Position Paper for the Panel on Theory

Abstract

(a) Experimental observations. Since the early days of the Schubauer and Skramstad experiment, most experimenters have approached the boundary layer transition problem by artificially exciting their flows with relatively two-dimensional, small amplitude, single frequency excitation devices, such as vibrating ribbons or acoustic speakers. They often go to great lengths to reduce the background disturbance level to an absolute minimum in order to make the external forcing in their experiments as small as possible. The initial disturbances should then exhibit harmonic time dependence, be well described by linear stability theory, and be nearly two-dimensional for the low Mach numbers at which most of the experiments have been carried out. This two-dimensional linear behavior can persist over long streamwise disturbances when the excitation levels are sufficiently small, but eventually becomes three-dimensional, as evidenced by the appearance of $\Lambda$-shaped structures in experiments where smoke-flow visualization is used. These structures, which are arranged in rows, can either be aligned or staggered in alternating rows. The unstaggered arrangement, which was originally observed in 1962 by Klebanoff, Tidstrom, and Sargent, is now commonly reffered to as “peak valley” splitting.