Formation Of Streamwise Vortices Research Articles

Air Interactions of Magnetically Driven Plasma Discharges in Crossflow

Motivated by the use of magnetohydrodynamic devices in fluid dynamic flow control research, the current work experimentally studied the actuation mechanism and the resulting aerodynamic interactions of magnetically driven low-current arc-plasma discharges. The investigation was conducted in the context of boundary-layer interactions in low-speed crossflow conditions through the use of coaxial and v-shaped geometries. Time-averaged velocity field measurements showed that the toroidal region of vorticity induced by the coaxial geometry in quiescent air resulted in the formation of a horseshoe vortex in crossflow. Similarly, the v-shaped actuator resulted in the formation of streamwise vortices. The resulting boundary layers had s-shaped streamwise velocity profiles as measured in the wall-normal direction characteristic for the flow downstream of a conventional vortex generator pair, due to the three-dimensional mixing induced by coherent vortex structures. A simplified model based on intermolecular momentum transfer through collisions is proposed to explain the fundamental discharge–air interactions and the induced flowfields. These results inform the applications of various magnetically driven discharges in the field of aerodynamic flow control.

Global and local analyses of the Görtler instability in hypersonic flow

The stationary Görtler instability in hypersonic flow over a concave wall is systematically investigated across a range of geometric and flow parameters using resolvent analysis, which seeks for the forcing and response pair that maximizes the energy amplification. The optimal forcing takes the form of streamwise vortices, while the optimal response is streamwise streaks. The growth of the optimal disturbance is contributed by both the lift-up and centrifugal mechanisms. The latter becomes dominant as the boundary layer develops, and its growth rate agrees well with that predicted by local stability analysis. In terms of changes in geometric parameters, an increase in curvature destabilizes the Görtler instability, as expected, while the effect of the angle subtended by the concave wall (the turning angle) is shown to be negligible. With respect to changes in flow parameters, the Görtler instability is stabilized at low Reynolds numbers, destabilized under the cold-wall effect, and insensitive to the change in Mach number. The most amplified spanwise wavelength scales with the boundary-layer thickness, which remains mostly unchanged when the freestream Mach number is varied from 3 to 10. A new dimensionless wavelength parameter is proposed to predict the wavelength of the most dangerous Görtler vortices in the compressible flow regime. The resolvent analysis results are confirmed by a three-dimensional numerical simulation, where the hypersonic flow is perturbed by a spatial white noise.

Turbulence suppression by streamwise-varying wall rotation in pipe flow

Direct numerical simulations of turbulent pipe flow subjected to streamwise-varying wall rotation are performed. This control method is able to achieve drag reduction and even relaminarize the flow under certain control parameters at friction Reynolds number $Re_\tau =180$ . Two control parameters, which are velocity amplitude and wavelength, are considered. It is found that increasing the wavelength rather than increasing the amplitude seems to be a better choice to improve the control efficiency. An annular boundary layer, called the spatial Stokes layer (SSL), is formed by the wall rotation. Based on the thickness of the SSL, two types of drag-reduction scenarios can be identified roughly. When the thickness is low, the SSL acts as a spacer layer, inhibiting the formation of streamwise vortices and thereby reducing the shear stress. The flow structures outside the SSL are stretched in the streamwise direction due to the increased velocity gradient. Within the SSL, the turbulence intensity diminishes dramatically. When the thickness is large, a streamwise wavy pattern of near-wall streaks is formed. The streak orientation is dominated by the mean shear-strain vector outside the viscous sublayer, and there is a phase difference between the streak orientation and local mean velocity vector. The streamwise scales of near-wall flow structures are reduced significantly, resulting in the disruption of downstream development of flow structures and hence leading to the drag reduction. Furthermore, it is found that it requires both large enough thickness of the SSL and velocity amplitude to relaminarize the turbulence. The relaminarization mechanism is that the annular SSL can absorb energy continuously from wall-normal stress due to the rotational effect, thereby the turbulence self-sustaining process cannot be maintained. For the relaminarization cases, the laminar state is stable to even extremely large perturbations, which possibly makes the laminar state the only fixed point for the whole system.

Numerical investigation on streamwise vortex generation by plasma actuator

To clarify vortex formation and development, numerical simulations of a streamwise vortex generation by a dielectric-barrier-discharge plasma actuator were performed. The simulations are set based on the experiment by Jukes and Choi [“Dielectric-barrier-discharge vortex generators: Characterisation and optimisation for flow separation control,” Exp. Fluids 52, 329 (2012)], which is a laminar boundary-layer flow with a plasma actuator aligned to the freestream direction (x). The input momentum by the plasma actuator varies in the low (CaseL), medium (CaseM), and high (CaseH) cases. The streamwise vortex distributions for CaseL and CaseM are similar, and the vortex developments qualitatively agree with the experimental result. In these cases, the single streamwise vortex with the negative x-vorticity is mainly generated by the plasma actuator. For CaseH, however, the generated vortex is made up of both the main streamwise vortex with negative x-vorticity and the helical vortices with positive x-vorticity surrounding the main vortex. These encircling vortices twist the main vortex. Scaling the vortex characteristics, as proposed by Jukes and Choi [“On the formation of streamwise vortices by plasma vortex generators,” J. Fluid Mech. 733, 370 (2013)], demonstrates that the vortex characteristics can be scaled for simulation results. The streamline visualization clarifies the streamwise vortex formation by the induced flow from the plasma actuator and entrainment of the freestream flow to the streamwise vortex. The transient behavior of the streamwise vortex formation is examined for CaseM and CaseH. CaseM and CaseH have distinct transient behavior in the formation of the main streamwise vortex and the surrounding vortices.

PIV Analysis Of Skin Friction And Coherent Structures In Turbulent Drag Reduction Regimes

In the present work, planar and tomographic PIV are used to investigate how the organisation of wall turbulence is altered when actuators are operated with the objective of reducing the skin friction drag. When the wall is mechanically oscillated in the spanwise direction, high-resolution planar PIV enable the direct measurement of wall shear and a 15% reduction of skin friction is observed. The use of tomographic PIV enables access to the threedimensional organisation of low- and high-speed streaks, along with ejection events and associated hairpin vortices. These observations help forming a conceptual model of the salient drag reduction mechanism, whereby hairpin autogeneration is inhibited through a tilting action at the tail of low-speed streaks. The second part of the study documents an effort to surrogate the mechanical oscillation by means of a densely distributed array of AC-DBD plasma actuators. The latter are first characterised in quiescent flow, where the induced velocity distribution is obtained and compared to the solution of the classical second Stokes problem that models the mechanical oscillation. The induced peak of spanwise velocity is found to surpass the velocity of the oscillating wall. However, the wall jet height develops further away from the wall. More importantly, the spatially inhomogeneous distribution of the unsteady body force produces an unwanted lattice of starting streamwise vortices. The latter are deemed to be detrimental for the purpose of drag reduction, compared to the orderly and homogeneous sideways motion induced by mechanical wall motion. The application of the AC-DBD actuator currently leads to a pronounced momentum deficit in the logarithmic region and increases skin friction. Finally, further research directions are anticipated, that potentially circumvent the formation of streamwise vortices by means of AC-DBD actuators operating in steady regime.

The Wake Structure of a Stationary and a Rotary Wall-Mounted Low-Aspect-Ratio Cylinder

This paper investigates the wake structure of a low-aspect-ratio rotating cylinder mounted wall-normal to a flat plate at a diameter-based Reynolds number of . The cylinder is driven at a constant rotational rate about its respective axial coordinate. Four nondimensional rotation rates were selected: 0.5, 1, 2, and 3, representing rates based on the static cylinder’s natural shedding frequency and corresponding harmonics. Two aspect ratios were investigated: i) and ii) . The case of represents a cylinder height equal to the local boundary-layer thickness. Both streamwise and cross-stream (transverse) particle image velocimetry (PIV) planes are presented, and the resulting time-averaged flowfield is examined. Results for the rotating cylinder reveal the development of a dominant streamwise vortex that persists for several length scales downstream of the cylinder. Accompanying the streamwise vortex formation is an enhancement in the wall-normal velocity component (downwash) behind the rotating cylinder. Additionally, the maximum downwash occurs immediately behind the rotating cylinder near 0.27 diameters downstream in contradistinction to a stationary cylinder with peak downwash near 2–3 diameters. The maximum downwash, being in line with the formation of the dominant streamwise vortex, shortens the recirculation region in the cylinder’s wake, thus shifting the plate reattachment point further upstream. In the case of , where the cylinder is submerged within the boundary layer, although the length of the recirculation region reflected by the mean flow is lengthened by the rotary motion, the maximum downwash for the rotating cylinder is enhanced by a factor of 10 from that of the maximum downwash experienced by a static counterpart.

Control of Fin Shock Induced Flow Separation Using Vortex Generators

This work demonstrates the control of the shock-induced separated flow generated by a sharp fin placed on a cylindrical surface (fin-on-cylinder shock-wave/boundary-layer interaction) with an incident Mach 2.5 flow. Well-quantified distortions in the form of streamwise vortices are embedded into the boundary layer using single sub-boundary-layer vortex generators (VGs). At four device heights downstream from the VG trailing edge, the boundary layer exhibits a peak deficit in the streamwise velocity from the undistorted boundary layer of 60%. At 15 device heights, the boundary layer was energized to a peak surplus of 25% in the streamwise velocity. These two VG streamwise placements (four and 15 device heights) were employed to emphasize the impact of direct vortex interactions and boundary-layer energization on separation vortex control. It was found that having an energized boundary-layer incident on the fin-on-cylinder SBLI made very little change to the separation vortex size and the associated pressure distribution. However, placing the vortex generator much closer significantly mitigated the separation vortex size, and several azimuthal placement locations showed an elimination of the separation vortex. Cross-sectional velocity and vorticity fields showed a strong reduction in spanwise velocity and streamwise vorticity above the separation vortex in the successful configurations.

Effect of undulating blades on highly loaded compressor cascade performance

Humpback whale’s flipper with leading-edge tubercles has been attracting aerodynamic and hydrodynamic researchers’ attentions by its stall-delayed characteristics. Inspired by this, the undulating configuration is used in a highly loaded compressor cascade as a new type of passive flow control technique. A new model of undulating compressor blade is studied in this paper. To investigate the effect of the undulating configuration on cascade performance without the impact of endwall, steady Reynolds-averaged Navier–Stokes simulations of infinite-span cascades are carried out with and without undulations at an inlet Mach number of 0.5. A parametric study is performed to conclude that, with a suitable wavelength, the undulating blade could achieve a rise in diffusion capacity, accompanied by 12.9% reduction in total pressure loss coefficient at a post-stall incidence angle of 8°, whereas it produces negligible impact in cascade performance at 0° incidence angle. Flow visualization further reveals that wavelength is a crucial parameter, determining the spanwise space for the formation of streamwise vortices. Undulating blades could produce positive effects with maximum magnitude when the counter-rotating streamwise vortices take dominant position along span with an appropriate size.

Evolution of elliptic synthetic jets at low Reynolds number

The influence of the nozzle aspect ratio ($AR=1$, 2 and 4), stroke length ($L_{0}=1.85$, 3.7 and 5.55) and Reynolds number ($Re=79$, 158, 316 and 632) on the behaviour of elliptic synthetic jets is studied experimentally. Laser-induced fluorescence and two-dimensional and stereoscopic particle image velocimetry are used to analyse the vortex dynamics and evolution mechanism. It is found that the fluid elements around the major axis of an elliptic vortex ring move downstream faster and tend to approach the centreline, while the fluid elements around the minor axis move downstream at a slower speed and away from the centreline, thereby resulting in the occurrence of the well-known axis-switching phenomenon for elliptic synthetic jets. During this process, a pair of arc-like vortices forms ahead of the primary vortex ring, and they are constituted by streamwise vortices in the leg part and spanwise vortices in the head part; two pairs of streamwise vortices form from the inside of the primary vortex ring and develop in the tails. The streamwise vortices are pushed away progressively from the centreline by the synthetic jet vortex rings that are formed during the subsequent periods. These additional vortical structures for non-circular synthetic jets show regular and periodic characteristics, which are quite different from the previous findings for non-circular jets. Their mutual interaction with the vortex ring causes significant changes in the topology of elliptic synthetic jets, which further results in the variation of the statistical characteristics. Increasing the aspect ratio, stroke length and Reynolds number will make the evolution of the synthetic jet become more unstable and complex. In addition, the entrainment rate of an elliptical synthetic jet is larger than that of a circular synthetic jet and it increases with the nozzle aspect ratio ($AR\leqslant 4$) and Reynolds number. It is indicated that the formation of streamwise vortices could enhance the entrainment rate. This finding provides substantial evidence for the potential application of elliptic synthetic jets for effective flow control.

Two-dimensionalization of a three-dimensional bluff body wake

The three-dimensional flow characteristics of a circular cylinder with synthetic jet control are numerically studied using large eddy simulation. The Reynolds number based on the diameter of the cylinder is Re = 500. The control effects and underlying mechanism are revealed to show how the synthetic jet changes the three-dimensional wake pattern. Analysis of the dynamic control process indicates that the blowing stroke helps the shear layer to assemble vorticity, and then, the suction stroke accelerates the detachment of the concentrated vorticity. The vortex shedding process will be gradually dominated by symmetric actuation of the synthetic jets. Thus, the asymmetric vortex shedding mode could be changed into a symmetric mode several periods after actuation at certain excitation frequencies, leading to significant suppression of lift fluctuations. A periodic pressure variation at the leeward surface of the circular cylinder caused by the changes of the separation point for the flow over a circular cylinder and recirculation region results in a large drag fluctuation. The excitation phase influences only the control process, but not the final state, while the excitation frequency plays an important role in the formation of different wake patterns. It is also found that the synthetic jet can completely suppress the formation of streamwise vortices due to the three-dimensional instability suppression and reduce the deformation of spanwise vortices, resulting in a conversion of the original three-dimensional flow into a two-dimensional one. Such two-dimensionalization can be achieved for both asymmetric and symmetric wake patterns, indicating that it is not influenced by the excitation phase and frequency as long as the actuation is two-dimensional.

Flow effects in the reattachment region of supersonic laminar separated flow

The structure of a laminar separation flow around a compression corner at a free-stream Mach number M∞ = 6 is considered. The existence of a new element in the reattachment area—a high total-pressure layer—is shown. This layer is located downstream from the reattachment line above the boundary layer, and its total pressure is higher than that in either the boundary layer beneath it or the supersonic flow above it. Its appearance is due to the difference in the total pressure losses of the streamlines passing through a compression-wave fan and a shock wave in the reattachment zone. The vortex structure in the area of attachment is considered and the formation of streamwise vortices is shown. The origin of these vortices is similar to that of Taylor–Goertler vortices in subsonic flow. The pressure-pulsation distribution in the reattachment zone is also analyzed. It is shown that the spectrum of wall-pressure pulsations in the separation zone has two local maxima, the first caused by longitudinal fluctuations in the separation region and the second by pulsations of the vortices.

Linear stability analysis of magnetohydrodynamic duct flows with perfectly conducting walls.

The stability of magnetohydrodynamic flow in a duct with perfectly conducting walls is investigated in the presence of a homogeneous and constant static magnetic field. The temporal growth and spatial distribution of perturbations are obtained by solving iteratively the direct and adjoint governing equations with respect of perturbations, based on nonmodal stability theory. The effect of the applied magnetic field, as well as the aspect ratio of the duct on the stability of the magnetohydrodynamic duct flow is taken into account. The computational results show that, weak jets appear near the sidewalls at a moderate magnetic field and the velocity of the jet increases with the increase of the intensity of the magnetic field. The duct flow is stable at either weak or strong magnetic field, but becomes unstable at moderate intensity magnetic field, and the stability is invariance with the aspect ratio of the duct. The instability of magnetohydrodynamic duct flow is related with the exponential growth of perturbations evolving on the fully developed jets. Transient growth of perturbations is also observed in the computation and the optimal perturbation is found to be in the form of streamwise vortices and localized within the sidewall layers. By contrast, the Hartmann layer perpendicular to the magnetic field is irrelevant to the stability issue of the magnetohydrodynamic duct flow.

Instability of MHD Flow in the Duct with Electrically Perfectly Conducting Walls

The stability problem of conducting fluid flow in a square duct with perfectly conducting walls is investigated. A homogeneous and constant static magnetic field is applied along the vertical direction of the flow. Nonmodal linear stability analysis is performed on this problem for the first time and the effect of the imposed magnetic field is also taken into account. The amplification and distribution of primary optimal perturbations are obtained by solving iteratively the direct and adjoint governing equations with respect of perturbations. Four modes of perturbations with different symmetries in the space are investigated. Computational results show that, the MHD duct flow is stable at either small or large Hartmann number, but unstable at moderate one. The primary optimal perturbations are in the form of streamwise vortices, which are located inside the thin sidewall layers parallel to the magnetic field. The size of the vortices is decreased with the growing of Hartmann number Ha, meanwhile the amplification of the perturbations is reduced due to the magnetic damping effect. The Hartmann layer perpendicular to the magnetic field seems to be irrelevant to the stability of the MHD duct flow. The most unstable perturbation is in the form of Mode I, which having co-rotating vortices at opposite sidewalls and the vortices tend to enhance each other.

Transition induced by linear and nonlinear perturbation growth in flow past a compressor blade

Flow past a NACA 65 blade at chord-based Reynolds number 138 500 is studied using stability analysis, generalized (spatially weighted) transient growth analysis and direct numerical simulations (DNS). The mechanisms of transition on various sections of the blade observed in previous work by Zaki et al. (J. Fluid Mech., vol. 665, 2010, pp. 57–98) are examined, with a focus on the pressure side around the leading edge. In this region, the linearly most energetic perturbation has spanwise wavenumber $40\unicode[STIX]{x03C0}$ (five boundary-layer thicknesses) and is tilted against the mean shear to take advantage of the Orr mechanism. In a DNS, the nonlinear development of this optimal perturbation induces $\unicode[STIX]{x1D6EC}$ structures, which are further stretched to hairpin vortices before breaking down to turbulence. At higher spanwise wavenumber, e.g. $120\unicode[STIX]{x03C0}$, a free-stream optimal perturbation is obtained upstream of the leading edge, in the form of streamwise vortices. During its nonlinear evolution, this optimal perturbation tilts the mean shear and generates spanwise periodic high- and low-speed streaks. Then through a nonlinear lift-up mechanism, the low-speed streaks are lifted above the high-speed ones. This layout of streaks generates a mean shear with two inflectional points and activates secondary instabilities, namely inner and outer instabilities previously reported in the literature.

Flow dynamics and enhanced mixing in a converging–diverging channel

An analysis of flows in converging–diverging channels has been carried out with the primary goal of identifying geometries which result in increased mixing. The model geometry consists of a channel whose walls are fitted with spanwise grooves of moderate amplitudes (up to 10 % of the mean channel opening) and of sinusoidal shape. The groove systems on each wall are shifted by half of a wavelength with respect to each other, resulting in the formation of a converging–diverging conduit. The analysis is carried out up to Reynolds numbers resulting in the formation of secondary states. The first part of the analysis is based on a two-dimensional model and demonstrates that increasing the corrugation wavelength results in the appearance of an unsteady separation whose onset correlates with the onset of the travelling wave instability. The second part of the analysis is based on a three-dimensional model and demonstrates that the flow dynamics is dominated by the centrifugal instability over a large range of geometric parameters, resulting in the formation of streamwise vortices. It is shown that the onset of the vortices may lead to the elimination of the unsteady separation. The critical Reynolds number for the vortex onset initially decreases as the corrugation amplitude increases but an excessive increase leads to the stream lift up, reduction of the centrifugal forces and flow stabilization. The flow dynamics under such conditions is again dominated by the travelling wave instability. Conditions leading to the formation of streamwise vortices without interference from the travelling wave instability have been identified. The structure and the mixing properties of the saturated states are discussed.

Turbulence attenuation in simultaneously heated and cooled annular flows at supercritical pressure

Heated or cooled fluids at supercritical pressure show large variations in thermophysical properties, such as the density, dynamic viscosity and molecular Prandtl number, which strongly influence turbulence characteristics. To investigate this, direct numerical simulations were performed of a turbulent flow at supercritical pressure (CO$_{2}$at 8 MPa) in an annulus with a hot inner wall and a cold outer wall. The pseudo-critical temperature lies close to the inner wall, which results in strong thermophysical property variations in that region. The turbulent shear stress and the turbulent intensities significantly decrease near the hot inner wall, but increase near the cold outer wall, which can be partially attributed to the mean dynamic viscosity and density stratification. This leads to decreased production of turbulent kinetic energy near the inner wall and vice versa near the outer wall. However, by analysing a transport equation for the coherent streak flank strength, it was found that thermophysical property fluctuations significantly affect streak evolution. Near the hot wall, thermal expansion and buoyancy tend to decrease streak coherence, while the viscosity gradient that exists across the streaks interacts with mean shear to act as either a source or a sink in the evolution equation for the coherent streak flank strength. The formation of streamwise vortices on the other hand is hindered by the torque that is the result of the kinetic energy and density gradients. Near the cold wall, the results are reversed, i.e. the coherent streak flank strength and the streamwise vortices are enhanced due to the variable density and dynamic viscosity. The results show that not only the mean stratification but also the large instantaneous thermophysical property variations that occur in heated or cooled fluids at supercritical pressure have a significant effect on turbulent structures that are responsible for the self-regeneration process in near-wall turbulence. Thus, instantaneous density and dynamic viscosity fluctuations are responsible for decreased (or increased) turbulent motions in heated (or cooled) fluids at supercritical pressure.

A method for analysis of stability of flows in ribbed annuli

A spectrally-accurate algorithm for the linear stability analysis of pressure-gradient-driven flows in corrugated annuli is presented. The algorithm is suitable to analyze three-dimensional disturbances in the form of spiral traveling waves as well as in the form of streamwise vortices. A separate algorithm for the analysis of axisymmetric disturbances is presented. The discretization method relies on Fourier expansions in the streamwise and circumferential directions and on Chebyshev expansions in the radial direction. The Immersed Boundary Conditions (IBC) method is used to enforce the physical boundary conditions at the corrugated walls. Numerous tests confirm the spectral accuracy of the results. The performance of the algorithm is consistent with the theoretical predictions.

Effects of turbulent eddies and Langmuir circulations on scalar transfer in a sheared wind-driven liquid flow

The effects of turbulent eddies and Langmuir circulations in liquid flow on scalar transfer across a sheared wind-driven gas-liquid interface are investigated by means of a direct numerical simulation of a gas–liquid two-phase turbulent flow with a wind-driven nonbreaking wavy interface. The wind-driven wavy gas-liquid interface is captured using an arbitrary Lagrangian-Eulerian method with boundary-fitted coordinates on moving grids. The results show that Langmuir circulations are generated on the liquid side below the sheared wind-driven gas-liquid interface. The marker particles on the gas-liquid interface, the turbulent eddies in the form of streamwise vortices on the liquid side (i.e., the typical horseshoe vortices associated with bursting motions), and the low scalar flux lines on the gas-liquid interface induced by the turbulent eddies on the liquid side tend to locally concentrate in the regions along the downward flows caused by the Langmuir circulations. It is suggested that the turbulent eddies on the liquid side mainly control the scalar transfer across the sheared wind-driven gas-liquid interface, and the effect of the Langmuir circulations is relatively small.

Numerical Evidence of Turbulence Generated by Nonbreaking Surface Waves

AbstractNumerical simulation of monochromatic surface waves propagating over a turbulent field is conducted to reveal the mechanism of turbulence production by nonbreaking waves. The numerical model solves the primitive equations subject to the fully nonlinear boundary conditions on the exact water surface. The result predicts growth rates of turbulent kinetic energy consistent with previous measurements and modeling. It also validates the observed horizontal anisotropy of the near-surface turbulence that the spanwise turbulent intensity exceeds the streamwise component. Such a flow structure is found to be attributed to the formation of streamwise vortices near the water surface, which also induces elongated surface streaks. The averaged spacing between the streaks and the depth of the vortical cells approximates that of Langmuir turbulence. The strength of the vortices arising from the wave–turbulence interaction, however, is one order of magnitude less than that of Langmuir cells, which arises from the interaction between the surface waves and the turbulent shear flow. In contrast to Langmuir turbulence, production from the Stokes shear does not dominate the energetics budget in wave-induced turbulence. The dominant production is the advection of turbulence by the velocity straining of waves.

The formation mechanism and impact of streamwise vortices on NACA 0021 airfoil's performance with undulating leading edge modification

Wings with tubercles have been shown to display advantageous loading behavior at high attack angles compared to their unmodified counterparts. In an earlier study by the authors, it was shown that an undulating leading-edge configuration, including but not limited to a tubercled model, induces a cyclic variation in circulation along the span that gives rise to the formation of counter-rotating streamwise vortices. While the aerodynamic benefits of full-span tubercled wings have been associated with the presence of such vortices, their formation mechanism and influence on wing performance are still in question. In the present work, experimental and numerical tests were conducted to further investigate the effect of tubercles on the flow structure over full-span modified wings based on the NACA 0021 profile, in the transitional flow regime. It is found that a skew-induced mechanism accounts for the formation of streamwise vortices whose development is accompanied by flow separation in delta-shaped regions near the trailing edge. The presence of vortices is detrimental to the performance of full-span wings pre-stall, however renders benefits post-stall as demonstrated by wind tunnel pressure measurement tests. Finally, primary and secondary vortices are identified post-stall that produce an enhanced momentum transfer effect that reduces flow separation, thus increasing the generated amount of lift.