Turbulence Characteristics of Trailing-Edge Flows on Thick and Thin Hydrofoils

Abstract

Turbulence characteristics of the boundary layers on both upper and lower surfaces immediately upstream of the trailing edges of air/hydrofoils are known to influence the turbulence structure in the vicinity of the edges and the nature of the vortex shedding process in the wake. Considerable theoretical and experimental research has been conducted on the two broad classes of trailing-edge flows. In the first class, flow over blunt trailing edges, i.e. cylinders, periodic symmetric vortex shedding is usually formed as the result of Helmholtz wake instability. The shedding Strouhal number has been experimentally determined. Its dependence on boundary-layer characteristics prior to flow separation is not fully understood. The vortex shedding causes pressure (hence lift) fluctuation near the trailing edge and therefore radiates tonal sound at the shedding frequency. In the second class, the trailing edges are thin with attached turbulent boundary layers on both upper and lower surfaces so that wake instabilities and periodic vortex shedding are inhibited by the two merging energetic turbulent boundary layers. The pressure fluctuations and sound generation by the thin trailing edges are found to have wider frequency range if the structural vibrations are not excited.