During take-off and landing, wings of planes need to generate an enormous amount of lift to reduce ground speeds and runway lengths. Instead of providing complex and heavy multi-element high-lift devices, single flaps without slat are desirable. Such flaps, however, can only be applied if flow-separation on the flap at high flap angles can be avoided. Recent experimental investigations by Nitsche and Tinapp [1] have shown that separation can be delayed by periodic excitation near the flap leading edge.
There are a number of experimental and numerical studies showing the general effectiveness of flow control for single airfoils. In most investigations, leading edge suction is applied for transition delay [2], nonetheless, jet flaps are also employed for lift increase and manoeuvering. Surface suction/blowing can be used to rapidly change lift and drag on rotary wing aircraft [3]. However, most control techniques considered in the past showed low or negative effectiveness.
Oscillatory suction and blowing is about 10 times more efficient with respect to lift than steady blowing. The process becomes extremely efficient if the excitation frequencies correspond to the most unstable frequencies of the free shear layer, generating arrays of spanwise vortices that are convected downstream and continue to mix across the shear layer. Suction and blowing can be applied tangential to the airfoil surface [4], rectangular [5], [6] or with cyclic vortical oscillation.
In the present study, periodic excitation is applied to delay flow separation on the flap of a two-element high-lift configuration resulting in enhanced lift and reduced drag. The objective of this investigation is to better understand the functionality mode of the periodic excitation on pressure-driven separated flow over a two-element high-lift configuration.
The numerical simulation of the unsteady turbulent flow is based on the two-dimensional Reynolds-averaged Navier-Stokes equations (RANS). One goal is to verify the applicability of statistical turbulence models for this kind of flow and to prove that all important flow features can be captured.