Shock Tube Investigations of Strong Shock Waves in a Convergent Channel

The behavior of the incident and reflected shock waves in a convergent channel is investigated in order to determine if such a geometrical device could be used as a means of producing high-enthalpy gas samples. A 10° half-angle conical convergence is mounted on the end of a pressure-driven, six-inch shock tube. Using argon at an initial pressure of 1.5 torr, initial shock Mach numbers are varied from 6.0 to 10.2. During each run local shock velocities at several positions along the cone axis are measured using a small, multi-crystal, axial piezoelectric probe inserted from the cone vertex. The incident shock velocity profiles show that the shock behavior is dominated by multiple diffraction processes which originate at the cone entrance. Sudden increases in shock velocity at certain positions along the axis are observed, corresponding to the intersection of stemshocks formed by Mach reflection on the cone wall. These increases are separated by regions of deceleration and acceleration, corresponding to the growth and decline of a center shock formed by Mach reflection on the cone axis. Near the vertex the shock velocity has increased by as much as a factor of three, indicating that high temperatures and pressures are generated. By varying the initial Mach number and pressure, real gas effects are found to influence the diffraction process only in a region near the vertex. Reflected shock profiles show that the shock velocity is nearly constant for much of the convergence length, in contrast to the power-law decline predicted by the similarity solution. During this period the shock propagates into fluid originally set into steady, uniform motion outside the cone entrance. Small variations in the velocity result from weak interactions with localized nonuniformities and secondary waves. Beyond the cone entrance the shock decelerates towards the velocity corresponding to reflection from a plane end wall. A departure from ionization equilibrium is likely near the vertex during the rapid expansion which occurs behind the reflected shock.