This paper describes an experimental investigation on the flow structure of the Mach 4 pseudo-shock wave (PSW) in a straight square duct with cross section of 80 × 80 mm², using the Mach 4 supersonic wind tunnel of Muroran Institute of Technology. The free stream Mach number, unit Reynolds number, and flow-confinement just upstream of the PSW were M_∞ = 3.98, Re = 2.6 × 10⁷ m^-1, and δ_∞/h = 0.39, respectively, where δ_∞ is the boundary layer thickness, and h is the half height of duct. The flow structure of the Mach 4 PSW and separated flow field were clarified by color schlieren photographs, wall surface oil flow observation, and detailed flow velocity measurements using a particle image velocimetry(PIV).

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Cavitation is one of the most important phenomena in the high-speed liquid flow. When cavitation appears in the flow, it causes some troubles like a performance decrement of the fluid machine, vibration, noise, and cavitation damage. So, it is necessary to observe the cavity behavior to solve such problems. In this study, an experimental research was performed about behavior of the cavitation inside a circular cylindrical orifice. The aspect of the cavitation developing on the inner wall was observed by the high-speed video camera with a laser sheet method. As a result, unsteady shedding behavior of vortex cavity was characteristically observed. In addition, pairing and coalescence behavior of vortex cavitation on the separated shear layer were clearly observed with the laser sheet method.

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