Asymmetric forcing of a turbulent rectangular jet with a piezoelectric actuator

A moving wall section attached to a piezoelectric actuator was used to perturb the airflow exiting a fully developed turbulent channel. The Reynolds number based on the channel width and centerline velocity was 4240. The maximum velocity of the moving wall section was 9.5 cm/s (2.3% of the mean centerline velocity), and the maximum displacement was 120 microns, corresponding to 1.84 wall units. The actuator frequency and displacement amplitude were tuned independently to generate different effects on the flow, and both quantities were documented to provide precise boundary conditions for numerical codes. Hot-wire measurements showed that actuation affects both the mean and rms velocity profiles downstream of the channel exit. In all cases, forcing yields mean profiles that are symmetric with respect to the centerline. However, forcing at low frequencies (St≤0.30) causes faster decay of the centerline velocity, higher spreading rates in the far field, and asymmetric rms profiles compared with unforced flow. The maximum rms value crosses from the actuator side to the nonactuator side as the flow moves downstream. This behavior is thought to be caused by staggered but asymmetric vortical structures developing in the opposing shear layers. Forcing from St=0.39 through 1.46 leads to altered but symmetric rms profiles and spectra compared with unforced flow. Forcing in the range St<0.50 yields centerline rms values that initially are larger than, but further downstream smaller than in the unforced flow.