渐缩平板间的缓变流(Ⅱ):C_(EB)的应用

本文用伯努利方程能量系数CEB判别了斜平面间渐缩流动的缓变流.对于用平均速度的动能和中点位置的压能构成的CEB,采用1%的相对偏差判据得到8.16?的缓变流临界角.考虑到截面平均压强和中点压强的差异以及计算中的少量误差,文中以平均速度引起的能量偏差为基础,用另外两种方式得到的缓变流临界角也在8?左右.这和前文用静力学方程能量系数得到的5.455?缓变流临界角略有不同.对于用平均速度的动能和截面平均压能构成的CEB,进行了动能校正以及动能校正、压能校正、和位能校正组合的校正.得到仅动能校正和三种校正同时考虑的结果接近理论值.如果以常用的忽略动能校正的做法作为缓变流的判别依据,20?的倾角也可以看作渐缩平板间缓变流的一个临界角.这个倾角实际上更直观地表现了常用的忽略动能校正所引起的偏差.     

板壳式换热器波纹倾角对换热和阻力性能影响

研究了波纹倾角（α=45°,60°,75°）对板壳式换热器单流道内单相流动与换热过程的影响,分析了波纹流道内的速度分布、湍动能分布、压力分布和温度分布;基于范宁摩擦阻力因子f和Nu与Re的关系,提出了板壳式换热器不同波纹倾角下换热特性和流动阻力特性的预测关联式;通过计算不同波纹倾角下的综合性能评价指标（performance evaluation criteria,PEC）和面积质量因子（j/f）综合评价了流动与换热性能。结果表明：波纹倾角是影响圆形板间流体流动形态的因素之一,随着波纹倾角增大,会出现“十字交叉流”向“曲折流”转变;所提出的关联式能够很好地预测板壳式换热器内阻力与换热性能,阻力特性偏差在±14%范围内,换热特性偏差在±7%范围内;α为45° 时,j/f较大,流道内阻力相对小;α为60° 时,PEC较大,流道内换热性能相对强。

     

屋面光伏板风荷载特性数值分析

风荷载在屋面光伏阵列结构体系设计中起控制作用。采用计算风工程的方法分析讨论了屋面光伏板的风荷载特性。数值算法采用分离涡模拟方法。数值计算结果与现有风洞实验数据的比较,验证了本文方法的正确性。考虑影响光伏板风荷载的因素主要有光伏板在屋面上的安装位置、安装倾角、光伏阵列之间的距离和风向等。计算结果表明,屋面处脱落的涡对安装在不同位置的光伏阵列风荷载的影响较明显。当倾角由15°增加到45°时,电池板受到的风荷载随着倾角的增加而增大。在一定阵列间距范围内,光伏板风荷载主要表现为前排对下游光伏板的遮挡影响。本文方法与结果能为屋面光伏建筑结构设计提供重要参考。     

同种材料球板斜碰带电量研究

报道了同种材料球板斜碰带电量实验和理论研究, 发现同种材料球和板斜碰后球板会带电,带电量的极性与材料有关,带电量的大小与板的倾角有关,随着倾角的增大先增加后减小,当倾角约50°时,碰撞电量达到最大,是正碰撞电量的6~8 倍. 通过球板斜碰过程的动力学分析,发现球板碰撞后球的带电量与碰撞过程中球与板的摩擦面积成正相关.     

岩桥拉张破裂数值模拟研究

为了研究岩桥拉张破坏规律, 采用有限元软件FEPG,对不同倾角岩桥拉张破裂过程进行数值模拟. 模拟结果显示, 在单轴压缩条件下, 对于同一裂纹角度、相等距离的两组平行预裂纹, 在不同的岩桥倾角条件下, 预裂纹扩展程度不同; 岩桥倾角越大, 岩桥越容易拉张破坏.     

SHPB试验中岩石试件的端面不平行修正

为研究短圆柱体岩石试件端面不平行对岩石动力学特性测试结果的影响,采用有限元分析软件LS-DYNA对9种端面不平行度和5种杨氏模量的岩石试件开展SHPB(split Hopkinson pressure bar)试验数值模拟,对岩石选用HJC(Holmquist-Johnson-Cook)本构模型。数值模拟结果表明,当端面不平行度在0.40%以内时,端面不平行对动态应力测试结果的影响可忽略不计;但对动态应变测试结果的影响较大。当杨氏模量一定时,平均应变率测试误差和峰值应变测试误差随端面不平行度增大呈线性增大;当端面不平行度一定时,平均应变率测试误差和峰值应变测试误差随杨氏模量增大也呈线性增大。对数值模拟得到的平均应变率测试误差和峰值应变测试误差实施二元线性回归分析,提出了SHPB试验中端面不平行岩石试件平均应变率和峰值应变的修正公式。     

弹体侵彻混凝土的临界跳弹

为了保证钻地战斗部打击防护层目标时不发生跳弹,需要对弹体侵彻目标的临界跳弹角度进行分析和估算。开展了一定大长径比弹体斜侵彻混凝土的跳弹实验,分析了在250~430 m/s速度下弹体侵彻30和60 MPa钢筋混凝土的临界跳弹角度,给出了弹体临界跳弹角度包络线。当靶板强度相同时,随着侵彻速度的增加,弹体的临界跳弹倾角增大,增大的趋势逐渐变缓;在相同侵彻速度下,随着靶板强度的增加,弹体的临界跳弹倾角减小;经验公式分析得到的弹体临界跳弹倾角偏低于实验,但偏差基本在3°以内。

     

两共面预制裂纹试样应力场特征与破坏模式研究

破坏模式决定着裂隙岩体的基本力学性质,通过对比分析典型两条共面裂纹的节理裂隙岩体试样的模型试验、模拟试验结果,系统地研究了预制裂纹倾角、侧压以及裂纹面摩擦系数等因素对试样应力场、破坏模式以及峰值强度的影响;进一步解释了不同裂隙分布条件下张拉次生裂纹、剪切次生裂纹的起裂、发展以及贯通路径与机理,系统地总结了两条共面裂纹试样贯通模式的演化规律。结果表明:随预制裂纹倾角从0°增大到90°,两共面裂纹试样第一主应力最大值呈现先增大而后减小的规律,倾角较小或较大时试样最大拉应力相对较小,而倾角为20°～70°时试样最大拉应力较大且45°～65°间达到最大;剪应力极值线绕预制裂纹尖端旋转,次生剪切裂纹起裂部位、发展方向均发生变化,接近0°和90°时试样内部最大剪应力值最小,倾角为20°～70°时试样最大剪应力相对较大且30°时剪应力达到最大;裂纹倾角较小时试样破坏路径由两预制裂纹中的一条与其次生裂纹贯通形成破坏路径,倾角较大时破坏路径通过两条预制裂纹以及其次生裂纹贯通形成;试样峰值强度变化曲线呈浅底"V"字形,倾角为0°、90°时峰值强度大体相同,倾角为45°时峰值强度达到最小且约为0°时峰值强度的40%～50%。     

涡轮增压器离心压气机非稳态工况流场分析

利用商用 CFD 软件对一小型车用离心压气机建立了数值模型,并将模拟结果与实验结果进行了对比:稳态的设计转速最高压比相差不超过 0.5%,最高效率相差不超过1.5%;非稳态模拟和实验得到的失速频率均为 3000Hz,模拟结果真实可信.主要利用设计转速下小流量工况时的非稳态数值模拟结果对喘振发生前离心压气机各部件的非稳态流动特点进行了详尽阐述.研究结果表明:小流量工况时离心压气机各部件均出现非稳态流动现象,这种非稳态效应在各部件中表现出不同的特点,且随着流量的减小这种非稳态效应会不断加剧.     

孔板空调风口送风射流的数值模拟

介绍Ｎ点风口模型用于数值模拟室内空气流动时描述孔板类送风口的入流边界条件．然后采用该风口模型对不同的孔板风口出流条件算例进行数值计算，并就轴心速度衰减、射流扩展角以及断面流速分布等射流特性与实验数据进行了对比．比较结果表明，Ｎ点风口模型用于描述数值模拟室内空气流动的孔板类风口入流边界条件，可以获得工程上足够满意的结果．     

Bagley correction: the effect of contraction angle and its prediction

The excess pressure losses due to end effects (mainly entrance) in the capillary flow of a branched polypropylene melt were studied both experimentally and theoretically. These losses were first determined experimentally as a function of the contraction angle ranging from 10° to 150°. It was found that the excess pressure loss function decreases for the same apparent shear rate with increasing contraction angle from 10° to about 45°, and consequently slightly increases from 45° up to contraction angles of 150°. Numerical simulations using a multimode K-BKZ viscoelastic and a purely viscous (Cross) model were used to predict the end pressures. It was found that the numerical predictions do agree well with the experimental results for small contraction angles up to 30°. However, the numerical simulations under-predict the end pressure for larger contraction angles. The effects of viscoelasticity, shear, and elongation on the numerical predictions are also assessed in detail. Shear is the dominant factor controlling the overall pressure drop in flows through small contraction angles. Elongation becomes important at higher contraction angles (greater than 45°). It is demonstrated in abrupt contractions (angle of 180°) that both the entrance pressure loss and the vortex size are strongly dependent on the extensional viscosity for this branched polymer. It is suggested that such an experiment (visualisation of entrance flow) can be useful in evaluating the validity of constitutive equations and it can also be used to fitting parameters of rheological models that control the elongational viscosity.     

An experimental study on jets issuing from elliptic inclined nozzles

This paper reports on an experimental flow visualisation and digital particle image velocimetry investigation on forced jets exhausting from aspect ratio equal to three elliptic nozzles with exits inclined at 30° and 60°. Flow images show that shear layer instabilities and subsequent vortex roll-ups are formed parallel to the inclined nozzle exits at 30° incline and that rapid re-orientation of the vortex roll-ups occurs at 60° incline. Flow observations also show that strong axis-switching occurs in a non-inclined elliptic nozzle. However, 30° and 60° elliptic inclined nozzles produce significant distortions to and suppression of the axis-switching behaviour, respectively. As a result, flow stresses and turbulent kinetic energy distributions become increasingly asymmetric. Their coherency and magnitudes along the shorter nozzle lengths also vary significantly. This can be attributed to the dissimilar formations of vortex roll-ups and rib structures, as well as unequal mutual interactions between them as the incline-angle increases. Lastly, results also show that unlike circular inclined nozzles, elliptic inclined nozzles do not produce serpentine-shaped jet columns nor lead to significant lateral jet-spread at large incline-angles.     

Squeezing flow of a highly viscous incompressible liquid pressed between slightly inclined lubricated wide plates

The theoretical force-height relationships of Newtonian and pseudo plastic liquids compressed between slightly tilted frictionless plates are compared with those produced when the plates are perfectly parallel. It is shown that a very small inclination angle can distort the flow curve to such an extent that a Newtonian liquid will appear as a pseudo plastic fluid, and a pseudo plastic liquid as having a flow index considerably smaller than its true one. The shape of the biaxial elongational viscosity vs biaxial strain rate relationship is also highly sensitive to the plates' inclination angle. Thus, if an experimental force-height relationship is used to determine a material's biaxial elongational viscosity, an unsuspected slight tilt will result in a considerable underestimate of the viscosity. A slight tilt will also produce an apparent strain rate dependency in a Newtonian liquid, which obviously does not exist. The mathematical model developed to reach these conclusions was tested with commercial mayonnaise, a self-lubricating fluid. A reasonable agreement was found between the predicted force-height relationships and those experimentally determined at tilts of 1°, 3°, and 5°. Received: 4 August 2000 Accepted: 21 August 2000     

Some observations in the vortex-turning behaviour of noncircular inclined jets

An experimental comparison was conducted for Re = 2,500, free elliptic and rectangular jets inclined at 30° and 60° along major or minor planes. Regardless of the jet base geometry, minor-plane inclined jets produced vortex roll-ups that remain inclined. In contrast, major-plane inclined jets produce significantly stronger vortex-roll-up turning behaviour. Interestingly, major-plane inclined rectangular jets exhibit strong vortex-overturning behaviour, where the vortex-roll-up inclination exceeds the 0° incline angle considerably. Vortex-turning extents and rates are compared between major-plane inclined elliptic and rectangular jets here and support present qualitative observations. Closer inspections reveal that the lack of axis-switching phenomenon in major-plane inclined rectangular jets allows vortex-overturning behaviour. In addition, jet centreline deflection is most sensitive in minor-plane inclined jets, where increasing the incline angle leads to a decrease and an increase in the elliptic and rectangular jet deflection, respectively.     

Three-dimensional numerical investigation of heat transfer for plate fin heat exchangers

In the present study, the potential of rectangular fins with 30° and 90° angle and 10 mm offset from the horizontal direction for heat transfer enhancement in a plate fin heat exchanger is numerically evaluated with conjugated heat transfer approach. The rectangular fins are mounted on the flat plate channel. The numerical computations are performed by solving a steady, three-dimensional Navier–Stokes equation and an energy equation by using Fluent software program. Air is taken as working fluid. The study is carried out at Re = 400 and inlet temperatures, velocities of cold and hot air are fixed as 300, 600 K and 1.338, 0.69 m/s, respectively. Colburn factor j versus Re design data is presented by using Fluent. The results show that the heat transfer is increased by 10 % at the exit of channel with fin angle of 30° when compared to channel without fin for counter flow. The heat transfer enhancement with fins of 30° and 90° for different values of Reynolds number with 300, 500 and 800 and for varying fin heights, fin intervals and also temperature distributions of fluids on the top and bottom surface of the channel are investigated for parallel and counter flow.     

带导管鱼雷层流与湍流绕流计算

用分块耦合求解方法和自编三维软件计算了带导管鱼雷整体流场无冲角和有冲角的绕流问题,湍流计算采用Baldwin-Lomax湍流模型.计算雷诺数从103到107.计算冲角0～20°.在各种雷诺数计算,头部滞止压力（无黏流理论值为1.5）误差在10％范围内,总流量误差在5％范围内.对于不同冲角计算表明,升力系数在冲角15°范围内基本上线性增加,超过15°后升力系数缓慢减少.而摩擦阻力系数几乎与冲角无关,但压差阻力随冲角的增加而迅速增长.     

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.     

Non-symmetric bi-stable flow around the Ahmed body

The flow around the Ahmed body at varying Reynolds numbers under yawing conditions is investigated experimentally. The body geometry belongs to a regime subject to spanwise flow instability identified in symmetric flow by Cadot and co-workers (Grandemange et al., 2013b). Our experiments cover the two slant angles 25° and 35° and Reynolds numbers up to 2.784 × 10⁶. Special emphasis lies on the aerodynamics under side wind influence. For the 35° slant angle, forces and moments change significantly with the yawing angle in the range 10° ≤ |β| ≤ 15°. The lift and the pitching moment exhibit strong fluctuations due to bi-stable flow around a critical angle β of ±12.5°, where the pitching moment changes sign. Time series of the forces and moments are studied and explained by PIV measurements in the flow field near the rear of the body.     

Squeeze flow of a power-law fluid between non-parallel plates

Squeeze flow in the gap between non-parallel circular plates of radius R is discussed. The test material is assumed to be a power-law fluid, with a no-slip boundary condition at the plates. If the mean separation between the plates is h, and the angle of inclination between the plates is ? ? h/R, the force on the plates is perturbed only at O(?²) and is increased by less than 10% if ? < 0.35h/R. A torque O(?) tends to return the plates to a parallel configuration.     

Ecoulement visqueux entre deux cones coaxiaux

A numerical method of resolution of laminar incompressible flows in cones of revolution is proposed by asymptotic expansions in powers of 1/r (r radius vector). Remarks on linearity allow to calculate all wanted terms, function after function, by fourth-order Runge-Kutta process. Two examples are selected: the flow between two symmetric cones and one between a cone and a plane. The study of the flow between two symmetric cones as a function of the aperture angle reveals the existence of two patterns separated by a discontinuity at approximately 156°.