Previous studies of cavity flows have focused on the phenomena which occur when a shockwave enters a cavity. These studies show that a region of high pressure and temperature is formed, which has been called the focus region [1]. The flow resulting from an expansion wave entering a cavity has not been investigated however. This study looks at the flow resulting from a plane expansion wave entering a cylindrical cavity. The research looks specifically at the expansion waves which occur in the driver section of a shock tube after a diaphragm burst. The research uses a tube with a 100mm by 100mm working section. A pressure ratio of 7 was used; this corresponds to an initial shock Mach number of 1.5. For this pressure ratio the flow between the contact surface and the expansion wave is subsonic, thus the tail of the expansion wave would move into the driver. The driver curves immediately at the diaphragm in order to ensure the expansion entering the cavity was as steep as possible. The initial numerical results show the expansion waves in the driver interact to create a region of very low pressure, density and temperature, however the location and formation of this region differs from the high pressure and temperature region found in shock focusing. These results also indicated that shocks form in the driver and thus follow the expansion focusing with a conventional shock focusing pattern as shown in Figure 1. These shocks start at the wall and grow towards the symmetry plane while moving into the driver. Once the shocks meet on the symmetry plane a high pressure and temperature region is created directly behind the intersection of the shocks. For some pressure ratios a second set of shocks are formed and behave in the same way, they are however much weaker. The result of this flow is a region in the driver, ahead of the shocks, which reaches extremely low pressures and temperatures. It also causes a stagnated region of flow which starts at the driver and then moves downstream. The contact surface is caught in this stationary region of flow and as a result doesn’t move down the tube. These numerical results will be compared with experimental data, from a facility which is in the final stages of manufacture, shown in Figure 2. The research will be expanded to various different driver geometries and pressures as well as the axisymmetric case.
Figure 1: Time series of expansion entering a cylindrical cavity
Figure 2: Experimental Facility
References
|
[1]
|
D.A. MacLucas, "Shock wave-induced flow features in concave profiles," PhD Unversity of Witwatersrand, Johannesburg, 2012.
|
