Based on experimental observations of pressure changes during hydraulic transients in pipe flow with column separation, a physical model was hypothesized and correlated with the data to provide a semiempirical “bubble shear stress” which can be used to predict the increased momentum loss observed under column separation conditions. The “bubble shear stress” arises from the nonadiabatic expansion and collapse of gas bubbles present throughout the low pressure column separation flow, and is found to be proportional to the strength of passing pressure waves, and a weak function of the amount of gas trapped on the pipe walls.

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