Open-loop Control Of A D-type Bluff-body Wake For Drag Reduction

The development of the turbulent wake behind a D-type bluff body were investigated ex-perimentally using an open-loop flow control method.The research is aimed at reducing the form drag resulting from the alternating von-Kármán vortex shedding in the wake of the D-type body.The experiment was conducted at a Reynolds number Re_H=47000,based on incoming free-stream velocity and the cylinder height H.Loud speakers embedded inside the bluff body producing two zero-net-mass-flux jets through span-wise slots with a width of 2 mm located along the upper and lower edges on the rear wall.The drag forces for different excitation frequencies St_A=0.11⁰.35 and excitation amplitudes c_?=0⁰.45%,which is the ratio between the momentum of the actuating jets and the moment deficit caused by the bluff body were examined.The effects of the phase difference between the velocity variations of the two jets??=0 when the velocities of the two jets varied in phase and?out of phase?were also studied.It was found that a drag reduction up to 5%was achieved when c_?was 0.1%and the velocities of the two jets varied in phase at a frequency of St_A=0.16,which was about 2/3 of the characteristic frequency St₀of the natural wake structures of the D-type body.When the two jets velocities varied out of phase,significant drag increase was observed,especially when excitation frequency approximately equivalent to natural frequency,a maximum drag increment of 22%was observed.Then the cylinder wake with perturbation is examined in detail and compared with that without perturbation,based on smoke-wire flow visualization,pressure transducer and hotwire rake measurements at a forcing amplitude c_?=0.1%.Methods of data analyses of spectra,tempo-spatial cross-correlation and proper orthogonal decomposition?POD?was applied to the measured velocities.Large-scale vortical structures in the cylinder wake are significantly modified by the syn-thetic jets perturbations,exhibiting symmetric or asymmetric patterns,depending on the pertur-bation frequency and phase relationship of the synthetic jets.These observations are internally correlated with the drag force variations.It was found that the forcing changed the mode shape when the two actuators worked in-phase for a forcing frequency of St_A=0.16,the symmetric motion?mode 2 when without per-turbation?in the wake was largely promoted,thus the anti-symmetric motion?mode 1 without perturbation?was suppressed.The lower-order modes contains most of the fluctuating energy,it can reappear the original wake field by reconstruction method using the first three modes.