Upstream condition effects on the evolution of symmetric and asymmetric near-wakes of a flat plate

摘要

An experimental study was conducted in order to investigate the influence of upstream conditions with respect to near-wakes evolution by using a two-dimensional flat plate. A position of x_(B.L)/C =0.98 for the flat plate with a blunt trailing edge was settled upon as an upstream condition point. Three upstream conditions (i.e., natural laminar, bypass transitional, and turbulent boundary layers) were applied by tripping wires that were placed on the upper surface of the plate. The free-stream velocity was 6.0 m/s and the local Reynolds numbers based on momentum thickness at the upstream conditions point were 259, 303, and 331. The boundary layers and the near-wakes were measured by hot-wire anemometry. It was observed that for the symmetric near-wakes, the local similarity profiles of the mean defect velocity matched well with the traditional similarity profile in the center part, whereas some difference in the outer part occurred due to the different trailing edge shape. For the asymmetric upstream conditions, the center of the local similarity profiles moved to one side, and the amplitude of the local similarity profiles over the wakes edge varied according to the momentum thickness of the upstream conditions. Also, the collapse of the normalized Reynolds shear stress depended on turbulent quantities included in the upstream conditions, and these results showed that the evolution of the near-wakes is clearly affected by upstream conditions.