Shock Train Unsteadiness Characteristics, Oblique-to-Normal Transition, and Three-Dimensional Leading Shock Structure

The structure and unsteadiness characteristics of a shock train in a constant-area ducted flow are studied experimentally as a function of duct back pressure, which is mechanically controlled by a valve. Increasing the back pressure pushes the shock train upstream where the approach conditions (including Mach number and boundary-layer momentum thickness) are different, as these quantities vary along the duct length. Despite nominally constant boundary conditions during a run, the shock train fluctuates about its time-averaged position. Schlieren imaging reveals that the unsteadiness characteristics, including the fluctuation amplitude and frequency content, are independent of back pressure. However, changes in the Mach stem length and shock angles demonstrate that the leading shock structure transitions from oblique to normal as back pressure increases. Time-averaged pressure profiles show that the shock train length decreases and the pressure rise across the shock train increases during this process, but these quantities are nominally constant when normalized by the approach conditions. Finally, stereo particle image velocimetry is used to study the three-dimensional structure of a normal leading shock. The velocity fields show a large separation region on the side-wall of the duct and a degree of axisymmetry that indicates a nominally conical shock structure.