Abstract:

The time-resolved particle image velocimetry (TR-PIV) method was used to measure the flow field of a jet impinging on a flat plate in two orthogonal plane at very close distance. The effects of impinging distance and Reynolds number on the flow characteristics and the vortex topology in the clearance were analyzed. The results show that there are three kinds of typical vortex structure in the clearance, namely, double vortex ring mode, single vortex ring mode and complete entrainment mode. However, under the condition of turbulent state with large flow rate, the jet may break through the vortex ring and form random high-speed outflow. The occurrence of each flow pattern is mainly related to the strength of wall constraints. The energy transportation and flow loss of three typical flow patterns are investigated by vorticity analysis. The results show that the energy of the jet is transmitted outward through the vortex-ring mode at close impingement. In the double vortex ring mode, the two vortex rings have opposite rotational direction. Due to the constraint of the end face, both vortex rings are strictly constrained within the end face of the jet nozzle. The strength of the primary vortex ring is significantly greater than that of the secondary vortex ring. Finally, the proper orthogonal decomposition (POD) method is used to analyze the flow mode and energy distribution of the impinging jet. The first ten modal analysis of single and double vortex mode show that the energy fluctuation occurs in a paired pattern at a lower order, which indicates that both the primary and secondary vortex rings have good symmetry. Meanwhile, in the double vortex ring mode, the primary vortex ring is the dominant large-scale flow structure. The first three modes of the complete entrainment mode show that the energy of the jet is concentrated in the upstream of the jet and decreases sharply with the turbulent diffusion.

Key words: impinging jet, PIV, close impingement, vorticity, POD, experiment