Shock wave and contact zone instabilities

Abstract

It is established that a turbulent mixing zone, rather than an infinitely thin contact surface , separates the test sample from the driver gas, over a wide range of shock tube operating conditions. Severe distortions of the shock front are observed when this mixing zone is co incident with the shock wave. It is shown that these shock distortions can be removed by separating the mixing zone from the shock. This separation can be accomplished by either increasing the mean molecular weight of the driver gas, or decreasing the mean molecular weight of the test gas. A model that predicts the mixing zone to become trapped inside the relaxation zone under certain conditions, is proposed. Good agreement is achieved with experiments for the case of vibrational non-equilibrium in sulfur hexafluoride. However, improvements to the model are necessary to obtain agreement with experiments at higher enthalpy conditions. The same model also accurately predicts the lengths of test samples for the case of boundary layer entrainment when similarity assumptions are satisfied and reconciles discrepancies observed with earlier predictions for test sample lengths. In addition a general linear analysis , that unifies presently existing theories on shock wave instability , is developed, and a criterion for spontaneous distortion of the shock wave is proposed. However, modifications to the analysis are necessary when radiation losses are important. Good agreement is achieved with experimental observations for cases when radiation losses are expected to be small. An experimental technique that allows spectral line absorption to be synchronized with interferometric flow visualisation is developed, and enables earlier observations of contact zone and shock wave instability to be unambiguously confirmed by the present investigation .