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When sonic nozzles of significantly smaller diameter are used as standard flow meters, the critical back pressure ratio is affected by the boundary layer at the nozzle throat. However, the effect of the boundary layer on choking criteria is still controversial. Then, the choking phenomenon of a convergent nozzle flow has been experimentally investigated using four convergent nozzles with the same diameter followed by a straight pipe of a variable length.As a result, it is shown that the critical back pressure ratio is smaller than that for the steady one-dimensional isentropic flow and decreases as the boundary layer thickness increases. Moreover, the main flow Mach number at the nozzle exit is supersonic when the back pressure ratio is equivalent to the choking condition, and the Mach number increases as the boundary layer thickness increases.

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Pseudo-shock is often encountered in a variety of internal flows, almost always entailing an increase in total pressure loss, hence leading to deterioration of the flow system performance. This pseudoshock is a very complex process still requiring a good deal of elucidation. In spite of the recent rapid progress that has been made in numerical computation methods, there is as yet no quantitative prediction of pseudo-shock flow. The present paper describes a simple pseudo-shock flow model using a mass-weighted averaging technique. The flow quantities across pseudo-shock were analysed for different Mach numbers, flow confinements and turbulent boundary layer Reynolds numbers. The analytical results agree well with the previous experimental data. The flow model presented can be used to estimate the important features of pseudo-shocks in engineering practice.

The slightly underexpanded free jet issued from a supersonic micro nozzle with a design Mach number of 1.5 and a square shape of 1 mm × 1 mm at the nozzle exit has been experimentally investigated using the rainbow schlieren deflectometry combined with the computed tomography where the nozzle operating pressure ratio is held constant at 4.0. Density fields in the free jet obtained from the rainbow schlieren deflectometry are the vortex sheet theory proposed by Tam. It is shown that there is good quantitative agreement between experiment and theory. It is shown that the rainbow schlieren deflectometry is a useful tool for studying the three-dimensional structure of shock containing free jets from micro-scale supersonic nozzles.

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