Erik Wassen

Active Drag Control for a Generic Car Model

Experimental and numerical investigations were carried out aiming at the reduction of the total aerodynamic drag of a generic car model by means of active separation control. For two different configurations separate control approaches were tested, taking into account the differences in the wake topology of the models. The targeted excitation of the respective dominant structures in the wake region leads to their effective attenuation. The experiments as well as the numerical simulations showed that a weakening of a spanwise vortex in the separated flow over the slant is strongly coupled with the occurrence of stronger streamwise vortices along the slant edges and vice versa.

Simulation of Active Drag Reduction for a Square-Back Vehicle

An active flow control approach was investigated in order to reduce the aerodynamic drag of a generic square-back vehicle. Using Large Eddy Simulations, it could be shown that steady blowing along the rear edges of the vehicle can reduce the drag by more than 10%. The blowing angle was varied, and a most effective angle of 45°. was found. The control method leads to a delay of shear layer vortex generation and to changes in the wake structure that cause a pressure increase on the rear surface of the vehicle. A simple estimation of the energy balance showed that the energy input needed for the active control is relatively large. Only for one case investigated in this study a small net power gain was found.

Drag Reduction for a Generic Car Model Using Steady Blowing

Drag reduction for a generic car model was investigated applying an active control approach of steady blowing. Using Large Eddy Simulations, it could be shown that blowing through a narrow slit along the vehicle’s rear edges can reduce the drag by 6.4%. The steady blowing increased the average pressure both on the slant and on the vertical base of the car model which had a slant angle of 25◦. On the slant, the side edge vortices above the model were weakened, reducing the effect of the low-pressure regions beneath the vortices. On the vertical base, blowing along the bottom edge increased the size of the lower one of two counter-rotating vortices downstream of the base and pushed the free stagnation point farther downstream.

Numerical Simulation of the Flow Around a Finite Cylinder with Ground Plate in Comparison to Experimental Measurements

Simulations and experiments were performed to capture the spatio-temporal flow field around a finite circular cylinder mounted on a ground plate. In order to provide a combined database and testcase for future simulations and experiments, the flow is investigated using state-of-the-art techniques with a high resolution in time and space, namely Large-Eddy Simulation and Detached-Eddy Simulation for the numerics and time-resolved PIV as well as LDA for the measurements. The predicted time-averaged and unsteady flow field from simulations corroborate well the experiments, giving new insights into the complex turbulent separated flow behind a quite simple geometry.

Turbulent Drag Reduction by Oscillating Riblets

A novel approach using laterally oscillating riblets is investigated to reduce the turbulent drag of wall-bounded flows. The new method is intended to combine the eect of the wellknown stationary riblets with the strong eect of lateral wall-oscillations. Experimental investigations in this study show only minor eects for the parameters investigated. DNS results demonstrate that the secondary flow induced by the riblet motion has a strong influence on the amount of drag reduction. The influence is not straightforward since stronger oscillations can lead to a higher drag under certain conditions and to a lower drag under dierent conditions. It is shown that the oscillating riblets are able to induce a similar lateral motion as an oscillating wall.

LES of Wake Control for a Generic Fastback Vehicle

The flow around an Ahmed car model with a slant angle of 25◦ was investigated using Large Eddy Simulations. Two active flow control approaches were applied. Both used constant blowing through slits in order to disturb the strong longitudinal vortices near the slanted edges that are characteristic for this car model. The control approaches were both able to weaken the vortices and to achieve a faster pressure recovery on the slanted surface. These positive effects were outweighed by areas of lower pressure that were induced near the blowing slits. As a result, both control configurations lead to a small net increase in total drag.

Numerical Investigation of Spatially Distributed Actuation on a Three-Element High-Lift Configuration

This paper presents a numerical investigation of active flow control cases conducted on the flap leading edge of a three-component high-lift configuration. A numerical method solving the unsteady Reynolds averaged Navier Stokes equations (URANS) is applied, combined with a numerical actuation boundary condition. Based on previous results, which have shown that the lift can be significantly enhanced by synthetic jet excitation, the effects of three-dimensionality are assessed. The influence of the spanwise width of the actuation segments is investigated, as well as the effect of varying the actuation angle. The active flow control mechanism is applied at a Reynolds number of Re = 750 000. As a test model, the industrially relevant Swept Constant Chord Half Model is used.

Active Separation Control on a High-Lift Configuration Using Segmented Actuation Slots

A numerical investigation of the actively controlled flow over the flap of a generic high-lift configuration is conducted solving the unsteady Reynolds Averaged Navier-Stokes equations (URANS). At a Reynolds number of Re = 750 000 an active flow control mechanism with periodic suction and blowing through slots near the flap leading edge is applied in order to achieve an extra gain in lift. Previous results have shown that the mean aerodynamic lift can be enhanced by excitation with continuous and segmented slots. On this basis it is assessed whether the vortex structures on the upper side of the flap can be positively influenced by dividing the slots into two segments with variable length in the spanwise direction. Those two segments are actuated with and without a phase shift. Furthermore, a variation of the actuation frequency and the blow out angle is conducted. Nomenclature bs actuation segment width in the spanwise direction c, ck ,c s clean chord length, flap length (ck =0 .254c), slat length (cs =0 .158c) cL lift coefficient Cμ momentum coefficient Cμ = H·b s c·s ua u∞ 2

Road Vehicle Drag Reduction by Combined Steady Blowing and Suction

Drag reduction for a generic fastback vehicle was investigated applying an active control approach combining steady blowing and suction. Using Large Eddy Simulations, it could be shown that blowing and suction through an array of small slits along the vehicle’s upper rear edge can reduce the drag by 9.4%. The active control increased the average pressure both on the slant and on the vertical base of the car model which had a slant angle of 25 ‐ . On the slant, the area of separated flow was significantly reduced, while on the vertical base the size of the recirculation bubble increased. The advantage of this approach is that there is no net mass flux, so that it can work locally without any long tubing system.

Flow Simulation around a Finite Cylinder on Massively Parallel Computer Architecture

Publisher Summary The purpose of the of the research project “Imaging Measuring Methods for Flow Analysis” funded by the German Research Foundation (DFG) is to develop flow measuring techniques and improve their performance. Because of the scarcity of experimental methods capable of producing similar results to simulations, the development of improved measurement methods for the analysis of complex flows is to be furthered in the scope of this program. The numerical simulations are to yield all flow quantities with a high resolution in time and space. The provision of the highly spatially and temporally resolved simulated flow field together with the unsteady experimental data will form a combined database for the verification of newly developed visualization methods and numerical turbulence models, and establish a reference test-case. The chapter also explains the detached-eddy simulation (DES) and the large-eddy simulation (LES).