Investigation On Characteristics Of Jets And Buoyant Jets Under The Effects Of The Flowing Environment And Vegetation

Due to public attention to ecological environment and increasing awareness of environmental protection year by year,there have been a lot of researches on turbulent jets or vegetated flow in recent years.However,studies on water ecology considering the interaction between the turbulent jets and vegetated flow are scarce.Based on the researches on the buoyant jets in counterflow and crossflow,considering the factors such as vegetation and type of open channel,the interactions between the jets and the receiving bodies are analyzed by the experiments and large-eddy simulation method,revealing the flow field in the near-field of the jets.This study provides a reasonable guidance for wastewater discharge,and reference bases for the planning and design of related engineering construction.For the buoyant jet in counterflow,the reliability and applicability of various uncertainty estimators in the numerical simulation are studied quantitatively.Through a series of verification and validation researches on the numerical simulation results with four sets of grid resolutions,they show that FS(Factor of safety method)and FS1(Factor of safety method modified version)methods have good reliability and applicability for the buoyant jet in counterflow among the seven uncertainty estimators.The area without validation at the validation level shows a strong relationship with the shear layer between the jet flow and the ambient fluid.The predicted concentration contours,coherent structures,and centerline concentration agree well with the experimental data when the grid resolution is improved.In particular,the predicted coherent structures reflect the strong shearing between the jet flow and the ambient fluid well only at a high level of grid resolution.For the buoyant jet in crossflow,the grid convergence analysis is conducted by the uncertainty estimators FS and FS1 methods.The predicted temperature distribution and velocity trajectory of the longitudinal profile of the channel centerline are consistent with the experimental data.With the increase of velocity ratio,the jet trajectory rises obviously.By comparing the temperature and velocity trajectories,it is found that the velocity trajectory is slightly higher than the temperature trajectory.In the development process of temperature field,two high temperature regions,namely the jet bifurcation phenomenon,appear,where the angle between the two high temperature regions roughly within 8~°-10~°.Meanwhile,the velocity distribution,development and evolution on the water surface are recorded in time and space.The vorticity distribution along the typical cross-section are investigated,and it is found that both positive and negative vorticities appear in the same cross-section.The shape of positive and negative vorticities are similar to the kidney shape,and the vortex pair is called as the counter-rotating vortex pair.For the jet in vegetated crossflow,the grid convergence analysis provides a reference for the determination of grid resolution in numerical simulations.The time-averaged resultant velocity distributions in the flow symmetry and spanwise planes are predicted.The presence of vegetation diminishes the channel velocity,resulting in a significant increase in jet penetration height as compared with the tests without cylinders.As for the coherent structures,the classical and well-documented vortical structures are found based on the mean flow field,like shear-layer vortices,wake vortices,counter-rotating vortex pair,windward vortex pair,etc.In particular,the sunken trails of the asterisk shape on the windward side of the counter-rotating vortex pair are associated with the wake vortices generated by the cylinders.Also,the evolution of the coherent structures is showed based on the instantaneous flow field,especially the wake vortices and the shear-layer vortices.In instantaneous flow field,the counter-rotating vortex pair is not a distinct vortex structure,but the streaky vortices originating from the jet flow along the jet trajectory is moving downstream in a counter-rotating manner.The mechanism of momentum transport is quantitatively elaborated by the quadrant analysis.The ejection event is dominant on the top of the vegetation,and the sweep event is dominant on the bottom.Spectral analysis is adopted to identify the vortex shedding frequencies of large-scale structures.The characteristic length of the vortex is approximately equal to the distance between the cylinders,and it indicates that the vortexes are mainly affected by the cylinders.For the dye concentration in compound open channel flows,the FS and FS1methods are adopted to analyze the convergence of grid.The simulated velocity distribution and second currents in the typical cross-section show a good agreement with the experimental data.The distribution of bed shear stress in typical cross-section presents a parabolic shape in the main channel.But in the transition from the main channel to the floodplain,the bed shear stress decreases,then increases sharply,and then gradually decreases,which is mainly related to the momentum transport and secondary currents from the main channel to the floodplain.Based on the concentration distribution and Reynolds flux data,the effect of the secondary currents on dye diffusion is analyzed.The effect of the secondary currents on the dye concentration shows that the peak position is significantly affected by the secondary currents and deviates from the dye release position.Owing that the direction of the secondary currents is opposite to that of the turbulent diffusion,the gradient of the concentration profiles near the peak is relatively steep.Due to the effect of the secondary currents on the concentration distribution,Fickian's law is not valid in the whole area of the compound open channel,especially in the area of strong secondary currents.For the jet in compound open channels with vegetated floodplain,the accuracy of the numerical model is validated by experimental data.The predicted mean velocity,turbulent kinetic energy and secondary currents are consistent with the experimental data.The momentum transport of the jets in the compound channel with vegetated floodplain is investigated by analyzing the bed shear stress.As the velocity ratios decrease,the bed shear stress decrease.The positions of the maximum bed shear stress shift with the location of the jet core.The bed shear stress in the vegetated floodplain presents a wave pattern,indicating that vegetation in the floodplain increases the total drag,reduces the bed shear stress.According to the quadrant analysis,the ejection and sweep events are dominant in the momentum transport,implying that the flows are blocked by vegetation and the secondary currents enhance the scalar mixing and make the concentration distribution more uniform.The analyses of the concentration and Reynolds flux show that the Fickian law is not valid in the whole region in the simulations,owing to the secondary currents skewing the concentration distribution in the compound open channels.Based on the investigation of the instantaneous flow field,the typical vortexes and spatiotemporal evolution of vortex structures in compound open channels are successfully reproduced,especially near the junction between the main channel and floodplain.