| With the advantages of convenience,efficiency and environmental friendliness,submerged impinging jets are widely used in reservoir dredging and waterway management.There is a large amount of energy exchange in the free jet shear layer,wall impact area and radial wall jet region of submerged impinging jet.At the same time,the formation,development and fragmentation of vortex structures occur in the flow field.The evolution characteristics of these complex flow structures are not clear,and they cannot provide clear direction guidance for practical application.In this paper,the submerged impinging jet is taken as the research object.A variety of numerical simulation methods were used to predict the distribution of the velocity,pressure and stagnation point in different zone of the impinging jet under a series of impact angles.The numerical simulation results were compared with the experimental results.Finally,RNG k-ε turbulence model and LES method were used to analyze the time-averaged flow field and transient flow field of submerged impinging jet,and the internal flow mechanism of submerged impinging jet is mastered.The main research contents are as follows:(1)The Wray-Agarwal,RNG k-ε,and SST k-ω turbulence models and LES large eddy simulations were applied to numerically calculate the submerged impinging jets at multiple impact angles.The RNG k-ε turbulence models and LES were selected for grid independence analysis and time step independence.Compared numerical simulation results with experimental data,it is found that in the free jet region,the prediction results of the W-A turbulence model matches the best with the experimental values,while in the wall jet region,RNG k-ε performs the best.When the impact angleθ=90°,LES has a high prediction accuracy in both regions,and in general,RNG k-εcan accurately predict the average characteristics of the submerged impact j et flow field.(2)Based on the two-equation RNG k-ε turbulence model and LES method,the time-averaged flow field of submerged impinging jet was analyzed.It is found that the radial root mean square velocity is suppressed in the free jet region.In the region of impact origin and jet shear layer,the axial root mean square velocity and radial root mean square velocity are quite different,and the flow is strong anisotropy.The influence of Reynolds number and impact height on the submerged impinging structure is analyzed.Under different Reynolds numbers,the distribution of radial velocity,axial velocity and pressure coefficient near the impinging wall are highly consistent,the dimensionless turbulent kinetic energy is only numerically different in the impact area,and the distribution at other locations is almost consistent.Axial velocity decreases faster in the scope of 0<z/d<0.75,and the falling rate increases with the decrease of impact height,while the descending tends to be stable in the region of z/d>0.75,so the blocking effect of the wall is within 0.75d.In the near-wall region,at x/d=1.8,the axial velocity is almost zero,and the jet completes the transition from axial to radial.The radial velocity is zero on the axis of the jet.Near wall at x/d=0.2,the jet has produced a substantial deflection.The radial velocity increases first and then decreases along the flow direction of the wall jet,reaching a peak in the range of 0.8<x/d<1.2,and its peak value decreases with the increasement of the impact height.The turbulent kinetic energy increases rapidly from the wall along the height direction and then decreases gradually.The increase or decrease rate increases with the increasement of impact height.The larger the impact height,the higher the peak value of turbulent kinetic energy.The distribution nephograms of axial and radial root mean square velocity show that the boundary layer thickness of free jet increases along the flow direction after the nozzle ejection.When the impact height increases to H/d=6,the boundary layer has penetrated inward to the axis of the jet.(3)The entrainment performance under different parameters was analyzed with the numerical calculation.According to the transverse velocity distribution,the entrainment ability of free j et fluid to the surrounding static environment fluid gradually decreases along the flow direction,but the influence range gradually expands,and finally the entrainment disappeared at z/d=0.375 near the wall surface.The expansion velocity of the wall jet in the impinging wall is significantly higher than that in the midsection of the jet,and the virtual source position in the two planes does not coincide.The maximum wall pressure is located at the impinging origin,and then decreases gradually along the radial direction.The decline rate decreases with the increasement of impact height.When the impact height is H/d=6,the low pressure zone penetrates the entire impact region.The wall shear stress increases rapidly from zero at the impact origin and then decreases slowly.The change rate increases with the impact height.The peak location at different heights basically coincide,corresponding to the region of maximum Ux,rms/Uj.Results of scouring under different jet parameters shows that the diameter of the sand trap increases with the increasement of Reynolds number.At a smaller impact height,the depth of the sand trap increases with the Reynolds number,but the depth of the sand trap increases first and then decreases with Reynolds number while H/d>6.After impinging on the wall,the wall jet is formed,then a pair of wall vortexes with the same size and opposite rotation direction are generated at the downstream near the impinging plate.With the interaction of the wall vortex and the vertical wall on both sides,two large annular recirculation zones are formed which provide energy for the mixing of impurities.The center of the wall vortices approaches the jet axis with the increase of the impact height,and the appearance of the wall vortices destroys the similarity of the velocity distribution in the nearby area.(4)The turbulent flow characteristics of submerged impinging jet were analyzed by LES method.The relationships between Reynolds stress,turbulent kinetic energy production factors and turbulent kinetic energy were studied.The results show that w’w’ has a high value in the free jet shear layer.(?)is the main source of the positive turbulent kinetic energy generation term.(?)contribute to the negative turbulent kinetic energy generation term.In the wall j et region,Reynolds normal stress u’u’ and velocity gradient ?Ux/?x are higher,so(?)、(?)became the main contributor to the turbulent kinetic energy generation term.The vorticity distribution shows that the free jet is affected by Kelvin-Helmholtz instability and bulges in the boundary layer.With the development of the jet,the K-H instability is amplified along the way,and finally the obvious large-scale vortex structure is rolled around the free jet.The analysis of the stream-wise vortex and spanwise vortex in the free jet stage shows that the spanwise vortex contributes greatly to the total vorticity,which generates within a certain distance after the nozzle exit and wraps in a circular shape outside the core area of the jet.Along the flow direction of the free jet,the spanwise vortex penetrates outward radially faster than inward.The streamwise vortex is distributed in the boundary layer in pairs,and its value is much smaller than that of the spanwise vortex.With the development of the jet,the scale of the streamwise vortex pair grows in the radial direction.The pressure criterion and the Q criterion are used to extract the coherent structure in the flow field.Compared with the Q criterion,the pressure criterion is obviously insufficient to capture the small coherent structure and the secondary vortex.Q criterion clearly shows the coherent structure of spanwise vortices and streamwise vortices in submerged impinging jets,which are generated by different mechanisms.The spanwise vortices appear in the free jet boundary layer in the form of vortex rings,and the generation time of streamwise vortices is later than that of spanwise vortex.The streamwise vortices are located between adjacent ring-like vortexes and promote the deformation and fragmentation of spanwise vortex.At the impinging wall,these orderly structures hit the wall at different times,resulting in asymmetric impact and shake of stagnation point.The flow field of submerged impinging jet becomes more complex due to the evolution,collision and interaction of the two vortex structures,which further dominates the turbulent characteristics of the flow field. |
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