不同雷诺数下方柱绕流的数值模拟

在不同雷诺数条件下,对流体绕经方形柱体的流动进行了数值模拟,计算雷诺数分别为100,1×103,1×104和2.2×104.当Re=100时,直接采用N-S方程进行计算;当Re=1×103,1×104和2.2×104时,则引入k-ε湍流模型进行计算.应用Galerkin有限元法对控制方程进行离散和求解,利用分离时间步长法处理控制方程中的非线性项.模拟计算得出了在不同雷诺数下的卡门涡街脱落形态.方柱后尾涡的形态会随雷诺数的变化而产生一定的变化.当雷诺数较低时,尾涡会拖得比较长,随着雷诺数的增加,尾涡长度会随之缩短.计算得到了方柱的受力系数和Strouhal数.将计算结果与文献上的实验和计算结果进行了比较,两者吻合较好.     

等离子抑制翼尖涡实验研究

该文对利用DBD等离子体注入能量抑制翼尖涡进行了风洞试验研究。通过采用PIV粒子成像测速技术测量了三种不同结构的等离子体激励器影响下的后掠翼下游的尾涡流场,并结合矩形机翼在等离子作用前后的气动力变化,来判断等离子体抑制翼尖涡的效果。试验结果表明:在机翼翼梢的上下表面处布置等离子体激励器,通过等离子体产生的定向诱导气流形成诱导涡对流场注入能量,可以有效地延缓翼梢上翼面流动分离,抑制翼尖涡,增加升力,提高升阻比。在翼尖处流动分离较小时,等离子体抑制翼尖涡强度的效果明显;在大攻角下仍具有一定的作用;其效果与等离子体发生器的结构有关。因此,通过优化等离子体激励器结构,选择合适的等离子体激励器在翼梢表面的安放位置,可以更好地提高抑制翼尖涡的效果。     

新型运输机机翼的颤振特性分析

应用高精度耦合计算方法,对带翼梢小翼和C型翼梢的运输机机翼颤振机理进行了研究.气动弹性模拟中,采用Euler方程描述非线性气动力,基于有限元方法获得结构响应.首先详细分析了基本机翼的颤振和极限环振荡(LCO),并与试验值进行比较.在此基础上进行带翼梢装置的机翼颤振特性计算,同时合理分离翼梢装置的质量和气动力作用.研究结...     

锂快离子导体Li1+2x+2yAlxMgyTi2-x-ySixP3-xO12系统的研究

以LiTi2(PO4)3为基以天然高岭石为起始原料,经高温固相反应(～900℃)制得了一系列新的锂快离子导体材料Li1+2x+ 2yAlxMgyTi2-x-ySixP3-xO1 2(以下简称Ti-Mg-Lisicon).系统的合成温度随x和y值的增大而降低.应用交流阻抗技术测定的电导率数据结果表明x=0.1,y=0.1的合成物的室温电导率最好为1.01×10-4S/cm,而400℃时x=0.1,y=0.3的合成物的电导率最大,为2.53×10-2S/cm.XRD分析结果表明在x=0.1,y≤0.8;x=0.2,y≤0.6的组成范围内均能得到空间群为R3c的合成物.     

血管切片的三维重建

本文讨论血管的三维重建问题.我们通过研究,证明了以下的定理.定理设C(i)是中轴线和平面Z=i的交点,那么存在以C(i)为中点且端点P1(i),P2(i)在ω(i)上的线段,并且在P1(i),P2(i)处ω(i)的切线相互平行.根据定理,我们找到利用求截面图象边界曲线的平行切线方法找到中轴线和100个截面的交点及管道的直径59.1238pixel.并用这100个交点的数据拟合出中轴线的方程:x(t)=-0.207806-0.610303t+0.206455t2-0.0144935t3+0.000517774t4-8.394241977754047×10-6t5+6.133353112035975×10-8t6-1.6673218267444805×10-10t7y(t)=158.211+1.86595t-0.266798t2+0.0141407t3-0.000325412t4+3.043275597680807×10-6t5-9.899171274615063×10-9t6z(t)=t然后我们用中轴线的方程重建了三维血管,并求出了重建血管在40个平面上的截面ω′(i)(30≤i≤69),并与原始截面ω(i)(30≤i≤69)进行比较,截面平均符合率高达96.8024%.     

计量测试技术集锦

19m三维测量机英国British航空公司生产的Air Bus(空中客车) A300系列飞机,其主翼过去均为人工组装式结构。为了缩短生产周期且提高机翼强度,该公司拟改变传统结构,采用无需组装的整体式结构,并用当今世界上最大的三维测量机来测量这种长18 m重935kg的零件。该测量机的最大测量范围为19000×1600×100mm,定位精度0.100×0.018×0.120mm,最小示值1μm(线性值)、1.5”(角度值);用于数据处理的32位Micro VAX2000,具有高速插补功能。 改进后的结构及其测量方法,便于修正制造误差,使空中客车生产的可靠性提高,生产成本降低。 编译自日刊《机…     

Magnetic Properties and Applications of Newly-developed Fe_(73.5)Cu_1Nb_(1.5)V_(1.5)Si_(13.5)B_9 Alloy

The synthetical soft magnetic properties including d.c. and a.c. magnetic properties and pulse magnetic property are reported for a newly-developed nanocrystalline Fe73.5Cu1 Nb1.5V1.5Si13.5B9 alloy The new alloy possesses high d.c. relative initial permeability of 12.5×104 and low coerciv ity of 0.54 A/m. Under the conditions of Bm=0.3 T, f=100 kHz and Bm=0.2 T, f=200 kHz the core losses of the new alloy are 543 kW·m-3 and 834 kW.m-3, respectively which can be compa rable with those of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy. The analyses of core losses have been carried out in the wider range of f=20～104 kHz and Bm=0.0025～0.8 T and the approxi mate expression P(kW·m-3)=1.803 B:f1.77 has been obtained. The analyses of core losses in the range of f=20～104 kHz and Bmf=(10～40)×103(T.Hz) have shown that the core loss and the corresponding amplitude permeability roughly vary as P = 2.347×10-6(Bmf)1.97f-0.2231 and μa = 9.56× 105f-0.7464, respectively for the given product B.f. Some practical applica tions have also been mentioned.     

(Zr0.7Sn0.3)TiO4陶瓷介电性能与工艺参数关系的回归分析

用回归分析方法得到了(Zr0.7Sn0.3)TiO4陶瓷的介电性能ε和δ与其工艺参数(CuO、ZnO)和玻璃的添加量(分别为x1、x2、x3 wt%)及预烧和烧结温度(分别为x4×103和x5×103℃)之间的定量关系ε＝10.9731-1.4559x1+9.9154x2+1.9776x3-3.3160x22-0.2286x23-200.1697x24-161.9102x25+375.1160x4x5;lg(tanδ)＝-38.5876-0.6452x2+0.1235x3+31.2221x4+30.3861x5+0.1100x21+0.2077x22-0.0106x23-27.4317x4x5.能对给定工艺参数下的介电性能进行预测,并能确定满足特定介电性能的工艺参数,有助于加快电子陶瓷材料的研究.     

垂直浸渍液相外延Hg1—xCdxTe的生长及性能

本文报导采用汞回流垂直浸渍液相外延方法,在CdTe或CdZnTe(111)面的衬底上生长Hg_(1-x)Cd_xTe单晶,其厚度为20μm,面积为1.5×2cm~2,组分x从0.18到0.7,组分均匀性Δx≈0.001。样品经热处理以后,x=0.2的n型样品电子浓度n≈1×10~(15)cm~(-3),电子迁移率μ≈10~5cm~2/v·s,p型样品空穴浓度p≈2×10~(16)cm~(-3),空穴迁移率μ≈300cm~2/v·s,双晶衍射显示样品的半峰宽为90arc sec,X光貌相分析表明外延层晶体结构优良。样品的红外光谱测量,电学参数测量以及载流子寿命测量表明样品具有优良的光电性质。     

KTN∶Cu晶体的铁电和光折变性能

用熔盐提拉法培育了掺 Cu~(2+)离子的 KTa_(1-x)Nb_xO_3(KTN∶Cu,x=0.32)晶体;在±160℃范围内测量了介电常数、介质损耗、自发极化强度和热释电系数;用 Ar 激光和二波混频法测量了室温下的光折变性能;扼要讨论了它的铁电相变特点和光折变性质.结果表明,该晶体的两个铁电-铁电相变为一级相变,而它的铁电相变已开始趋向于二级相变;室温立方相的衍射效率已高达52%,光折变灵敏度为3.2×10~(-6)cm~3/J,光折变动态范围Δn 为2.33×10~(-5),表现出良好的光折变应用前景.     

Typical flow between enclosed corotating disks and its dependence on Reynolds number

Abstract

The flow in an enclosed co‐rotating disk pair is investigated by Laser Doppler Velocimetry (LDV) measurements and flow visualizations. First, the typical flow structure at Re = 5.25 × 10⁵ and S = 0.09 is clarified. The flow fields in the r – θ and the r – z planes are both investigated and then divided into several flow regions based on the distinct flow types observed. The flow regions found in the two different planes are also compared and integrated. Second, with S fixed, the dependence of the flow field structure upon the Reynolds number is discussed. Three regimes of the r – θ plane flow with different Reynolds numbers are identified based on the measured mean velocity and spectral intensity. When Re < 1.6 × 10⁵, no solid body region is found and the flow is in a laminar regime. In the range 1.6 × 10⁵ ≤ Re ≤ 2.0 × 10⁶, the solid body region and the outer region vortices coexist, and an empirical equation is developed to estimate the number of vortices. When Re > 2.0 × 10⁶, the flow becomes turbulent. As Re increases from 9.3 × 104 to 5.25 × 10⁵, the spectral intensity initially increases and then decreases before increasing again to an even higher level, resulting in an increasing sawtooth pattern.     

Numerical simulation of flow over delta wing with trailing‐edge jet at high angle of attack

The incompressible Reynolds‐averaged Navier–Stokes equations, together with a modified mixing length algebraic turbulence model, are solved to simulate the flow over a delta wing with trailing‐edge jets at high angles of attack. An artificial compressibility method and a Beam–Warming implicit approximate factorization scheme are employed to discretize the equations. The computed results indicate that trailing‐edge jets tend to not only decrease the pressure but also increase the velocity at the cores of the streamwise primary vortices. This moves the vortex breakdown locations aft and stabilizes the vortical flow. As a result, the jets lead to a decreased pressure on the upper wing surface and an increased pressure on the lower wing surface, thereby increasing the overall lift. Computations further show that as the exit area of the trailing‐edge jets is enlarged, or, as the jets are deflected downward, the above‐mentioned effects become more pronounced. Copyright © 2004 John Wiley & Sons, Ltd.     

等离子体射流控制机翼气动力矩的实验研究

为考察火花放电等离子体射流控制机翼气动力矩的效果,在NACA0021平直机翼模型上安装火花放电等离子体射流发生器,通过改变射流发生器安装位置、射流角度及加载电参数,研究其控制机翼模型气动力矩的性能及机理。在NACA0021机翼模型近前缘处,布置2个火花放电等离子体射流发生器,采用气动力测量技术,在来流风速为20 m/s时测得,攻角-4°~10°时,滚转力矩系数最大减小了0.0024,攻角为12°~16°时,滚转力矩系数最大增加了0.0021;偏航力矩系数最大减小了0.00097。实验研究结果表明:等离子体射流可改变机翼模型横航向气动力矩,并可通过改变射流角度和加载电压频率调节等离子体射流控制横向气动力矩的效果。     

The effect of submergence ratio on flow pattern around short T-head spur dike in a mild bend with rigid bed using numerical model

Spur dikes are hydraulic structures frequently applied to protect rivers’ banks. Since flow pattern around a spur dike located in a rivers’ bend is particularly complicated due to the existence of secondary flow and helical flow, the effect of submergence ratio on flow patterns around a short T-head spur dike located in a 90° bend has been monitored numerically in this study. The numerical data are verified with experimental data obtained under non-submerged conditions, which indicate the numerical results are in accordance with experimental data. In this research, the numerical simulation based on various submergence ratios is done using FLOW-3D software to analyze. The results showed an increase in dimensions and the number of vortices associated with an increase in submergence ratio so that for the amount of 50% submergence, two vortices form around outer wall and an area downstream of the wing of spur dike, at bed level. Moreover, the length and width of separation zones were estimated to be 1.6 and 1.5 times larger compared to non-submerged condition, respectively.     

Power reduction and the radial limit of stall delay in revolving wings of different aspect ratio

Airplanes and helicopters use high aspect ratio wings to reduce the power required to fly, but must operate at low angle of attack to prevent flow separation and stall. Animals capable of slow sustained flight, such as hummingbirds, have low aspect ratio wings and flap their wings at high angle of attack without stalling. Instead, they generate an attached vortex along the leading edge of the wing that elevates lift. Previous studies have demonstrated that this vortex and high lift can be reproduced by revolving the animal wing at the same angle of attack. How do flapping and revolving animal wings delay stall and reduce power? It has been hypothesized that stall delay derives from having a short radial distance between the shoulder joint and wing tip, measured in chord lengths. This non-dimensional measure of wing length represents the relative magnitude of inertial forces versus rotational accelerations operating in the boundary layer of revolving and flapping wings. Here we show for a suite of aspect ratios, which represent both animal and aircraft wings, that the attachment of the leading edge vortex on a revolving wing is determined by wing aspect ratio, defined with respect to the centre of revolution. At high angle of attack, the vortex remains attached when the local radius is shorter than four chord lengths and separates outboard on higher aspect ratio wings. This radial stall limit explains why revolving high aspect ratio wings (of helicopters) require less power compared with low aspect ratio wings (of hummingbirds) at low angle of attack and vice versa at high angle of attack.     

Modeling Dense-Phase Pneumatic Conveying of Powders Using Suspension Density

This article presents results of an investigation into the modeling of pressure drop in horizontal straight pipe section for fluidized dense-phase pneumatic conveying of powders. Suspension density and superficial air velocity have been used to model pressure drop for two-phase solids-gas flow. Two applicable models formats (developed by other researchers using two different definitions of suspension density) were used to represent the pressure drop due to solids-gas flow through straight pipe sections. Models were generated based on the test data of conveying power-station fly ash and electrostatic precipitator (ESP) dust (median particle diameter: 30 and 7 µm; particle density: 2300 and 3637 kg m^?3; loose-poured bulk density: 700 and 610 kg m^?3, respectively) through a relatively short length of a smaller diameter pipeline. The developed models were evaluated for their scale-up accuracy and stability by using them to predict the total pipeline pressure drop (with appropriate bend model) for 69 mm I.D. × 168 m; 105 mm I.D. × 168 m and 69 mm I.D. × 554 m pipes and comparing the predicted versus with experimental data. Results show that both the models with suspension density and air velocity generally provide relatively better prediction compared to the conventional use of solids loading ratio and Froude number. For fly ash, the two formats result in considerable different predictions, whereas they provide relatively similar results for ESP dust.     

CFD study of the thermochemical characteristics of mesoscale bubbles in a BFB gasifier

Using the computational fluid dynamic model based on the multiphase particle-in-cell approach, reactive gas-solid flows in a three-dimensional bubbling fluidised bed gasifier regarding coal gasification is simulated. The predicted results of gas components are firstly validated with experimental measurements. Subsequently, the dynamics (e.g. chord length, aspect ratio, velocity, volume) together with the essential thermal features (e.g. temperature, pressure, density, viscosity, conductivity, specific heat capacity) of mesoscale bubbles are explored. Bubbles in the central part of the gasifier have a large lateral chord length and volume. The restriction effect of the gasifier wall elongates the vertical chord length and enlarges the aspect ratio of bubbles close to the wall. The density and pressure of bubbles decrease along with the bed height, and the thermochemical properties of bubbles are strongly related to bubble temperature. A large content of the CO₂ exists for those bubbles close to the bed surface. The gas velocity, operating pressure and gasifying agent obviously affect the bubble property, while the effects of inlet gas temperature and coal flow rate are relatively weak. The results obtained provide meaningful insight regarding the mesoscale structures involved in fluidised bed coal gasification.     

Investigation of the effect of a pylon and a wing with flaps on the flow within an exhaust jet of a double-flow turbojet engine by a simulation method for large eddies

The effect of a pylon and a wing with deflected flaps on the flow and turbulence characteristics in the exhaust jet of a double-flow turbojet engine (TJE) is investigated with the use of the combined Reynolds Averaged Navier-Stokes/Implicit Large Eddy Simulation (RANS/ILES) method with high resolution. In addition to assessment of the accuracy of the method and comparability, the flow in an axisymmetric nozzle with the same geometry and in its exhaust jet is calculated. All computations are carried out for the flow mode corresponding to the take-off. In the case of the axisymmetric nozzle, good agreement between the computation and the experiment on the distribution of the averaged longitudinal velocity and the turbulence energy in jet cross sections is observed. It is shown that the flap angle and the gap between their internal tips, which the exhaust jet of the TJE passes through, significantly affect the flow and the turbulence level in the jet. Because of the interaction between the jet and tip eddies coming out from the flaps, jet deformation occurs, and a heterogeneous azimuth appears in the distribution of the flow parameters in jet cross sections. The peak turbulence energy increases by a factor of 1.5–2 in comparison with the basic axisymmetric variant. Computations are carried out with meshes containing 2.224 × 10⁶ cells in the case of the axisymmetric nozzle and 3.34 × 10⁶ cells in the case of the nozzle with the pylon and the wing.