Spreading of Turbulent Round Jet in Coflow

Abstract

The scalar concentration and velocity field of an axisymmetric turbulent jet in a coflow are measured using laser-induced fluorescence and laser Doppler anemometry techniques. Both the time-averaged and turbulence data suggest the use of the top-hat average velocity as the characteristic velocity in this shear flow. The visual boundary and the 25% maximum concentration (0.25C_m) contour are found to correspond closely to the edge of the top-hat profile, where r = B = $\sqrt{2}{\mathit{b}}_{\mathit{g}}$ (B and b_g are the nominal half-width of the top-hat and Gaussian profiles, respectively). This jet edge, defined by the 25%C_m contour, also matches with the 50% contour of the intermittency factor. An integral model of the coflowing jet has been formulated using a Lagrangian jet spreading hypothesis for turbulent closure. The model predictions are in good agreement with the present concentration, velocity data obtained in the near field (strong jet), and the velocity data of a recent study in the far field (weak jet).