ON RAYLEIGH INSTABILITY IN DECAYING PLANE COUETTE-FLOW

The inflexion point criterion of Rayleigh is one of the most well-know n results in hydrodynamic stability theory but cannot easily be demons trated experimentally in wall bounded flows. For plane Couette flow, w here both walls move with equal speed in opposite directions, it is po ssible to establish a (time-dependent) inflectional velocity profile i f both walls are brought momentarily to rest. If the Reynolds number i s high enough a growing stationary instability develops. This situatio n is ideally suited for flow visualization of the instability. In this paper we show flow visualization experiments and stability calculatio ns of the developing transverse roll cell instability in such a flow a t low Reynolds numbers. Although the stability calculations are based on a quasi-stationary velocity profile, the measured and most amplifie d wave length obtained from the calculations are in excellent agreemen t.