CONTROL OF 3-DIMENSIONAL PHASE DYNAMICS IN A CYLINDER WAKE

Recently there has been a surge of new interest in three-dimensional w ake patterns. In the present work, we have devised a method to control the spanwise end conditions and wake patterns using ''end suction'', which is both continuously-variable and admits transient control. Clas sical steady-state patterns, such as parallel or oblique shedding or t he ''chevron'' patterns are simply induced. The wake, at a given Reyno lds number, is receptive to a continuous range of oblique shedding ang les (theta), rather than to discrete angles, and there is excellent ag reement with the ''cos theta'' formula for oblique-shedding frequencie s. We show that the laminar shedding regime exists up to Reynolds numb ers (Re) of 205, and that the immense disparity among reported critica l Re for wake transition (Re = 140 - 190) can be explained in terms of spanwise end contamination. Our transient experiments have resulted i n the discovery of new phenomena such as ''phase shocks'' and ''phase expansions'', which can be explained in terms of a simple model assumi ng constant normal wavelength of the wake pattern. Peter Monkewitz (La usanne) also predicts such transient phenomena from a Guinzburg-Landau model for the wake.