Hydraulic characteristics of co-planar jets
Date of Issue2013
School of Civil and Environmental Engineering
Maritime Research Centre
The mixing of turbulent jets can be applied in a wide variety of fields. For the sake of effective mixture, the jets configuration needs to be properly addressed. From an environmental aspect, it is vital to design a jets configuration so that the efflux flows can mix rapidly and effectively with the ambient fluid. Previous studies have suggested that jets interactions with inclination configuration to be an optimal scheme in terms of improving mixture. However, most previous studies only concern the side jets as zero mass flux compared to the central jet, i.e.,1~5%, which might not be suitable for most engineering applications. In this study, the writer investigated the behaviour of three submerged round jets injecting convergingly to stationary recipient water in the laboratory. The three jets are set in the same vertical plane and have intersectant axes. Four parameters were investigated in this investigation: the inclined (converging) angle of the side jets with respect to the central jet, the momentum coefficient (velocity ratio) of the side jets, the Reynolds number of the central jet and the jet spacing ratio. Two incline angles (30° and 45°), four momentum coefficients (1⁄16, 1⁄9, 1⁄4 and 1⁄1), three Reynolds numbers (4240, 6400 and 8000) and three jet spacing ratios (2, 3 and 4) were applied. The complex interactions had been investigated by employing the advanced laser diagnostic techniques of particle image velocimetry (PIV). With the PIV system, both ensemble-averaged and instantaneous characteristics of the complex flow fields were acquired. Geometric features and gross mixing characteristics were addressed by the ensemble-averaged (long time average) results, e.g., the location of the merging point, combining point and necking point. The mixing behaviour of the converging jets was also investigated through the analysis of the longitudinal and cross-sectional velocity distributions. The cross sectional distribution of the mean streamwise and lateral velocity on both the X-Y and X-Z planes, respectively,were found to attain self-similarity after the flow was fully developed and the merging processes were complete. The jet boundary was elucidated by the jet half-width based on the cross-sectional velocity distribution. The converging jets were observed to grow linearly on the X-Y plane and shaped like a parabola curve with open rim on the X-Z plane. The growth rate of the jet half width for converging jets was marginally enhanced than that of a single pipe jet on the X-Y plane but significantly enlarged on the X-Z plane. Based on the experimental results, the cross-sectional profiles of the streamwise velocities were found to show strong resemblance on both the central planes and the parallel offset planes. The mean streamwise velocity for converging jets might be prescribed by the corresponding values on the two central planes (X-Y and X-Z).It was also demonstrated by the velocity measurements that the entrainment (mixture) from ambient fluid to the jets were enhanced when suitable parameters had been chosen based on jets spreading and the lateral velocity gradient near the jets edges. The dynamic processes in the interactions among the three converging jets are revealed by time series of instantaneous flow fields. Galilean decomposition of the velocity vectors and calculation of λ_2(complex eigenvalues) in conjunction with the vorticity fields are used to discriminate the swirling and the shearing in the evolution of the flow.The free outer shear layers of the side jets depict the standard K-H roll-ups similar with that of a single pipe jet flow in the merging region. These K-H structures become unstable gradually and the instability is characterized by “sinuous” flow motions. The two shear layers interact with each other, forming large-scale discrete vortices. The periodic nature of the interactions between the negative and positive Karman-like vortices enhances mixing among three converging jets. The translational velocity of these discrete vortices is estimated to be about 60~80% of the average efflux velocity〖 U〗_0. Based on the experimental results in the present study, it has been found that the large-scale mixing of jets would be maximised when the jets momentum flux is matched (1:1:1). The geometrical features of the flow field are altered at different jet spacing ratios and inclination angles. The merging and combining of the coplanar jets take place earlier with smaller jet spacing ratios or bigger inclination angles. In these situations, the 45° jets at jet spacing ratios of 3 are found to be more receptive to the influence from the side jets because the side jets penetrate the central jet at a location close enough to the nozzle where the central jets’ instabilities have not yet developed, but not so close as in those cases with smaller jet spacing where there are still sufficient room of the evolution of the mixing layer for the jets.