Experimental investigation of the wake behind an axisymmetric bluff body

Abstract

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, or into the wake at the base of the body. This setup allowed the excitation of azimuthal mode numbers up to four. The resulting flow field was evaluated using flow visualization, single wire hot film anemometry, and direct drag force measurements. Four different helical mode combinations were used to force the wake, ±1, ±2, ±3, and ±4. The ±1 modes are dominant in the natural wake. When forcing the ±1 modes it was possible to lock their frequency and phase 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. Forcing the ±2 modes, the lock-in frequency range was significantly smaller and was centered at somewhat higher frequencies. The mean flow in this case was distorted to a four-lobed polygon, and the drag increased by more than 60%. The ±3 forcing yielded a flow response that involved neighboring modes with significant amplitudes, which was most likely caused by the decreased quality of the spatial representation of the forcing input due to the limited number of pistons. The combination of the different modes resulted in a mean flow distortion and amplitude distribution with five lobes. The frequency range for which lock-in could be observed was further reduced when compared to the ±2 case. For forcing modes ±4, the flow responded only locally to the forcing, and the decay of the forced modes in downstream direction was very rapid, for example, at three diameters downstream the forced modes were no longer detectable.