Effect of forcing on the wake drag of an axisymmetric bluff body

The wake of an axisymmetric bluff body was investigated using water tunnel experiments. The parameters common to all investigations were a Reynolds number of 1000 or 1500 based on the body diameter, and a boundary layer thickness entering the body base of 30 % of the base diameter. Harmonic forcing was accomplished using eight individual piston pump actuators providing blowing and suction disturbances into the boundary layer close to the body base. The flow field was evaluated using flow visualization, single wire hot film anemometry, and direct drag force measurements. Due to the low velocities and small model cross sectional areas, the resulting drag forces are difficult to measure. A high precision balance was developed to measure the drag force. The balance is able to measure forces in the Millinewton range with Micronewton resolution. While the ±1 modes are dominant in the natural wake, the associated vortices are nonperiodic in time and irregular in size and orientation. When forcing the ±1 modes it was possible to lock their frequency, phase and orientation to the forcing over a relatively large frequency range. Within the lock-in range, the wake drag increased by up to 40 %. The mean flow of the wake was axisymmetric.