A low-dimensional Galerkin model is proposed for the flow around a high-lift configuration, describing natural vortex shedding, the high-frequency actuated flow with increased lift and transients between both states. The form of the dynamical system has been derived from a generalized mean-field consideration. Steady state and transient URANS (unsteady Reynolds-averaged Navier–Stokes) simulation data are employed to derive the expansion modes and to calibrate the system parameters. The model identifies the mean field as the mediator between the high-frequency actuation and the low-frequency natural shedding instability.
This paper describes a joint experimental and numerical investigation of the control of the flow over the flap of a three-element high-lift configuration by means of periodic excitation. At Reynolds numbers between 0.3 × 10 6 and 1 × 10 6 the flow is influenced by periodic blowing or periodic blowing/suction through slots near the flap leading edge. The delay of flow separation by periodic vertical excitation could be identified in the experiments as well as numerical simulations based on the Unsteady Reynolds-averaged Navier-Stokes equations (URANS). As a result, the mean aerodynamic lift of this practically relevant wing configuration could be significantly enhanced. By investigating different excitation frequencies and intensities optimum control parameters could be found. The behaviour of the aerodynamic forces with varying flap deflection angle are measured on a finite swept wing. Scientific visualisation of the numerical simulations of an infinite swept wing allows a detailed analysis of the structures in this complex flow field and the effect of flow control on these. Nomenclature c, c k clean chord length, flap length (c k = 0.254c) c L , c Lmax lift coefficient, maximum lift coefficient C µ momentum coefficient C µ = 2 H c ua u∞ 2
The concept of active flow control is applied to the steady flow around a NACA4412 and to the unsteady flow around a generic high-lift configuration in order to delay separation. To the former steady suction upstream of the detachment position is applied. In a series of computations the suction angle β is varied and the main flow features are analyzed. A gradient descent method and an adjoint-based method are successfully used to optimize β. For the unsteady case periodic blowing and suction is employed to control the separation. Various calculations are conducted to obtain the dependency of the lift on the amplitude and frequency of the perturbation and the amplitude is optimized with the gradient descent method.
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