STUDY OF THE DISCHARGE OF WEAK SHOCKS FROM AN OPEN END OF A DUCT

Abstract

Shock discharge from a duct exit is often encountered in many engineering practices and usually gives rise to an annoying noise problem similar to a sonic boom. Such a noise can have hazardous effects on human beings, as well as structures in the vicinity, unless proper control strategies are developed. The objective of the current work is to characterize the impulse wave caused by a weak shock discharged from an open end of a duct. Experiments were performed using an open-ended shock tube for shock Mach number of 1·02–1·45. Computational analysis was applied to model the unsteady flow field by application of axisymmetric, inviscid, compressible equations. A total variation diminishing (TVD) numerical scheme was used to solve the conservation equation system. The effect of a baffle plate, installed at the duct exit, on the impulse wave was investigated both experimentally and by numerical calculation. The results of the experiments were in good agreement with the results of numerical calculations. The results showed that the baffle plate affects the strength of the impulse wave only when its diameter is less than 3 times the duct diameter. With the diameter of the baffle plate less than 3 times the duct diameter, the strength of an impulse wave for a given shock Mach number increases when the baffle plate becomes larger. The impulse wave has a directivity to the centerline of the duct. It was found that for prediction of the impulse wave aero-acoustical theory should be used only at distances larger than four times the duct diameter.