On the mixing of a rectangular jet

Abstract

Results of hot-wire measurements in an incompressible rectangular jet issuing into a quiet surrounding at ambient conditions are presented. The quantities measured include distributions of mean velocity and the turbulence shear stresses in the two central planes of the jet at stations up to 115 widths (small dimension of the nozzle) downstream of the nozzle exit. The flow field of the jet was found to be characterized by the presence of three distinct regions, defined by the axial mean velocity decay, which are referred to as: a potential core region, a two-dimensional-type region, and an axisymmetric-type region. The onset of the axisymmetric region occurs at a downstream location where the two shear layers from the short edges of the nozzle meet. In the central plane which contains the small dimension of the nozzle, similarity was found both in the mean velocity and shear stress profiles beyond 30 widths downstream of the nozzle exit; however, profiles of r.m.s. velocity show similarity in the second but not the third region. The mean velocity, shear stress and r.m.s. velocity profiles in the central plane containing the long dimension of the nozzle do not show geometrical similarity.