Experimental Study of Helium and Argon Diffusion in the Wake of a circular Cylinder at M=5.8

Abstract

Experimental measurements of the diffusion of helium and argon in the wake of a porous cylinder were made in the GALCIT hypersonic wind tunnel at Mach number 5. 8. The cylinder was mounted perpendicular to the flow and small quantities of tracer gas were pumped through the model walls into the flow. The thermal conductivity method of gas analysis was used to determine the concentration of sample gases extracted from points in the wake. The transverse and axial distribution of concentration appeared to follow theoretical estimates of "similarity behavior". Injection of tracer gas was found to have a measurable effect on stagnation pressure and this effect was taken into account during computations. Numerical values of diffusion coefficients along the wake centerline were computed from the experimental data and then compared with theoretical values for laminar flow. Close agreement between experimental and theoretical values at Re_d= 18,000 verified that the inner wake was laminar as far downstream as measurements could be made (15 diameters). At Re_d= 72,000, the data showed that mixing processes were 3 times more rapid for helium, and 10 times more rapid for argon, than those expected in laminar flow. This result confirmed the presence of turbulence at this flow condition.