An experimental investigation of propagation of weak shock waves in a random medium

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. An apparatus has been constructed for generating a 25 cm cube of randomly inhomogeneous gas in the GALCIT 17-inch diameter shock tube. It consists of a two-dimensional 7 x 8 array of fine jets alternately of helium and freon-12 behind coarse grids located on opposite sides of a box. The two other side walls of the box are fitted with optical windows for diagnostics. The up-stream and downstream faces of the box are movable and can be opened rapidly just before shock arrival. This process is automated, and the arrival time of the shock wave relative to the decay of the turbulent density field can be varied. The gas mixture is made neutrally buoyant so that the mean interface between the scattering region and the uniform air in the shock tube is parallel to the plane of the incoming wave. Furthermore, the mean acoustic impedance of the mixture is matched to that of the quiescent air in the shock tube to minimize the effect of the air-gas mixture interface on the shock wave. In this experiment shock waves of strengths varying from [...] = 1.007 to 1.1 scatter from random variations of acoustical impedance and index of refraction (defined as the ratio of the sound speed in air to the local, variable, sound speed in the scattering medium) which occur during the turbulent mixing of the two different gases. The scale and amplitude of the fluctuations before interaction with the shock wave are obtained from optical and point density measurements; the method of Uberoi and Kovasznay (Ref. 10) has been used to obtain length scales of the flow from shadowgraph and schlieren pictures, and the Brown-Rebollo (Ref. 11) density probe is used to measure local mean and rms density fluctuations, and space- and cross-correlation functions. To study the interaction of the shock with the turbulence, spark shadowgraph and schlieren pictures have been taken and pressure measurements have been made. Arrays of pressure transducers located in a false endwall downstream of the scattering volume record the shock front topology and the spatial variation of shock amplitude. The pressure measurements indicate a substantial modification of the unperturbed shock profile. Data are presented which indicate the effect of the Mach number on the scattering process. Optical measurements show that length scales in the fluid, which is processed by all but the weakest shocks, have changed due to the interaction process