| An experimental study was conducted of a forced backward-facing step water ow, and the design of 16 actuators for creating the perturbations used to force the ow. The 16 actuators allowed for variant forcing in the spanwise direction with a resolution of 0.5 times the step height h. They are capable of producing unique perturbation waveforms of forcing velocity amplitudes 0 < u'/U infinity ≤ 2 and either single or multiple forcing Strouhal numbers in the range 0 < Sth ≤ 1.0. These forcing amplitudes are larger than ever used in any previous forced backward-facing step ow experiments. For measurement of the reattachment length in the wake of the backward-facing step, a novel hydro-tuft was designed which can indicate ow direction for local ow velocities less than 5 cm/s. A set of images taken of an array of hydro-tufts was computationally processed using a MATLAB program to calculate a time-averaged reattachment line. The effect of spanwise-invariant forcing for amplitudes u'/U infinity ≤ 2 and forcing Strouhal numbers 0 < Sth ≤ 0.5 was investigated. The results show an optimal Sth which shifts to a lower value with increasing forcing amplitude, and a non-monotonic shortening of the reattachment length. As a function of forcing amplitude, reattachment reaches a pronounced minimum at u '/Uinfinity ≈ 0.3 -- 0.4, and then rises to a peak at u'/ Uinfinity ≈ 0.5 -- 0.6. Any further increase in forcing amplitudes up to our maximum at u'/ Uinfinity = 2 results in more shortening. None of these behaviours have been previously noted in the literature. |
