Shock-Wave Curvature at Low Initial Pressure

The shape of the primary shock wave in a shock tube has been measured by recording the time of arrival of the shock in argon at an array of piezoelectric detectors located along a diameter of a 28.6-mm i.d. shock tube operated at an initial expansion chamber pressure of 0.34 mm Hg. The following conclusions were drawn from the results: (1) Shock-wave tilt was negligibly small under the conditions of these experiments; (2) shock-wave curvature is independent of shock strength for shock Mach numbers from 1.8 to 6.3; (3) the shock-front shape is nearly spherical; (4) the axial extent or the bulge of the curved shock is approximately 1 mm. The axial extent of a shock in argon was also measured over the range of initial pressure from 0.1 to 20 mm Hg. It varies approximately as the inverse square root of the initial pressure.