Measurements of the acceleration of reflected shock waves by means of a new heat transfer gauge

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A small heat transfer probe, operating in the free molecule tow regime, was developed with the purpose of obtaining accurate x, t diagrams of a reflected shock wave close to the end wall of the GALCIT 17-inch shock tube. The sensitive element of the probe consists of a .005 diameter filament of non-conducting material coated with a thin metallic film. The sensitivities of a filament probe and a conventional cold wire are compared analytically and it is found that the filament probe has favorable characteristics for measurement times of the order of a few microseconds. This is confirmed by the experiments. The probe was mounted in the end wall of the 17-inch shock tube and x, t diagrams of reflected shock waves were measured at three levels of the initial pressure. The initial motion of the reflected shock is governed by heat loss to the reflecting wall; the wave velocity approaches its ideal value only asymptotically. The asymptotic approach agrees closely with the results of a boundary layer theory. The measurements indicate that the trajectory of the reflected shock close to the end wall is characterized by three different regions; the region in which boundary layer theory is valid, a region closer to the wall in which deviations from boundary layer theory are observed, and a non-continuum region adjacent to the wall in which the reflected shock is formed. The reflected shock wave is found to leave the formation region with a velocity which is approximately 20 per cent below the ideal velocity. It then accelerates toward this ideal velocity, approaching within 3 per cent at distances from the end wall of about 1000 mean free paths. The boundary layer approximation is found to be valid for distances greater than about 70 mean free paths. Accordingly, a non-uniform temperature profile is to be expected in a layer of approximately 100 mean free paths from the end wall.