Efficient Prediction of Aerodynamic Response Behavior for Control Surfaces using the Linear Frequency Domain

Abstract

The prediction of aerodynamic forces and moments caused by dynamic flap motions using time-marching techniques like the Unsteady Reynolds-averaged Navier-Stokes equations is extremely time-consuming - they do offer accurate results for the simulation of unsteady flows, however due to their high computational costs they are impractical for the extensive analysis of flight conditions, flap geometries and dynamic flap motions in a design context and for load alleviation functionality. In this paper a surrogate model is presented, which enables the fast and accurate prediction of the time-accurate behavior for different flap systems in a wide parameter space. The model is based on the assumption that the aerodynamic response behavior of an arbitrary flap movement can be decomposed into its components in the frequency domain. For each frequency the TAU linear frequency domain solver computes the first harmonic of the lift derivatives for sinusoidal excitations with small-amplituded flap angles. These complex spectra of lift derivatives are defined as frequency responses, which vary for different flight conditions and flap geometries. The aerodynamic response for an arbitrary flap movement in the time domain is then reconstructed by superposition of the frequency response and the frequency components. In the surrogate model samples of frequency responses are precomputed for a defined parameter space, so that the frequency response for a new case can be determined by mere interpolation. Once the samples for the surrogate model are generated, the method achieves reduction in computational time of up to 6 orders of magnitude for the prediction in comparison to time-marching simulations. The surrogate model is shown and validated for dynamic flap movements on 2D sections of a transonic airfoil. Mach number, Reynolds number, angle of attack, flap chord length and the mean-state flap deflection span out the parameter space in this study. For demonstration of the broad applicability of the methodology for control surfaces a plain flap and a fowler flap are examined and the surrogate model is validated in subsonic as well as in transonic flow regions. The prediction of lift responses using the surrogate model shows a very good agreement with the reference data provided by unsteady Reynolds-averaged Navier-Stokes simulations. Thereby, even unsteady aerodynamic effects, for instance induced by a fast flap deflection, are covered accurately in the model.