Aerodynamic drag of ridge arrays in adverse pressure gradients.

Drag measurements for excrescence arrays of square section mounted on a smooth wall and subjected to two adverse pressure gradients in equilibrium are obtained. Differences in drag which arise when the excrescences are uniformly distributed and when tending to isolation are shown. Flow visualization photographs using the surface oil flow technique are presented to illustrate different flow patterns around arrays of varying spacing. A prediction model for the drag of excrescence arrays based on the results obtained was devised. Its range of application could be extended to excrescences of varying shape providing they are sufficiently small to be immersed in the logarithmic part of the boundary layer. Drag results are determined by both a momentum defect and a pressure distribution technique. These results are compared and the difference between them is partially attributed to the change in surface friction between the excrescences and partially to some lack of two-dimensionality in the test procedure. Corrections for the latter effect are made to the data obtained.