Statistical Properties of Round, Square, and Elliptic Jets at Low and Moderate Reynolds Numbers

An experimental study was conducted to investigate the effect of nozzle geometries on the statistical properties of free orifice jets at low and moderate Reynolds numbers. The studied cross sections were round, square, and ellipses with aspect ratios of 2 and 3. For each jet, detailed velocity measurements were made using a particle image velocimetry (PIV) system at Reynolds numbers of 2500 and 17,000. The results showed that at both Reynolds numbers, the elliptic jets had relatively higher velocity decay and jet spreading; however, the nozzle geometry effects were more pronounced at Re = 17,000 than at Re = 2500. Analysis of the swirling strength revealed that the rotational motions induced by vortices within the minor planes of the elliptic jets were stronger than observed in the major planes, square and round jets which were consistent with the relatively higher spreading observed in the minor planes. It was observed that the streamwise locations of the switchover points were independent of Reynolds number but are a strong function of aspect ratio. Based on the present results and those documented in the literature, a linear correlation was proposed for the location of axis-switching in orifice jets. Due to the axis-switching phenomena, a sign change was observed in the distribution of the Reynolds shear stress in the major planes of the elliptic jets. This results in the existence of regions with negative eddy viscosity in the near field regions, an observation that has an important implication for the predictive capabilities of standard eddy viscosity models.