The experimental investigations presented in this paper deal with an active flow control approach at the rear end of two different three-dimensional generic car configurations. Periodic forcing is used to influence the recirculation area behind these models based on the Ahmed body geometry with slant angles of 25°. and 90°. and hence to achieve a reduction in the total drag. Several forcing parameters (jet angle, forcing frequency and intensity) and their influence on the static pressure distribution at the rear end of the models and the total drag are examined. A model predictive closed-loop actuation approach is presented for one steady blowing excitation configuration. The interaction between the periodic compressed air actuator jet and the shear layer is depicted by means of stereo time-resolved Particle Image Velocimetry. The different actuation concepts lead to a drag reduction of up to 5.7% and demonstrate a good correlation with the static pressure distribution at the rear end of the model.

Experimental investigations were carried out aiming at the reduction of the total aerodynamic drag of a generic car model by means of active flow control. Considering the complexity of the near wake of the car model the actuation system consists of steady blowing to influence streamwise vortices, while periodic perturbations were used to control shear layer vortices. This study describes a successful approach for active drag reduction of a generic car model using these dierent types of actuation simultaneously. A slope-seeking controller is used to find the optimal forcing parameters in order to reduce the overall drag.

To speed up gradient estimation in a slope-seeking controller two different modifications are proposed in this study. In a first approach, the gradient estimation is based on a locally identified black-box model. A further improvement is obtained by applying an extended Kalman filter to estimate the local gradient of an input-output map. Moreover, a simple method is Outlined to adapt the search radius in the classical extremum- and slope-seeking approach to reduce the perturbations near the optimal state. To show the versatility of the slope-seeking controller for flow control applications two different wind tunnel experiments are considered, namely with a two-dimensional bluff body and a generic three-dimensional car model (Ahmed body).

Experimental investigations aiming at the reduction of the total aerodynamical drag of a generic car model are conducted. Since open-loop control concepts are lacking the possibility of compensation for uncertainties, such as uncertain ow conditions, closed-loop control concepts are used. Steady blowing is applied to attenuate dominant structures in the wake region and thus decrease the overall drag. Two dierent control concepts, namely a model predictive controller (MPC) and a robust H1-controller are implemented. Both controllers exploit pressure measurements at the rear end of the car model since the pressure is an indication for the dominant structures in the wake and directly correlates with the aerodynamical drag. The controllers are applied successfully in wind-tunnel tests and their performance is compared.

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.

The experimental investigations in the present paper deal with the excitation of fundamental instability mechanisms in separated free shear layers on a bluff body and downstream of a diffuser by means of periodic forcing in order to reduce the expansion of flow separation. The experiments focus on a unique approach to separation control using fundamental frequencies for local forcing in two different shear layer configurations (inner and outer diffusers). Each separation process is characterized by the periodic occurrence of large spanwise vortex structures. These vortices scale with the difference in height between the ramp ends. The excitation of these large scale vortex structures by periodic forcing intensifies the momentum transfer between the separation region and the outer flow, resulting in a substantial reduction of the reattachment length. For the inner and outer diffuser configurations, a universal value for the optimum forcing frequency was established.

The experimental investigations described in the present paper deal with the reduction of the total aerodynamic drag of a generic car model (Ahmed-Body) by means of periodic forcing. The experiments carried out in this study focus on a unique approach to separation control using fundamental frequencies for local forcing of the shear layer separated from the rear end of the car model. The excitation of large scale vortex structures by periodic forcing intensifies the primary momentum transfer between the separation region and the outer flow, resulting in a substantial reduction of the separation length. A total drag reduction of 27% was achieved using the flow control method described in this study.

Das am DLR in Gottingen (AS-EV) entwickelte DISKON-Flugelkonzept zur Beeinflussung der Wirbelschleppe im Nachlauf eines Flugels wird erstmalig experimentell auf seine Wirksamkeit und Effektivitat hin untersucht. Die Messungen wurden am Wasserschleppkanal des DLR Gottingen (WSG) durchgefuhrt. Durch Zacken an der Hinterkante werden Wirbel in der Scherschicht (DISKONtinuitatsflache) induziert, die gegenlaufig zum Randwirbel an der Flugelspitze sind. Veranderungen in der Wirbelstruktur werden uber einen Vergleich der Ergebnisse von DISKON-Modell und einem Referenzmodell bestimmt. Die beiden Flugelmodelle haben das gleiche Profil und eine Halbspannweite von s = 0,5 Meter. Die Wirbelschleppe wird uber einen Bereich von bis zu 30 Spannweiten hinter dem Flugel mit der Stereo Particle Image Velocimetry (Stereo-PIV) untersucht. Parallel dazu werden die aerodynamischen Krafte mit einem Dreikomponenten- Dynamometer (Piezo-Waage) erfasst, uber welche die aerodynamische Ahnlichkeit der beiden Flugelmodelle nachgewiesen wird. Aus den Stromungsfeldmessungen werden unter anderem die Wirbelposition, die tangentiale Geschwindigkeit und die Zirkulationsverteilung uber dem Wirbelradius bestimmt. Der Vergleich dieser Parameter ergibt eine Beeinflussung der Wirbelschleppe durch das DISKON-Flugelkonzept, die sich in einer Positionsveranderung, einer Umverteilung der Wirbelstarke in den auseren Bereich und in einer Abschwachung der Tangentialgeschwindigkeit innerhalb des Wirbelzentrums verdeutlicht.

The experimental investigations in the present paper deal with the excitation of fundamental instability mechanisms in separated free shear layers on a bluff body and downstream of a diffuser by means of periodic forcing in order to reduce the expansion of flow separation. The experiments focus on an unique approach to separation control using fundamental frequencies for local forcing in two different shear layer configurations (inner and outer diffusers). Each separation process is characterized by the periodic occurrence of large spanwise vortex structures. These vortices scale with the difference in height between the ramp ends. The excitation of these large scale vortex structures by periodic forcing intensifies the momentum transfer between the separation region and the outer flow, resulting in a substantial reduction of the reattachment length. For the inner and outer diffuser configurations, a universal value for the optimum forcing frequency was established.

The experimental investigations in the present paper deal with the excitation of fundamental instability mechanisms in separated free shear layers on a bluff body and downstream of a diffuser by means of periodic forcing in order to reduce the expansion of flow separation. The experiments focus on an unique approach to separation control using fundamental frequencies for local forcing in two different shear layer configurations (inner and outer diffusers). Each separation process is characterized by the periodic occurrence of large spanwise vortex structures. These vortices scale with the difference in height between the ramp ends. The excitation of these large scale vortex structures by periodic forcing intensifies the momentum transfer between the separation region and the outer flow, resulting in a substantial reduction of the reattachment length. For the inner and outer diffuser configurations, a universal value for the optimum forcing frequency was established.