Simulation of two-phase flows of a droplet impacting on a liquid film with an improved lattice Boltzmann model

Based on the lattice Boltzmann method (LBM), an improved pseudo-potential model, combined with a method of adding force term, is used to simulate the two-phase flows caused by a liquid droplet impacting on a liquid film. In this model, the different phases are treated as one fluid, and the interfaces between the vapor and liquid phases can be obtained by density value of the fluid. This variant of the LBM allows one to obtain the densities of vapor and liquid with high accuracy. The model is validated by an example of phase separation. The early stage of the impact of droplet on liquid film is simulated, and the results are qualitatively consistent with physical phenomena.     

Parallel communication optimization based on least square method for LBM

Efficient communication is important to every parallel algorithm. A parallel communication optimization is introduced into lattice Boltzmann method (LBM). It relies on a simplified communication strategy which is implemented by least square method. After testing the improved algorithm on parallel platform, the experimental results show that compared with normal parallel lattice Boltzmann algorithm, it provides better stability, higher performance while maintaining the same accuracy.     

Numerical investigation on thermal effects by adding thin compartmental plates into cooling enclosures with heat-leaking walls

Adding thin compartmental plates near the internal walls of enclosures has been numerically modeled using the lattice Boltzmann method. This practice was found to be an effective way to further suppress the disadvantageous effects of heat leak, along with the application of insulation materials on the external surfaces. A modified extrapolation scheme for handling the thermal boundary of the thin plate was proposed and verified by comparison with the conventional coupled boundary scheme. The simulation of the natural convection during the cooling down processes and at steady states in the enclosure indicates that the existence of the plates leads to a higher cooling rate and a more favorable temperature uniformity. For a typical case, the one with plates takes 6% less time to reach the halfway point of the steady state and has 26% less temperature variance. Effects by the plates’ positions and sizes were parametrically investigated, in order to find an optimal geometrical configuration. In addition, the fluid’s intrinsic characteristics and the relative heat leak by using the Rayleigh number and Nusselt number, respectively, have been discussed in detail through hydrodynamic and convective heat transfer analyses.     

Comparision of DdQm models in image optical flow analysis based on LBM

The optical flow analysis of the image sequence based on the formal lattice Boltzmann equation, with different DdQm models, is discussed in this paper. The algorithm is based on the lattice Boltzmann method (LBM), which is used in computational fluid dynamics theory for the simulation of fluid dynamics. At first, a generalized approximation to the formal lattice Boltzmann equation is discussed. Then the effects of different DdQm models on the results of the optical flow estimation are compared with each other, while calculating the movement vectors of pixels in the image sequence. The experimental results show that the higher dimension DdQm models, e.g., D3Q15 are more effective than those lower dimension ones.     

Modified surface tension model for free surface flow with single-phase lattice Boltzmann method

A single-phase lattice Boltzmann model with modified surface tension is developed in this paper to solve the problem of high-density-ratio free surface flow.The computational efficiency and accuracy are both enhanced.The restriction to the relaxation factor （which needs to be smaller than 1） is circumvented by the new surface tension algebra,due to its rational physical nature compared with the treatment of Xing,Buther and Yang in their paper （Comp.Mater.Sci.,2007,39（2）：282-290）.The proposed stable surface tension scheme is applied to simulate the free deformation of a square droplet with surface tension effect and the process of a droplet impinging on a liquid film.The numerical solution for free deformation of a droplet agrees well with thermodynamic principles,and also achieves high accuracy in comparison with Xing,et al.＇s model.Three typical impinging modes are successfully obtained with the new scheme,and another particular mode found by Wang and Chen is also successfully simulated.The evolutions of liquid crown agree well with the power law related to time.     

微通道中甲醇蒸汽重整反应过程的LBM模拟

采用格子-Boltzmann方法对平行平板间甲醇水蒸气重整反应过程进行模拟,其中包括反应、对流、扩散。模拟中假设化学反应对流场没有影响,流场、温度场和组分场可分别利用演化方程进行求解,利用多尺度技术可分别将演化方程回归到对应的宏观方程。在低Ma的条件下格子-Boltzmann方法模拟的结果与传统Navier-Stokes方程数值计算结果进行对比,吻合得较好,证明了格子-Boltzmann方法的可靠性。     

Numerical investigation of flow characteristics around two side-by-side cylinders by immersed boundary-lattice Boltzmann flux solver

In this paper, to study the characteristics of the flow in a laminar regime, an immersed boundary-lattice Boltzmann flux solver (IB-LBFS) is applied to numerically simulate the unsteady viscous flows around two fixed and rotating circular cylinders in side-by-side arrangement. This method applies finite volume discretization to solve the macroscopic governing equations with the flow variables defined at cell centers. At the cell interface, numerical fluxes are physically evaluated by a local lattice Boltzmann solution. In addition, the no-slip boundary condition is accurately imposed by using the implicit boundary condition-enforced immersed boundary method. Due to the simplicity and high efficiency of IB-LBFS on non-uniform grids, it is suitable for simulating fluid flows with complex geometries and moving boundaries. Firstly, numerical simulations of laminar flow past two side-by-side cylinder are performed with different gap spacings at Reynolds numbers of 100 and 200. The simulation results show that a small gap spacing induces a biased flow and forms an irregular big wake behind two cylinders at a low Reynolds number. As the gap spacing increases, an in-phase or anti-phase flow is observed. Then, the effects of the main important parameters on flow characteristics are analyzed for flow past two side-by-side rotating cylinders, including the rotational speed, Reynolds number, and gap spacing. As the rotational speed is increased, the numerical results illustrate that unsteady wakes are suppressed and the flow becomes steady. As the gap spacing is increased, two separate vortex streets behind each cylinder are formed with a definite phase relationship and single shedding frequency.     

Numerical simulation of wind flow fields around buildings using the lattice Boltzmann method

This paper presents the simulation results of the wind environment around a single high-rise building and that around two tall buildings in tandem arrangement by using the lattice Boltzmann method with an aim to understand the ventilation issues around high-rise buildings in an urban environment. We analyzed the velocity distribution around the buildings and performed numericl simulations to reveal the formation and evolution law of the complex vortex system around the high-rise buildings. Numerical simulation results manifest a periodicity phenamenon in the process of the vortex evolution. For the case of two high-rise buildings, wind velocity in the space between the two buildings is very small, which is nearly a silent regime. Wind velocity above the front building is relatively larger and the maximum wind velocity is approximately 2.5 times the incoming wind velocity. The numerical results can be used in layout planning of high-rise residential buildings to create better environment for ventilation purpose in an urban area.     

组合KdV-Burgers方程的预校算法及其数值仿真

非线性偏微分方程的有限差分算法存在两大难点,一是求解高阶非线性方程组消耗太多的时间和内存,二是计算过程极不稳定,以至在很短暂的时间步内产生爆破现象.为了改善数值稳定性和提高计算效率,针对KdV-Burgers方程,提出一种预校算法及其改进技巧：多次校正的PCM算法,Gauss-Seidel算法和正反交替校正算法.通过这个预校算法,可以求解许多一般的非线性偏微分方程,包括KdV方程,修正KdV方程,组合KdV-MKdV方程,Burgers方程,KdV-Burgers方程等.在一定条件下,这种算法收敛速度快、稳定性好、计算复杂度保持为O（1/h.1/τ）;相比Fourier拟谱方法和线性隐式格式,该算法无需求解高阶方程组,编程统一,内存消耗很少.数值实验表明所构造的格式能长时间模拟不同孤立波解的传播与碰撞过程,验证了算法的有效性和稳定性.     

Study on One-Dimensional Model of Traffic Flow with Stochastic Deceleration via Lattice Boltzmann Method

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A meshless method based on moving least squares for the simulation of free surface flows

In this paper, a meshless method based on moving least squares (MLS) is presented to simulate free surface flows. It is a Lagrangian particle scheme wherein the fluid domain is discretized by a finite number of particles or pointset; therefore, this meshless technique is also called the finite pointset method (FPM). FPM is a numerical approach to solving the incompressible Navier-Stokes equations by applying the projection method. The spatial derivatives appearing in the governing equations of fluid flow are obtained using MLS approximants. The pressure Poisson equation with Neumann boundary condition is handled by an iterative scheme known as the stabilized bi-conjugate gradient method. Three types of benchmark numerical tests, namely, dam-breaking flows, solitary wave propagation, and liquid sloshing of tanks, are adopted to test the accuracy and performance of the proposed meshless approach. The results show that the FPM based on MLS is able to simulate complex free surface flows more efficiently and accurately.     

Buckling and multiple equilibrium states of viscoelastic rectangular plates

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Study on One—Dimensional Model of Traffic Flow with Stochastic Deceleration via Lattice Boltzmann Method

One-dimensional traffic model was studied with the consideration of stochastic deceleration by using the lattice Boltzmannmethod.The evolution equation for vehicle density was derived,which has the form of the Burgers equation exhibiting the effect of viscosity corresponding to the deceleration.The simulation with the model shows that the variation of vehicle density in space tends to a periodic one,which implies the existence of kinematic waves in the 1D traffic flow and coincides with theoretical prediction.     

Time allocation scheme in IEEE 802.15.3 TDMA mechanism

In network with a shared channel in TDMA mechanism, it is a core issue to effectively allocate channel time to provide service guarantees for flows with QoS requirements. This paper proposes a simple and efficient time allocation scheme called MES-ESRPT (MCTA at the End of Superframe-Enhanced Shortest Remaining Processing Time) for delay-sensitive VBR traffic in accordance with IEEE 802.15.3 standard. In this algorithm, PNC (piconet coordinator) allocates one MCTA (Management Channel Time Allocation) for each stream which is the process of communication at the end of superframe. During the MCTA period, each transmitter should report current fragments number of the first MSDU (MAC Service Data Unit) and the fragments number of the remainder MSDUs to PNC. In the next superframe, PNC firstly allocates part CTAs (Channel Time Allocation) for each stream based on the remainder fragments number of the first MSDU by SRPT rule, then allocates remainder CTAs for each stream based on all fragments number of remainder MSDUs by the same SRPT rule. Simulation results showed that our proposed MES-ESRPT method achieves significantly better performance in QoS for multimedia streams compared to the existing schemes.     

一维纳米线的热学性质

根据边界条件中声子散射的不同情况，用Boltzmann方程研究了计算一维纳米材料－线的晶格热导率，结果发现，与体热导率相比， 线的热导率显著减小。     

木材干燥过程中一个非线性模型的二阶收敛差分格式

针对描述木材干燥过程中的一个非线性微分方程模型,用降阶法对其建立了一个差分格式.此模型是由一个非线性常微分方程和一个非线性抛物方程组成的耦合微分方程组.首先引进一个新变量把原问题转化为一阶微分方程组问题,然后对此一阶微分方程组建立了一个线性化差分格式,应用能量方法证明了差分格式的可解性、稳定性和收敛性,并给出了误差估计式.差分格式关于时间步长和空间步长均为二阶.在实际计算时,将引入的新变量分离开,得到仅含原变量的差分格式,降低了计算量.数值计算结果验证了理论结果的可靠性.     

Pore-Scale Investigation of Coupled Two-Phase and Reactive Transport in the Cathode Electrode of Proton Exchange Membrane Fuel Cells

A three-dimensional multicomponent multiphase lattice Boltzmann model(LBM) is established to model the coupled two-phase and reactive transport phenomena in the cathode electrode of proton exchange membrane fuel cells.The gas diffusion layer(GDL) and microporous layer(MPL) are stochastically reconstructed with the inside dynamic distribution of oxygen and liquid water resolved,and the catalyst layer is simplified as a superthin layer to address the electrochemical reaction,which provides a clear...     

Fascinating world of shock waves

This article introduces the reader to the exhilarating world of Shock Waves. With their unique ability to enhance the pressure and temperature instantaneously in any medium of propagation, shock waves are now being used for many innovative applications in the industry. The origin of shock waves, their characteristics, laboratory tools used in their study, along with a few interesting industrial applications of shock waves developed at the Indian Institute of Science are described. G Jagadeesh is an Associate Professor in the Department of Aerospace Engineering, IISc, Bangalore. His areas of interest include hypersonic flows, biomedical and industrial applications of shock waves. He has five international patents on various discoveries on industrial applications of shock waves.     

Performance investigation of 2D lattice Boltzmann simulation of forces on a circular cylinder

The lattice Boltzmann method (LBM) is employed to simulate the uniform flow past a circular cylinder. The performance of the two-dimensional LBM model on the prediction of force coefficients and vortex shedding frequency is investigated. The local grid refinement technique and second-order boundary condition for curved walls are applied in the calculations. It is found that the calculated vortex shedding frequency, drag coefficient and lift coefficient are consistent with experimental results at Reynolds nu...     

Recent development of vortex method in incompressible viscous bluff body flows

INTRODUCTION A number of problems involve flows of gases orliquids over bluff bodies such as air flowing over carsand airplanes and wind blowing over bridges andbuildings. Many experiments or simulations havefocused on the above problems. In those phenomena,flows will not follow the solid bodies completely, butseparate from them and create wakes. The generationand shedding of large coherent vortex structures dueto flow separation make the bluff body flows complexand very difficult …