Turbulent separated flow over a rounded divergent trailing edge

Measurements have been obtained to characterize the flow structure in the vicinity of the divergent trailing edge airfoil that has a rounded base instead of the conventional blunt base, in a subsonic flow, using a large-scale windtunnel model. This flow configuration was designed such that the flow over the aft 20% of the model was similar to that downstream of the shock on a supercritical airfoil in transonic cruise, albeit without compressibility effects. Calculated distributions of surface pressure and boundary-layer development matched measured distributions over the aft 20% of the windtunnel model. Measured results were obtained in the fully turbulent regime at chord Reynolds number of 2.5 million. A combination of surface taps, impact probes and hot wire anemometry provided measured distributions of mean pressure, mean velocity and turbulence stresses in the two-dimensional boundary-layer and wake flow. Emphasis was placed on the near wake region to provide insights into the significance of differences between flow around rounded and blunt bases. Separation with a blunt base occurs at the two geometric discontinuities, whereas that on a rounded trailing edge occurs due to adverse pressure gradient. Results confirm that the flow remains attached on both the suction and pressure sides upstream of start of curvature in the rounded base. Vortex shedding occurred at a Strouhal number of about 0.21 with both the round and blunt trailing edges. The rate of mixing downstream of the rounded trailing edge was less than that aft of the blunt base because of lower turbulence production in the shear layers surrounding the recirculation region. Two implications for airfoil design are: (1) round trailing edges have lower drag, and (2) have a narrower wake, and so may be preferred when flaps are extended during take-off and landing.