Investigation of the Transmission of a Shock Wave through an Orifice

Abstract

A shock wave propagating in air in a shock tube was reflected from an orifice plate, and the strength or Mach number of the transmitted wave was measured for a range of incident shock Mach numbers from 3 to 9 for several types of orifices. Also schlieren photographs of the starting flow pattern were made for some of the orifices investigated. The measured values of transmitted shock strength are compared with predicted values based on a theoretical one-dimensional flow model for both an ideal gas and a real gas. The agreement between the measured values of transmitted wave Mach number and the theoretically predicted values is extremely good in the Mach number range investigated for a wedge type orifice at an ambient shock tube pressure of 5.0 mm Hg, and also for a conical type orifice at an ambient shock tube pressure of 2.5 mm Hg. For both orifices the ratio of outlet area to inlet area is 7.67. The data also indicate that for a wedge type orifice of area ratio of 23.0 and for a plate (free expansion) type orifice of area ratio 23.0 possible boundary layer and shock wave interactions downstream of the orifice result in measured values of transmitted wave Mach number somewhat greater than that predicted by the one-dimensional flow model. Investigation of the conical orifice with an area ratio 7.67 at a low ambient pressure in the shock tube (0.4 mm Hg) also yields measured values of transmitted wave Mach number greater than that predicted by the one-dimensional flow model, indicating the probable development of a thick boundary layer behind the transmitted wave downstream of the orifice.