THE DRAG OF SPHERES IN RAREFIED HYPERVELOCITY FLOW

Drag of spheres has been measured under hypersonic, cold-wall, support-free conditions in a nonreacting flow in which molecular vibration was frozen. Data were obtained for a nominal freestream Mach number of 11 and for Reynolds numbers from 1 to 10 based on conditions immediately downstream of the normal shock and sphere diameter. These data were supplemented by measurements at a nominal Mach number of 10 where a conventional balance was used, and Reynolds numbers downstream of the shock as high as 104 were investigated in the cold-wall condition. The experimental results have been analyzed both from the viewpoint of continuum flow with second-order viscous effects and from the standpoint of a noncontinuum concept, taking account of first collisions between re-emitted and freestream molecules. In both cases, useful, semiempirical expressions for drag coefficient are derived. The first-collision analysis is numerically indeterminant because of the lack of a method for explicit calculation of mean free path. However, the form of the derived equation for drag coefficient in noncontinuum flow is particularly suitable for modification to permit its use in linking freemolecule solutions and available experimental data. This possibility is suggested because the limiting drag coefficient at high Reynolds numbers is on the order of the accepted value for continuum, inviscid flow.