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

No abstract

No abstract

There is a rapidly growing interest in using general-purpose CFD codes based on second-order finite volume methods for Large-Eddy Simulation (LES) in a wide range of applications, and in many cases involving wall-bounded flows. However, such codes are strongly affected by numerical dissipation and the accuracy obtained for typical LES resolutions is often poor. In the present study, we approach the problem of improving the LES capability of such codes by reduction of the numerical dissipation and use of an anisotropycapturing subgrid-scale (SGS) stress model. The latter is of special importance for wall-resolved LES with resolutions where the SGS anisotropy can be substantial. Here we use the Explicit Algebraic (EA) SGS model [Marstorp L, Brethouwer G, Grundestam O, et al. Explicit algebraic subgrid stress models with application to rotating channel flow. J Fluid Mech. 2009;639:403-432], and comparisons are made for channel flow at friction Reynolds numbers up to 934 with the dynamic Smagorinsky model. The numerical dissipation is reduced by using an OpenFOAM based custom-built flow solver that modifies the Rhie and Chow interpolation and allows to control and minimise its effects without causing numerical instability (in viscous, fully turbulent flows). Different resolutions were used and large improvements of the LES accuracy were demonstrated for skin friction, mean velocity and other flow statistics by use of the new solver in combination with the EA SGS model. By reducing the numerical dissipation and using the EA SGS model the resolution requirements for wall-resolved LES can be significantly reduced.

No abstract