Effect of aspect ratios on potential core length for cruciform jet

Abstract

This paper documents an experimental study of the mean flow and turbulence characteristics for three-dimensional turbulent free jets of air issuing into still air surroundings from cruciform nozzles with four aspect ratio values (AR = 3,5,12.5, and 50). The measured quantities comprise streamwise, spanwise, and lateral mean velocities obtained with a Pitot-static pressure tube and a five-hole pressure tube, and the turbulent intensity with both an X-type hot-wire probe and a linearized constant-temperature anemometer. The experimental results reveal the “saddleback” shape of both the streamwise mean velocity and turbulent intensity distributions in the near flowfield. Potential core lengths nondimensionalized by jet nozzle width d for the cruciform jets are larger than those for two-dimensional and axisymmetric jets. Furthermore, a relationship between the aspect ratio and the potential core length is examined and it is clearly shown that the potential core length could be significantly controlled by the variation of the magnitude of the inward secondary flow velocity on both the y and z axes for the cruciform jets.