (${\rho }_1,{\rho }_2$), an intermediate density plume (${\rho }_1<{\rho }_m<{\rho }_2$) combined with topography successfully mimicked the interaction between cold fronts and terrain, leading to cyclonic-anticyclonic dipole formation. Their experiments demonstrated that intermediate-layer flow instabilities spontaneously generate dipolar vortices when the Rossby number ($\mathrm{R}\mathrm{o}={\displaystyle \frac{\mathrm{U}}{\mathrm{f}\mathrm{L}}}$) and Burger number ($\mathrm{B}\mathrm{u}={\displaystyle \frac{{\mathrm{g}}^{\prime }\mathrm{h}}{{\mathrm{f}}^2{\mathrm{L}}^2}}$) fall within critical thresholds, with horizontal and vertical scales resembling atmospheric frontal systems observed in Alpine lee cyclones.

Eddy stability remains a key focus of laboratory studies. Lazar et al. (2013) validated theoretical instability criteria by generating anticyclones in strongly stratified thin layers [16]. While the classical Rayleigh criterion predicts instability when absolute vorticity ($\mathsf{\zeta }+f$) becomes negative, experiments revealed that anticyclones retain stability even at low core vorticity ratios (${\displaystyle \raisebox{1ex}{${\zeta }_0$}\!\left/ \!\raisebox{-1ex}{$f$}\right.}$ ≈ −3.5) under high Burger numbers (Bu > 30). This aligns with theoretical predictions that strong stratification ($N/f>10$) suppresses vertical motion scales, enhancing the viscous damping of inertial modes. Stability thus depends jointly on Ro, Bu, and the Ekman number ($\mathrm{E}\mathrm{k}={\displaystyle \frac{\mathsf{\nu }}{\mathrm{f}{\mathrm{h}}_{\mathrm{c}}^2}}$), underscoring the interplay between eddy strength, stratification intensity, and dissipation in submesoscale anticyclone dynamics.

The energy decay pathways of rotating turbulence govern eddy longevity. Moisy et al. (2011) identified a three-phase decay process in grid-generated turbulence within the Coriolis platform: (1) an initial Saffman-type decay (Ro > 0.25), (2) a transitional slowing phase (Ro ≈ 0.25) marked by vertical shear layer formation, and (3) a final stage (Ro < 0.1) where shear-layer instabilities restore vorticity symmetry [17]. These findings highlight how background rotation suppresses 3D turbulence cascade while promoting quasi-2D structures, ultimately mimicking energy transfer pathways in oceanic quasi-geostrophic turbulence and informing parameterization schemes.

Island wake dynamics were systematically explored by Han et al. (2011) using a large-scale rotating platform [12]. An analysis of an anticyclone’s complete lifecycle (Figure 1) revealed two distinct dissipation phases: (i) a slow decay governed by centrifugal instability and (ii) a rapid decay driven by vortex–vortex interactions. Such experimental insights quantify how eddy interactions mediate energy redistribution and structural breakdown.

Inter-eddy interactions fundamentally shape oceanic eddy fields. Early experiments established critical merger thresholds for like-signed vortices. Griffiths and Hopfinger (1986, 1987) demonstrated that uniform-vorticity anticyclones merge when the separation-to-radius ratio ($d^{\ast }/R$) reaches 3.3 ± 0.2, contrasting with cyclones exhibiting larger critical distances ($d^{\ast }/R=4.5$) [18,19]. Refining these criteria, Meunier and Leweke (2001) introduced a universal critical threshold ${\left(\mathrm{a}/\mathrm{b}\right)}_{\mathrm{c}\mathrm{r}}\approx 0.24$ for non-uniform vortices, where a and b denote the vortex core radius and separation distance, respectively. For parabolic vorticity profiles, their experimental-theoretical synthesis yielded ${\left(\mathrm{a}/\mathrm{b}\right)}_{\mathrm{c}\mathrm{r}}\approx 0.218\pm 0.001$ [20].

Fu et al. (2024) further dissected merger dynamics in multi-vortex systems using the Okubo-Weiss parameter, revealing asymmetric stretching patterns and antisymmetric vorticity filamentation during coalescence [11]. Their work identified convective processes induced by antisymmetric vorticity as a key merger mechanism, advancing a quantitative understanding of eddy interaction physics.

2.2. Internal Waves

Internal waves, as ubiquitous phenomena in the ocean, significantly influence vertical shear flows and turbulent mixing, with profound implications for marine engineering, ecosystems, and military operations [21,22,23]. Notable events, such as the deformation of an offshore oil platform in the Davis Strait in 1980s and the vessel damage observed in the South China Sea in 1990s, highlight the destructive potential of internal waves. Beyond hazards, these waves modulate underwater acoustic propagation, enabling submarine stealth operations while posing risks of destructive thermocline shifts. Ecologically, internal waves drive vertical nutrient transport, sediment resuspension, and benthic-pelagic coupling, underpinning marine biodiversity and geological evolution.

Pioneering experiments focused on elucidating internal wave generation under various topographies. Renouard and Baey (1993) utilized a large rotating platform to study tidal interactions with complex bathymetry, revealing nonlinear wave transformations induced by seafloor irregularities [24]. Ramirez and Renouard (1998) systematically examined slope-dependent internal wave generation over continental shelves, demonstrating that steeper slopes enhance wave amplitudes by intensifying topographic forcing [25]. Mercier et al. (2013) replicated M2 internal tides in Luzon Strait in laboratory settings, correlating ridge-and-canyon bathymetry with observed in situ tidal dynamics [26]. Their work identified localized flow-topography interactions as critical drivers of internal tide generation, validating theoretical models against field data.

Innovative imaging technologies have revolutionized experimental capabilities. Connan et al. (1998) reconstructed high-resolution internal wave signatures using millimeter-wave radar, establishing quantitative relationships between radar parameters and wave kinematics (velocity, wavelength) [27]. Guizien et al. (1998) investigated nonlinear internal waves in a two-layer rotating system, linking wave amplitude to generator incidence angles through comparative analysis with field measurements [28]. However, early dye-tracing methods, though effective for visualizing wave structures (Figure 2), lacked the resolution to quantify turbulence from wave breaking.

The integration of particle image velocimetry (PIV) has enabled unprecedented insights into energy cascades and mixing processes. Swart et al. (2010) resolved wave attractor-driven turbulence in sloped terrains, demonstrating how ray convergence amplifies diapycnal mixing and triggers secondary mean flows and topographic Rossby waves [30]. Contrasting numerical predictions, Savaro et al. (2020) observed discrepancies in horizontal velocity spectra between laboratory experiments and oceanic Garrett-Munk models under low Froude number conditions [31]. Shmakova et al. (2021) explored inertial gravity wave focusing mechanisms, revealing nonlinear harmonic generation and amplitude suppression in high-oscillation regimes [32]. Rodda et al. (2022, 2023) characterized stratified turbulence by modulating geometries and forcing frequencies, identifying distinct regimes—discrete wave turbulence (1 ≤ Reb ≤ 3.5) and strongly nonlinear turbulence (Reb > 3.5)—based on buoyancy Reynolds numbers (Figure 3) [33,34]. Their refractive index-matched alcohol addition technique significantly enhanced PIV accuracy, opening new avenues for quantifying wave-breaking mixing.

2.3. Ekman Currents

The Ekman layer, a cornerstone of rotating fluid dynamics, governs momentum transfer in geophysical flows and laboratory-scale systems. Ekman’s theory describes steady flow velocity decaying spirally with depth $u\left(z\right)=u_0{e}^{-z/{\delta }_E}\mathrm{c}\mathrm{o}\mathrm{s}(z/{\delta }_E-\pi /4)$, where the Ekman layer thickness ${\delta }_E=\sqrt{2\nu /f}$ depends on the eddy viscosity ($\nu$) and the Coriolis parameter ($f$). While this model assumes idealized conditions, real-world complexities—including bottom friction, unsteady forcing (e.g., tidal oscillations), and variable boundary conditions—necessitate laboratory validation. Sous and Sommeria (2012) pioneered stereoscopic particle image velocimetry (SPIV) in rotating tanks, achieving a <1% spatial resolution error in quantifying turbulent Ekman layer dynamics [35]. Their work revealed instantaneous velocity fields and Reynolds stress distributions, providing benchmark data for modeling turbulent energy dissipation in oceanic boundary layers.

Ekman layer coupling with coastal boundary layers (CBLs) drives long-term material transport in coastal regions. D’Hieres et al. (1991) identified baroclinic instabilities arising from vertical mass flux mismatches between oscillatory Ekman and CBL flows [36]. Their rotating flume experiments demonstrated peak instability growth rates at transitional Rossby numbers ($R{o}_T=U/\left(fL\right)$), where nonlinear advection dominates. Aelbrecht et al. (1993) further quantified residual currents (5–10% of instantaneous velocities) under oscillatory forcing with dimensionless frequencies $S=\omega /f$ [37]. These currents, driven by phase-lagged momentum transport, critically influence estuarine and shelf-scale particulate fluxes, informing parameterizations for coastal numerical models.

The transition to turbulence in Ekman layers depends critically on flow unsteadiness. Aelbrecht et al. (1999) delineated stability thresholds for steady versus oscillatory flows, showing that low-frequency oscillations suppress turbulence while high frequencies enhance inertial instabilities [38]. Sous et al. (2013) advanced turbulence closure models for rotating boundary layers using high-resolution SPIV data [39]. Their modified wall law,

$$u^{+}={\displaystyle \frac{1}{\kappa }}\ln z^{+}+B-\mathsf{\Delta }{u}_{rot}^{+},$$

(1)

incorporates a rotation-dependent term $\mathsf{\Delta }{u}_{rot}^{+}\propto (z^{+}{)}^{1/2}R{o}_{\tau }^{-1}$, which diminishes with increasing rotational Reynolds numbers. Despite rotational deviations in outer-layer velocity profiles, near-wall regions retain universal logarithmic behavior. Centrifugal effects were shown to redistribute turbulent kinetic energy (TKE), reducing outer-layer TKE intensity by about 20% compared to non-rotating scenarios.

Emerging studies explore resonant energy transfer in wind-driven Ekman layers. Vincze et al. (2019) observed resonance amplification when forcing frequencies approached half the system’s inertial frequency (f/2), tripling kinetic energy density and cascading efficiency [40]. Resonance-induced mixing enhanced vertical tracer fluxes by orders of magnitude, potentially explaining submesoscale vortex genesis (100 m–1 km scales) and intermittent turbulence in ocean interiors. Critically, resonance activation requires precise frequency matching and nonlinear energy transfer pathways—a dual criterion with implications for mesoscale–submesoscale coupling in thermohaline circulation models.

While laboratory studies excel in isolating physical mechanisms, such as rotation-turbulence interactions, scaling to oceanic conditions necessitates integrating real-world complexities—bathymetric gradients, thermohaline stratification, and transient forcing. Future experiments should couple high-resolution measurements with data-assimilative numerical frameworks to refine predictive models of Ekman-driven transport and climate-scale ocean dynamics.

2.4. Gravity Currents

Gravity currents, driven by density contrasts, play critical roles in oceanic and atmospheric transport processes [41]. Their dynamics are governed by a synergistic framework integrating laboratory experiments, theoretical models, and numerical simulations across multiple scales [42].

Key advancements include the validation of boundary layer dynamics and axisymmetric current theories. D’Hieres et al. (1991) experimentally reconstructed the stability of surface boundary currents analogous to the Algerian Current, demonstrating Burger number (Bu) dominance [36]. At Bu = 0.15, laboratory flows closely mimicked field observations, with instability growth rates varying systematically across $Bu\in [0.15,0.82]$. Hallworth et al. (2001) compared rotating versus non-rotating gravity currents at high Reynolds numbers ($Re>{10}^4$), revealing that numerical models resolved fine-scale flow features neglected by shallow-water approximations [43]. Thomas and Linden (2007) unified geostrophic theory with multi-scale experiments (1 m to 13 m diameters), correlating current length (L), width (W), and propagation velocity (U) via the dimensionless parameter $\mathsf{\Pi }=\mathsf{\Omega }{g^{\prime }}^{-1/2}{q}_0^{1/2}$ (Figure 4) [44]. While geostrophic predictions matched velocity and height measurements within 5%, width discrepancies (~10%) stemmed from unaccounted boundary layer effects. Aubourg et al. (2017) identified three-wave nonlinear interactions in laboratory turbulence, contrasting with oceanic four-wave processes—a disparity attributed to weak nonlinear energy convergence or excessive dissipation in controlled settings [45].

Recent studies emphasize boundary roughness, sediment dynamics, and slope interactions. Maggi et al. (2022) quantified roughness impacts using PIV/PLIF, defining relative roughness height $\lambda$ (protrusion-to-current depth ratio) [46]. For $\lambda <0.1$, flows resembled smooth-bottom behavior; at $\lambda >0.3$, front velocities decreased by 15–25% due to enhanced recirculation and counter-rotating vortices (Figure 5). Systematic tests with LEGO^®-based configurations (R0–R4) revealed inverse correlations between roughness and downstream momentum fluxes. Maggi et al. (2024) further linked mobile sediment beds to vertical velocity reversals, proposing a suspended-load model validated against concentration gradients [41].

Slope-driven propagation mechanisms were dissected by Martin et al. (2020) investigated gravity currents transitioning from horizontal to sloped beds exhibited time- and distance-dependent acceleration, with Richardson number ($R_i$) decay triggering interfacial instabilities [47]. Nascent front velocities lagged behind internal maxima by 20–30%. De Falco et al. (2021) extended this to upslope currents, validating depth-averaged momentum models incorporating slope-parallel gravity components and bed friction ($slope\le 1$) [48]. Flow morphology depended solely on the slope angle, independent of ambient stratification.

2.5. Rossby Waves

Rossby waves, driven by planetary or topographic β-effects, govern large-scale, quasi-geostrophic adjustments in global oceans and atmospheres. With phase speeds (~10 m/s) comparable to wind velocities and horizontal scales spanning hundreds of kilometers, these waves dominate oceanic mass and momentum transport [49]. Despite extensive theoretical frameworks, unresolved dynamics persist, including nonlinear wave interactions, scale-dependent propagation, and topographic coupling. Advancements in rotating-tank experiments, numerical models, and oceanic observations now enable systematic investigations of these processes.

Previous studies have modeled Rossby waves and jet dynamics using rotating tank experiments. Sommeria et al. (1989) generated a strong eastward jet with embedded Rossby waves and showed that the jet acted as a barrier to tracer transport [50]. Behringer et al. (1991) found that adding multiple wave modes could induce chaotic particle motion, but cross-jet transport remained limited unless wave amplitudes were large [51]. Uleysky et al. (2010) further interpreted these results using a theoretical model and showed that cross-jet transport occurs when central invariant curves (CICs) break due to wave interactions [52].

Rotating platforms uniquely replicate topographic Rossby normal modes (TRMs), which dictate localized tidal responses [53]. Miller et al. (1996) experimentally identified TRMs with a 1.8-day period, validating shallow-water models [54]. Early studies by Pedlosky and Greenspan (1967) and Beardsley (1975) established analytical and experimental linkages between TRMs and planetary Rossby modes (PRMs), demonstrating universal dynamics under linear sloping boundaries [55,56].

Pierini et al. (2022) conducted barotropic experiments on the LEGI-Coriolis platform, generating TRMs via potential vorticity conservation (Figure 6) [57]. A 4.3 × 2 m linear slope, bounded by vertical walls, produced boundary-reflected Rossby waves. Adjusted channel lengths and rotation periods (Ω = 0.05–0.2 rad/s) consistently excited the first barotropic mode, matching analytical predictions within 5%.

Cohen et al. (2010) mechanized wave generation in cylindrical tanks, controlling wavelengths via adjustable rods and frequencies (0.1–1.0 Hz) [58]. Experimental dispersion relations aligned with theoretical phase speeds, particularly under non-harmonic conditions (broad channels, steep slopes), resolving discrepancies in narrow geometries.

Pirro et al. (2024) pioneered scaled modeling of Eastern Mediterranean eddies (Cyprus Eddy [CE], South/North Shikmona Eddies [SSE/NSE]) on a β-plane (slope s = 0.1) [59]. Key parameters (

Table 1. Experimental parameters and initial conditions scaled to real oceanic conditions. Key parameters include d, representing the ratio of the bottom layer thickness to the total water depth (H) under stratified conditions, and the Rossby number (${Ro}_e$), defined as ${Ro}_e={\displaystyle \frac{V_{max}}{R_{max}}}$, where $V_{max}$ denotes the maximum flow velocity and $R_{max}$ denotes the deformation radius (Pirro et al., 2024 [59]).

Case	H/L	h0′/L	U0 cm/s	f0 s−1	β (ms−1)	b	${\mathit{R}\mathit{o}}_{\mathit{e}},\mathit{R}\mathit{o}$	N s−1	s	d	λobs m	λth, λMC m
EXP-H1	0.68	0.45	1.4	0.314	0.12	1.0	0.21, 0.13	-	-	-	2.0	2.1, -
EXP-H2	0.68	0.45	0.79	0.314	0.12	1.77	0.18, 0.07	-	-	-	1.5	1.5, -
EXP-S1	0.68	0.45	1.4	0.314	0.12	1.04	0.2, 0.13	1.1	0.56	0.91	2.2	1.9, 2.6
EXP-S2	0.68	0.45	0.83	0.314	0.12	1.68	0.16, 0.08	1.1	0.56	0.91	1.5	1.46, 1.3
Ocean	0.03	0.4	2–5	7.2e−5	2e−11	1.6–0.65	e−2	3e−3	0.96	0.9	(1.8–2.5)e5	(24, 19–28)e4

) were derived from oceanic data.

In the EXP-H2 and EXP-H1 experiments (Figure 7a,b), anticyclonic vortices formed over the seamount-like topographic obstacle, with stationary lee wakes observed downstream. These wakes transitioned westward only under unsteady flow conditions, such as reduced inflow velocities. Furthermore, closed cyclonic and anticyclonic vortices developed within the wake meander zones, exhibiting enhanced prominence under lower flux conditions, such as EXP-H2. Theoretical calculations yielded barotropic Rossby wavelengths of 1.53 m (EXP-H2) and 2.16 m (EXP-H1), consistent with homogeneous flow scenarios. In weakly stratified cases, vortices remained defined by closed streamlines over bathymetric protrusions, a feature preserved in wake dynamics. Rossby wave theory robustly aligned with both laboratory and oceanic conditions, particularly in the EXP-S1 and EXP-S2 experiments (Figure 7c,d), where measured wavelengths closely matched field-derived estimates and exhibited explicit dependence on the b-parameter. Specifically, for EXP-S2—representing low inflow velocities and elevated b—the dimensionless wavelength (λ_obs/L) of 2.2 corresponded to an oceanic value of ~2.25 (observed λ_obs = 180 km and L = 40 km). For EXP-S1 (low b and higher velocity), λ_obs/L = 3.2 mirrored field-derived values (λ_obs = 250 km and L = 40 km). These results quantitatively validate the dynamical similarity between laboratory-scale Rossby waves and their oceanic counterparts.

Figure 6. (a) Schematic diagram of the experimental setup, including top and side views of the Coriolis platform, positions of measurement instruments, and definitions of key variables. The top-view inset depicts a detailed geometry of the bathymetric protrusion employed in the experiments. (b) Instantaneous image from the EXP-H2 (homogeneous flow) experiment, showing fluorescein dye injected upstream (white arrows highlighting flow direction) and the topographic feature location marked by a red dashed circle. Rossby waves and associated vortices are distinctly visible (Adapted from Pirro et al., 2024 [59]).

Figure 6. (a) Schematic diagram of the experimental setup, including top and side views of the Coriolis platform, positions of measurement instruments, and definitions of key variables. The top-view inset depicts a detailed geometry of the bathymetric protrusion employed in the experiments. (b) Instantaneous image from the EXP-H2 (homogeneous flow) experiment, showing fluorescein dye injected upstream (white arrows highlighting flow direction) and the topographic feature location marked by a red dashed circle. Rossby waves and associated vortices are distinctly visible (Adapted from Pirro et al., 2024 [59]).

While the present study focuses on short-period Rossby waves generated and modulated in a laboratory setting, recent studies have proposed the existence of long-period, multi-frequency Rossby waves—so-called Gyral Rossby Waves (GRWs)—which form around subtropical gyres and are thought to be resonantly forced by subharmonic modes related to solar and orbital cycles [60,61]. These GRWs may play a role in large-scale oceanic and climatic variability, including the Atlantic Multidecadal Oscillation and long-term shifts in geostrophic circulation. Although replicating such long-period phenomena in laboratory conditions remains technically challenging, future developments in experimental design may enable controlled investigations of subharmonic resonance and coupled oscillator dynamics, thereby offering complementary insights for observational and theoretical studies of GRWs.

2.6. Bottom Boundary Currents

Boundary layer stability represents a classical problem in fluid mechanics, particularly in rotating systems where factors such as the rotation rate, density stratification, and bottom topography collectively influence stability. Jacobs et al. (1999) experimentally investigated boundary layer instabilities in rotating fluids and demonstrated that these instabilities can be characterized using dimensionless parameters [62]. Their results revealed that weaker stability occurs at small Burger numbers (Bu), while the combined effects of the Ekman (Ek) and Rossby (Ro) numbers govern the type and intensity of instability. Notably, seabed topography significantly alters the boundary layer structure and stability. In shallow boundary layers, the ratio of fluid depth to total water depth plays a critical role: when the boundary layer occupies a substantial portion of the water column, fluid transport through the bottom Ekman layer enhances offshore advection, further destabilizing the system.

Ferrero et al. (2005) extended this work by examining neutral boundary layers in a rotating tank, simulating atmospheric boundary layer (ABL) dynamics using PIV to quantify turbulent properties [63]. Their experiments demonstrated strong correlations between turbulent kinetic energy (TKE), turbulent diffusivity, surface roughness, and rotation speed. While surface Rossby number similarity holds under varying roughness conditions, the Prandtl mixing length hypothesis exhibited limitations in certain regimes—a finding pivotal for refining turbulence closure models.

Sous et al. (2013) explored the transition from laminar to turbulent flow in Ekman boundary layers, validating empirical scaling laws for turbulent growth rates against ABL theory [39]. As shown in Figure 8, experimental friction velocity ($U/u_{\ast }$) versus friction Rossby number ($\sqrt{{Ro}_f}$) aligns with theoretical predictions (using constants A = 3.3 and B = 3.0), confirming the validity of Ekman friction laws under laboratory conditions. The transition from turbulence to laminar flow occurs at $U/u_{\ast }\approx 15$ (corresponding to Re ≈ 150). Figure 9 further illustrates the universal relationship between $U/u_{\ast }$ and $\sqrt{{Ro}_f}$ across vertical positions and depths, though slightly elevated $U/u_{\ast }$ values in shallow regimes suggest depth-dependent friction effects. Their modified friction law, replacing the Coriolis parameter f with local absolute vorticity (f + ω), significantly advances the understanding of seabed boundary layer dynamics.

Davarpanah Jazi et al. (2020) investigated Coriolis effects on density-driven bottom boundary layers in deep channels using a large rotating platform [64]. Coriolis forces dominantly redirect near-bed velocities at bends when Ro < +0.8, overriding centrifugal effects. This mechanism highlights the role of planetary rotation in governing sediment erosion/deposition patterns in high-latitude channels. Their observations also revealed decoupling between density and velocity fields at bends, driving asymmetric left- and right-turning flow behaviors.

Inertial wave propagation in rotating fluids has long intrigued researchers. Manders and Maas (2003) experimentally demonstrated wave focusing in a rectangular tank with sloping boundaries, where reflections concentrate wave energy into quasi-periodic structures [65]. While infinite channels approach limit cycles, finite-domain experiments showed wave attractor evolution constrained by boundary conditions and three-dimensional wavefield adjustments near vertical endwalls.

Shi et al. (2024) examined the near-field evolution of shallow, neutrally buoyant planar jets over sloping beds [66]. Compared to horizontal boundaries, sloping beds induce pronounced three-dimensionality: jets contract laterally near the outlet before expanding due to transverse entrainment. Kelvin–Helmholtz coherent structures dominate shear layers, enhancing turbulent momentum exchange and TKE production. As jet thickness increases, Kelvin–Helmholtz coherent structure vertical scales grow, triggering flapping instabilities when opposing shear layers interact. Slope-induced effects accelerate flapping onset, positioning instabilities closer to the jet source—critical insights for coastal management of buoyant discharges.

3. Conclusions

Large-scale rotating experimental platforms serve as pivotal tools in physical oceanography, enabling the precise replication of Coriolis-dominated flows to investigate multi-scale dynamical coupling mechanisms in the ocean. Recent advancements have surpassed traditional single-factor simulations by integrating coupled physical fields, such as wind stress, topographic forcing, thermohaline gradients, and biogeochemical processes, yielding critical insights into energy cascades of internal tides, interactions of mesoscale eddies, and turbulence-driven mixing mechanisms. Notably, these platforms have elucidated the generation of submesoscale motions during inertia-gravity wave breaking, advancing parameterization schemes for vertical mixing in ocean models. However, significant challenges persist. Environmental realism remains constrained by limitations in reconstructing continuously stratified thermohaline structures alongside complex seabed topography. Observational techniques face spatiotemporal resolution gaps, as conventional PIV systems struggle to simultaneously resolve millimeter-scale turbulent features and basin-scale circulation patterns. Furthermore, the integration of experimental data with high-resolution numerical models is hindered by underdeveloped dynamic data assimilation frameworks, restricting the synergistic use of multi-source datasets.

Looking ahead, the integration of artificial intelligence promises transformative advancements. Machine learning-driven optimization of experimental parameters, combined with ultra-high-speed 3D-PIV systems, could unlock the unprecedented visualization of submesoscale frontal dynamics. Concurrently, digital twin platforms that bridge physical simulations and numerical extrapolation may transcend traditional experimental spatiotemporal limitations, supporting interdisciplinary solutions for carbon flux quantification under marine carbon neutrality and coupled typhoon-ocean forecasting systems. Continued innovation in rotating platform technologies will deepen the integration of fundamental oceanic dynamics research with applications critical to global sustainability challenges.