Results are presented for flow patterns over backward facing steps covering a wide range of geometric variables. Velocity profile measurements are given for both single and double steps. The stall region is shown to consist of a complex pattern involving three distinct regions. The double step contains an assymmetry for large expansions, but approaches the single-step configuration with symmetric stall regions for small values of area ratio. No effect on flow pattern or reattachment length is found for a wide range of Reynolds numbers and turbulence intensities, provided the flow is fully turbulent before the step.

In the calculation of turbulent flows near walls, a substantial amount of mesh points and computational effort is required to adequately resolve the intense velocity profile variations in the near-wall region. In this study, a profile approximation is obtained for the mean velocity that is based upon consideration of the coherent structure of the time-dependent wall-layer flow. The profile approximation is an analytical formula that satisfies all of the required compatibility conditions at the wall and the logarithmic behavior at the edge of the wall layer. A general method for utilizing the present wall-layer profile model in a prediction method is indicated.

The four common optimum problems in diffuser design are defined. These optima are located in relation to the over-all flow regimes in terms of geometrical parameters for straight-walled units. Using an empirically derived transformation of variables between the conical and two-dimensional geometries, all available data for optimum recovery at constant ratio of wall length to throat width are correlated by a single straight line. This line lies slightly above and parallel to the line of onset of large transitory stall on the chart of over-all flow regimes. The correlated results are based on a literature survey. The range of conditions for each investigation is tabulated for convenient future reference.

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