Control of global instability in a non-parallel near wake

The effect of base suction on a plane wake was found to produce significant changes in wake dynamics. The wake is produced by merging two boundary layers from the trailing edge of a splitter plate in a two-stream water tunnel. A threshold suction speed exists which is approximately equal to half of the free-stream velocity. If the suction speed is below the threshold, the wake flow is unstable. If the suction speed is above the threshold, the wake becomes stable and no vortex shedding is observed. In the present experiment, the suction technique can stabilize a wake at a maximum tested Reynolds number of 2000. The suction significantly reduces the length of the absolutely unstable region in the immediate vicinity of the trailing edge of the splitter plate and produces a nonparallel flow pattern, resulting in the breakdown of global instability. The global growth rate changes from positive (unstable how) to negative (stable flow) at the suction speed equalling 0.46 of the free-stream velocity. The threshold suction speed can be accurately predicted by the global linear theory of Monkewitz et al. (1993) with a non-parallel flow correction.