Compressible Coanda wall jet: predictions of jet structure and comparison with experiment

Abstract A two-dimensional underexpanded jet blowing tangentially over a cylindrical surface was studied experimentally and reported in a previous paper. The curved jet has a shock cell structure similar to that of a plane jet, but with a separated region on the curved surface. This region grows with increasing upstream blowing pressure until reattachment fails to take place and the jet breaks away from the surface. This paper reports progress in calculating the jet structure so that predictions may be made of the jet development and of breakaway conditions. The inviscid core of the jet was calculated by the method of characteristics: the outer shear layer and surface boundary layer were ignored. When compared with experiments, this gave good predictions for the structure of the first shock cell for low blowing pressures. However, the neglect of the separated region on the surface caused increasing error in the predictions as blowing pressures increased. The program was modified to replace the wall condition by a specified pressure boundary, the pressures being obtained experimentally. The predictions then agreed well with experimentally observed flow patterns for the first one or two shock cells. After that the growth of the shear layer encroaches into the core so that the shock cell structure disappears. Although of wide application, this work is particularly related to the design of Coanda flares where the jet is axisymmetric. As well as extension of the method to axisymmetric geometry, further work is required to develop methods to predict the outer shear layer and the separated region. This would enable predictions of complete jet development and breakaway conditions to be made.