Bert Günther

A generalized mean-field model of the natural and high-frequency actuated flow around a high-lift configuration

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.

Control of Separation on the Flap of a Three-Element High-Lift Configuration

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 dierent 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 eect of flow control on these.

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.

Turbulence Control Based on Reduced-Order Models and Nonlinear Control Design

We present a closed-loop flow control strategy for experiments and simulations. This strategy is based on low-order Galerkin models and nonlinear control. One key enabler is a partitioning of the flow in low-, dominant- and high-frequency components, i.e. a base flow, coherent structures and stochastic fluctuations. Another enabler is a control design exploiting the nonlinearities distilled by the model. Examples are presented for the actuated flow around a high-lift configuration and the controlled bluff body wake.

Numerical Investigation of Segmented Actuation Slots for Active Separation Control of a High-Lift Configuration

The control of the flow over the flap of a three-element high-lift configuration is investigated numerically by solving the unsteady Reynolds-averaged Navier-Stokes equations (URANS). At a Reynolds number of Re = 750 000 the flow is perturbed by periodic blowing/suction through slots near the flap leading edge. The main focus is on comparing the manner with wich the flow field reacts dierently if this actuation is segmented in the spanwise direction. Previous results show that the mean aerodynamic lift can be enhanced by excitation with continuous slots in the spanwise direction. On this basis an assessment of the additional gain in lift provided by segmented excitation slots is conducted. Furthermore, the influence of a phase shift of the actuation on the flow field is investigated.

Numerical Study of the Optimization of Separation Control

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.

Numerical Investigation of Active Flow Control Applied to an Airfoil with a Camber Flap

This paper gives an overview of numerical flow control investigations for a high-lift airfoil. The flow around a real glider airfoil with a deflected camber flap at stall conditions was simulated with the Reynolds-averaged Navier-Stokes equations in combination with the LLR-k-ω turbulence model. For this configuration flow separation can be delayed by periodic excitation through a slot close to the leading edge of the camber flap. By simulating different excitation positions, modes, frequencies, intensities and blow-out directions, a set of control parameters suitable for delaying separation and enhancing the lift could be identified.

Simulation Study of the Robust Closed-Loop Control of a 2D High-Lift Configuration

The investigation focuses on the closed-loop separation control of a two dimensional high-lift configuration in a numerical simulation study. The lift is to be controlled by adjusting the non-dimensional intensity of the harmonic excitation near the leading edge of the single slotted flap. Since control laws based on a high-dimensional discretisation or low-dimensional description of the Navier-Stokes equations are not applicable in real-time, this investigation presents a fast and efficient controller synthesis methodology employing robust methods. This offers real-time capability for future experimental implementations. In spite of the nonlinear and infinite-dimensional Navier-Stokes equations, it is surprising to observe that the dynamic behaviour appears very simple. This input-output behaviour in the vicinity of set points can be empirically approximated by stable linear black-box models of second order. Based on these, a simple robust controller is synthesised that autonomously adjusts the excitation such that a desired lift is obtained.

Feature-based Comparison of Flow Fields around a Three-element High-lift Configuration with Active Flow Control

The control of the flow over the flap of a three-element high-lift configuration is investigated numerically by solving the unsteady Reynolds-averaged Navier-Stokes equations (URANS). At a Reynolds number of Re = 1 · 10 6 the flow is perturbed by periodic blowing/suction through a slot near the flap leading edge. The main focus is on the mechanisms of separation control, for which flow field structures at two dierent excitation parameters are studied. The simulations are conducted using a swept wing of infinite span in order to study the impact of dierent excitation parameters. The method of feature-based extraction will be used to identify dominant large scale structures in the unforced and excited flow field.