Session E16: Aerodynamics: Control

Modeling bias in ensemble-based state estimators for aerodynamic flows

Ensemble-based estimators have been shown to be an efficient way of estimating the state of the high-dimensional systems that arise from the discretization of fluid flows. Since the computational cost of these models has a significant impact in the runtime, dealing with modeling errors is a practical inevitability. When these errors have non-zero mean and are left unaccounted for, the introduced bias can severely impair the estimator performance. In this work, we propose a low-rank representation for the modeling error and use colored-noise processes to represent the dynamics of the slow-varying portion of bias. The Ensemble Kalman Filter is then employed to simultaneously correct both the state and bias parameters. The methodology is demonstrated using the twin-experiment strategy: the state of a fine-grid 2D low-Re flow simulation past an inclined flat plate is estimated using an ensemble of coarse-mesh simulations and pressure measurements taken on the surface of the plate. This scheme is shown to improve the estimator accuracy by 70% when compared to a bias-blind strategy.   

Numerical simulation of active flow control in a symmetrical airfoil using oscillating synthetic jets

The analysis of an active flow control based on synthetic jets in a symmetrical airfoil profile is presented. The physical model consists of a NACA0025 airfoil profile with several slots along the upper surface. A synthetic jet with an injection/suction harmonic function is located at each slot. The 2D Navier-Stokes equations for an incompressible fluid  are solved by using the OpenFOAM free source computational code. The purpose of the study is to analyze the influence of the out of phase synthetic jets (with different frequency and amplitude) on the development of the boundary layer and the lift and drag coefficients. We observe that varying the set of parameters of the active flow control system, the streamlines in the neighborhood of the solid symmetrical airfoil profile are modified in such a way that we are able to investigate low Reynolds number flows over a virtual shape with harmonic motion.   

Friction drag reduction on an airfoil using passive blowing

Friction drag reduction of a passive blowing on a Clark-Y airfoil is investigated experimentally. The passive blowing is driven by the pressure difference on the surface between suction and blowing sections. The experiment is carried out in a JAXA 0.65 m×0.55 m low turbulence wind tunnel. The Reynolds number based on the chord length is set to 0.65×10⁶ and 1.54×10⁶. The angle of attack is 0° and 6°. The streamwise mean velocity profile in the blowing section is shifted away from the wing surface, which suggests friction drag reduction by the passive blowing. We quantitatively assess the effect of blowing using log-law by taking into account the effects of pressure gradient and the roughness of the wall caused by the perforated metal plate used for blowing. In order to obtain the friction velocity, the slope of the velocity profile corrected through the modified log-law which takes into account the blowing effect is fitted to the slope of the theoretical profile with pressure gradients. As a result of this quantitative assessment, we estimate that the passive blowing reduces the local friction drag by 5% - 25%.