Tests were performed to study the mechanism of shock formation in supersonic flow
in long orifices to gain insight into the leakage flow of turbine tip gaps. The flow was
modeled on a water table using a sharp-edged rectangular channel. The hydraulic
analogy between free surface water flows and compressible gas flows was used to
study the implications of the water table flow on tip leakage flows.
The flow on the water table exhibited oblique hydraulic jumps starting on the channel
sidewall near the channel entrance. This flow was analyzed using the oblique
hydraulic jump relations developed by classical hydraulic theory. The results of this
analysis suggested a model for the formation of the jump. As the flow accelerates
around the corner of the channel entrance, supercritical free stream flow is turned
as it intersects the sidewall. The abrupt change in flow direction results in the
formation of the oblique hydraulic jump.
An acceptable hydraulic analogy of compressible gas flows with shocks was obtained
by reducing the surface tension of the water and using a large model size. The
modified analogy for non-isentropic flow then allowed quantitative evaluation of the
modeled shock structure in a compressible flow field. The predicted shock formation
in such a flow has possible implications for both the efficiency of a gas turbine and
the useful life of the turbine blade.
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