A new viscous-inviscid interaction procedure of the semi-inverse type has been developed to predict two-dimensional separated flows. The method is applied to incompressible flow over an external backward-facing step, using linearized potential theory for the inviscid region and a simple modification of Pohlhausens’ momentum-integral method in the viscous region. The modified Pohlhausen method, which approximates the reverse flow region with a region of “dead-air,” is first tested without the viscous-inviscid procedure to predict fully developed laminar and turbulent flow in a plane symmetric sudden expansion. Comparisons are made with experimental data, other calculation methods, and finite difference predictions using a modified version of an elliptic code (TEACH-II). Reasonable predictions of the sudden expansion and backward-facing step flows are obtained, provided that the step-height to boundary-layer thickness ratio is large enough for the Pohlhausen type velocity profiles to be effective. The relative simplicity of the zonal equations coupled with the viscous-inviscid interaction procedure makes the present calculation method computationally attractive. The method should also prove useful in more complex separated flow situations, such as bluff-body aerodynamics.

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