LDA measurements in the turbulent round jet

摘要

In the present paper, LDA was used to measure the velocity field of turbulent round air jet flows. Two cases were investigated; a jet issuing vertically upward and freely in the laboratory surrounding environment, and a jet issuing vertically upward but out of wall section setting 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 respectively. For each Reynolds number, profile measurements of the mean velocity, turbulence intensity, skewness and flatness factors were made 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 and characteristics is the objective of the present study. The experimental results indicate that the wall, placed at the exit plane, limits the interaction of the jet flow with the surroundings, and consequently results in a reduction in the velocity spread rate, kinematic momentum flux, and kinematic mass flux. Further, the flatness and skewness factors distributions across the jet flow registered relatively higher values in the outer region of the jet when the wall was used. This indicates a more intermittent behavior of the jet flow in that region due to the existence of the wall. (C) 1997 Elsevier Science Ltd.