Streamwise Vortices Research Articles

Designing Vortex Generators for Tidal Turbine Blades

Increasing tidal turbine performance through innovation is crucial if the cost of tidal energy is to become competitive compared to other sources of energy. The present investigation deals with the application of Vortex Generators (VGs) on tidal turbines in view of increasing their performance. The more mature wind energy industry uses passive VGs either as a retrofit or in the blade design process to reduce separation at the inboard part of wind turbine blades. Tidal turbine blades also experience flow separation and here we examine whether passive vane VGs can be used to reduce or suppress that separated flow.
 Vortex generators (VGs) in various forms have been used and studied for flow separation control on wings since the 1940s [1]. Their working principle is relatively simple: they generate streamwise vortices that energise the boundary layer on the surface they are attached to, by bringing high momentum fluid closer to the surface [2]. This mechanism has been described by various researchers [3–6], while a number of studies have provided optimization guidelines under a variety of flow conditions [7–13].
 In the present investigation, a VG configuration is selected following a thorough wind tunnel campaign. It is found that sizing parameters for the tidal turbine profile are very similar to the wind turbine relevant literature [13,14]. The best performing vane VG configuration had a height of 0.007c, which corresponded to half the local boundary layer height (0.5δ) for operational Reynolds numbers. The results are also used to validate a Reynolds Averaged Navier Stokes (RANS) VG modelling approach using the BAY model [15]. The validated method is used to simulate the flow past a tidal turbine in both model size (1:8) and full scale, see Figure 1. The results show that VGs do suppress flow separation in both cases. However, and importantly, it is revealed that the significance of rotational effects is such that when deciding VG placement locations, only the full size blade should be considered. In the interest of brevity, the performance increase caused by a standard VG configuration is show in Figure 2, where a power coefficient improvement of 1.05% is predicted at λ=3. Figure 3 shows the effect on the normal and tangential forces on the blade. In the final paper and presentation, the results for different VG locations will be included and analysed in detail.

Supercell Low-Level Mesocyclones: Origins of Inflow and Vorticity

Abstract The development and intensification of low-level mesocyclones in supercell thunderstorms have often been attributed, at least in part, to augmented streamwise vorticity generated baroclinically in the forward flank of supercells. However, the ambient streamwise vorticity of the environment (often quantified via storm-relative helicity), especially near the ground, is particularly skillful at discriminating between nontornadic and tornadic supercells. This study investigates whether the origins of the inflow air into supercell low-level mesocyclones, both horizontally and vertically, can help explain the dynamical role of environmental versus storm-generated vorticity in the development of low-level mesocyclone rotation. Simulations of supercells, initialized with wind profiles common to supercell environments observed in nature, show that the air bound for the low-level mesocyclone primarily originates from the ambient environment (rather than from along the forward flank) and from very close to the ground, often in the lowest 200–400 m of the atmosphere. Given that the near-ground environmental air comprises the bulk of the inflow into low-level mesocyclones, this likely explains the forecast skill of environmental streamwise vorticity in the lowest few hundred meters of the atmosphere. The low-level mesocyclone does not appear to require much augmentation from the development of additional horizontal vorticity in the forward flank. Instead, the dominant contributor to vertical vorticity within the low-level mesocyclone is from the environmental horizontal vorticity. This study provides further context to the ongoing discussion regarding the development of rotation within supercell low-level mesocyclones. Significance Statement Supercell thunderstorms produce the majority of tornadoes, and a defining characteristic of supercells is their rotating updraft, known as the “mesocyclone.” When the mesocyclone is stronger at lower altitudes, the likelihood of tornadoes increases. The purpose of this study is to understand if the rotation of the mesocyclone in supercells is due to horizontal spin present in the ambient environment or whether additional horizontal spin generated by the storm itself primarily drives this rotation. Our results suggest that inflow air into supercells and low-level mesocyclone rotation are mainly due to the properties of the environmental inflow air, especially near the ground. This hopefully provides further context to how our community views the development of low-level mesocyclones in supercells.

Turbulent boundary layer over porous media with wall-normal permeability

Porous walls are a widely used passive flow control technique, which shows potential in reducing skin friction and mitigating flow-introduced noise. In the present study, porous media with wall-normal permeability is applied to a flat plate to investigate its interaction with the turbulent boundary layer at the Reynolds number based on friction velocity of Reτ=225. Time-resolved planar and tomographic particle image velocimetry were employed to identify the impact on mean statistics and coherent structures. An overall skin friction reduction of 22% is achieved. The porous wall induces counter-rotating streamwise vortex pairs at the spanwise sides of each pore, leading to momentum transport and the generation of alternative low- and high-speed regions close to the wall. Slip velocity is obtained, associated with the reduction in turbulent fluctuations and Reynolds shear stress. The streamwise velocity streaks and the hairpin vortices are significantly distorted compared with the smooth wall condition due to the downwash and upwash motion, featuring a notable reduction in the number and scale of the coherent structures, in which the skin friction reduction mechanism is related to. The proper orthogonal decomposition analysis returns the most energetic unsteady modes. Although the wall-coherent mode type remains to dominate the production of turbulent fluctuations, the scale and energy content of wall-incoherent modes increase, confirming the modification of the distribution and scale of near-wall turbulent structures.

Flow and heat transfer of supercritical hydrogen in a regenerative cooling channel with the arc ribs of a rocket engine

This paper reports on the heat transfer characteristics and flow performance of supercritical hydrogen in a regenerative cooling channel with arc-shaped ribs. A 3-D numerical model of hydrogen flowing inside the cooling channel has been established and the data are compared with experimental results. The effect of rib parameters such as the number of ribs n, rib angle θ, rib size (hr × e), and rib orientation (convex or concave) on the cooling channel performance has been explored. The results indicate that arc ribs effectively mitigate the heat transfer deterioration and thermal stratification phenomena by reducing the wall temperature; nevertheless, the pressure loss due to friction would considerably increase. In studied cases, the wall temperature significantly decreases when the rib angle θ increases from 0° to 90°. The results have also shown that increasing the size of square arc ribs while maintaining the same rib spacing p/hr and length of the ribbed section Lr is beneficial for the overall heat transfer performance. Compared to smooth channel, a channel with large 90° arc ribs oriented either convex or concave to the flow direction enhanced the thermohydraulic performance η by up to 45%. This enhancement in performance of channel due to arc ribs is 1.95 and 1.384 times higher than the previously known designs; V-ribs (23%) and cylindrical ribs (32.5%), respectively. The flow field has shown that the evolution and morphology of rib-induced streamwise vortex considerably improve the wall temperature reduction by intense fluid mixing. In addition, it has been shown that the direct impact of the coolant stream on the ribs, the reattachment of the separated shear layer, and the intermittent growth of the boundary layer helps to enhance heat transfer.

High-Frequency Pulsed Coaxial Injectors for High-Speed Flow Mixing and Control

Efficient and controlled mixing of fuel with fast-moving air is a challenging physical problem relevant to hypersonic systems. Although mixing happens at the molecular level through diffusion, the macroscopic phenomenon such as entrainment and vorticity dynamics resulting from the shear layer instabilities of the mixing fluids play a significant role in the overall efficiency of the process. With a focus on improving mixing at extreme flow conditions, this paper presents a fundamental study of a novel, high-speed, pulsed coflow system integrated with ultrahigh-frequency actuators (11–20 kHz). This injection system consists of a supersonic actuation air jet at the inner core that provides large mean and fluctuating velocity profiles in the shear layers of a fluid stream injected surrounding the core through an annular nozzle, with pulsing occurring at a designated frequency. The high-frequency streamwise vortices and shockwaves tailored to the mean flow significantly enhanced supersonic flow mixing between the fluids compared to a steady coaxial configuration operating at the same input pressure. Experiments also indicate a strong connection between the frequency and unsteady amplitude of the actuation jet to the supersonic flow mixing phenomenon. This paper reports the design details of the injector assembly and flow mixing characteristics captured using phase-locked microschlieren and planar laser-induced fluorescence techniques.

Development and evolution mechanism of streamwise vortex in an inward-turning inlet

The three-dimensional flows in an inward-turning inlet were numerically investigated at different incoming flow conditions. When the incoming flow conditions change, the shock angle and the shock interaction form of the external compression wave change, and the development of the near-wall low-energy fluid and the streamwise vortex is also affected. The impingement of the shock wave leads to a sharp increase in the vorticity of the low kinetic energy fluid. Under the pressure gradient caused by the shock wave, the high-vorticity fluid migrates from the cowl to the ramp and entrains the mainstream fluid to form a streamwise vortex, for which the velocity gradient ( ∂v/ ∂y + ∂w/ ∂z) along the vortex axis can accurately determine the rotation direction and the Hopf bifurcation position. By considering high Reynolds number flows, the pressure gradient along the vortex axis is developed to estimate the simplified dilation term (velocity gradient) due to its ease of measurement. However, the pressure gradient ( ∂p/ ∂x) along the vortex axis can lead to bias when evaluating the cross-flow state of the streamwise vortex, with the shock wave structure and high-vorticity fluid leading to under- and overestimation, respectively. This study provides a theoretical basis for an accurate determination of the flow state of a streamwise vortex in an inward-turning inlet and thus lays the foundation for effective vortex control.

Effects of hump deflection angle on streamwise vortex instability over a yawed cone at Mach 6

This paper investigates the influence of three-dimensional smooth humps with varying deflection angles (φ) on the linear stability of streamwise vortices over a yawed cone with a 7° half-angle at a 6° angle-of-attack, free-stream Mach number of 6, and unit Reynolds number of 1.0×107/m. The steady laminar flow is obtained using direct numerical simulations. The eN method based on global stability theory is used to predict the transition location of the streamwise vortices along the centerline on the leeward side of the cone. The results reveal that φ plays a significant role in the outward vortex generation location, with smaller values of φ effectively delaying the outward vortex generation. Moreover, there is a qualitative relationship between the instability of the streamwise vortices and the inward/outward vortex structure characteristics of the base flow over a yawed cone. Namely, the transition delay effect of the streamwise vortices is proportional to the delay in the generation location of the outward vortex, which provides insights into the control of the transition induced by streamwise vortices. In particular, the configuration based on a hump with φ=9° and a height of 0.57 times the local boundary layer thickness delays the transition by approximately 38.2% at the critical N-factor (Ntr=12.5) without significantly increasing the instability of the inner mode.

Regeneration mechanisms of oblique and streamwise vortices in oblique transition

Oblique transition can be triggered by introducing componentwise optimal perturbations into a plane Couette flow. It is shown that in the middle stage of the transition, oblique vortices are generated again through transient spanwise velocity energy growth in the presence of developed streamwise streaks. In the following stage, streamwise independent kinetic energies of cross-streamwise velocities are intensified due to the nonlinear contribution caused by amplified oblique vortices together with the pressure transport, and the increased cross-stream velocity fluctuations can strengthen the lift-up effect. As a result, the streamwise vortices are formed again, and the streamwise streaks are even enhanced prior to their final breakdown to turbulence.

Manipulating near-wall flow instability and transport characteristics by the airfoil probe: Investigation on a transonic compressor cascade

The flow field exhibits complex features, such as shock waves, wakes, and end wall vortices in a transonic cascade. Installation of airfoil probes exacerbates the multi-scale and unsteady behavior of the internal passage flow. Apart from inducing measurement errors, it also generates extra flow loss inevitably and further affects the measuring accuracy. This paper investigates the impact of airfoil probes on a transonic compressor cascade's unsteady behavior and transport characteristics. Using high-fidelity numerical simulations, the influence of pipe layouts on the flow field of instrumented blades is visualized, revealing highly radial asymmetry. Loss analysis uncovers entropy transport induced by the streamwise vortices, primarily manifested by large-scale angular deformation at the outlet. The vortex structures in the wake region are dominated by momentum transport, displaying regional evolution and momentary equilibrium. The vortex expansion plays a leading role in the global vortex transport process, which is strengthened by the presence of the probes. Spatiotemporal analysis of the unsteady flow field can reveal some features overlooked by conventional fluid mechanics analysis. Using proper orthogonal decomposition, wake vortex pairs' high-frequency oscillations and shedding behaviors are captured in adjacent modes for the first time. The proposed approach can provide a theoretical basis for in-depth investigations of instrumented blade flow fields at the transonic regime. Furthermore, corresponding research can promote the refinement of instrument design by enabling experimentalists to understand the effects of intrusive instruments on transonic flow fields.

On the association of kinematics, spanwise instability and growth of secondary vortex structures in the wake of oscillating foils

Three-dimensional wake of an oscillating foil with combined heaving and pitching motion is numerically evaluated at a range of chord-based Strouhal number (0.32≤Stc≤0.56) and phase offset (90∘≤ϕ≤270∘) atRe=8000. The changes inϕandStcreflect a unique route of transition in mechanisms that govern the origin of spanwise instabilities and growth of secondary wake structures. At lowerStc, heave-dominated kinematics demonstrates a strong secondary leading edge vortex (LEV) as the source of growing spanwise instability on the primary LEV, followed by an outflux of streamwise vorticity filaments from the secondary LEV. With increasing heave domination, the origin of stronger spanwise instability is governed by a counter-rotating trailing edge vortex and LEV that leads to growth of streamwise secondary structures. A decreasing heave domination ultimately coincides with an absence of strong LEV undulations and secondary structures. The consistent transition routes are represented on a phase-space map, where a progression of spanwise instability and growth of secondary structures becomes evident within regimes of decreased heave domination. The increasing strength of circulation for the primary LEV, with increasingStc, provides a crucial reasoning for this newly identified progression.

Numerical Analysis of the Flow Structure around Inclined Solid Cylinder and Its Effect on Bed Shear Stress Distribution

The flow-inclined cylinder interaction is an application area in the industry (i.e., offshore wind turbines and pile-supported near-shore structures). Findings of recent studies have revealed the significance of eco-friendly coastal structures that needs the utilization of inclined cylinder. The primary purpose of this study was to better understand the influence of inclination on flow, turbulence, and bed shear stress character. To achieve this objective, a three-dimensional numerical code (the Reynolds-averaged Navier-Stokes model) was used. The numerical model was calibrated based on eleven velocity profiles obtained by point measurements data of the wake region of the inclined cylinder. The mean flow, turbulence, and secondary flow characteristics around the bodies were extensively investigated, particularly at points where experimental measurements are inapplicable with intrusive turbulence measurement devices. The findings of the study revealed that as the inclination of the cylinder increased, the coherent structures that largely control the flow dynamics in the wake zone became stable rather than cyclical. Specifically, it was determined that although vorticity couples underpinned the flow field behind the vertical cylinder, large-scale streamwise vortices replaced the visible coherent structures when the cylinders were inclined (LSCSVs). When the cylinder inclined 42 degrees, the reduction in amplification factor (τ0 / τ∞) over the bed was roughly fifty percent in terms of quantity. This finding shows that inclination is a streamlined form for a cylinder and may reduce the collapse risk due to scour.

Numerical Investigation on the Effect of Geometric Parameters of Chevron Nozzle on Generation of Steamwise Vortices in High Subsonic Jets

Chevron nozzle is gaining popularity as a noise control device for jet noise reduction. Though it is the only noise control technique currently in use for commercial airliners, complete design methodology of chevrons is not available. In the present work, attempt is made to understand the effect of various geometric parameters of a chevron nozzle on the jet flow using commercial RANS based CFD solvers. Enhancement in mixing, which affects the acoustic signature of the jet, can be attributed to the strength of the streamwise vortices produced by chevrons. Hence major emphasis is given on analyzing the streamwise vortices formed in the jet flow from chevron nozzle. Various geometric parameters like different number of chevrons, their length and tip angles are employed on a 30 mm diameter nozzle producing M = 0.8 jet for further investigation.

A Boundary-Vorticity Perspective on Lift Enhancement at Ultra-Low Reynolds Numbers

The ultra-low Reynolds number flow over airfoils is considered from the perspective of boundary vorticity dynamics in this paper. The dynamic balance between the streamwise pressure gradient and vorticity creation rate at the wall is highlighted by the boundary vorticity flux. It is a localized feature with a high peak generally indicating impending boundary layer separation. Two interesting features of ultra-low Reynolds number flows are considered. Firstly, the higher values of lift coefficient with decreasing Reynolds number is seen to be due to delayed boundary layer separation. The peak in boundary vorticity flux clearly predicts the separation event. Secondly, the lift enhancement achieved by certain optimal airfoils due to a vortex trapped in the airfoil cavity is considered. An interesting double-peak in boundary vorticity flux seen for the optimal airfoil enhances the lift. An alternative interpretation of the lift enhancement is also provided by examining the Lamb vector-field.

Flow and heat transfer in a meandering channel

Fluid flows occur due to internal or external forces such as wind, gravity, pressure gradients, side-wall motion, MHD, and free convection. This study examines how meanders impact heat transfer by studying the behavior of viscous fluid flow with streamwise vortices in a sinusoidal wavy meandering channel of non-uniform radius. The study simplifies the motion and energy equations governing the fluid flow using novel transformations and a regular perturbation method. By plotting graphs for different parameter values, such as Pr, Re, and Ec, it reveals that decreasing the wavelength leads to flow separation near the channel surface. However, the stream moves forward with a sudden meander disturbance, causing the flow to become rectilinear and independent of vertex-generating centrifugal forces. The study identifies a stream function using standard and established relations. The fluid flow patterns and temperature distribution behavior are shown in various plots, highlighting the significant impact of meanders on fluid flow.

Dynamics of cavitating tip vortex

The three-dimensional dynamic behaviour of a tip vortex generated by a NACA $66_2$ -415 hydrofoil is investigated under wetted flow and cavitating conditions using time-resolved tomographic particle image velocimetry. Two main cavitation modes are studied, namely the breathing and double-helical modes. The time-averaged flow field consists of a system of two streamwise vortices for all three conditions. The shape of the tip vortex resembles that of the cavitation mode, instead of a circular cylinder. Multi-scale vortical structures are captured in the instantaneous flow field. The surface oscillation of the cavity contributes to the growth of Kelvin–Helmholtz instability over the tip vortex, leading to the onset of hairpin vortices. Stronger spanwise interaction between the tip vortex and flow separation of the hydrofoil is produced by cavitation, further intensifying the perturbation growth. The proper orthogonal decomposition analysis gives insight into the relationship between cavity oscillation and vortex instability. Two major types of unstable modes of the tip vortex are obtained, leading to serpentine centreline displacement and elliptical deformation motion. The selection of the dominant unstable mode is associated with cavity surface oscillation. For the wetted flow condition, the displacement mode dominates the growth of vortex perturbation, while for breathing mode cavitation, the most energetic unstable mode changes into an elliptical deformation pattern, the disturbance energy of which is negligible in the wetted flow condition. Consistency is found between the peak frequency of the deformation mode and the cavity resonance frequency, indicating the contribution of cavity oscillation to the disturbance growth and breakdown of the tip vortex.

Hypersonic Fluid–Thermal–Structural Interaction of Cone–Slice–Ramp: Computations with Experimental Validation

The paper presents an investigation of fluid–thermal–structural interaction (FTSI) on a complex geometry in hypersonic flows. The objective is to assess high-fidelity numerical tools for FTSI by validating two numerical frameworks against wind-tunnel data. We choose a cone–slice–ramp geometry with a wedge angle of 30 deg subjected to a laminar inflow at Mach 8. First, the time-averaged and the time-resolved laminar flow solutions are validated and highlight the mean flow features and their unsteady characteristics, including low-frequency bubble breathing and shear-layer flapping at a higher frequency. The numerical analysis reveals the presence of upstream and downstream legs of streamwise vortices. Next, FTSI analysis is performed at a tractable computational cost by decomposing the numerical frameworks into a quasi-steady FTSI (QS-FTSI) analysis and a transient fluid–structure interaction (FSI) analysis based on the disparity of the characteristic timescales. The QS-FTSI results highlight the shear-layer reattachment as the dominant source heating the structure, with nonuniform spanwise variations due to the streamwise vortices. The temperature increase drives the decrease in the structural modal frequencies. The FSI analysis was performed for an unheated, undeformed structure and a nominal heated and deformed structure. The average error in the predicted FSI frequency of the unheated panel is 2.65% in comparison to the experimental data; however, this value increases to 7.42% for the heated panel. The frequency peaks are associated with the structural mode shapes via dynamic mode decomposition analysis and show a close resemblance to natural modes, indicating weak FSI coupling. The FTSI study concludes that for a cone–slice–ramp configuration under the present inflow conditions, the interaction mostly occurs in a one-way coupled manner, where the flowfield drives the thermal and structural response of the panel.

Numerical investigation of a Mach 6 laminar shock-wave/boundary-layer interaction on a two-dimensional ramp with 3D controlled surface roughness

Direct Numerical Simulations (DNS) are performed to investigate roughness effects on a Mach 6 hypersonic compression-corner flow. Different variants of a roughness patch placed upstream of the ramp are considered in order to study their impact on the steady base flow characterized by a large separation bubble. The patches consist of sinusoidal elements, with modifications in position and spanwise extent of the roughness. Under adiabatic-wall conditions, compared to a smooth-wall case, the rough cases where the patch is upstream of the smooth-separation point cause a delayed boundary-layer separation and hence a smaller recirculation bubble. Moreover, the roughness-induced streamwise vorticity convected downstream by the detached shear layer generates mushroom-like structures once the flow reattaches on the ramp. The properties of such structures, although similar, reflect the character of the patch upstream of the bubble. The roughness inducing the largest local flow distortion is responsible for the widest structure on the ramp, characterized by the highest increase in wall-temperature. The patch occupying the entire width of the domain instead generates the weakest vortices on the ramp. No noticeable effects on the recirculation region can be noticed if the patch is placed downstream of the separation point for smooth conditions. Regarding the cold-wall case, the patch increases the heat-transfer rate on the ramp downstream of the roughness location, but with minimal effects on the bubble length. The analysis of the shear magnitude in the mushroom-shaped structures at reattachment suggests that, under adiabatic-wall conditions, the most upstream patch might have the highest transition potential.

On the origin of streamwise vortices in braid regions for compressible mixing layers

The origin of initial streamwise vortices in braid regions and their relationship with deformed spanwise vortices are numerically studied via direct numerical simulation (DNS) in the compressible mixing layer with a convective Mach number (Mc) of 0.4. Through an analysis of fast Fourier transform on DNS data, two low-rank velocity models (vd and vs) are developed to demonstrate that both deformed spanwise vortices and streamwise vortices are all originated from the linear superposition of a fundamental norm mode [mode (1, 0)], a pair of fundamental oblique modes [modes (1, ±1)], and a mean mode. Further investigations reveal that, downstream of spanwise vortices, the increase in amplitude ratio (Ao/An) between modes (1, ±1) and mode (1, 0) leads to the formation of deformed spanwise vortices in vortex regions. As the amplitude ratio Ao/An further increases, reaching the threshold that the streamwise rotation motion from modes (1, ±1) exceeds the spanwise irrotational deformation from mode (1, 0), streamwise vortices are generated in braid regions. The aforementioned formation path for deformed spanwise vortices and streamwise vortices provides a mechanism support for our flow visualization results that the emergence of deformed spanwise vortices precedes that of streamwise vortices in the flow field.

The effect of spanwise wall oscillation on spatially developing compressible transitional boundary layers

Spanwise wall oscillation is an easily implemented active control scheme. It can delay the transition to turbulent flows in incompressible boundary layers under appropriate control parameters. However, this delayed transition in compressible boundary layers has not yet been confirmed numerically. In this paper, we perform a series of direct numerical simulations of a spatially developing subsonic boundary layer with different spanwise wall oscillation periods and amplitudes. We numerically confirm the delay in the transition under appropriate control parameters. The delayed transition can be explained by the Stokes boundary layer induced by the spanwise wall oscillation affecting the streamwise vortices. The superposed spanwise velocity and streamwise vorticity observed in the Stokes boundary layer can weaken the development of instability in the original streamwise vortices under appropriate control parameters, leading to a delayed transition. Furthermore, the spanwise wall oscillation changes the flow compressibility, as can be inferred from the equation for the velocity divergence. The enhanced compressibility around the pre-transition regions also stabilizes the development of the streamwise vortices and causes a delayed transition.

Water surface response to turbulent flow over a backward-facing step

The water surface response to subcritical turbulent flow over a backward-facing step (BFS) is studied via high-resolution large-eddy simulation (LES). The LES method is validated first using data of previously reported experiments. The LES-predicted water surface is decomposed into different types of gravity waves as well as turbulence-driven forced waves. Analysis of the LES data reveals the interplay between low-frequency large-scale turbulence structures, which are the result of flow separation from the step and reattachment behind the step, and the dynamics of the water surface. The water surface deformation is mainly the result of freely propagating gravity waves and forced waves, owing to turbulence in the form of rollers and/or hairpin vortices. Gravity waves with zero group velocity define the characteristic spatial and temporal scales of the surface deformations at higher frequencies, while large eddies determine their low-frequency modulation. These deformations are mainly confined in lateral bands that propagate downstream following the advection of the near-surface streamwise vortices (rollers) that are shed from the step. Steeper surface waves are observed in regions of negative perturbation velocity gradient and down-welling, downstream of the larger rollers, and are associated with thin isolated regions of high vorticity near the surface. The investigation of such a complex flow has shown that the decomposition of the water surface fluctuations into its different physical components may be used to identify the dynamics of the underlying flow structure.