IMAGES OF DISSIPATION LAYERS TO QUANTIFY MIXING WITHIN A TURBULENT JET

Images were obtained that visualize the structure of the mixture fract ion field and the structure of the scalar dissipation layers that exis t near the base of a turbulent jet. To characterize the local mixing r ate, measured profiles of the mean dissipation rate were compared with a general scaling analysis. In addition, the joint probability densit y function (zeta, chi) was measured. Some effects of adding coaxial ai r and swirl are discussed. The especially Large values of the mean dis sipation rate (up to 30 s(-1)) and the instantaneous dissipation rate (up to 175 s(-1)) that were measured in the base region are due to the targe local values of the mean velocity gradient in this region. It w as found that the dissipation layers in the base region have a unique structure; the layers tend to be aligned at approximately a 45-deg ang le to the flow and thus differ from layers in the far field, which ten d to be oriented in an isotropic pattern, as was shown previously. Str ong dissipation layers exist at the boundary of the entrained air, and the observed 45-deg alignment in the base region is believed to be du e to the ordered vortex pattern in the shear Layer. Dissipation layers are typically 0.3 mm thick. Dissipation rates varied in a manner cons istent with the general scaling analysis, and the proper seating const ant was measured to be 9300.