Trailing Edge Shear Layer Research Articles

Numerical investigation of self-sustained oscillations of stall cells around a leading edge-separating airfoil

The flow around a stalled airfoil is investigated using zonal detached-eddy simulation (mode 2), including transition effects through a coupling with the γ−Reθ,t framework. The airfoil exhibits mixed trailing edge-leading edge stall type properties. The chord length-based Reynolds number and Mach number, respectively, amount to 1·106 and 0.16. Two computations with different initial conditions are performed for 40 and 120 chord-passing durations, respectively (or equivalently 0.23 and 0.67 s), allowing the capture of several periods of the low frequency dynamics of the flow—compared to typical von Kármán vortex shedding. A stall hysteresis is observed: the computation initiated from an attached flow remains thus, but the computation which starts from a separated flow yields a quasi-permanent low-frequency oscillatory behavior, which bifurcates to the previously attached topology after 90 chord-passing durations (0.45 s). The oscillatory phase displays events of emergence and disappearance of stall cells. The partly- and fully attached flows are validated against experimental data. The oscillatory bistable flow is then analyzed with regard to the characteristics and frequency contents of both massive separation and partial transient reattachments. It is shown that the low-frequency separated shear layer flapping at the leading edge is forced by high-frequency fluctuations, which travel from the trailing edge upstream, close to the wall in the separated flow. The flapping phenomenon displays a Strouhal number based on the front-section height of the airfoil around St=fc sin (α)/u∞≃0.02. Conversely, the high-frequency fluctuations have Strouhal numbers closer to 3, which is in close agreement with leading-edge shear-layer instability frequencies. The spectral content of the flow is then explored in search of the source of these high-frequency fluctuations. It is proposed that they stem from the instability of the trailing edge shear layer between the pressure side boundary layer and the separated flow from the suction side. Finally, a scenario describing a cycle of the low-frequency oscillation of a stall cell is proposed.

Control of leading-edge separation on bioinspired airfoil with fluttering coverts.

In this work, the aerodynamic role of the artificial covert feathers (i.e., coverts) on an airfoil is experimentally studied in a wind tunnel to investigate the flow control effect on the leading-edge separation. We apply flexible featherlike devices on a high-angle-of-attack airfoil. We use a hot-wire anemometer to measure the velocity profiles and turbulent fluctuations in the downstream wake flow. As a baseline of flow separation, a two-dimensional NACA 0018 airfoil model is set at the angle of attack of 15 ° at the chord-based Reynolds number of 1.0×10^{5}, causing strong leading-edge and trailing-edge shear layers and a low-speed wake flow area in between as large as 0.35 chord length. When deployed on the upper wing surface, the flexible coverts adaptively flutter under the influence of the local unsteady airflow. Hot-wire measurement results show that the leading-edge coverts effectively suppress the flow separation and reduce the size of the wake flow area. The change of power spectral density shows that the predominant peaks as the fundamental and harmonic frequencies are both attenuated due to the suppression of unsteady motions of the shear layers. On the other hand, the fluttering coverts at the trailing edge modify the trailing-edge shear layer by redistributing the turbulent kinetic energy to the high-frequency components. By simultaneous double-point measurement, we find that the leading-edge and trailing-edge shear layers are drawn closer to each other, and the two shear layers show an increased peak in the coherence spectrum. Further multiscale wavelet analysis shows that the perturbations at the 60% chord length increase the large-scale amplitude modulation of small-scale turbulence and therefore they stabilize the leading-edge and trailing-edge shear layers. Meanwhile, the flow intermittency outside of the wake flow area is attenuated as well. The effective flow control effects in the present work are in good agreement with the previous direct observations of bird flight in literature that the coverts on the upper wing surface play an important role in flow separation control during high-angle-of-attack flight. These findings advance the understanding of aerodynamic contribution of the covers on bird wings and reveal the engineering potential of bioinspired coverts for flow separation control of aircrafts and unmanned air vehicles.

Investigation of large-scale structures in supersonic planar non-reactive shear layer with a base flow region

Large-scale structures in supersonic planar non-reactive shear layer with a base flow region are experimentally investigated. By utilizing the recently developed nanoparticle-based planar laser scattering (NPLS) method, the fine transient streamwise and spanwise flow fields are acquired, and the large-scale structures in the reattachment region, initial turbulent shear region, and developing turbulent shear region are analyzed. The predominant instability is convective for the two separated shear layer, whereas the instability in the base flow zone is still absolute according to the linear instability analysis. From the initial to developing turbulent shear layer region, the predominant instability transited gradually from absolute to convective instability, and the corresponding scales of the large-scale structures turn from about half to whole of the shear layer thickness. The entire streamwise mean flow topology and the streamwise and transverse velocity fluctuations are compared with different pressure ratios and with incompressible blunt trailing-edge shear layer. Linear instability analysis shows further evidence to the instability evolution of the supersonic shear layer with a base flow along the streamwise. It implied that instability induced by unmatched pressure in the wake zone can increase the absolute instability and improve the receptivity to inflow perturbations. The reattachment points are approximated with the intersection points of the reattachment shock pairs, which can be obtained from the transient NPLS images. The distribution of the reattachment points suggests that: the principal axis of the reattachment point distribution is dominated by the high-speed flow, and unmatched pressure increases the perturbation in the wake zone, result in a high absolute instability.

On the mechanism of high-incidence lift generation for steadily translating low-aspect-ratio wings

High-incidence lift generation via flow reattachment is studied. Different reattachment mechanisms are distinguished, with dynamic manoeuvres and tip vortex downwash being separate mechanisms. We focus on the latter mechanism, which is strictly available to finite wings, and isolate it by considering steadily translating wings. The tip vortex downwash provides a smoother merging of the flow at the trailing edge, thus assisting in establishing a Kutta condition there. This decreases the strength/amount of vorticity shed from the trailing edge, and in turn maintains an effective bound circulation resulting in continued lift generation at high angles of attack. Just below the static lift-stall angle of attack, strong vorticity is shed at the trailing edge indicating an increasingly intermittent reattachment/detachment of the instantaneous flow at mid-span. Above this incidence, the trailing-edge shear layer increases in strength/size representing a negative contribution to the lift and leads to stall. Lastly, we show that the mean-flow topology is equivalent to a vortex pair regardless of the particular physical flow configuration.

Study of a stall cell using stereo particle image velocimetry

The structure of Stall Cells (SCs) on wings is analyzed on the basis of stereo particle image velocimetry measurements. All experiments regard a Reynolds number 0.87 × 106 flow around a rectangular wing with endplates and an aspect ratio of 2.0. The inherently unstable stall cell is stabilized by means of a localized spanwise disturbance. Velocity, vorticity, and Reynolds stress data above the wing and in the wake are presented and discussed, also in combination with Computational Fluid Dynamics data. The present study completes and clarifies the previously suggested models regarding the SC structure. The SC emerges in between the separation and trailing edge shear layer where three different types of vortices are identified: (a) the stall cell vortices that start normal to the wing surface and continue downstream aligned with the free stream, (b) the separation line vortex, and (c) the trailing edge line vortex that both run parallel to the wing trailing edge and grow significantly at the center of the stall cell. Analysis of the Reynolds stress data reveals high anisotropy. Concentration of high streamwise shear stress values is connected to the two shear layers and high cross shear Reynolds stresses are connected to vortex stretching. High normal Reynolds stress values are observed (a) in the separation but not in the trailing edge shear layer indicating the flapping of the former and (b) along the stall cell vortices indicating their wandering motion. The eddy viscosity based Reynolds averaged Navier-Stokes simulations are found in good qualitative agreement with the experiments in terms of the type and position of the identified vortex structures, an agreement which is linked to the correct trend in the predicted shear Reynolds stresses distributions. Quantitative deviations of the numerical results from the measurements are attributed to the isotropic definition of the turbulence model. Therefore, use of large eddy simulation is suggested for better prediction of the flow.

Three-dimensional vortex wake structure of flapping wings in hovering flight

Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity.