16MnR钢缺口试样解理断裂行为研究

研究了低合金热轧钢16MnR缺口试样在$-196\,{^\circ}$C和$-130\,{^\circ}$C的解理断裂机 理. 拉伸试验、单、双缺口四点弯曲实验、断口形貌观察以及有限元分析结果表明, 缺口试 样发生解理断裂时均起裂于夹杂物粒子, 一种位于缺口根部前端(IC型), 另一种位于距缺口 根部较远的条形裂纹前端(SIC型); 且随温度升高, 起裂源的类型从$-196\,{^\circ}$C下的IC 型转变为$-130\,{^\circ}$C下的SIC型. 微裂纹均形核于夹杂物, 最终的断裂由铁素体晶粒尺 寸的微裂纹扩展控制. 缺口试样IC型解理断裂遵循裂纹形核条 件$\varepsilon_{\rm p} \ge \varepsilon_{\rm pc}$和裂纹扩展条件$\sigma_{yy} \ge \sigma_{f}$, 而SIC型解理断裂条件则演化为$\varepsilon_{\rm p}+\varepsilon_{\rm ps} \ge \varepsilon_{\rm pc}$和$\sigma_{yy} +\sigma_{yy{\rm s}} \ge \sigma_{f}$.     

等速上仰翼型动态失速现象研究

翼型大迎角绕流的静态失速将造成升力突降和气动性能急剧恶化，但利用非定常运动所产生 的动态失速效应，可以大大地延缓气流分离和失速现象的发生. 采用Rogers发 展的双时间步Roe格式，求解拟压缩性修正不可压N-S方程. 数值模拟了低雷诺数 ($Re=4.8 \times 10^{4}$)条件下NACA0015翼型作等速上仰($\alpha =0^{\circ} \sim 60^{\circ}$)的动态失速过程，同Walker的试验结果比 较，验证了计算结果的正确性. 研究了该过程中主涡、二次涡和三次涡的发展，升 力系数随攻角变化，以及不同上仰速度对动态失速效应所造成的影响.     

线圈绕Y轴倾斜时方腔内空气磁力与重力耦合对流的数值模拟

数值分析了微重力与重力环境下圆形载流线圈绕Y轴倾斜时方腔内空气热磁对流。磁场计算采用毕奥萨伐定律;控制方程基本变量采用控制容积法离散,求解采用SIMPLE算法。计算过程中Ra数的变化范围为104~105,线圈倾斜角yeuler的变化范围为-90°~90°,磁场力数γ数的变化范围为0~200。获得了空气热磁对流的流场和温...     

混凝土双K断裂参数计算的半解析有限元法

混凝土裂缝扩展的双$K$断裂准则，用于描述混凝土结构裂缝的起裂、稳定扩 展和失稳断裂. 其相应的双$K$断裂参数(起裂断裂韧度$K_{\rm IC}^{\rm ini} $和失 稳断裂韧度$K_{\rm IC}^{\rm un}$)一般通过简便的试验和基于虚拟裂缝扩展粘 聚力的解析方法确定. 利用平面扇形域哈 密顿体系的方程，通过分离变量法及共轭辛本征函数向量展开法，以解析的方法推导出基于混 凝土虚拟裂缝扩展线性粘聚力模型的平面裂缝解析元列式. 将该解析元与有限元相结合，构成 半解析的有限元法，可求解任意结构几何形状的混凝土平面裂缝双$K$断裂参数的计算问题. 数值计算结果表明半解析有限元法对该类问题的求解是十分有效的.     

不同壁面取向下超疏水平面直轨道上的气泡滑移

利用特定几何分布的超疏水表面实现气泡定向输运在矿物浮选和生物孵化等领域具有广阔的应用前景, 对平面直线超疏水轨道而言, 其壁面取向是相关工程结构的关键参数, 但超疏水壁面取向对倾斜壁面气泡滑移的影响尚不明确. 本文采用高速阴影成像系统研究了不同壁面取向($-90^\circ\leqslant \beta \leqslant 90^\circ$)及轨道倾角($45^\circ\leqslant \alpha \leqslant 75^\circ$)下, 气泡($D_{eq}=2.4$ mm, $Re=500$ $\sim$ 700, $We=7$ $\sim$ 13)在轨道宽度为2 mm的超疏水直线轨道上的运动特性. 气泡在轨道上的滑移近似为匀速, 形状为具有多脊的半子弹型. 根据气液界面波动程度的不同, 滑移气泡可分为波动型和稳定型, 稳定型气泡只在较小倾角且较大方位角时出现($45^\circ\leqslant \alpha < 70^\circ$, $| \beta | \geqslant 45^\circ$). 根据倾角不同, 滑移速度关于$\beta $有2种变化规律: 当$\alpha \leqslant 65^\circ$, 气泡滑移速度近似为关于$\beta =0^\circ$ 的单峰分布($\beta =0^\circ$时, 气泡滑移速度最大); 当$\alpha \geqslant 70^\circ$, 气泡滑移速度在不同的方位角下基本保持稳定. 气泡的最大滑移速度可达0.66 m/s ($\beta =0^\circ$, $\alpha =70^\circ$), 远大于相同尺度的自由上升气泡($\approx0.25$ m/s), 这主要是壁面浸润性分布和惯性力的耦合效应所致. 轨道取向(方位角$\beta )$及轨道倾角($\alpha )$通过改变气泡沿轨道方向的驱动力和气泡迎风面积影响气泡的滑移速度和气液界面稳定性.      

砂-膨润土混合屏障材料渗透性影响因素研究

建立了一个新的结构-尾流振子耦合模型. 流场近尾迹动力学特征被模化为非线性阻尼 振子,采用van der Pol方程描述. 以控制体中结构与近尾迹流体间受力互为反作 用关系来实现流固耦合. 采用该模型进行了二维结构涡激振动计算,得到了合理的 振幅随来流流速的变化规律和共振幅值,并正确地预计了共振振幅值$A_{\max}^\ast$ 随着质量阻尼参数$\left( {m^\ast + C_A } \right)\zeta $的变化规律,给出了预测$A_{\max }^\ast $值的拟合公式. 采用该模型计算了三维柔性结构在均匀来流和简谐波形来流作用下的VIV 响应. 结构在均匀来流作用下振动呈现由驻波向行波的变化过程, 并最后稳定为行波振动形态. 在简谐波形来流作用下,结构呈现混合振动形态,幅值随时间呈周期变化.     

考虑翘曲效应的圆柱螺旋弹簧的振动分析

以空间曲梁理论为基础对簧丝截面为矩形的圆柱螺旋弹簧的自由振动特性进行了研究. 在弹簧的运动微分方程中, 所有的位移函数和广义翘曲坐标均定义在横截面的形心主轴上, 同时考虑了翘曲变形对弹簧固有频率的影响. 通过精确地应用符号运算软件MATHEMATICA可以得到振动模态的显式表达式, 固有频率则可用搜索的方法来确定. 在较宽的范围内, 给出了各种参数变化, 如簧丝截面的宽高比(a/ b = 0.6 ~ 1.7)、螺旋角(\bar {\alpha } = 5^\circ ~ 12.5^ \circ)、弹簧工作圈数(n = 6 ~ 12)和圆柱螺旋线半径(R = 4~ 10mm)对固有频率的影响. 为了证明解析法的有效性, 对两端固支和一端固支、一端自由矩形截面圆柱螺旋弹簧的固有频率和振动模态进行了求解, 并同ANSYS三维实体单元(Solid45)的结果和文献的结果进行了比较. 计算表明: 用解析方法得到的解和用数值方法得到的结果吻合得很好.      

微型热驱动回路的脉动及位差对热性能的影响

以甲醇为工质，采用高速数据采集系统测定了微型热驱动回路在不同运行参数下的压力 及温度脉动，其脉动周期及脉动幅度随蒸发段热流密度的增加而减小. 实验发现，在蒸发段 热流密度较低的情况下，蒸气管中是泡状流或弹状流交替存在，而在蒸发段热流密度较高时， 蒸气管中为环状流. 就位差对热性能的影响进行了详细的实验研究，并在冷凝器空气自 然对流和强迫对流情况下，以加热块温度90${^\circ}$C为上限，得出微通道蒸发器和冷凝 器在不同位差下的最大蒸发段热流密度. 通过对实验现象的观察及分析，以期开发出适用于 未来电子产品高功率需求的微型化电子冷却器.     

粗糙表面接触力学问题的重新分析

为了克服基于统计学参数的接触模型的尺度依赖性以及现有接触分形模型推导过程中初始轮廓表征受控于接触面积或取样长度的不足,基于粗糙表面轮廓分形维数$D$、尺度系数$G$ 和最大微凸体轮廓基底尺寸$l$,建立了新的粗糙表面接触分形模型,探讨了微凸体变形机制、粗糙表面的真实接触面积和接触载荷的关系,揭示了接触界面的孔隙率和真实接触面积随端面形貌、表面接触压力等参数变化的规律,给出了不同形貌界面被压实的最大变形量. 结果表明:微凸体变形从弹性变形开始,并随着平均接触压力$p_{\rm m}$ 的增大逐步向弹塑性变形和完全塑性变形转变;接触界面的初始孔隙率$\phi_{0}$ 随$D$ 的增大而增大,压实孔隙所需要的最大变形量$\delta $ 也随之增大;接触压力$p_{\rm c}$ 增大,孔隙率$\phi$ 减小,并随着$D$ 的增大和$G$ 减小,$\phi$ 快速减小,直至填实,变为零;$D$ 较小时,$G$ 的增大对真实接触面积的增大影响较小;$D$ 较大时,$G$ 的增大对真实接触面积的增大作用明显. 研究成果为端面摩擦副的润滑与密封设计提供了理论基础.      

黏弹性液滴热毛细迁移的对流不稳定

液滴在温度分布不均的固壁面上产生的热毛细迁移广泛存在于微流控、喷墨印刷等应用中, 对其流动进行稳定性分析对液滴迁移的精准控制具有重要意义. 本文采用线性稳定性理论研究了附壁黏弹性液滴在热毛细迁移中的对流不稳定性, 得到了不同Prandtl数($Pr$)下的临界Marangoni数($Ma_{\rm c})$与弹性数的函数关系, 并分析了临界模态的流场和能量机制. 研究发现: 流体弹性激发了更多不稳定模态, 小$Pr$的临界模态为斜波和流向波, 而中高$Pr$的临界模态为斜波和展向稳态模态. 强弹性使得$Ma_{\rm c}$显著下降, 而弱弹性略微增强了流动稳定性. 在中$Pr$下, $Ma_{\rm c}$随$Pr$的增大而增大. 对于斜波模态, 扰动温度的振幅可存在于流场中间区域, 而其他两种模态的温度振幅只存在于自由表面上, 并且在高$Pr$下的流线分布几乎是对称的. 能量分析表明: 随着弹性数增大, 基本流做功由正变负; 在小$Pr$中, 扰动应力做功既可能耗散能量又可能提供能量; 在高$Pr$中, 基本流做功可忽略不计. 对于同向流向波, 扰动速度和扰动应力做功在垂直方向上均存在多次振荡. 将液滴迁移与热毛细液层进行对比发现, 由于基本流和边界条件的不同, 两者在临界模态和扰动流场中均存在较大差异.      

Magnetic and Gravitational Convection of Air in a Porous Cubic Enclosure with a Coil Inclined Around the Y Axis

The natural convection heat transfer of air in a porous media can be controlled by gradient magnetic field. Thermomagnetic convection of air in a porous cubic enclosure with an electric coil inclined around the $Y$ axis was numerically investigated. The Biot–Savart law was used to calculate the magnetic field. The governing equations in primitive variables were discretized by the finite-volume method and solved by the SIMPLE algorithm. The flow and temperature fields for the air natural convection were presented and the mean Nusselt number on the hot wall was calculated and compared. The results show that both the magnetic force and coil inclination have significant effect on the flow field and heat transfer in a porous cubic enclosure, the natural convection heat transfer of air can be enhanced or controlled by applying gradient magnetic field.     

MHD natural convection in porous media-filled enclosures

In this work, the magnetohydrodynamics （MHD） natural convection heat transfer problem inside a porous medium filled with inclined rectangular enclosures is investigated numerically. The boundary conditions selected on the enclosure are two adiabatic and two isothermal walls. The governing equations, continuity, and Forchheimer extension of the Darcy law and energy are transformed into dimensionless forms by using a set of suitable variables, and then solved by using a finite difference scheme. The governing parameters are the magnetic influence number, the Darcy Rayleigh number, the inclination angle, and the aspect ratio of the enclosure. It is found that the magnetic influence number and the inclination angle have pronounced effects on the fluid flow and heat transfer in porous media-filled enclosures.     

Numerical investigation of natural convection inside an inclined parallel‐walled channel

Steady two‐dimensional natural convection in an inclined parallel‐walled channel was investigated numerically. The full elliptic forms of conservation equations were solved together and the velocity vectors, temperature contours and local and average Nusselt number distribution were obtained. The comparisons of local and average Nusselt number with published experimental and numerical results indicate very good agreement. Results are presented for a single aspect ratio, L/b=24, over the range of Rayleigh number of 3–1000 and angle of inclination 0–90°. The results indicate that the overall channel average Nusselt number is reduced as the inclination angle is increased. Significant reductions in the overall Nusselt number are exhibited at high angle of channel inclination. Copyright © 2005 John Wiley & Sons, Ltd.     

矩形倾斜腔体中的两种对流斑图和分区

为了研究矩形倾斜腔体中普朗特数Pr=0.72的流体对流斑图和分区,本文基于流体力学方程组进行了数值模拟。在相对瑞利数r=6.0的情况下,观察了倾角θ=10°和θ=60°时对流斑图随着时间的发展,发现系统存在单圈型对流和多圈型对流两种斑图。流线随着倾角的变化说明:随着倾角增加,对流圈数逐渐减少,对流波长逐渐增加,对流波数减小;然后,随着对流圈数显著地减少,系统由多圈型对流过渡到单圈型对流。根据模拟计算结果,给出了多圈型对流过渡到单圈型对流的临界倾角θc随着相对瑞利数r变化的关系曲线。对流在θ-r平面上分为两个区域:θ<θc时系统是单圈型对流,θ>θc时系统是多圈型对流。对流最大振幅A和努塞尔数Nu随着倾角θ的变化曲线被临界倾角θc分成两段,它们有不同的变化规律。因此,临界倾角也可以由对流最大振幅A或努塞尔数Nu的变化曲线来确定。     

Interaction of turbulent natural convection and surface thermal radiation in inclined square enclosures

Two-dimensional numerical studies of flow and temperature fields for turbulent natural convection and surface radiation in inclined differentially heated enclosures are performed. Investigations are carried out over a wide range of Rayleigh numbers from 10⁸ to 10¹², with the angle of inclination varying between 0° and 90°. Turbulence is modeled with a novel variant of the k–ε closure model. The predicted results are validated against experimental and numerical results reported in literature. The effect of the inclination of the enclosure on pure turbulent natural convection and the latter’s interaction with surface radiation are brought out. Profiles of turbulent kinetic energy and effective viscosity are studied to observe the net effect on the intensity of turbulence caused by the interaction of natural convection and surface radiation. The variations of local Nusselt number and average Nusselt number are presented for various inclination angles. Marked change in the convective Nusselt number is found with the orientation of enclosure. Also analyzed is the influence of change in emissivity on the flow and heat transfer. A correlation relevant to practical applications in the form of average Nusselt number, as a function of Rayleigh number, Ra, radiation convection parameter, N _RC and inclination angle of the enclosure, φ is proposed.     

Influence of inclination angle on buoyancy-driven convection in triangular enclosure filled with a fluid-saturated porous medium

The present investigation deals with the numerical analysis of steady-state laminar buoyancy-driven convection in an inclined triangular enclosure filled with fluid saturated porous media using the Darcy law equation. One wall of the enclosure is isothermally heated and the other is cooled, while the remaining wall is adiabatic. The effect of inclination angle on natural convection is investigated by varying the angle of inclination (φ) between 0° and 360°. The governing transformed equations are solved numerically using a finite-difference method. Obtained results are shown in the form of streamlines, isotherms, mean Nusselt numbers and dimensionless stream function for different values of the Rayleigh number Ra in the range 100 ≤ Ra ≤ 1,000. It is found that the values of the maximum and minimum mean Nusselt number are reached for φ = 330° and φ = 210° , respectively. However, the lowest flow strength is formed at φ = 240° for all values of Ra.     

MHD natural convection with Joule and viscous heating effects in iso-flux porous medium-filled enclosures

In this study, the magnetohydrodynamics （MHD） natural convection heat transfer with Joule and viscous heating effects inside an iso-flux porous medium-filled inclined rectangular enclosure is studied numerically. An iso-heat flux is applied for heating and cooling the two opposing walls of the enclosure while the other walls are adiabatic. The Forchheimer extension of Darcy-Oberbeck-Boussinesq and energy equations is transformed into a dimensionless form using a set of suitable variables instead of a finite difference scheme. The governing parameters are the magnetic influence number, the modified Rayleigh number, the inclination angle, and the aspect ratio of the enclosure. The results show that viscous and Joule heating effects decrease heat transfer rates.     

Natural convection in an inclined rectangular porous cavity with uniform heat flux from the side

The problem of natural convection in an inclined rectangular porous layer enclosure is studied numerically. The enclosure is heated from one side and cooled from the other by a constant heat flux while the two other walls are insulated. The effect of aspect ratio, inclination angle and Rayleigh number on heat transfer is studied. It is found that the enclosure orientation has a considerable effect on the heat transfer. The negative orientation sharply inhibits the convection and consequently the heat transfer and a positive orientation maximizes the energy transfer. The maximum temperature within the porous medium can be considerably higher than that induced by pure conduction when the cavity is negatively oriented. The peak of the average Nusselt number depends on the Rayleigh number and the aspect ratio. The heat transfer between the two thermally active boundaries is sensitive to the effect of aspect ratio. For an enclosure at high or low aspect ratio, the convection is considerably decreased and the heat transfer depends mainly on conduction.     

Heat transfer from a flat surface to an inclined impinging jet

An experimental study was carried out to investigate the effect of the inclination jet on convection heat transfer to horizontal flat plate. Local heat transfer determined as a function is of three parameters including inclination angle of the air jet relative to the plate in range of 90° ≤ θ ≤ 45°, jet-to-plate spacing in range of 2 ≤ L/D ≤ 8 and Reynolds number in range of 1,500 ≤ Re ≤ 30,000. The results show that the maximum heat transfer point moves towards the uphill side of the plate and the maximum heat transfer decreases as the inclination angle decreases. The correlations were conducted to predict maximum and local Nusselt number as a function of Re, θ, L/D, and x/D for a specific three regions.     

Fully developed mixed convection in horizontal and inclined tubes with uniform heat flux using nanofluid

Fully developed laminar mixed convection of a nanofluid consists of water and Al₂O₃ in horizontal and inclined tubes has been studied numerically. Three-dimensional elliptic governing equations have been solved to investigate the flow behaviors over a wide range of the Grashof and Reynolds numbers. Comparisons with previously published experimental and numerical works on mixed convection in a horizontal and inclined tube are performed and good agreements between the results are observed. Effects of nanoparticles concentration and tube inclinations on the hydrodynamics and thermal parameters are presented and discussed. It is shown that the nanoparticles concentration does not have significant effects on the hydrodynamics parameters. Heat transfer coefficient increases by 15% at 4 Vol.% Al₂O₃. Skin friction coefficient continually increases with the tube inclination, but the heat transfer coefficient reaches a maximum at the inclination angle of 45°.