As an important lightning protection device in substations,lightning rods are susceptible to vibration and potential structural damage under wind loads.In order to understand their vibration mechanism,it is necessary ...As an important lightning protection device in substations,lightning rods are susceptible to vibration and potential structural damage under wind loads.In order to understand their vibration mechanism,it is necessary to conduct flow analysis.In this study,numerical simulations of the flow field around a 330 kV cylindrical lightning rod with different diameters were performed using the SST k-ωmodel.The flow patterns in different segments of the lightning rod at the same reference wind speed(wind speed at a height of 10 m)and the flow patterns in the same segment at different reference wind speeds were investigated.The variations of lift coefficient,drag coefficient,and vorticity distribution were obtained.The results showed that vortex shedding phenomena occurred in all segments of the lightning rod,and the strength of vortex shedding increased with decreasing diameter.The vorticity magnitude and the root mean square magnitudes of the lift coefficient and drag coefficient also increased accordingly.The time history curves of the lift coefficient and drag coefficient on the surface of the lightning rod exhibited sinusoidal patterns with a single dominant frequency.For the same segment,as the wind speed increased in a certain range,the root mean square values of the lift coefficient and drag coefficient decreased,while their dominant frequencies increased.Moreover,there was a proportional relationship between the dominant frequencies of the lift coefficient and drag coefficient.The findings of this study can provide valuable insights for the refined design of lightning rods with similar structures.展开更多
The flow past two tandem circular cylinders of different diameters was simulated using the finite volume method. The diameter of the downstream main cylinder (D) was kept constant, and the diameter of the upstream c...The flow past two tandem circular cylinders of different diameters was simulated using the finite volume method. The diameter of the downstream main cylinder (D) was kept constant, and the diameter of the upstream control cylinder (d) varied from 0.1D to D. The studied Reynolds numbers based on the diameter of the downstream main cylinder were 100 and 150. The gap between the control cylinder and the main cylinder (G) ranged from 0.1D to 4D. It is concluded that the gap-to-diameter ratio (G/D) and the diameter ratio between the two cylinders (d/D) have important effects on the drag and lift coefficients, pressure distributions around the cylinders, vortex shedding frequencies from the two cylinders, and flow characteristics.展开更多
In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) nu...In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) numerical simulation,the mean and fluctuating forces,Strouhal number(St) and vortex shedding pattern in the wake for each cylinder were analyzed with the spacing ratio(L /D) ranging from 1.5 to 6.0.It was found that,four different vortex modes(viz.,flip-flopping,shielding anti-phase-synchronized,in-phasesynchronized and anti-phase-synchronized) gradually appear with the increase of the L/D ratio.The average drag coefficient of the upstream cylinders is larger than that of the downstream cylinders,while the downstream cylinders usually undergo serious fluctuating forces.When the L/D ratio ranges from 3.0 to 4.0,the dominant frequency of the drag coefficient is equal to the value of St of upstream cylinders.This indicates that a simultaneous resonance in the in-flow and cross-flow directions may occur for some single structures of a multi-body oscillating system.For the 3-D numerical simulation,the L/D and aspect ratios are kept constant as 5.0 and 10,respectively.It was found that some vortices are formed in the wake of the upstream cylinders.Besides,with the same spacing ratio,the calculated drag coefficient and lift coefficient fluctuation are slightly larger than the 2-D results,but with a phase difference.展开更多
Using boundary-fitted coordinate system, the Navier-Stokes equations in terms of stream function and vorticity are solved for the flow across a semicircular-sectioncylinder. The flow structure is analyzed for the Reyn...Using boundary-fitted coordinate system, the Navier-Stokes equations in terms of stream function and vorticity are solved for the flow across a semicircular-sectioncylinder. The flow structure is analyzed for the Reynolds number Re=100. Vortex shedding behind the cylinder wake can be observed clearly.展开更多
In the design of rock sheds for the mitigation of risk due to rapid and long landslides, a crucial role is played by the evaluation of the impact force exerted by the flowing mass on the rock sheds. This paper is focu...In the design of rock sheds for the mitigation of risk due to rapid and long landslides, a crucial role is played by the evaluation of the impact force exerted by the flowing mass on the rock sheds. This paper is focused on the influencing factors of the impact force of dry granular flow onto rock shed and in particular on the evaluation of the maximum impact force. The coupled DEM-FEM model calibrated with small-scale physical experiment is used to simulate the movement of dry granular flow coupled with impact forces on the rock-shed. Based on the numerical results, three key stages were identified of impact process, namely startup streams slippery, impact and pile-up. The maximum impact force increases linearly with bulk density, and the maximum impact force exhibits a power law dependence on the impact height and slop angle respectively. The sensitivities of bulk density, impact height, and slope angle on the maximum impact force are: 1.0, 0.496, and 2.32 respectively in the benchmark model. The parameters with high sensitivity should be given priority in the design of the rock shed. The results obtained from this study are useful for facilitating design of shed against dry granular flow.展开更多
The influence of buoyancy on vortex shedding from a heated square cylinderand its effects on the heat transfer of mixed convection were simulated. The flow equations, basedon the velocity and the pressure, were solved...The influence of buoyancy on vortex shedding from a heated square cylinderand its effects on the heat transfer of mixed convection were simulated. The flow equations, basedon the velocity and the pressure, were solved along with the energy equation by a modified SIMPLERalgorithm - the Quick Scheme and small Control Volume (QSCV) algorithm developed by the authors ofthe present paper. A set of optimized computational domain and artificial lateral boundaryconditions were used along with the QSCV algorithm. Several cases were simulated for the Grashofnumbers up to 1. 8 X 10~5, the Reynolds numbers up to 500, and a range of angles between free streamvelocity and gravity from 0 to 1.5π. In opposing flow, the results distinguish two different flowpatterns: periodic flow for Gr < Gr_c and steady flow with attached twin vortices for Gr > Gr_c, anda precise correlation between Gr_c and Re expressed as Gr_c = 0. 00166(Re)^(2.97795) for Re up to500 was firstly obtained. In cross flow, it could be seen that the Ri = Gr/Re^2 plays a great rolein the wake vortex patterns and the temperature fields. In aiding flow, it is found that the mainflow currents cause a large expansion of the streamlines and isotherms in the direction normal tothe free stream velocity. These changes in the wake vortex patterns and the temperature fieldsgreatly modify the heat flux along the surface of the square cylinder and consequently, the heattransfer rate is strongly dependent upon the Reynolds numbers, the Grashof numbers, and the gravitydirection.展开更多
基金supported by State Grid Ningxia Electric Power Co.,Ltd.under Grant 5229CG220006Natural Science Foundation of Ningxia Province under Grant 2022AAC03629.
文摘As an important lightning protection device in substations,lightning rods are susceptible to vibration and potential structural damage under wind loads.In order to understand their vibration mechanism,it is necessary to conduct flow analysis.In this study,numerical simulations of the flow field around a 330 kV cylindrical lightning rod with different diameters were performed using the SST k-ωmodel.The flow patterns in different segments of the lightning rod at the same reference wind speed(wind speed at a height of 10 m)and the flow patterns in the same segment at different reference wind speeds were investigated.The variations of lift coefficient,drag coefficient,and vorticity distribution were obtained.The results showed that vortex shedding phenomena occurred in all segments of the lightning rod,and the strength of vortex shedding increased with decreasing diameter.The vorticity magnitude and the root mean square magnitudes of the lift coefficient and drag coefficient also increased accordingly.The time history curves of the lift coefficient and drag coefficient on the surface of the lightning rod exhibited sinusoidal patterns with a single dominant frequency.For the same segment,as the wind speed increased in a certain range,the root mean square values of the lift coefficient and drag coefficient decreased,while their dominant frequencies increased.Moreover,there was a proportional relationship between the dominant frequencies of the lift coefficient and drag coefficient.The findings of this study can provide valuable insights for the refined design of lightning rods with similar structures.
基金supported by the National Natural Science Foundation of China(Grant No.40871050)
文摘The flow past two tandem circular cylinders of different diameters was simulated using the finite volume method. The diameter of the downstream main cylinder (D) was kept constant, and the diameter of the upstream control cylinder (d) varied from 0.1D to D. The studied Reynolds numbers based on the diameter of the downstream main cylinder were 100 and 150. The gap between the control cylinder and the main cylinder (G) ranged from 0.1D to 4D. It is concluded that the gap-to-diameter ratio (G/D) and the diameter ratio between the two cylinders (d/D) have important effects on the drag and lift coefficients, pressure distributions around the cylinders, vortex shedding frequencies from the two cylinders, and flow characteristics.
基金Sponsored by the National Natural Science Foundation of China(Grant No.50538020)
文摘In the present paper,two-and three-dimensional numerical simulations of the flow interference between four cylinders in an in-line square arrangement at Re = 200 are performed.Assisted with the two-dimensional(2-D) numerical simulation,the mean and fluctuating forces,Strouhal number(St) and vortex shedding pattern in the wake for each cylinder were analyzed with the spacing ratio(L /D) ranging from 1.5 to 6.0.It was found that,four different vortex modes(viz.,flip-flopping,shielding anti-phase-synchronized,in-phasesynchronized and anti-phase-synchronized) gradually appear with the increase of the L/D ratio.The average drag coefficient of the upstream cylinders is larger than that of the downstream cylinders,while the downstream cylinders usually undergo serious fluctuating forces.When the L/D ratio ranges from 3.0 to 4.0,the dominant frequency of the drag coefficient is equal to the value of St of upstream cylinders.This indicates that a simultaneous resonance in the in-flow and cross-flow directions may occur for some single structures of a multi-body oscillating system.For the 3-D numerical simulation,the L/D and aspect ratios are kept constant as 5.0 and 10,respectively.It was found that some vortices are formed in the wake of the upstream cylinders.Besides,with the same spacing ratio,the calculated drag coefficient and lift coefficient fluctuation are slightly larger than the 2-D results,but with a phase difference.
文摘Using boundary-fitted coordinate system, the Navier-Stokes equations in terms of stream function and vorticity are solved for the flow across a semicircular-sectioncylinder. The flow structure is analyzed for the Reynolds number Re=100. Vortex shedding behind the cylinder wake can be observed clearly.
文摘In the design of rock sheds for the mitigation of risk due to rapid and long landslides, a crucial role is played by the evaluation of the impact force exerted by the flowing mass on the rock sheds. This paper is focused on the influencing factors of the impact force of dry granular flow onto rock shed and in particular on the evaluation of the maximum impact force. The coupled DEM-FEM model calibrated with small-scale physical experiment is used to simulate the movement of dry granular flow coupled with impact forces on the rock-shed. Based on the numerical results, three key stages were identified of impact process, namely startup streams slippery, impact and pile-up. The maximum impact force increases linearly with bulk density, and the maximum impact force exhibits a power law dependence on the impact height and slop angle respectively. The sensitivities of bulk density, impact height, and slope angle on the maximum impact force are: 1.0, 0.496, and 2.32 respectively in the benchmark model. The parameters with high sensitivity should be given priority in the design of the rock shed. The results obtained from this study are useful for facilitating design of shed against dry granular flow.
文摘The influence of buoyancy on vortex shedding from a heated square cylinderand its effects on the heat transfer of mixed convection were simulated. The flow equations, basedon the velocity and the pressure, were solved along with the energy equation by a modified SIMPLERalgorithm - the Quick Scheme and small Control Volume (QSCV) algorithm developed by the authors ofthe present paper. A set of optimized computational domain and artificial lateral boundaryconditions were used along with the QSCV algorithm. Several cases were simulated for the Grashofnumbers up to 1. 8 X 10~5, the Reynolds numbers up to 500, and a range of angles between free streamvelocity and gravity from 0 to 1.5π. In opposing flow, the results distinguish two different flowpatterns: periodic flow for Gr < Gr_c and steady flow with attached twin vortices for Gr > Gr_c, anda precise correlation between Gr_c and Re expressed as Gr_c = 0. 00166(Re)^(2.97795) for Re up to500 was firstly obtained. In cross flow, it could be seen that the Ri = Gr/Re^2 plays a great rolein the wake vortex patterns and the temperature fields. In aiding flow, it is found that the mainflow currents cause a large expansion of the streamlines and isotherms in the direction normal tothe free stream velocity. These changes in the wake vortex patterns and the temperature fieldsgreatly modify the heat flux along the surface of the square cylinder and consequently, the heattransfer rate is strongly dependent upon the Reynolds numbers, the Grashof numbers, and the gravitydirection.