Supersonic air jets from the D'Laval nozzle of designed Mach number 2.0 were experimentally investigated. Some typical flow patterns of overexpanded, designed and underexpanded flows were observed by the schlieren system and the various pressure measurements. Then, the normal shock wave standing just at the nozzle exit in the overexpanded flow was cleared as to be recognized as a pseudo-shock wave, because the normal shock wave had interacted with the shear layer of jet boundary and the compression region of the shock wave was lengthened to some diameters of the nozzle exit. The pitot and static pressure along the jet axis were varied up and down alternately influenced by the shock waves and expansion waves in the pseudo-shock wave. Both of the pitot and static pressure distributions normal to the flow axis had two or more peaks in the region of pseudo-shock, because of the existence of λ type or X type shock waves. After the region, static pressure became to have the value of the atmospheric pressure, and then the pitot pressure coincided with that of the ordinary incompressible circular free jet asymptotically. The static pressure ratio across the pseudo-sheck wave was only 38% of the theoretical value of the normal shock wave, and even the maximam peak pressure value measured in the front part of the pseudo-shock was not more than about 52%.
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