A study on increase of duration of a free piston shock tunnel

In the present study, performance of a free-piston shock tunnel driver section, which is equipped with a convergent-divergent diaphragm section, has been investigated analytically, experimentally and numerically. Measurements of shock Mach number, pressure and holding time of the constant pressure in stagnation region have been conducted under the conditions of non-over and over drive operation. Experimental results show qualitatively good agreements with analytical study. And the results show that a disadvantage by using the convergent-divergent diaphragm section has been improved by applying the over drive operation, and duration of constant pressure in the stagnation region is increased. Axisymmetric full Navier-Stokes equations have been solved to study the starting process of the driver section. Numerical results show that vortex and secondary shock wave are generated at the inlet of convergent-divergent diaphragm section, which was not assumed in the analytical study. And the quantitative differences between experimental results and analytical ones are shown to be caused by the complicated flow field. Consequently, investigation of the flow field and increase of duration by use of the convergentdivergent diaphragm section for the small size free piston shock tunnel have been achieved.