90°弯管内流动的理论模型及流动特性的数值研究

从三维不可压缩雷诺时均Navier-Stokes方程出发,对90°弯曲管道内湍流流动进行数值模拟。网格划分采用六面体网格,湍流模型为RNGk-ε模型,在近壁区采用两层壁面模型进行修正,流场的计算结果与实验数据吻合较好。在此基础上,本文数值研究了来流方向对流场结构和流动特性的影响。得出在弯管流场中发生了分离现象,且随着来流侧滑角的增大,分离区范围增大。此外,随着来流从同一侧滑角变换至同一攻角时,横截面的二次流图像中也从具有两个对称主涡变成只具有一个主涡的现象。     

NUMERICAL SIMULATION OF THREE DIMENSIONAL GAS-PARTICLE FLOW IN A SPIRAL CYCLONE

The three-dimension gas-particle flow in a spiral cyclone is simulated numerically in this paper. The gas flow field was obtained by solving the three-dimension Navier-Stokes equations with Reynolds Stress Model (RSM). It is shown that there are two regions in the cyclone, the steadily tangential flow in the spiral channel and the combined vortex flow in the centre. Numerical results for particles trajectories show that the initial position of the particle at the inlet plane substantially affects its trajectory in the cyclone. The particle collection efficiency curves at different inlet velocities were obtained and the effects of inlet flow rate on the performance of the spiral cyclone were presented. Numerical results also show that the increase of flow rate leads to the increase of particles collection efficiency, but the pressure drop increases sharply.     

NUMERICAL SIMULATION OF THREE DIMENSIONAL TURBULENT FLOW IN SUDDENLY EXPANDED RECTANGULAR DUCT

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三维波纹型可延展结构振动特性的辛分析

基于三维组装技术的可延展结构具备优异的延展性和可调控性,使其成功应用于各类可延展电子器件的制备中。为了评估该类电子器件的稳定性,本文研究三维波纹型可延展结构的振动行为。首先,基于非线性的Euler-Bernoulli梁理论、Kelvin-Voigt粘弹性理论和考虑压电材料的表面压电效应,建立三维波纹结构的理论分析模型;其次,基于能量原理和扩展拉格朗日运动原理,推导出该结构的动力学控制方程;然后采用二级四阶辛Runge-Kutta求解该动力学方程。通过数值仿真实验验证了辛算法的优越性,同时,还发现随着三维波纹型可延展结构外界激励及其结构参数的变化,该结构的振动特性会从倍周期向分岔和混沌转化;本文结果为三维波纹型可延展结构的优化设计及应用提供理论基础。     

Variational formulation and stability analysis of a three dimensional superelastic model for shape memory alloys

This paper presents a variational framework for the three-dimensional macroscopic modelling of superelastic shape memory alloys in an isothermal setting. Phase transformation is accounted through a unique second order tensorial internal variable, acting as the transformation strain. Postulating the total strain energy density as the sum of a free energy and a dissipated energy, the model depends on two material scalar functions of the norm of the transformation strain and a material scalar constant. Appropriate calibration of these material functions allows to render a wide range of constitutive behaviours including stress-softening and stress-hardening. The quasi-static evolution problem of a domain is formulated in terms of two physical principles based on the total energy of the system: a stability criterion, which selects the local minima of the total energy, and an energy balance condition, which ensures the consistency of the evolution of the total energy with respect to the external loadings. The local phase transformation laws in terms of Kuhn–Tucker relations are deduced from the first-order stability condition and the energy balance condition.The response of the model is illustrated with a numerical traction–torsion test performed on a thin-walled cylinder. Evolutions of homogeneous states are given for proportional and non-proportional loadings. Influence of the stress-hardening/softening properties on the evolution of the transformation domain is emphasized. Finally, in view of an identification process, the issue of stability of homogeneous states in a multi-dimensional setting is answered based on the study of second-order derivative of the total energy. Explicit necessary and sufficient conditions of stability are provided.     

Investigation of three dimensional air flow characteristic and effect of heated obstruction within room

This study reports the results of a numerical investigation of three-dimensional turbulent buoyant recirculating flow within rooms with heated obstruction. The study involves the solution of partial differential equations for the conservation of mass, momentum, energy, concentration, turbulent energy and its dissipation rate. These equations were solved together with algebraic expressions for the turbulent viscosity and heat diffusivity using k-ε turbulence model by performing simulations on FLUENT 6.3. The CFD method was validated via comparing with the available experimental data. A comparison with experimental results shows good agreement. This means that the present computer code has a good capability to simulate 3D airflow and effect of obstruction within room. The present study demonstrates the flow behavior, thermal distribution and CO₂ concentration inside the room in the presence of heat flux obstruction. Two different configurations of ventilation system have been studied. Mixing and Displacement ventilation system have been used in two geometries depending on location of opening inlet. The ventilation effectiveness for heat removal (E_T) is used to evaluate the indoor climate and average temperature is an important parameter in designs the ventilation systems. Two notable points are presented; first, mixing ventilation is depending on throw of jet. CO₂ concentration and temperature distribution have been effected in upper zone more than occupied zone with presence the obstruction. Second notable points are presented; in displacement ventilation buoyancy effect is considerable. Vertical temperature gradient above the obstruction implies that both fresh air and CO₂ concentration.     

Axisymmetric and three dimensional flow modeling within thermal vapor compressors

Thermal vapor compressor (TVC) is a device for compressing vapor in water–steam cycles and frequently used in desalination systems. Large amounts of useless vapor can be compressed by this device and the efficiency of a desalination unit is effectively enhanced through this process. Motive steam is injected into the TVC through a convergent–divergent nozzle and accelerated to supersonic velocities. The low pressure steam is entrained at the upstream zone and mixed with this highly compressible motive flow within the TVC. In the current study, the flow field of an experimental TVC is scrutinized in both axisymmetric and three-dimensional approaches and compared with experimental measurements. Since the steam collector at the suction surface of the TVC has a curved shape and may undermine the symmetry of the flow on either side of the central axis, the second objective of this study is to reveal the deviation of the symmetric assumption from the real non-symmetric condition of entering steam flow into the TVC. Results show that the presence of a bending at the inlet side has approximately negligible effects on the mixing phenomenon and the flow remains symmetric around the central axis. Hence, there is no need to consider the collector geometry in further simulations and the performance parameters of the TVC would be sufficiently obtained through an axisymmetric method with a substantial reduction in the computational cost and time.     

三维复杂流动的间断有限元方法模拟

本文基于三维可压缩Euler方程,采用基于Runge-Kutta时间离散的间断有限元方法(RKDG方法),对三维前台阶、三维Riemann问题和球Riemann等问题进行了模拟。结果表明,本文的RKDG方法能够在很少的网格内清晰地捕捉到三维复杂流场中的激波和接触间断;同时,将球Riemann问题中z=0.4平面压强沿到对称轴距离的分布与文献中的近似精确解相比,吻合较好,这也验证了本文的RKDG方法不仅能够进行三维复杂流场的定性描述,也能够应用于三维复杂流场的定量计算。     

Numerical analysis of interfacial two‐dimensional Stokes flow with discontinuous viscosity and variable surface tension

A fluid model of the incompressible Stokes equations in two space dimensions is used to simulate the motion of a droplet boundary separating two fluids with unequal viscosity and variable surface tension. Our theoretical analysis leads to decoupled jump conditions that are used in constructing the numerical algorithm. Numerical results agree with others in the literature and include some new findings that may apply to processes similar to cell cleavage. The method developed here accurately preserves area for our test problems. Some interesting observations are obtained with different choices of the parameters. Copyright © 2001 John Wiley & Sons, Ltd.     

超空泡形态及其流动特性的数值模拟

对圆盘空化器分别采用CFD、"1/3法则"和空泡截面独立扩展原理三种不同方法数值模拟了水下航行体定常自然超空泡外形及其流动特性。应用CFD方法基于粘性多相流的空泡捕捉法,采用六面体网格,选择Singhal空化模型和SST湍流模式,数值求解均质超空泡流场RANS方程。研究表明:泡形态时变特性是一种行之有效的工程估算方法,应用空泡截面独立扩展原理其计算结果与CFD方法吻合较好,说明了CFD方法用于超空泡流动仿真计算的可行性和独立性原理快速估算超空泡形态的准确性;同时超空泡外形主要与头部空化器有关,空泡长度会由于航行体本身存在变长;可优先选择空泡截面独立扩展原理对水下航行体超空泡外形进行快速估算。     

Numerical investigation of the deflector effect on the aerodynamic and acoustic characteristics of turbulent cavity flow

The effect of a deflector on the aerodynamic and acoustic characteristics of turbulent flow in a cavity is numerically modeled. The computation method working on unstructured grids is described. The immersed boundary method is applied to model the deflector. The adequacy of the computation method is checked against some similar problems for which there are plausible experimental data. The results of the calculations of turbulent flow around deflectors of different shape are presented and their influence on the flow structure is analyzed. The results of the large-scale calculations of flow near a cavity are also considered in the presence and the absence of a deflector near the leading edge. On the basis of the numerical data obtained the deflector effect on the aerodynamic and acoustic characteristics of cavity flow is analyzed, including the effect on the acoustical loads on the cavity surface.