Study on the effects of stroke ratio, velocity ratio, and duty cycle on the diffusion characteristics of pulsed buoyant jets in crossflow

Pulsed buoyancy jet diffusion dilution is characterized by low cost and high efficiency, so it is widely used in wastewater discharge dilution projects. This study, based on three-dimensional numerical simulations, investigates the effects of stroke ratio (1.71 < L/D < 8.57), velocity ratio (2.4 < R < 12), and duty cycle (0.17 < eta < 0.83) on the diffusion of low-frequency pulsed buoyant triple jets. A large number of simulations were conducted for each parameter, and the concentration field, vorticity field, and flow field at different sections during the initial stage of the jet were analyzed. The results reveal the evolution patterns of the vortex structure in pulsed triple buoyant jets and their diffusion characteristics. The vortex structure at the initial stage of the jet exhibits continuous vortex ring characteristics, which later evolve into a vortex street and counter-rotating vortex pair (CVP) combination structure. Meanwhile, the buoyancy effect enhances the development and diffusion of the vortex structure in both the X and Z directions. As the stroke ratio decreases, the wastewater concentration distribution becomes more concentrated, and the dynamic interaction between the jets intensifies. The increase in duty cycle enhances the interaction between the jets, significantly improving the horizontal diffusion capacity, but the vertical penetration depth is less than that of continuous jets. The Strouhal number (St) is composed of the stroke ratio, velocity ratio, and duty cycle. These three parameters control the motion state of pulsed jets in the crossflow. The established relationships and equations arising from this research offer vital reference values for predicting sewage leakage and wastewater diffusion in engineering applications, contributing substantively to the theoretical framework governing multiple buoyant jets.