Turbulence and drag control in jet and wake flows

Structures of plane and circular jets were modified by using active, passive and an active-passive (hybrid) combination to investigate the phenomenon of turbulence control using hot-wire anemometry. Active control was achieved by exciting the flow by using either an acoustic speaker or vibrating piezoceramic elements, whereas passive control was achieved by placing a slender object such as a thin wire or a ring, a mesh or a honeycomb section downstream of the nozzle exit plane. Turbulence enhancement and suppression in plane-and circular-jet mixing layers could be achieved by employing active techniques, whereas passive techniques essentially yielded turbulence reduction. For a circular cylinder, a significant reduction in drag was obtained by placing a cylinder of small diameter upstream of the main cylinder. The total drag of the two cylinders was smaller than that of a single cylinder alone. Hot-wire measurements indicate significant changes to the downstream flow structure associated with the observed drag reduction. Our investigations indicate that different types of modification and excitation can yield different but useful outcomes.