Asymmetric forcing of a turbulent rectangular jet with a piezoelectric actuator

A moving wall section attached to a piezoelectric actuator was used to pert urb the airflow exiting a fully developed turbulent channel. The Reynolds n umber based on the channel width and centerline velocity was 4240. The maxi mum velocity of the moving wall section was 9.5 cm/s (2.3% of the mean cent erline velocity), and the maximum displacement was 120 microns, correspondi ng to 1.84 wall units. The actuator frequency and displacement amplitude we re tuned independently to generate different effects on the flow, and both quantities were documented to provide precise boundary conditions for numer ical codes. Hot-wire measurements showed that actuation affects both the me an and rms velocity profiles downstream of the channel exit. In all cases, forcing yields mean profiles that are symmetric with respect to the centerl ine. However, forcing at low frequencies (St less than or equal to0.30) cau ses faster decay of the centerline velocity, higher spreading rates in the far field, and asymmetric rms profiles compared with unforced flow. The max imum rms value crosses from the actuator side to the nonactuator side as th e 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 prof iles and spectra compared with unforced flow. Forcing in the range St <0.50 yields centerline rms values that initially are larger than, but further d ownstream smaller than in the unforced flow. (C) 2001 American Institute of Physics.