Existence of mach cones and helical vortical structures around the underexpanded circular jet in the helical oscillation mode

Existence of Mach cone and helical vortical structure in the helical oscillation mode of an underexpanded circular jet was confirmed by using schlieren instantaneous photographs and drawing of the envelopes of the Mach cones by the superposition of spherical sound waves radiated from two moving sound sources about the jet axis at a supersonic speed. Existence of such structures was conjectured in our earlier paper [Umeda and Ishii, J. Acoust. Soc. Am. 110, 1845-1858 (2001)]. The envelopes of a Mach cone are observed as a V-shaped pattern composed of a pair of clear fine lines starting from a prominent point, which rotates about the jet axis. The helical vortical structure is observed as a bright pattern of the gathering of the tiny specks around the jet. It always appears to overlap on the envelopes of the moving Mach cones.