Controlling unsteady separation on a cylinder with a driven flexible wall

摘要

A driven flexible wall consisting of an array of strip-shaped capacitive transducers is used to defer unsteady Bow separation over a circular cylinder. The transducer elements, according to the need for detection and control of separation, are employed as wall pressure fluctuation sensors and vibrating-wall actuators. The spectrum of wall pressure fluctuations, sensed by a transducer element immediately upstream of the separation point, displays a dominant instability frequency. This frequency is believed to correspond to how unsteadiness resulting from a passive interaction of the separating Bow with the flexible wall. Actuation at this sensed frequency produces submicron-level vibration amplitudes of the wall normal to the boundary-layer flow. The mean separation point on the cylinder could be moved farther downstream by driving transducer elements slightly upstream of the sensing location. The control was effective at a frequency of 2.25 kHz for untripped and tripped boundary layers in the Reynolds number range from 1.2 x 10(5) to 1.5 x 10(5). This also reduced the mean pressure drag on the cylinder by 12 to 20, reduced the size of the wake, and lowered velocity fluctuations inside the attached boundary layer and the separated shear layer. References: 15