Influence of isolated backward-facing steps on boundary layers.
2015-11-19T08:59:04Z (GMT) by
The drag of backward-facing steps was experimentally measured in a series of turbulent boundary layer flows and in a laminar flow. The main objective of the investigation was to study the influence of the freestream pressure gradients on the drag values for backward-facing steps in turbulent boundary layer flows. The measured boundary layer characteristics of all the established flows are in very good agreement with other published results and their two-dimensionality was ascertained. Two different techniques have been used to measure the drag of such steps. The momentum thickness technique was not suitable for determining the drag of such small irregularities, since the incremental increase in the momentum thickness due to the irregularities was small and the method of calculation was crude. The pressure distribution technique was found to be a more suitable method of estimating this drag. When applied appropriately the measurement uncertainty was small. The drag results published by Gaudet and Johnson for backward-facing steps in a zero-pressure gradient have been considerably extended for a number of turbulent boundary layer flows with values of pressure gradient parameter, a, from -0.97 X 10-3 to 9.14 X 10-3. For small steps results covered a range of irregularity Reynolds number of 150 to 900. For large steps in zero-pressure gradient flow this range was increased up to 3500, and up to 2000 in a mild adverse pressure gradient flow. The drags of backward-facing steps in a laminar boundary layer flow on a flat plate have been measured by the pressure distribution technique. No correlation was made for the drag of such steps in the laminar boundary layer, however a comparison between the total drag of the plate with and without the largest step showed that the total drag of the plate increased by only some 65 % of the clean surface drag value.