Cyclic wave action in the stable operation of a Hartmann-Sprenger tube

An explanation, believed to be the first, is given of the way in which stable oscillation of the air column in a Hartmann-Sprenger tube is initiated and maintained. Shadowgraphs of the external flow field obtained by synchronous shadow-photography and the numerical simulation of the flow in the tube clarify the behavior of the shock wave system in the space between the forcing nozzle and driven tube; they also show how the movement of the shock wave system is related to flow within the tube. The fractions of the periodic time occupied by well-defined and quasi-steady inflow and outflow phases are shown to be about 22% and about 32%, respectively. Two transient phases that precede quasi-steady inflow and quasi-steady outflow are distinguished, occupying about 33% and 13% of a period, respectively. It is noted that the process of inflow is extremely stable, whereas that of outflow is not always stable. Instability of the outflow is found to derive primarily from a succession of weak shock waves emerging from the tube as a result of internal wave action contained between the contact surface, on the one hand, and the closed end, on the other.