LDA MEASUREMENTS IN THE TURBULENT ROUND JET

In the present paper, LDA was used to measure the velocity field of tu rbulent round air jet flows. Two cases were investigated; a jet issuin g vertically upward and freely in the laboratory surrounding environme nt, and a jet issuing vertically upward but out of wall section settin g flush horizontally at the nozzle exit. Data were collected for three exit Reynolds numbers of 1.32 x 10(4), 2.64 x 10(4) and 3.96, x 10(4) , which correspond to exit velocities of 10 m/s, 20 m/s and 30 m/s res pectively. For each Reynolds number, profile measurements of the mean velocity, turbulence intensity, skewness and flatness factors were mad e at 8 downstream stations up to 30 nozzle-exit diameter. The relative influence of using a wall at the jet exit plane on the jet behavior a nd characteristics is the objective of the present study. The experime ntal results indicate that the wall, placed at the exit plane, limits the interaction of the jet flow with the surroundings, and consequentl y results in a reduction in the velocity spread rate, kinematic moment um flux, and kinematic mass flux. Further, the flatness and skewness f actors distributions across the jet flow registered relatively higher values in the outer region of the jet when the wall was used. This ind icates a more intermittent behavior of the jet flow in that region due to the existence of the wall. (C) 1997 Elsevier Science Ltd.