Markus Schatz

Drag Reduction on Gurney Flaps by Three-Dimensional Modifications

Miniflaps at the trailing edge of airfoils, that is, Gurney flaps, change the Kutta condition and thereby produce higher lift. Unfortunately, because of the flow separation downstream of such trailing edges, the drag also increases. Investigations are described with the aim to stabilize the wake flow to achieve drag reduction. When hot-wire anemometry is used, a tonal component in the spectrum of the velocity fluctuations downstream of the Gurney flap is shown. This points to the existence of a von Karman vortex street. Modifications of the Gurney flap can reduce this flow instability, which results in a drag reduction. Trailing-edge modifications, such as slits or holes in Gurney flaps and vortex generators, were tested in experiments. The experiments were carried out using straight wings and a swept wing at a Re = 1 × 10 6 At lower angles of attack of the airfoils with geometrical modifications a drag reduction was observed. This drag reduction was determined through force measurements. The flowfield behind the Gurney flaps was also investigated numerically, using methods based on Reynolds averaged Navier-Stokes and detached eddy simulation

Separation Control by Self-Activated Movable Flaps

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.

Separation Control by Periodic Excitation and its Application to a High Lift Configuration

In a joint experimental and numerical study, the effect of periodic excitation on a twocomponent high lift configuration is investigated. At Reynolds numbers between 160, 000 and 2 × 10 the flow is influenced by periodic blowing and suction through a slot near the flap leading edge. The effects the excitation frequency and intensity are investigated using PIV measurements and numerical simulations based on the Unsteady Reynoldsaveraged Navier-Stokes equations (URANS) for different Reynolds numbers. Comparison of measured aerodynamic forces and flow visualisation to the numerical simulations allows a detailed analysis of the dominant structures in the flow field and the effect of flow control on these. The mean aerodynamic lift can be significantly enhanced by active flow control whilst the mean detachment on the flap is delayed.

Computational Investigation of Separation Control for High-Lift Airfoil Flows

This paper gives an overview of numerical flow control investigations for high-lift airfoil flows carried out by the authors. Two configurations at stall conditions, a generic two-element setup with single flap and a second configurationwith slat and flap of more practical relevance are investigated by simulations based on the Reynolds-averaged Navier-Stokes equations and eddy-viscosity turbulence models. For both cases flow separation can be delayed by periodic vertical suction and blowing through a slot close to the leading edge of the flap. By simulating different excitation modes, frequencies and intensities optimum control parameters could be identified. Comparison of aerodynamic forces computed and flow visualisations to experiments allows a detailed analysis of the dominant structures in the flow field and the effect of flow control on these. The mean aerodynamic lift can be significantly enhanced by the active flow control concepts suggested here.

Computational Modeling of the Unsteady Wake behind Gurney-Flaps

The effect of small micro-tabs (Gurney-flaps) mounted at the lower trailing edge of a HQ17 airfoil has been studied using numerical simulation. The focus of attention has been placed on the unsteady flow structures in the wake of the Gurney-flap, as these are responsible for increased induced drag. At a Reynolds number of 106, the flow is investigated using steady and unsteady simulations based on the Reynolds-averaged Navier-Stokes equations (URANS) as well as Detached Eddy Simulations (DES). In order to reduce the occuring unsteady flow structures, further simulations have been performed using alternative trailing edge shapes. The results show that the unsteadyness can successfully be suppressed and the drag can be reduced substantially using advanced flap concepts.

Numerical Study of Separation Control by Movable Flaps

The effect of movable flaps on the upper surface of an airfoil is studied by an unsteady numerical RANS simulation of the flowfield around static flaps at different flap angles. The self-adjusting flap remains in an equilibrium position if the sum of all resulting aerodynamic forces is zero. In this case, the lift increases significantly compared to the clean airfoil. The numerical results, however, indicate that in equilibrium position, the flap does not yield the optimal performance. Additionally, the effect of an oscillating flap as a means of flow-control is studied to investigate the effect of different excitation frequencies.

Increasing Lift by Means of Active Flow Control on the Flap of a Generic High-Lift Configuration

This paper demonstrates the use of active separation control on a high-lift configuration in order to enhance the aerodynamic performance in terms of lift and drag. The aim is to delay boundary layer separation on the flap’s upper surface by periodic excitation using a pulsed wall jet The experimental and numerical results show a massive improvement of almost all aerodynamic coefficients over a wide range of angles of attack and flap deflection angles. By actuating with correct excitation parameters, the jet formed by the single slotted flap can be reattached or kept attached, depending on the conditions, to the surface. Lift is increased by up to 12% while drag is reduced by the same amount. As a result, the lift to drag ratio defining the aerodynamic quality is improved by up to 25%. A numerical calculation on the basis of unsteady Reynolds-averaged Navier Stokes equations is also presented to determine the influence of different excitation parameters.