Streamwise Evolution of a Square Jet Cross Section

Results of an experimental study of a turbulent free jet of air issuing from a sharp-edged square slot into still air surroundings are presented. The study was done to shed some light on the stream wise development of the jet cross section. The quantities measured directly, using hot-wire anemometry, include the three components of the mean velocity vector, the three Reynolds normal stresses, and the two Reynolds primary shear stresses. It was found that the near region of the jet is dominated by four sets of counter-rotat ing streamwise vortices. These vortices, which represent Prandtl's secondary flow of the first kind, are generated from distributed vorticity shed from the four corners of the slot by skewing of the shear layers as a result of the vena contracta effect. Mean streamwise velocity off-center peaks were also found in the very near region; such mean streamwise velocity off-center peaks may be the result of the self-induction of the streamwise vortices. Furthermore, the higher numerical values of the Reynolds normal and primary shear stresses in the present square jet, compared with those found in a round jet, indicate faster mixing of the square jet.