Energy concentration by spherical converging shocks generated in a shock tube

Authors: Kjellander, M. K., Tillmark, N.T., Apazidis, N.
Document Type: Article
Pubstate: Published
Journal: Physics of Fluids
Volume: 24   126103
Year: 2012


Spherical converging shock waves are produced in a conventional shock tube with a circular cross-section. Initially plane shocks are transformed into the shape of a spherical cap by means of a smoothly convergent cross-section. The wall shape in the transformation section is designed to gradually change the shape of the shock wave until it approaches a spherical shape. Thereafter the shock enters a conical section where it converges towards the apex of the cone. Numerical calculations with the axisymmetric Euler equations show that the spherical form is only slightly dependent on the initial Mach number of the plane shock within the range 1.5 < M < 5.5 and is preserved to a close vicinity of the focal point. The test gas is heated to very high temperatures as a result of shock convergence and emits a bright light pulse at the tip of the test section. The light radiation is collected by optical fibers mounted at the tip of the convergence chamber and investigated by photometric and spectroscopic measurements. Experiments are performed with argon and nitrogen and with different initial Mach numbers. The radiation of the shock-heated argon closely resembles blackbody radiation. Fits to the experimental data result in apparent blackbody temperatures in argon of up to 27,000 K some 250 ns after the focusing instant. The initial Mach number in these spectrometric runs is M=3.9, indicating an efficient amplification of the shock wave strength.