FAR WAKE BEHIND CYLINDERS AT HYPERSONIC SPEEDS .2. STABILITY

At Reynolds numbers below 104 the inner viscous wake behind cylinders at Mach 6 is entirely laminar. At these low Reynolds numbers the outer wake caused by the bow shock becomes transitional thousands of diameters downstream. Flowfield measurements (Part I) indicated strong deviations from the behavior of laminar steady flow from a certain point on, suggesting a division into linear and nonlinear wake instability regions. Fluctuation measurements confirm this supposition at four Reynolds numbers. Wake instabilities were investigated experimentally up to a distance of 12,000 diameters behind the cylinder in a Reynolds number range of 200 to 4,000. Additional measurements were performed in the regime where both inner and outer wake instabilities occur. In the linear region close correspondence with linear inviscid stability theory was found. The nonlinear instability region is a long stretchedout region in comparison with the wake at low speeds behind a flat plate. A further increase of the fundamental instability component and a sudden growth of the first harmonic is observed. A tentative picture is given of the onset of nonlinearity and/or transition in the inner and outer wakes behind blunt bodies at hypersonic speeds. © 1968 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.