A finite-difference algorithm is proposed for numerical modeling of hydrodynamic flows with rarefaction shocks, in which the fluid undergoes a jump-like liquid-gas phase transition. This new type of flow discontinuity...A finite-difference algorithm is proposed for numerical modeling of hydrodynamic flows with rarefaction shocks, in which the fluid undergoes a jump-like liquid-gas phase transition. This new type of flow discontinuity, unexplored so far in computational fluid dynamics, arises in the approximation of phase-flip(PF) hydrodynamics, where a highly dynamic fluid is allowed to reach the innermost limit of metastability at the spinodal, upon which an instantaneous relaxation to the full phase equilibrium(EQ) is assumed. A new element in the proposed method is artificial kinetics of the phase transition, represented by an artificial relaxation term in the energy equation for a "hidden"component of the internal energy, temporarily withdrawn from the fluid at the moment of the PF transition. When combined with an appropriate variant of artificial viscosity in the Lagrangian framework, the latter ensures convergence to exact discontinuous solutions, which is demonstrated with several test cases.展开更多
Without using any turbulent model, direct numerical simulation of a three-dimensional gas-solid two-phase turbulent jet was performed by finite volume method. The effects on dispersion of particles with different Stok...Without using any turbulent model, direct numerical simulation of a three-dimensional gas-solid two-phase turbulent jet was performed by finite volume method. The effects on dispersion of particles with different Stokes numbers by the transitional behavior of turbulent structures were investigated. To produce high-resolution results and reduce the computation and storage, the fractional-step projection algorithm was used to solve the governing equations of gas phase fluid. The low-storage, three-order Runge-Kutta scheme was used for time integration. The governing equations of particles were solved in the Lagrangian framework. These numerical schemes were validated by the good agreement be-tween the statistical results of flow field and the related experimental data. In the study of particle dis-persion, it was found that the effects on particle dispersion by the spanwise vortex structures were prominent. The new behaviors of particle dispersion were also observed during the evolution of the flow field, i.e. the transitional phenomenon of particle dispersion occurs for the particles with small and intermediate Stokes numbers.展开更多
Phase changes in the precipitation processes of early winter and late spring in midlatitude regions represent challenges when forecasting the timing and magnitude of snowfall.On 4 April 2018,a heavy snow process occur...Phase changes in the precipitation processes of early winter and late spring in midlatitude regions represent challenges when forecasting the timing and magnitude of snowfall.On 4 April 2018,a heavy snow process occurred in Beijing and northwestern Hebei Province,becoming the most delayed occurrence of heavy spring snow ever recorded over Beijing in the last 30 years.This paper uses observational and numerical simulation data to investigate the causes for the rapid rain-to-snow(RRTS)phase transition during this process.The following results are obtained.(1)Return flows(RFs),an interesting type of easterly wind,including those at 1000,925,and 800 hPa,played an important role in this heavy snow process and presented a characteristic"sandwich"structure.The RFs,complex topography,and snow particles that dominated the clouds,were the three key factors for the RRTS transition.(2)The RRTS transition in the plains was directly related to the RF at 925 hPa,which brought about advective cooling initiated approximately 4-6 h before the onset of precipitation.Then,the RF played a role of diabatic cooling when snow particles began to fall at the onset of precipitation.(3)The RRTS transition in the northern part of the Taihang Mountains was closely related to the relatively high altitude that led to a lower surface temperature owing to the vertical temperature lapse rate.Both immediately before and after the onset of precipitation,the snow particles in clouds entrained the middle-level cold air downward,causing the melting layer(from surface to the 0℃-isotherm level)to become very thin;and thus the snow particles did not have adequate time to melt before falling to the ground.(4)The rapid RRTS over the Yanqing mountainous area in the northwest of Beijing could have involved all the three concurrent mechanisms:the advective cooling of RF,the melting cooling of cloud snow particles,and the high-altitude effect.Compared with that in the plain area with less urbanization the duration of the RRTS in the plain area with significant urbanization was extended by approximately 2 h.展开更多
为了进一步探究纳米流体热管的模拟仿真,运用Fluent软件中的VOF(volume of fluid)模型,以单一纳米流体(TiO_(2)-H_(2)O)和混合纳米流体(Al_(2)O_(3)+TiO_(2)-H_(2)O)为工质,从热管内部蒸发冷凝过程的可视化分析以及整体热力参数结果两...为了进一步探究纳米流体热管的模拟仿真,运用Fluent软件中的VOF(volume of fluid)模型,以单一纳米流体(TiO_(2)-H_(2)O)和混合纳米流体(Al_(2)O_(3)+TiO_(2)-H_(2)O)为工质,从热管内部蒸发冷凝过程的可视化分析以及整体热力参数结果两个维度综合分析利用Fluent模拟纳米流体热管的可行性。结果表明,从可视化角度看,模拟结果云图整体可以反映热管内部随着时间变化所发生的相变过程。但具体的模拟数值与修正实验数值仍存在3%的误差,说明存在非常规因素影响模拟结果。利用VOF模型模拟纳米流体热管在数值规律以及可视化分析上是可行的,但对于具体的精细化原理研究仍需结合其他模拟方法。展开更多
为准确高效地模拟高压CO_(2)管道泄漏的瞬态特性,基于Fluent仿真平台,利用用户自定义真实气体模型(User Defined Real Gas Model,UDRGM)和用户自定义函数(User Defined Function,UDF),结合查表法和双线性插值法建立CO_(2)的真实气体模型...为准确高效地模拟高压CO_(2)管道泄漏的瞬态特性,基于Fluent仿真平台,利用用户自定义真实气体模型(User Defined Real Gas Model,UDRGM)和用户自定义函数(User Defined Function,UDF),结合查表法和双线性插值法建立CO_(2)的真实气体模型,并将压力驱动的Lee模型通过用户自定义函数嵌入Fluent求解器来模拟CO_(2)的非平衡相变过程,建立了高压CO_(2)管道泄漏的非平衡相变数值模型。通过与Botros等的试验数据进行对比分析,验证了该模型的准确性。在此基础上,对比了上述模拟方法与编译S-W(Span-Wagner)状态方程模拟方法的精度和效率,最后使用本模型研究了不同初始压力对高压CO_(2)管道泄漏瞬态特性的影响。结果表明:两种模拟方法精度接近,最大相差为7.37%,但提出的模拟方法效率明显优于编译S-W状态方程的模拟方法,计算时间相较缩短约86.9%;初始压力为11.27 MPa的最大总出口质量流量比4.36 MPa的大7.24 kg/s,而对应的气相CO_(2)出口质量流量却低0.14 kg/s;初始压力为11.27 MPa和4.36 MPa对应的近场最大射流流速分别为155.12 m/s和175.50 m/s;管内初始压力越低,泄漏后到达亚稳态时过热度越大,管内及泄漏口附近相变程度越剧烈,气相出口质量流量越大,近场射流峰值速度越大。这项研究可为工业规模管道泄漏的模拟和管道泄漏的三维模拟提供更高效的方式。展开更多
文摘A finite-difference algorithm is proposed for numerical modeling of hydrodynamic flows with rarefaction shocks, in which the fluid undergoes a jump-like liquid-gas phase transition. This new type of flow discontinuity, unexplored so far in computational fluid dynamics, arises in the approximation of phase-flip(PF) hydrodynamics, where a highly dynamic fluid is allowed to reach the innermost limit of metastability at the spinodal, upon which an instantaneous relaxation to the full phase equilibrium(EQ) is assumed. A new element in the proposed method is artificial kinetics of the phase transition, represented by an artificial relaxation term in the energy equation for a "hidden"component of the internal energy, temporarily withdrawn from the fluid at the moment of the PF transition. When combined with an appropriate variant of artificial viscosity in the Lagrangian framework, the latter ensures convergence to exact discontinuous solutions, which is demonstrated with several test cases.
基金the National Natural Science Foundation of China (Grant No. 50506027)
文摘Without using any turbulent model, direct numerical simulation of a three-dimensional gas-solid two-phase turbulent jet was performed by finite volume method. The effects on dispersion of particles with different Stokes numbers by the transitional behavior of turbulent structures were investigated. To produce high-resolution results and reduce the computation and storage, the fractional-step projection algorithm was used to solve the governing equations of gas phase fluid. The low-storage, three-order Runge-Kutta scheme was used for time integration. The governing equations of particles were solved in the Lagrangian framework. These numerical schemes were validated by the good agreement be-tween the statistical results of flow field and the related experimental data. In the study of particle dis-persion, it was found that the effects on particle dispersion by the spanwise vortex structures were prominent. The new behaviors of particle dispersion were also observed during the evolution of the flow field, i.e. the transitional phenomenon of particle dispersion occurs for the particles with small and intermediate Stokes numbers.
基金Supported by the National Natural Science Foundation of China(41475051 and 42075008)Beijing Natural Science Foundation(8192019)Civil Aviation Administration of China Security Capacity Building Project(20600822)。
文摘Phase changes in the precipitation processes of early winter and late spring in midlatitude regions represent challenges when forecasting the timing and magnitude of snowfall.On 4 April 2018,a heavy snow process occurred in Beijing and northwestern Hebei Province,becoming the most delayed occurrence of heavy spring snow ever recorded over Beijing in the last 30 years.This paper uses observational and numerical simulation data to investigate the causes for the rapid rain-to-snow(RRTS)phase transition during this process.The following results are obtained.(1)Return flows(RFs),an interesting type of easterly wind,including those at 1000,925,and 800 hPa,played an important role in this heavy snow process and presented a characteristic"sandwich"structure.The RFs,complex topography,and snow particles that dominated the clouds,were the three key factors for the RRTS transition.(2)The RRTS transition in the plains was directly related to the RF at 925 hPa,which brought about advective cooling initiated approximately 4-6 h before the onset of precipitation.Then,the RF played a role of diabatic cooling when snow particles began to fall at the onset of precipitation.(3)The RRTS transition in the northern part of the Taihang Mountains was closely related to the relatively high altitude that led to a lower surface temperature owing to the vertical temperature lapse rate.Both immediately before and after the onset of precipitation,the snow particles in clouds entrained the middle-level cold air downward,causing the melting layer(from surface to the 0℃-isotherm level)to become very thin;and thus the snow particles did not have adequate time to melt before falling to the ground.(4)The rapid RRTS over the Yanqing mountainous area in the northwest of Beijing could have involved all the three concurrent mechanisms:the advective cooling of RF,the melting cooling of cloud snow particles,and the high-altitude effect.Compared with that in the plain area with less urbanization the duration of the RRTS in the plain area with significant urbanization was extended by approximately 2 h.
文摘为了进一步探究纳米流体热管的模拟仿真,运用Fluent软件中的VOF(volume of fluid)模型,以单一纳米流体(TiO_(2)-H_(2)O)和混合纳米流体(Al_(2)O_(3)+TiO_(2)-H_(2)O)为工质,从热管内部蒸发冷凝过程的可视化分析以及整体热力参数结果两个维度综合分析利用Fluent模拟纳米流体热管的可行性。结果表明,从可视化角度看,模拟结果云图整体可以反映热管内部随着时间变化所发生的相变过程。但具体的模拟数值与修正实验数值仍存在3%的误差,说明存在非常规因素影响模拟结果。利用VOF模型模拟纳米流体热管在数值规律以及可视化分析上是可行的,但对于具体的精细化原理研究仍需结合其他模拟方法。
文摘为准确高效地模拟高压CO_(2)管道泄漏的瞬态特性,基于Fluent仿真平台,利用用户自定义真实气体模型(User Defined Real Gas Model,UDRGM)和用户自定义函数(User Defined Function,UDF),结合查表法和双线性插值法建立CO_(2)的真实气体模型,并将压力驱动的Lee模型通过用户自定义函数嵌入Fluent求解器来模拟CO_(2)的非平衡相变过程,建立了高压CO_(2)管道泄漏的非平衡相变数值模型。通过与Botros等的试验数据进行对比分析,验证了该模型的准确性。在此基础上,对比了上述模拟方法与编译S-W(Span-Wagner)状态方程模拟方法的精度和效率,最后使用本模型研究了不同初始压力对高压CO_(2)管道泄漏瞬态特性的影响。结果表明:两种模拟方法精度接近,最大相差为7.37%,但提出的模拟方法效率明显优于编译S-W状态方程的模拟方法,计算时间相较缩短约86.9%;初始压力为11.27 MPa的最大总出口质量流量比4.36 MPa的大7.24 kg/s,而对应的气相CO_(2)出口质量流量却低0.14 kg/s;初始压力为11.27 MPa和4.36 MPa对应的近场最大射流流速分别为155.12 m/s和175.50 m/s;管内初始压力越低,泄漏后到达亚稳态时过热度越大,管内及泄漏口附近相变程度越剧烈,气相出口质量流量越大,近场射流峰值速度越大。这项研究可为工业规模管道泄漏的模拟和管道泄漏的三维模拟提供更高效的方式。
