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.