A closed-loop active flow control strategy to influence drag and yaw moment using steady blowing through Coanda actuators along the rear edges of a bluff body is presented. The relevant flow characteristics at cross-wind angles up to 5° are discussed for Reynolds numbers from 30000 to 50000. Based on black-box models identified from experimental data a robust multivariable controller is synthesized using the mixed-sensitivity loop-shaping approach. Experimental results show the successful reduction of drag and yaw moment and the suppression of disturbances during cross-wind gusts. The active flow control strategy with steady blowing through Coanda actuators along the rear edges of the investigated 2-dimensional bluff body reduces the drag coefficient by approximately 50 % at straight oncoming flow, corresponding to a net power saving of up to 37.1 %. Furthermore, the yaw moment under side-wind conditions can be decreased by actuating stronger on the leeward side. Using the described methods of modelidentification and control synthesis, a multivariable controller is designed that simultaneously adjusts the actuation pressures to achieve desired values for drag coefficient and yaw moment and rejects disturbances with a bandwidth of 0.87 Hz. The capability to suppress deviations of drag coefficient and yaw moment during a simulated cross-wind gust is demonstrated. This would greatly improve the driving stability when applied to a real car. However, there remain a couple of tasks to be addressed in future research. First of all, the designed controller uses unnecessary large actuation amplitudes during cross-wind gusts. This could be circumvented by implementing a control strategy that switches to a SISO controller for the yaw moment at larger side-wind angles. Furthermore, the performance of the controller could be improved by taking the nonlinearities into account, which are especially seen in the steady-state maps of the drag coefficient. Since the aerodynamic forces and moments acting on the body cannot be measured driving a real car, these variables would have to be estimated based on pressure readings at suitable locations. Furthermore, real cars experience much larger Reynolds numbers and higher turbulence levels with length scales larger than the vehicles itself. I is therefore planned to investigate more realistic cross-wind gusts using a special cross-wind facility.
Robust closed-loop flow control of drag and yaw moment for a bluff body under cross-wind conditions
2012
15 Seiten, 14 Bilder, 16 Quellen
Aufsatz (Konferenz)
Englisch
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