Wake Topology Research Articles

Large-Scale Side By Side Stereo-PIV Measurements In An Automotive Wind Tunnel: Aerodynamic Influence Of Ride Height Variations.

In this investigation, we conducted large-scale Stereo PIV experiments in the flow around a Volkswagen ID.4. within the Aerodynamic and Aeroacoustic Wind Tunnel (AAK) at Volkswagen AG, Wolfsburg. The primary objective of these experiments was to investigate the effects of ride height variation on the near wake of the passenger vehicle using Stereo PIV, while minimizing wind tunnel occupancy. The measurements aimed to create datasets for CFD validation to improve virtual aerodynamic development capabilities of passenger vehicles. The experiment was performed on a 1:1 scale vehicle and multiple cross-sectional planes were scanned along the longitudinal axis of the vehicle. The impact of ride height alterations on the wake flow topology were evaluated. Average velocity vector fields were computed per case, providing comprehensive insights into the aerodynamic effects of diverse vehicle setups. A small change in the ride height has shown to have a significant impact on the wake topology. The results show that the electric SUV ID.4 can yield a wake topology similar to the estate back and fast/notchback type of vehicle, depending on the ride height. The normal ride height of the ID.4 shows to have a broader but lower wake region, with a significant downwash. The PIV results of this configuration show significant similarities to the wake topology of a fast back vehicle. Lowering the ride height by 15 mm has shown already to be enough to cause a global effect on the wake topology and to exhibit a narrower wake and recirculation region, like an estate back vehicle. Additionally, the association between the drag/lift coefficients and the wake topology of both configurations was discussed. Results have shown that sole measurements in the wake are not sufficient to fully explain changes in the determined drag or front lift coefficients. The rear lift coefficients however, indicate a stronger relationship with the near wake topology of the vehicle.

Effects of Reynolds Number on the Wake Characteristics of a Notchback Ahmed Body

Abstract This paper investigates the effects of Reynolds number on the wake characteristics of a notchback Ahmed body with effective backlight angle, βe=17.8 deg. The Reynolds number based on the body height was varied from 5×103 to 5×104. Prior to the Reynolds number investigation, a Reynolds-averaged Navier–Stokes (RANS) model assessment was performed using nine turbulence models consisting of one- and two-equation eddy-viscosity models and second moment closure models. The standard Spalart–Allmaras model was the only model that accurately predicted the asymmetric time-averaged wake topology, as reported in previous studies, for the βe=17.8 deg notchback Ahmed body at Re=5×104. The drag coefficient decreased with increasing Reynolds number, while the lift coefficient remained constant for Re≥1×104. The wake structure exhibited three regimes: symmetric (Re≤1×104), transitionally asymmetric (1×104<Re≤3.5×104), and fully asymmetric (Re>3.5×104) states. The wake asymmetry was attributed to an imbalance in entrainment from the sides and asymmetric separation from the roof and the C-pillars of the body. The tilting and stretching terms in the vorticity transport equation were used to provide insight into the source of asymmetry in the vorticity field around the body.

Interaction between the helical vortices shed by contra-rotating propellers

Large eddy simulation is adopted to analyze the interaction between the tip vortices shed by two contra-rotating propellers, by using a computational grid consisting of 4.6 × 109 points. Despite the complexity of the wake topology, the results of the computations show an excellent agreement with the measurements from an earlier experimental study on the same system. The interaction between the tip vortices shed by the two propellers produces vortex rings. Each of them consists of six helical sides, which are connected by U-shaped vortex lobes. The three upstream lobes of each vortex ring move to outer radial coordinates, as a result of their shear with the downstream lobes of the upstream vortex ring. In contrast, the downstream U-shaped lobes move to inner radial coordinates, as a result of their shear with the upstream lobes of the downstream vortex ring. This interaction results in an overall expansion of the wake of the contra-rotating propellers. The regions of shear between the U-shaped lobes of consecutive vortex rings are the areas of the largest turbulent stresses, which achieve higher levels than those produced in the wake of the two front and rear propellers working alone. This complex flow physics also triggers a faster instability of the wake system, breaking its coherence at more upstream coordinates, in comparison with the isolated propellers.

Particulate dispersion in turbulent wake of Ahmed body and experimental investigation of impact of rear slant angle

This study investigates the particle dispersion characteristics in the turbulent wake of a simplified vehicle model (Ahmed body) for two values of the rear slant angle ϕ (25° and 40°) to study the effect of flow separation. In the experiments (Reynolds number Rel=1.90 ×105), smoke particles were released from a source and visualized with a laser sheet. Concentration fields were analyzed to calculate the vertical (Sy) and lateral (Sz) smoke spread. The findings indicate that the flow topology and concentration fields in the wake are highly dependent on ϕ. In the ϕ=40° case, separation on the rear slant disrupts the trailing vortices originating on the rear slant edges and significantly alters the wake topology. The growth of vertical smoke spread saturates after the recirculation region. The uniform mixing and absence of trailing vortices concentrate the smoke particles in the model midplane. In the ϕ=25° case, the signatures of trailing vortices were observed in the concentration fields behind the model. The vertical smoke spread is less, and the lateral smoke spread is more compared to the ϕ=40° case. The growth of the smoke spread (Sy, Sz) driven by the trailing vortices persists for a long distance, even after the recirculation region. The disruption of trailing vortices brought about by the flow separation appears to be an important effect driving the vertical smoke spread in the wake. The connection between the turbulent velocity structures and concentration structures will need to be explored with combined velocity and concentration measurements in the wake.

Evolution of wake structure with aspect ratio behind a thin pitching panel

The evolution of wake structures behind thin pitching panels at Reynolds number 1000 is studied numerically for a wide range of aspect ratios (0.54≤AR≤16). At the maximum pitching angle (θmax) of 5° and 15°, simulations are performed for pitching frequency St=0.5,1. The higher pitching angle and frequency promote thrust generation for all the AR. Although the propulsive power grows with AR in the thrust cases, the propulsion efficiency remains insensitive to the AR. Higher AR imparts stabilizing effect on the flow in the drag regime, though it triggers instabilities with asymmetric vortex pairing in the thrust cases. Wake topology for the drag cases reveals the emergence of horseshoe vortices and bridgelets-type vortex structures within the range of AR studied. The wake consists of entangled vortices undergoing subsequent reconnections in the thrust cases. A smaller θmax and St induces organized 3D structures for AR≤4, although the end instabilities diffuse for increasing AR, resulting in a 2D wake signature at larger AR. The spanwise circulation data show the dominant influence of primary instability in rendering a high horizontal velocity-compressed separated region in the drag regime.

Mean wake evolution behind low aspect-ratio wall-mounted finite prisms

The evolution in the mean wake of wall-mounted prisms with varying depth-ratio (length-to-width) between 0.016 and 4 is examined numerically over a range of Reynolds numbers (Re = 50 - 500). The aspect-ratio (height-to-width) of the prism is limited to 1. This study aims to ascertain the mechanism of the evolution of mean wake topology due to changing depth-ratio. The mean wake topology is classified into Dipole, Multipole, and Quadrupole types as functions of Reynolds number and depth-ratio. The threshold depth-ratio for the wake evolution changes with Reynolds number. The Multipole-type wake appears as an evolutionary or intermediate wake pattern between Dipole and Quadrupole-type wakes. Increasing the depth-ratio leads to the enhancement of the downwash flow, resulting in the mean wake evolving from Quadrupole to Dipole wake. This suggests that the downwash flow dominates in dictating the wake topology downstream of the prism. Enhancing the downwash flow (with increase in depth-ratio) results in the reattachment of the flow on the prism surfaces, which further results in the suppression of velocity gradients and streamwise momentum transfer downstream of the prism trailing edge. This suppression of gradients and momentum leads to the deterioration of the base vortex, which evolves the wake from Quadrupole (at a small depth-ratio) to Dipole-type (at a large depth-ratio).

Effect of upstream flow characteristics on the wake topology of a square-back truck

The influence of upstream flow characteristics on the bi-stable flow structure in the wake region of a simplified square-back heavy vehicle model at a Reynolds number of 2.7 × 104 was investigated by using the improved delayed detached eddy simulation method. The asymmetric wake structure of this model and its corresponding aerodynamic response were examined, aiming to identify the effect mechanism of three inlet profiles on the asymmetric wake structure of the named ground transportation system (GTS) model in simulations. The accuracy of the numerical method used in this study was validated by comparison with wake structure data, including the flow states, vortex core's location, and aerodynamic drag obtained from previous large eddy simulations and water channel experiments. The numerical results show that different turbulent inlet velocity profiles lead to different wake topologies. When the turbulent velocity profile with a turbulence intensity of 15% generated by TurbSim, a stochastic inflow turbulence tool for generating turbulent velocity inlet on an atmospheric boundary layer profile, is used, the expected bi-stable flow topology is still observed, but it is not shown in the case by means of the turbulence generator incorporated into ANSYS Fluent. Those turbulent inlet velocity profiles contribute to the increase in GTS model's aerodynamic drag forces. Compared to the uniform velocity profile, the TurbSim velocity profile can achieve a drag increase in 7.23%. In addition, this turbulent profile intensifies the flow fluctuations in the wake region and enhances the transient response frequency of the wake region. Thus, when assessing the vehicle aerodynamic performance in open air, especially under crosswinds, the real turbulence velocity profile, e.g., the profile generated by TurbSim in the current study, is recommended to be used for a more accurate prediction in numerical simulations.

Performance improvement of a wing with a controlled spanwise bending wingtip

This paper aims to investigate the unsteady force characteristics and performance improvement of a wing with a controlled morphing wingtip. Taking the geometry nonlinearity of morphing into consideration, a new predictor-corrector method is developed to get the updated coordinates of nodes on the morphing wing surface. The effects of morphing frequency and amplitude are investigated. The results indicate that the unsteady force amplitude increases with the morphing frequency and amplitude. Meanwhile, the morphing amplitude affects the phase and harmonic characteristics of the force. The wingtip morphing influence on the unsteady force is divided into the shape effect and the additional angle of attack effect. The force can be described by a simple scaling law using a generalized Duffing-van-der-Pol nonlinear model. This scaling law depicts the nonlinear effect of large amplitude bending in essence. Afterward, the in-depth wake structures analysis of the flow field finds that the morphing frequency does not affect the essential feature of wake topology, while the morphing amplitude does. As the morphing amplitude increases, the vortical structures shift from vortical tubes to ring-like vortices, which induces the change of the phase and harmonic characteristic of force response.

Experimental study on energy harvesting maximization from the flow-induced vibration of a highly confined bluff body

Flow-induced vibrations of rigid prisms supported elastically were studied experimentally in a free-surface water channel with a high blockage (2/5). The study focused on finding the prism cross-sectional shape that maximizes the efficiency of energy harvesting. Seven cross-sectional shapes were tested: square, circular, 45° tilted square, equilateral triangle, isosceles 120° triangle, D-section, and C-section. All other dimensionless parameters of the problem, mass ratio, damping, blockage ratio, reduced velocity range, and the Reynolds (Re) number (characteristic velocity times characteristic length divided by kinematic viscosity) range (400–1070), were kept unchanged. By doing so, the effect of the cross-sectional shape was isolated. D-section proved to be the geometry with the highest values of energy transfer efficiency. A hysteresis loop was present in its oscillatory response (dimensionless oscillation amplitude vs reduced velocity). This loop was characterized by two branches, (+) and (−), meaning a bi-valued amplitude response for each reduced velocity. Regarding temporal patterns of wake topology and body motion, it was found that synchronization occurs in the (+) branch, but not in the (−). Regarding vortex shedding modes, particle image velocimetry was used for identification purposes, and it was found that the 2P mode is the dominant mode in the (+) branch, while the 2S mode pervades the (−). Finally, a new relative reduced velocity definition was introduced, and, when re-plotting the experimental results, it was found that the hysteresis loop disappears, thereby providing a more compact mathematical description of the observed phenomena.

Modelling of two tandem floating offshore wind turbines using an actuator line model

The aerodynamic and wake recovery dynamics of floating offshore wind turbines differ from fixed turbines due to the platform motions. Understanding tandem rotor interactions is essential for both turbines as well as wind farm design. This paper investigates the wake interactions in offshore wind farms by studying the effect of the upstream turbine motion on the downstream wind turbine loads and performance. A previously developed and validated Navier-Stokes actuator line model is used and implemented in the OpenFOAM® solver. The NREL 5 MW turbine is selected as a reference, and the upstream turbine is prescribed both surging and pitching motions (of different amplitude) while the downstream turbine is maintained fixed. Results for the turbine loading, wake and flow development are presented. It was found that the peak-to-peak thrust and power variations depend on modelling the discrete nature of the blades. Although the discrete tip vortices in fixed conditions diffuse within the first two diameters, downstream of the rotor, the platform motion can transform them into a new wake topology form with discrete ring shapes. The frequency spectra of the parameters showed a significant impact from these motion-induced discrete rings. The results indicate the need for higher fidelity modelling approaches when studying floating wind turbine interactions.

Impact of the train heights on the aerodynamic behaviour of a high-speed train

The impact of train heights on train aerodynamic performance is studied by using an improved delayed detached-eddy simulation (IDDES) method. The correctness of the numerical method has been verified by the existing wind tunnel and moving model experiments data. The aerodynamic drag, lift, slipstream, and wake flow are compared for three train heights. The results presented that the drag and lift increased by 6.2% and 23.8% respectively, with an increase in train height from 3.89 m to 4.19 m. Compared with the 3.89 m case, the maximum time-averaged slipstream at the platform location for 4.04 and 4.19 m cases are increased by 2.0% and 4.3% respectively. Meanwhile, the wake topology for three cases is described and analyzed quantitatively. The downwash angle of the wake longitudinal flow is increased with the increasing train height, resulting in the mixing of the downwash flow and the ground flow in advance. The wake in the higher trains tends to develop outward and downward. Besides, the higher trains will also bring greater transient aerodynamic loads to the equipment above the train. It’s recommended to shorten the maintenance period of the electrical equipment above the higher trains to ensure the devices’ safety.Abbreviations: CFL: Courant–Friedrichs-Lewy; COT: Center of the track; FDR: Flow development region; FFT: Fast Fourier transform; GF: Ground-fixed reference system; ICE3: Intercity Express 3; IDDES: Improved delayed detached-eddy simulation; LES: Large-eddy simulation; LV: Longitudinal vortex; MME: Moving model experiments; NBL: Negative bifurcation line; PBL: Positive bifurcation line; PSD: Power spectral density; RANS: Reynolds averaged Navier – Stokes; SF: Stable focus; SP: Saddle point; STBR: Single-track ballast and rails; SV: Spanwise vortex; TF: Train-fixed reference system; TOR: Top of the track; TSI: Technical specification for interoperability; UN: Unstable node; WPR: Wake propagation region

Scalar transport in flow past finite circular patches of tall roughness

Simultaneous particle image velocimetry-planar laser-induced fluorescence (PIV-PLIF) measurements are conducted on the flow around and downstream of patches of tall cylinders with various solidity ϕ≡Nc(d/D)2, where Nc is the number of cylinders in a patch, d and D are the diameters of the cylinder and the patch, respectively. Results suggest that the bleeding and scalar concentration in the wake and above the patches are dependent upon ϕ. Recirculation bubble appears at downstream of the patch boundary, limiting the flow from convecting downstream and forcing it (and the scalar) to escape vertically from the top of the patch. Above the wake, a shear layer generated by the velocity difference between the free flow and the wake itself shields the near-wake region. The net effect of the solidity on the wake topology is the appearance of an elongated patch as ϕ decreases and a taller patch as ϕ increases. In terms of scalar transport, both advective and turbulent vertical scalar transport are enhanced as ϕ increases. High advective vertical scalar transport CV is concentrated around the canopy (above the canopy due to vertical bleeding and at the TE due to high scalar concentration), while high turbulent transport c′v′¯ largely coincides with the shear layer. Octant analysis of the conditionally averaged fluctuating components (u′, v′, and c′) shows an enhanced mixing inside the shear layer and entrainment of turbulent flows between the shear layer, the wake region below and the freestream above this layer. In terms of eddy diffusivity models, the complexity of the eddy diffusivity constant increases with ϕ.

Validation of PANS and effects of ground and wheel motion on the aerodynamic behaviours of a square-back van

This paper presents a numerical investigation of the effects of the moving ground and rotating wheels on the turbulent flow around a 1/10 scaled square-back van model. A comprehensive comparison among the partially averaged Navier–Stokes (PANS), large eddy simulation (LES) and particle image velocimetry (PIV) involving the aerodynamic drag, the wake topology, the velocity and the Reynolds stress profiles in the wake region is conducted. The proper orthogonal decomposition (POD) and fast Fourier transform (FFT) are applied to the shear layers shedding from the trailing edges to comment on the coherent structures and their frequency content. The Reynolds number for both simulations and experiments is set to Re = 2.5 × 105 based on the inlet velocity ${U_{inf}} = 9\;\textrm{m}\;{\textrm{s}^{ - 1}}$ and the width of the model W = 0.17 m. The results show that PANS accurately predicts the flow field measured in experiments and predicted by a resolved LES, even with a low-resolution grid. The superiority of the PANS approach could provide good guidance for industrial research in predicting the turbulent flow around the square-back van model with affordable computational grids. The ground and wheel motion mechanism on the aerodynamic forces has been revealed by analysing the surface pressure distribution, the wheels’ surrounding flow, the underbody flow characteristics and the turbulent wake structures. The effects of the ground and wheel motion on the frequency, evolution and development characteristics of the wake shear layers are analysed, thus providing relevant insights for future experimental investigations of square-back van models.

Hydrodynamic Performance and Wake Topology of Schooling Fish in Three-Dimensional Formations

Photo Gallery Hydrodynamic Performance and Wake Topology of Schooling Fish in Three-Dimensional Formations Yu Pan, Yu Pan Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA e-mail: yp8xd@virginia.edu Search for other works by this author on: This Site PubMed Google Scholar Haibo Dong Haibo Dong Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA e-mail: haibo.dong@virginia.edu Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information Yu Pan Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA Haibo Dong Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA e-mail: yp8xd@virginia.edu e-mail: haibo.dong@virginia.edu J. Fluids Eng. Jun 2023, 145(6): 060905 (1 pages) Paper No: FE-23-1066 https://doi.org/10.1115/1.4057072 Published Online: March 10, 2023 Article history Received: February 10, 2023 Revised: February 26, 2023 Accepted: February 27, 2023

Comparison of flow characteristics behind squareback bluff-bodies with and without wheels

The wake dynamics of two referenced variations of the squareback Windsor model with and without wheels is numerically studied by performing improved delayed detached eddy simulation. Numerical assessments are validated against publicly available experimental data. The focus of this study is on the wake states influenced by the wheels and the thick oncoming floor boundary layer. Results show that the addition of the wheels significantly changes the aerodynamic forces, the underbody flow, and the wake topology. The wake bi-stability is also enhanced with wheels in place due to the increased curvature of lateral shear layers in the near wake. However, the bi-stable behavior is largely suppressed when immersed in a thick boundary layer. These alterations depend on the degree of interaction between the wake recirculation and the bottom flow, and such degree is strongly affected by the underbody flow momentum. The evolution of low-order flow organizations and complementary spectral analysis highlight the differences in the coherent dynamics of the wake. The finding of this present work suggests that the wake bi-stability behind the squareback body can exist not only for a simplified geometry but also for a more realistic car with wheels in real-world upstream conditions.

Numerical study of viscoelastic flow around an oscillating circular cylinder

The viscoelasticity-induced fluid–structure interaction studies have a significant influence on practical applications. To clarify the lock-in phenomenon and the wake topology of the vibrating cylinder placed in the viscoelastic flow, the Oldroyd-B fluid flows around an oscillating circular cylinder have been numerically investigated at Re = 10 and Re = 60, respectively. The governing equations are solved by the coupling of the square-root-conformation representation approach and the discontinuous Galerkin method in framework of the high-order dual splitting scheme. In addition, the arbitrary Lagrangian–Eulerian formulation is implemented in the coupling procedure in order to account for the interaction between the fluid and the oscillating body in the flow field. With this, complex boundary movements can be tackled simply and efficiently. In numerical simulation, the force coefficients and the wake structures of vortex and stress are discussed in some detail. At Re = 10, when the frequency of cylinder is small, it is obvious that the vortex shedding takes place in the wake. As the frequency increases, almost no obvious vortex shedding is observed. Also, the wake still oscillates at the same frequency of the cylinder for all cases, even for high Wi numbers. However, different wake modes of vortex and stress are found for various frequencies at Re = 60 and Wi = 0.1. In the lock-in region, the 2S mode of wake type are observed. Beyond the lock-in region, the wake type is no longer 2S, but the formation of vortex shedding and stress distribution in the far wake recovers to its natural mode. These numerical results open up a new field of study for viscoelastic fluids.

On the wake deflection of vertical axis wind turbines by pitched blades

AbstractWake losses are a critical consideration in wind farm design. The ability to steer and deform wakes can result in increased wind farm power density and reduced energy costs and can be used to optimize wind farm designs. This study investigates the wake deflection of a vertical axis wind turbine (VAWT) experimentally, emphasizing the effect of different load distributions on the wake convection and mixing. A trailing vortex system responsible for the wake topology is hypothesized based on a simplified vorticity equation that describes the relationship between load distribution and its vortex generation; the proposed vorticity system and the resulting wake topology are experimentally validated in the wind tunnel via stereoscopic particle image velocimetry measurements of the flow field at several wake cross‐sections. Variations in load distribution are accomplished by a set of fixed blade pitches. The experimental results not only validate the predicted vorticity system but also highlight the critical role of the streamwise vorticity component in the deflection and deformation of the wake, thus affecting the momentum and energy recoveries. The evaluation of the various loading cases demonstrates the significant effect of the wake deflection on the wind power available to a downwind turbine, even when the distance between the two turbines is only three diameters.

Assessment of a RANS Transition Model with Flapping Foils at Moderate Reynolds Numbers

Numerical simulations based on a high-order discontinuous Galerkin solver were performed to investigate two-dimensional flapping foils at moderate Reynolds numbers, moving with different prescribed harmonic motion laws. A Spalart–Allmaras RANS model with and without an algebraic local transition modification was employed for the resolution of multiple kinematic configurations, considering both moderate-frequency large-amplitude flapping and high-frequency small-amplitude pure heaving. The propulsive performance of the airfoils with the two modelling approaches were tested by referring to experimental and (scale-resolving) numerical data available in the literature. The results show an increase in effectiveness in predicting loads when applying the transition model. This is particularly true at low Strouhal numbers when, after laminar separation at the leading edge, vorticity dynamics appears to have a strong effect on the forces exerted on the profile. Specifically, the transition model more accurately predicts the wake topology emerging in the flow field, which is the primary influence on thrust/drag generation.

Flow characteristics and wake topology of two-seat convertibles

This study aims to clarify the flow characteristics and the wake structure of convertible vehicles. Numerical simulations are performed to obtain a preliminary visualization, and the potential vortical motion characteristics are investigated by examining the Q-criterion across multiple cross sections. Comparisons between numerical and experimental results validate the reasonableness of our numerical model. The predominant wake topology of a two-seat convertible is obtained in terms of the location, shape, and spin direction of the vortices. We observe a “nook” vortex that is triggered by the flow acceleration induced by the pressure gradient near the windshield step, provoking undesirable aeroacoustic noise and degrading the cabin comfort. Complicated A-pillar vortex dynamics are revealed, with small vortices that are shed into the cabin and impinge the seats, eventually forming a long tail structure above the back of the vehicle. Moreover, periodic fluctuations of the windshield vortex are induced by the Kelvin–Helmholtz instability, significant impacting the streamwise wake. Ultimately, the combined motion characteristics of the A-pillar and windshield vortices exert undesirable effects on the aeroacoustic noise and drag, suggesting fundamental mechanisms for achieving optimal energy-saving and acoustic convertibles in the future. Based on the wake topology and the vortical generating mechanism, some approaches are proposed to reduce the drag and aeroacoustic noise by impeding the flow over the door into the cabin, modifying the shape of windshield step, and lengthening the windshield in stream direction.

The effect of eroded leading edge to the turbulent wake topology studied by PIV

The effect of leading edge erosion to the flow is studied experimentally by standard method of Particle Image Velocimetry. Three airfoil models are manufactured: the reference one, the homogenously eroded one, which is modeled by putting a sheet of sand-paper to its leading edge and the airfoil with large erosion, which is modeled by regular spikes and a valley in sub-stagnation area. The damaged airfoils support earlier transition to turbulence; in fully turbulent regime, their wake is wider than in the smooth case and, especially in the case of regular spikes, the spanwise fluctuations are reduced.