Separation Control by Self-Activated Movable Flaps

Abstract

Separation control is an important issue in the physiology of birdflight. Here, the adaption of the separation control mechanism by bird feathers to the requirements of engineering applications is described in detail. Self-activated movable flaps similar to artificial bird feathers represent a high-lift system for increasing the maximum lift of airfoils. Their effect on the unsteady flow around a two-dimensional airfoil configuration is investigated by a joint numerical and experimental study. First, attention is paid to the automatic opening and closing mechanism of the flap. Following this, its beneficial effect on lift is investigated for varying incidences and flap configurations. In-depth analysis of experimental and numerical results provides a detailed description of the important phenomena and the effect of self-adjusting flaps on the flow around the airfoil. In the second part of this paper, a contribution is made to verification of the applicability of unsteady Reynolds-averaged approaches using statistical turbulence models for unsteady flows with particular attention to turbulent time scales with comparison to the results of a hybrid simulation based on unsteady Reynolds-averaged Navier-Stokes equations and large-eddy simulation. Finally, flight experiments are described using an aircraft with movable flaps fitted on its laminar wing. Nomenclature A = amplitude of oscillation b = wing span c = chord length c F = flap-moment coefficient c G = gravity-moment coefficient c L ; c D = lift and drag coefficients c R = reverse flow parameter k = turbulent kinetic energy Lt = turbulent length scale l F = flap length M F = flap moment due to fluid force M G = flap moment due to gravity Re = Reynolds number based on chord length Sr = Strouhal number based on flap length u 0 = inflow velocity x d = detachment position = angle of attack , max = flap deflection angle, maximum angle = conventional flap angle = density ! = specific turbulent dissipation

Numerical modeling of flow control on a symmetric aerofoil via a porous, compliant coating

“…Thus, this feature of automatic activation, as opposed to active control techniques which require an energy input to come into action, provides industry with yet another reason to study ways to mimic this mechanism. Much experimental [11][12][13][14][15] as well as computational work [16][17][18] has been done simulating this behaviour and the results obtained indeed prove the effectiveness of a poro-elastic coating over a surface. For instance, a substantial drop in the mean drag, lift, and drag fluctuations, as well as a noteworthy increase in mean lift, has been observed.…”

Lift Enhancement at Low Reynolds Numbers Using Self-Activated Movable Flaps

“…The airfoil chord length is 60 mm. The flap length is 10 mm and is mounted flush with the trailing edge of the airfoil, as Schatz et al [2,3] reported this as the location of the flap in their measurements. With a flow speed of 0:51 m=s, this translates to a Reynolds number of Re 30; 600.…”

“…Schatz et al [2,3] reported that the use of a passive flap near the trailing edge results in a lift increase at high angles of attack. The use of a passive flap is inspired by the feather structure of a bird on the upper side of the bird's wing [4,5].…”

“…1). Schatz et al [2,3] investigated the use of a passive flap to emulate this behavior. Their force measurements at Reynolds numbers above Re 1; 000; 000 showed an increase of lift of about 10%.…”

Feedback flow control of a low-Re airfoil by flap actuators

“…Shan et al (2008) numerically showed that the active vortex generators controlled by a duty cycle with an excitation frequency equal to the natural frequency of the separation region are more effective than the passive ones for evading the flow separation. Meyer et al (2007) demonstrated that the self-activated movable pop-up flaps fitted on the airfoil enhance the lift by more than 10%, though this device is driven by flow itself, meaning exactly a passive device with no external energy. Similar passive but movable devices, self-adaptive hairy flaps, were investigated by Brücker and Weidner (2014), and they indicated that the hairy flaps of an appropriate flap length can delay the stall without external energy.…”