Large-eddy simulations of a turbulent Coanda jet on a circulation control airfoil

Abstract

Large-eddy simulations are performed of a turbulent Coanda jet separating from a rounded trailing edge of a simplified circulation control airfoil model. The freestream Reynolds number based on the airfoil chord is 0.49×106, the jet Reynolds number based on the jet slot height is 4470, and the ratio of the peak jet velocity to the freestream velocity is 3.96. Three different grid resolutions are used to show that their effect is very small on the mean surface pressure distribution, which agrees very well with experiments, as well as on the mean velocity profiles over the Coanda surface. It is observed that the Coanda jet becomes fully turbulent just downstream of the jet exit, accompanied by asymmetric alternating vortex shedding behind a thin (but blunt) jet blade splitting the jet and the external flow. A number of “backward-tilted” hairpin vortices (i.e., the head of each hairpin being located upstream of the legs) are observed around the outer edge of the jet over the Coanda surface. These hairpins create strong upwash between the legs and weak downwash around them, contributing to turbulent mixing of the high-momentum jet below the hairpins and the low-momentum external flow above them. The probability density distribution of velocity fluctuations is shown to be highly asymmetric in this region, consistent with the observation that the hairpin vortices create strong upwash and weak downwash. Turbulent structures inside the jet, its spreading rate, and self-similarity are also discussed.