Wind tunnel measurements of sphere drag at supersonic speeds and low Reynolds numbers

Sphere drag has been measured in a low-density supersonic wind tunnel by a simple displacement technique. A conical nozzle of fixed geometry, operated at a constant supply temperature of about 300 °K was used. Test section Mach numbers ranged between 3.8 and 4.3 depending on supply pressure. The Reynolds number and Knudsen number range, based on free-stream conditions and sphere diameter, were 50 < Re∞ < 1000 and 0·106 < Kn∞ < 0·006, respectively. This range was achieved by varying sphere size and supply pressure. The drag coefficient was found to increase from a value of C D = 1·17 at Re∞ = 1000 to CD = 1·73 at Re∞ = 50.