Steady and unsteady transonic flow in a duct with a sudden enlargement

Flow structure in a rectangular or circular duct, following an abrupt change in section, were studied. Density measurement was with a Mach-Zehnder interferometer and of velocity with a laser Doppler anemometer. The flow structure was controlled either by a single normal shock wave or by a series of reflected oblique shocks. In the case of the normal shock wave structure, one-dimensional theory is found to apply adequately to the overall duct. Within the duct, the flow is not one-dimensional, but has a minimum velocity in the center and four shear layers. One-dimensional theory is not successful in explaining the results of the oblique shock wave structure. An empirical approach is adopted to the task of determining the characteristic base pressure oscillation frequency and of estimating the harmonic which occurs. The phase relationship between the pressure downstream at the wall and the base pressure was investigated. It is found that throughout the ducts there is always a phase change of 180 deg between the center and the duct walls.