Large Eddy Simulation Of Turbulent Characteristics Of An Offset Jet Impinging On A Flat Plate

Offset jets are widely found in engineering,such as dam discharging in hydraulic engineering,industrial discharge energy dissipation ponds,automotive mist eliminators and dehumidifiers in car washes,etc.Therefore,it is important to study the flow characteristics of offset jets.There are few reports on the study of turbulence characteristics of offset jets based on numerical simulations.Therefore,this paper uses large-eddy simulations to study the turbulent characteristics of offset jet impinging on a flat plate,and the following main results are obtained:（1）The time-averaged characteristics of the offset jet using large-eddy simulation are in good agreement with the model tests of previous authors.In the wall jet region,the maximum velocity decay is consistent with the-1/2 power law of the classical wall jet,and its characteristic width increases linearly,and the self-similarity of the velocity distribution is also in good agreement with that of the classical wall jet.（2）The statistical characteristics of turbulence of the offset jet were analyzed,and it was found that there was a reflux in the recirculation region,which led to a higher value of axial turbulence intensity in the outer shear layer than in the inner shear layer,and a higher value of radial turbulence intensity in the inner shear layer than in the outer shear layer.In the wall jet region,along the radial direction,the axial turbulence intensity has an obvious bimodal structure,while the radial turbulence intensity shows a single peak structure.The Reynolds shear stress is larger in the recirculation and impingement regions,and the Reynolds stress in the wall jet region is smaller,and the cross-sectional distribution is very similar.（3）The large-scale structure of the offset jet flow field was studied by proper orthogonal decomposition.The small-scale structure in the recirculation region is mainly concentrated near the main flow,and the small-scale structure in the wall jet region is more dispersed.In the impingement and wall jet regions,the contribution of POD low-order modes to Reynolds shear stress and axial turbulence strength is greater than that of radial turbulence strength.In the recirculation region,the contribution of POD low-order mode to the axial turbulence strength and radial turbulence strength is greater than that of Reynolds shear strength.（4）The energy spectrum analysis reveals that u’and v’are mainly dominated by lower-order POD modes（i.e.,large scale structures）at the beginning of the wall jet region,and lower-energy modes（i.e.,small scale structures）have an important role in v’as the wall jet develops.（5）Based on the turbulent kinetic energy transport equation to study each energy term,it is found that the magnitudes of each energy term are larger in the recirculation and impingement regions,and the magnitudes in the wall jet region are smaller.In the potential core region,the turbulent production term shows a bimodal structure,and the turbulent kinetic energy loss caused by the turbulent dissipation term is mainly compensated by the turbulent production term.In the near-wall area,the turbulent kinetic energy loss caused by the viscous diffusion term and dissipation term is compensated by the turbulence production term and the pressure diffusion term,and away from the wall it is mainly a balance of the turbulence production term,pressure diffusion term and turbulence diffusion term.（6）Based on the component analysis of each energy term,it is found that the Reynolds stress and the pulsating viscous stress have different effects on the turbulent energy in different regions.For example,in the core region of the potential flow,with the development of the jet,initially the Reynolds normal stress feeds the energy of the time-averaged field into the pulsating field through the positive strain rate of the time-averaged motion,and then gradually changes to the Reynolds shear stress feeds the energy of the time-averaged field into the pulsating field through the shear strain rate of the time-averaged motion.In the near-wall region 0≤Y/d≤1,the viscous dissipation term exists only in the near-wall region with large values,and the transport of turbulent energy by the pulsating viscous stress is dominated by the tangential pulsating viscous stress.The dissipative term is dominated by the tangential pulsating viscous stress resisting pulsation deformation in the near-wall region,and the work done by the normal pulsating viscous stress and the tangential pulsating viscous stress resisting pulsation deformation after moving away from the wall is almost equal.The dissipative term is dominated by the tangential pulsating viscous stress resisting pulsation deformation in the near-wall region,and the work done by the normal pulsating viscous stress and the tangential pulsating viscous stress resisting pulsation deformation after moving away from the wall is almost equal.（7）Through the two-point correlation function,it is found that the turbulent structure in the wall jet region is larger than that in the impingement and recirculation regions.In the wall jet region,the contour line of R_uu suddenly decreases at X/d（28）35,indicating that the turbulent structure breaks upstream of here.