线性波作用下紊流边界层特性的数值模拟

采用经修正的k-ω模型,即基线k-ω模型对线性波作用下边界层的流速、紊动能、涡粘系数分布及底床剪应力进行了数值模拟,验证了模型的合理性,进而采用该模型模拟了粗糙底床摩阻流速及具有不同相对粗糙度的粗糙底床紊流边界层特性,发现边界层内流速与自由流速的相位差随计算点至底床的距离和相对粗糙度的增大而减小。     

低流速净蒸汽产生点模型预测过冷沸腾空泡率

空泡率是汽液两相流动的基本参数之一,而已有过冷沸腾空泡率计算方法研究以高质量流速为主,且大量文献报道现有空泡率模型难以适用于低流速过冷沸腾工况。本文基于低流速过冷沸腾净蒸汽产生点(NVG)理论模型,进一步建立了计算过冷沸腾空泡率的分布拟合模型。在较宽广的压力、质量流速、热流密度和流道尺寸范围内将模型计算结果与现有空泡率实验数据进行了比较,低流速工况下该模型与实验数据符合良好,表明该模型可适用于低流速过冷沸腾工况。     

千瓦级质子交换膜燃料电池电堆的实验研究

空泡率是汽液两相流动的基本参数之一,而已有过冷沸腾空泡率计算方法研究以高质量流速为主。且大量文献报道现有空泡率模型难以适用于低流速过冷沸腾工况。该文基于低流速过冷沸腾净蒸汽产生点(NVG)理论模型,进一步建立了计算过冷沸腾空泡率的分布拟合模型。在较宽广的压力、质量流速、热流密度和流道尺寸范围内将模型计算结果与现有空泡率实验数据进行了比较,低流速工况下该模型与实验数据符合良好,表明该模型可适用于低流速过冷沸腾工况。     

用流速对数律研究陡坡上紊流边界层的发展

本文用流速分布的对数律,从卡门动量积分关系式出发,导出了光滑陡坡及粗糙陡坡上紊流边界层发展的计算式:根据国内外五个工程的原型观测资料,求得粗糙陡坡边界层发展的拟合式为并求得粗糙陡坡上流速分布对数律中A值的计算式为     

溃坝波数值模拟及其在坡面水流问题中的应用

为探讨不同方法模拟溃坝波的适用性和准确度,采用基于Roe格式的近似黎曼解方法与追踪自由表面的VOF方法模拟了二维对称矩形溃坝模型。结果表明,两种方法均能较好地模拟出溃坝波的传播过程,但与实测值的对比分析发现,采用Roe格式模拟的结果与实测值拟合较好,VOF方法对网格尺寸的要求较Roe格式更加精细。由此,基于既考虑降雨强度的变化,又考虑降雨动量和风的影响的缓坡和陡坡都适用的坡面径流基本方程,构建了基于Roe格式的坡面水流水动力模型,模拟了自然降雨条件下的坡面流运动情况,并通过相应的模型试验进行对比验证。模拟流速与实测流速之间拟合度较好,表明模型具备了模拟大坡度、浅水(深水)流的能力,为坡面流的水动力学机制研究提供了参考。     

转捩点预测对风力机翼型气动性能分析的影响

分别用带转捩模型的边界层耦合计算方法和基于RANS方程的通用CFD方法对风力机常用翼型S809进行了气动分析.通过自由转捩、强制转捩和全湍流模式的比较,着重阐述边界层转捩点预测对风力机翼型气动性能计算分析的影响.     

HCCI发动机多区燃烧模型的比较研究

多区模型作为现阶段均质压燃（HCCI）发动机高效准确的数值模型得到了世界范围的广泛关注。讨论了不同子模型对多区模型预测性能的影响。以实验为基准,比较了多区模型中区间划分、缸壁传热模型、区间热量交换模型、区间质量交换模型和边界层模型对HCCI发动机燃烧和排放模拟结果的影响,全部计算均基于异辛烷的详细化学动力学机理。结果表明：在区间划分时对温度较低的区域细化可以提高排放的计算效果,而对高温区域的细化对计算结果影响不大;改进的Woschni传热模型更准确地模拟了缸壁的传热过程;区间的质量和热量交换对计算结果影响显著,特别是质量交换模型的加入使CO排放的预测与实验值更为接近;而边界层厚度模型对整个结果影响不大。     

下降液膜流动模型及稳定性分析

建立了自由下降液膜流动的完备的数学模型。采用边界层理论对模型进行分析简化,得到液膜流动的二阶边界层模型,并对二阶界层模型进行稳定性分析,计算获得下降液膜波的增长率和中性稳定曲线,将计算结果与其它模型的计算结果进行了比较,证实了二阶边界层模型具有更好的预测效果,其形式便于作进一步的非线性分析。     

考虑颗粒边界层气相反应的炭粒气化过程模型预报

建立了描述炭粒气化过程的移动火焰锋面模型(MFF-G 模型),模型考虑了颗粒边界层气相反应(CO 燃烧和水煤气置换反应)的影响,以及 CO 和 H2 对气化的抑制作用、颗粒孔隙变化的影响等.新模型被用于气流床气化炉的一维数值计算中,预报结果显示在燃烧/气化的连续过程中炭粒与环境之间有着明显的温度差异,考虑颗粒边界层气相...     

裂隙通道内流固换热系数解析解及敏感性分析

以增强型地热系统中岩体裂隙通道内换热问题为核心,应用局部非热平衡法,针对圆柱岩体裂隙内流动换热理论模型,依据岩体截面二维导热方程推导出单裂隙流固换热系数解析解。通过将可测参数带入计算方程中,得出在不同工况条件下的换热系数,并与文献中将岩体半圆截面等效为矩形一维导热模型所计算数据进行对比。结果表明在相同裂隙开度条件下,当流速较低时其结果与文献得出的结果吻合较好,但随着流速的增大误差逐渐增大。利用敏感性分析法,在外壁温变化条件下,分析裂隙开度与流速对换热系数的影响,表明流速的影响略大于裂隙开度。     

柴油机近气缸盖壁面气流速度和热边界层的研究

研究了倒拖工况下直喷式柴油机气缸盖近壁气流速度场的分布规律，并利用一维传热模型计算了气缸盖近壁的热边界居，得出了气缸盖近壁不同位置的气流速度和热边界层是不同的，速度边界层基本上在压缩中期形成并达到一定厚度，以及热边界层和速度边界层数量级相同等结论。     

进口湍流对透平叶栅内部流动影响的数值研究

通过求解N S方程,分别采用Spalart Allmaras与k eps 湍流模型,对VKI低速轴流透平叶栅内部的湍流进行了数值分析。与实验结果比较,两种湍流模型计算结果与实验是一致的。叶栅流动损失的构成、分布以及计算准确性的主要因素是来流边界层状态。入口边界层是端部损失的主要来源。在高速流动条件下,叶栅损失主要是吸力面边界层径向串流及其尾迹。图14参8     

CFD boundary conditions at ablative surface in gas flow with arbitrary condensation and accommodation coefficients

We develop an analytical Knudsen layer model at the ablative surface in gas flow; this model takes into account the temperature and velocity gradients in the bulk gas and the rebounding of gas molecules by the ablative wall back into the gas region. In addition, this model uses a bimodal velocity distribution function which preserves the laws of conservation of mass, momentum and energy within the Knudsen layer and converges to the Chapman–Enskog velocity distribution function at the outer boundary of the Knudsen layer. This model enables us to obtain the boundary conditions at the ablative surface in gas flow for arbitrary condensation and accommodation coefficients, which can be used for computational fluid dynamics simulation of ablation avoiding “micro” modelling of the evaporation process at the mean free path scale.     

Improved velocity and temperature profiles for integral solution in the laminar boundary layer flow on a semi‐infinite flat plate

One of the most important problems in Mechanical Engineering is the determination of laminar boundary layer thickness over a flat plate. Integral solution and similarity solutions are two well‐known methods for calculation of boundary layer thickness. However, integral solution method is a computational cost‐effective method rather than the similarity solution method. Velocity and temperature profiles must be determined for the integral solution method. Velocity boundary layer thickness can be determined by the velocity profile whereas for determination of thermal boundary layer thickness both velocity and temperature profiles must be used. Available velocity profiles do not give an exact value for velocity boundary layer thickness, while the Nusselt number is affected by these profiles. In this study, a new velocity profile is proposed which gives an exact value for laminar boundary layer thickness on a flat plate. In addition, two temperature profiles are proposed that give the exact values of the Nusselt number over a flat plate for uniform temperature and uniform heat flux boundary conditions. The calculated constants in the velocity boundary layer thickness equation and the Nusselt relations are validated with the results of the similarity solution method. Excellent agreement between the results of the two methods is observed.     

Turbulence characteristics and vortical structures in combined-convection boundary layers along a heated vertical flat plate

Time-developing direct numerical simulations are performed for the combined-convection boundary layers created by imposing aiding and opposing freestreams to the pure natural-convection boundary layer in air along a heated vertical flat plate to clarify their structural characteristics. The numerical results reveal that with a slight increase in freestream velocity, the transition region moves downstream for aiding flow and upstream for opposing flow. This fact correlates well with the existing experimental results showing that the transition delays for aiding flow and quickens for opposing flow in the practical space-developing boundary layer. Thereby, for aiding flow, turbulence characteristics indicate the behavior proceeding to the laminarization of the boundary layer. On the other hand, for opposing flow, the large scale fluid motions are apparent and become larger than those for the pure natural-convection boundary layer with increasing freestream velocity. For the occurrence of such fluid motions, the budgets of turbulent energy and two-point spatial correlations in the turbulent combined-convection boundary layers are also examined. Consequently, it is found in the spatial correlations that the turbulence structures are mainly controlled by fluid motions in the outer region of the boundary layer.     

Nonlinear convective boundary layer flow of micropolar‐couple stress nanofluids past permeable stretching sheet using Cattaneo‐Christov heat and mass flux model

The present work aims to examine the effects of viscous dissipation and unsteadiness parameters on nonlinear convective laminar boundary layer flow of micropolar‐couple stress nanofluid past a permeable stretching sheet with non‐Fourier heat flux model in the presence of suction/injection variable. The unsteadiness in the flow, temperature, and concentration profile is caused by the time‐dependence of the stretching velocity, surface temperature, and surface concentration of the boundary layer flow. Similarity transformation is applied to transform the time‐dependent boundary layer flow equations into the corresponding highly nonlinear coupled ordinary differential equations with appropriate boundary conditions. The robust numerical technique called Galerkin finite element method is used to solve the obtained dimensionless governing equations of the flow. The effects of Eckert number, unsteadiness parameter, suction/injection parameter, mixed convection parameter, material parameter, Schmidt number, and couple stress parameter on linear velocity, angular velocity, temperature, concentration, local skin friction coefficient, local wall couple stress, local Nusselt number, and local Sherwood number is analyzed with the help of graphical and tabular form. Under special conditions, the present result is compared with the existing literature and revealed good agreement. Our result shows that as unsteadiness parameter boost, both heat and mass transfer rate rises. The present study has a significant application in material processing technology.     

Direct numerical simulation for a time-developing combined-convection boundary layer along a vertical flat plate

Time-developing combined-convection boundary layers induced by imposing aiding and opposing flows to the natural-convection boundary layer in air along a hot vertical flat plate have been examined with a direct numerical simulation. As the freestream velocity increases, the transition from laminar to turbulence delays for aiding flow and quickens for opposing flow, corresponding well to actual observations in space-developing flows. The calculated profiles of mean streamwise velocity, mean temperature, intensities of streamwise velocity fluctuation and temperature fluctuation in the laminarization process of the boundary layer for aiding flow agree relatively well with the existing data. Also, the distributions of turbulent statistics and instantaneous fluid motions in the combined-convection boundary layers with adding and opposing flows are displayed, and the regimes of the boundary layer flows obtained from the calculations are compared with those observed in the experiment.     

Significance of Hall effect on heat and mass transport of titanium alloy-water-based nanofluid flow past a vertical surface with IMF effect

In this mathematical presentation, we examined the significance of Hall current on MHD buoyancy-driven boundary layer flow of a Ti₆Al₄V-H₂O based nanofluid past a vertical surface implanted in a uniform permeable region. The vertical surface is considered to be magnetized and induced magnetic field (IMF) impacts are also considered. The nondimensional flow model is solved with the assistance of the two-term perturbation scheme. Various results are obtained by numerical computation for different significant parameters. These results are presented and analyzed in graphical and tabular form. In the boundary layer domain, the transpiration velocity across the surface tends to diminish the main flow, IMF along the main flow, fluid temperature, and concentration. It is remarkably noted that IMF along the main flow grows for incrementing values of volume fraction coefficient of nanofluid. In the magnetic boundary layer domain, the main flow and IMF along the main flow grow with Hall current. Furthermore, it is seen that for the progressing values of magnetic Prandtl number, the main flow reduces while normal flow and IMF along the main flow is induced in the boundary layer domain.     

A computational study of unsteady radiative magnetohydrodynamic Blasius and Sakiadis flow with leading‐edge accretion (ablation)

The present study investigates the influence of the magnetic field, thermal radiation, Prandtl number, and leading‐edge accretion/ablation on Blasius and Sakiadis flow. The convective boundary condition is employed to investigate the heat transfer. The nondimensional governing boundary layer equations have been solved by the homotopy analysis method for different values of the pertinent parameters. The effects of these parameters on the dimensionless velocity, temperature, skin friction, and Nusselt number are also investigated for various values of relevant parameters affecting the flow and heat transfer phenomena. The most relevant outcomes of the present study are that enhancement in magnetic field strength undermines the flow velocity establishing thinner velocity boundary layer for both Blasius and Sakiadis flows while an increase in accretion/ablation effect at leading‐edge manifests in a deceleration in velocity for Blasius case and the opposite trend is observed for Sakiadis flow. Another important outcome is that an increase in radiation and accretion/ablation at leading‐edge upsurges the fluid temperature leading to enhancement in the thermal boundary layer. For both Blasius and Sakiadis flow, the skin friction coefficient and the heat transfer rate decline with the enhancement of the leading‐edge accretion parameter. The results are compared with the existing data and are found in good agreement.     

A linearized numerical model of wind‐farm flows

A fast and reasonably accurate numerical three‐dimensional wake model able to predict the flow behaviour of a wind farm over a flat terrain has been developed. The model is based on the boundary‐layer approximation of the Navier–Stokes equations, linearized around the incoming atmospheric boundary layer, with the assumption that the wind turbines provide a small perturbation to the velocity field. The linearization of the actuator‐disc theory brought additional insights that could be used to understand the behaviour, as well as the limitations, of a flow model based on linear methods: for instance, it is shown that an adjustment of the turbine's thrust coefficient is necessary in order to obtain the same wake velocity field provided by the actuator disc theory within the used linear framework. The model is here validated against two independent wind‐tunnel campaigns with a small and a large wind farm aimed at the characterization of the flow above and upstream of the farms, respectively. The developed model is, in contrary to current engineering wake models, able to account for effects occurring in the upstream flow region, thereby including more physical mechanisms than other simplified approaches. The conducted simulations (in agreement with the measurement results) show that the presence of a wind farm affects the approaching flow far more upstream than generally expected and definitely beyond the current industrial standards. Despite the model assumptions, several velocity statistics above wind farms have been properly estimated providing an insight into the transfer of momentum inside the turbine rows. Copyright © 2016 John Wiley & Sons, Ltd.