A vorticity budget diagnoses the growth or decay of a vortex from advective transport,or non-advective fluxes such as those due to friction or vortex tilting.However,when a budget calculation is performed with respect...A vorticity budget diagnoses the growth or decay of a vortex from advective transport,or non-advective fluxes such as those due to friction or vortex tilting.However,when a budget calculation is performed with respect to a fixed coordinate,errors may result from time-dependence of the flow,leading to disagreement between the vorticity tendency and the observed vorticity field.An adaptive Lagrangian coordinate resolves this problem,provided that the resulting Lagrangian structure does not become too complicated.In this study,a numerical simulation of Hurricane Nate(2011),the vorticity tendency is evaluated along distinguished material curves.There can be no net advective flux along a closed material curve,therefore,the total circulation tendency for a material region includes only the non-advective uxes acting along its boundary.A distinguished set of material curves(DMCs)associated with a distinguished hyperbolic trajectory(DHT)form a Lagrangian topology similar to that of a cat’s eye flow or"pouch"at each Eulerian snapshot.The time-dependence of velocities allows additional regions called lobes,which are formed by the intersections of DMCs,to exchange fluid across the vortex boundary by redefining the boundary.Because the vortex boundary changes,we refer to this redefinition of material boundary as"topological rearrangement".The method is useful for unsteady cat’s-eye flows and more complex interactions of multiple waves,vortices and background shear.All advective changes of the vortex circulation are identified by exchanges of the lobes,and all non-advective uxes act between the vortex and either the lobes or environmental flow.The Lagrangian topology and combination of advective and non-advective uxes relative to the topology is used to describe the evolution of the circulation of Nate during its time of formation.展开更多
Suspended vegetation in rivers,lakes,reservoirs and canals can change flow structure,which will in turn affect the sediment transport and cause the variation of water ecological environment.In order to study the chara...Suspended vegetation in rivers,lakes,reservoirs and canals can change flow structure,which will in turn affect the sediment transport and cause the variation of water ecological environment.In order to study the characteristics of bend flow through suspended vegetation,three-dimensional numerical simulations are carried out by using the multi-relaxation-time lattice Boltzmann method(MRT-LBM).The drag force induced by vegetation is added in the velocity correction in the equilibrium distribution and a hybrid format combined bounce and specular reflection scheme is applied in the solid-fluid boundaries.After the validation of this model,six cases are designed to conduct the numerical simulations according to the root depth and the arrangement of vegetation.The simulated results show that the suspended vegetation can redistribute the flow structure in curved open channels.After the arrangement of suspended vegetation,the main flow moves to the side without vegetation,and the distribution of velocity tends to be balanced when vegetation is arranged on the entire cross section,the range of circulating current is reduced from the whole cross section to the local position without vegetation,however,the circulating current can still exist in the curve where the suspended vegetation enters less than half of the water depth.In addition,it can also be concluded that the suspended vegetation can affect the lateral gradient of flow velocity,and the bed shear stress in the curved channel.展开更多
基金National Science Foundation grants 1439283 and 1656156.
文摘A vorticity budget diagnoses the growth or decay of a vortex from advective transport,or non-advective fluxes such as those due to friction or vortex tilting.However,when a budget calculation is performed with respect to a fixed coordinate,errors may result from time-dependence of the flow,leading to disagreement between the vorticity tendency and the observed vorticity field.An adaptive Lagrangian coordinate resolves this problem,provided that the resulting Lagrangian structure does not become too complicated.In this study,a numerical simulation of Hurricane Nate(2011),the vorticity tendency is evaluated along distinguished material curves.There can be no net advective flux along a closed material curve,therefore,the total circulation tendency for a material region includes only the non-advective uxes acting along its boundary.A distinguished set of material curves(DMCs)associated with a distinguished hyperbolic trajectory(DHT)form a Lagrangian topology similar to that of a cat’s eye flow or"pouch"at each Eulerian snapshot.The time-dependence of velocities allows additional regions called lobes,which are formed by the intersections of DMCs,to exchange fluid across the vortex boundary by redefining the boundary.Because the vortex boundary changes,we refer to this redefinition of material boundary as"topological rearrangement".The method is useful for unsteady cat’s-eye flows and more complex interactions of multiple waves,vortices and background shear.All advective changes of the vortex circulation are identified by exchanges of the lobes,and all non-advective uxes act between the vortex and either the lobes or environmental flow.The Lagrangian topology and combination of advective and non-advective uxes relative to the topology is used to describe the evolution of the circulation of Nate during its time of formation.
基金Project supported by the National Natural Science Foundationof China (Grant No. 11861003)supported by the Natural Science Foundation of Ningxia (Grant Nos.2023AAC02049,2021AAC03208)+2 种基金the Support Plan for Innovation Team of North Minzu University,China (Grant No.2022PT_S02)the Support Plan for Leading Personnel of State Ethnic Affairs Commission,China (Grant No.113114000706)the Leading Academic Discipline Project of North Minzu University.
文摘Suspended vegetation in rivers,lakes,reservoirs and canals can change flow structure,which will in turn affect the sediment transport and cause the variation of water ecological environment.In order to study the characteristics of bend flow through suspended vegetation,three-dimensional numerical simulations are carried out by using the multi-relaxation-time lattice Boltzmann method(MRT-LBM).The drag force induced by vegetation is added in the velocity correction in the equilibrium distribution and a hybrid format combined bounce and specular reflection scheme is applied in the solid-fluid boundaries.After the validation of this model,six cases are designed to conduct the numerical simulations according to the root depth and the arrangement of vegetation.The simulated results show that the suspended vegetation can redistribute the flow structure in curved open channels.After the arrangement of suspended vegetation,the main flow moves to the side without vegetation,and the distribution of velocity tends to be balanced when vegetation is arranged on the entire cross section,the range of circulating current is reduced from the whole cross section to the local position without vegetation,however,the circulating current can still exist in the curve where the suspended vegetation enters less than half of the water depth.In addition,it can also be concluded that the suspended vegetation can affect the lateral gradient of flow velocity,and the bed shear stress in the curved channel.
