EXPERIMENTAL INVESTIGATIONS OF CAVITATION EFFECTS ON FLOW CHARACTERISTICS OF SMALL ORIFICES AND VALVES IN WATER HYDRAULICS

The flow characteristics and cavitation effects of water passing throughsmall sharp-edged cylindrical orifices and valves of different shapes in water hydraulics areinvestigated. The test results using orifices with different aspect ratios and different diametersshow that the flow coefficients in the case of non-cavitating flow are larger than that of flow inthe case of cavitation occurrence. The flow coefficients of flow with cavitation initially decreaseas Reynolds number increases and ultimately tend to be of constant values close to contractioncoefficient. Large aspect ratio has an effect of suppressing cavitation. The experimental resultsabout disc valves illustrate that the valves with sharp edge at large opening are less affected bycavitation than that at small opening. Throttle with triangle notch has better anti-cavitationability than that with square notch. The flowrate of the throttle with square notch is significantlyaffected by the flow direction or the flow passage shape.

Sedimentation motion of sand particles in moving water(Ⅰ):The resistance on a small sphere moving in non-uniform flow

In hydraulics,when we deal with the problem of sand particles moving relative to the surrounding water,Stokes'formula of resistance has usually been used to render the velocity of sedimentation of the particles.But such an approach has not been proved rigorously,and its accuracy must be carefully considered.In this paper,we discuss the problem of a sphere moving in a non-uniform flow field,on the basis of the fundamental theory of hydrodynamics.We introduce two assumptions:i)the diameter of the sphere is much smaller than the linear dimension of the flow field,and ii)the velocity of the sphere relative to the surrounding water is very small.Using these two assumptions,we solve the linearized Navier-Stokes equations and equations of continuity by the method of Laplace transform,and finally we obtain a formula for the resistance acting on a sphere moving in a non-uniform flow field.

A Higher-Efficient Three-Dimensional Numerical Model for Small Amplitude Free Surface Flows

A higher-efficient three-dimensional non-hydrostatic model is developed to simulate small amplitude free surface flows based on a staggered unstructured grid. In this model, a fractional step algorithm is adopted to solve the Navier-Stokes equations in two major steps. A top-layer pressure method is proposed to minimize the number of vertical layers and subsequently the computational cost. Three classical examples of small amplitude free surface flows are used to demonstrate the capability and efficiency of the model. The satisfactory results demonstrated the capability and efficiency of modelling a range of small amplitude free surface flows with only a small number of vertical layers.

A New Efficient Finite Volume Modeling of Small Amplitude Free Surface Flows with Unstructured Grid

A staggered finite-volume technique for non-hydrostatic, small amplitude free surface flow governed by the incompressible Navier-Stokes equations is presented there is a proper balance between accuracy and computing time. The advection and horizontal diffusion terms in the momentum equation are discretized by an integral interpolation method on the orthogonal unstructured staggered mesh and, while it has the attractive property of being conservative. The pressure-correction algorithm is employed for the non-hydrostatic pressure in order to achieve second-order temporal accuracy. A conservative scalar transport algorithm is also applied to discretize k - c equations in this model. The eddy viscosity is calculated from the k-c turbulent model. The resulting model is mass and momentum conservative. The model is verified by two examples to simulate unsteady small amplitude free surface flows where non-hydrostatic pressures have a considerable effect on the velocity field, and then applied to simulate the tidal flow in the Bohai Sea.

Examining the model dependence of the determination of kinetic freeze-out temperature and transverse flow velocity in small collision system

The transverse momentum distributions of the identified particles produced in small collision systems at the Relativistic Heavy Ion Collider(RHIC) and Large Hadron Collider(LHC) have been analyzed by four models. The first two models utilize the blast-wave model with different statistics. The last two models employ certain linear correspondences based on different distributions.The four models describe the experimental data measured by the Pioneering High Energy Nuclear Interaction eXperiment, Solenoidal Tracker at RHIC, and A Large Ion Collider Experiment collaborations equally well. It is found that both the kinetic freeze-out temperature and transverse flow velocity in the central collisions are comparable with those in the peripheral collisions. With the increase of collision energy from that of the RHIC to that of the LHC,the considered quantities typically do not decrease. Comparing with the central collisions, the proton–proton collisions are closer to the peripheral collisions.

A numerical simulation of unsteady flow in small diameter helical grafts

The application of small diameter arterial grafts is limited due to the fact of relatively poor long-time patency which is caused by thrombosis formation in the short term and intimal hyperplasia(IH) in the medium and long term.Thrombosis,obstructing the flow of blood

Numerical Study on Internal Flow of Small Axial Flow Fan with Splitter Blades

The splitter blades are widely used in axial compressors and play an active role in the improvement of the overall performance of compressors. However, little research on the application of splitter blades to small axial flow fans is conducted. This paper designs a splitter blade small axial flow fan (model B) with a small axial flow fan as the prototype fan (model A) by adding short blades at the second half part of the passageway among long blades of model A. The steady simulation for the two models was conducted with the help of RNG k-ε turbulence model provided by software Fluent, and static characteristics and internal flow characteristics of the two models were compared and analyzed. Results show that splitter blades can improve the unsteady flow in the small flow rate region and also have a positive role to increase static pressure rise and efficiency in the higher flow rate region. The variation of static pressure gradient on the meridian plane in model B is well-distributed. The static pressure on the blade surface of model B distributes more uniformly. Splitter blades can suppress the secondary flow from pressure side to suction side in the leading edge because the pressure difference between suction side and pressure side in model B is generally lower than that of model A. And it also can restrain the vortex shedding and flow separation, and further it may be able to get the aerodynamic noise lower because static pressure gradient on the blade surface is well-distributed and the vortex shedding is not developed. Therefore, the performance of the fan with splitter blades is better than that of the prototype fan. The findings of this paper can be a basis for the design of high performance small axial flow fans.

Portal inflow preservation during portal diversion in small-for-size syndrome

AIM:To investigate the impact of portal inflow on liver remnants in a stable pig model of small-for-size syndrome.METHODS:Twenty pigs underwent mesocaval shunt(MCS)surgery followed by 85%-90%hepatectomy.The control group had no shunt placement;the S1group had portal flow maintained at an average of 2.0times the baseline values;and the S2group had portal flow maintained at an average of 3.2 times the baseline flow.The effect of portal functional competition on the liver remnant was investigated for 48 h postoperatively.Data were presented as mean±SD.Statistical significance was determined using Student’s t test(SPSS,Chicago,IL,United States).Values of P<0.05 were considered statistically significant.RESULTS:At 24 h after hepatectomy,biochemical and histological changes were not significantly different between the S1and S2groups,but changes in both sets of variables were significantly less than in the control group.At 48 h,biochemical and histological changes were significantly less in the S2group than in the S1or control group.The regeneration index was significantly higher in the S2group than in the S1group,and was similar to that in the control group.Apoptosis index,serum lipopolysaccharide,and bacterial DNA levels were significantly lower in the S2group than in the other two groups.CONCLUSION:Diversion of portal inflow using MCS reduces portal overflow injury.Excessive diversion of portal inflow inhibits liver regeneration following major hepatectomy.Maintaining portal inflow at an average of 3.2 times above baseline helps promote hypertrophy of the liver remnant and reduce apoptosis.

Effect of swirling flow on platelet concentration distribution in small-caliber artificial grafts and end-to-end anastomoses

Platelet concentration near the blood vessel wall is one of the major factors in the adhesion of platelets to the wall.In our previous studies,it was found that swirling flows could suppress platelet adhesion in small-caliber artificial grafts and end-to-end anastomoses.In order to better understand the beneficial effect of the swirling flow,we numerically analyzed the near-wall concentration distribution of platelets in a straight tube and a sudden tubular expansion tube under both swirling flow and normal flow conditions.The numerical models were created based on our previous experimental studies.The simulation results revealed that when compared with the normal flow,the swirling flow could significantly reduce the near-wall concentration of platelets in both the straight tube and the expansion tube.The present numerical study therefore indicates that the reduction in platelet adhesion under swirling flow conditions in small-caliber arterial grafts,or in end-to-end anastomoses as observed in our previous experimental study,was possibly through a mechanism of platelet transport,in which the swirling flow reduced the near-wall concentration of platelets.

Vibrational shear flow of anisotropic viscoelastic fluid with small amplitudes

Using the constitutive equation of co-rotational derivative type for anisotropic viscoelastic fluid-liquid crystalline(LC),polymer liquids was developed.Two relaxation times are introduced in the equation:λn represents relaxation of the normal-symmetric stress components;λs represents relaxation of the shear-unsymmetric stress components.A vibrational rotating flow in gap between cylinders with small amplitudes is studied for the anisotropic viscoelastic fluid-liquid crystalline polymer.The time-dependent constitutive equation are linearized with respect to parameter of small amplitude.For the normal-symmetric part of stress tensor analytical expression of the shear stress is obtained by the constitutive equation.The complex viscosity,complex shear modulus,dynamic and imaginary viscosities,storage modulus and loss modulus are obtained for the normal-symmetric stress case which are defined by the common shear rate.For the shear-unsymmetric stress part,two shear stresses are obtained thus two complex viscosities and two complex shear modulus(i.e.first and second one) are given by the constitutive equation which are defined by rotating shear rate introduced by author.The dynamic and imaginary viscosities,storage modulus and loss modulus are given for each complex viscosities and complex shear modulus.Using the constituive equation the rotating flow with small amplitudes in gap between two coaxial cylinders is studied.

竖直互连小通道内流动沸腾传热强化机理分析

为推动流动沸腾技术在燃料电池散热领域的应用,该文以R141b为工质,对不同流速下的并联直通道及互连小通道内流动沸腾情况进行三维瞬态数值模拟。利用换热系数、基底温度及综合换热因子等参数对比各结构的换热性能,并探究互连小通道的换热强化机理。研究表明:换热趋势随气泡行为呈周期性变化,气泡脱离时的换热效果最佳;由连通口引发的二次流促进热量传递与壁温升高,使通道可提前2 ms满足成核条件,且气泡在10^(-4) m/s以上的二次流速作用下率先脱离;五连通通道的换热系数较并联直通道可提升19.6%~23.3%,继续增加连通口数,换热系数的相对增长率均不足3.5%;涡结构使压降随连通口数增加而增大,且增势并不减弱,八连通通道的压降较并联直通道增加29.5%~42%。可见,二次流在影响气泡行为、强化换热的同时也带来压力损失,综合换热性能与连通口数目不成正相关。

叶顶间隙对低比转数小流量泵性能的影响

为建立叶顶间隙对低比转速小流量泵性能的影响关系,采用数值计算的方法,以叶顶间隙宽s和叶轮出口宽b的比值(s/b)表征间隙大小,对不同间隙下泵的性能展开对比分析。分析发现,随着间隙的增加,泵的扬程和效率均逐渐降低,最大和最小间隙下的扬程和效率差分别达15.4%,5.5%。当间隙较小即s/b<0.2时,扬程曲线在小流量工况会出现不稳定的“驼峰”现象,在s/b>0.3后,设计工况附近效率的降幅显著增大。从流动特性来看,随着叶顶间隙的增大,间隙泄漏流对叶轮内主流区影响加剧,会诱导严重的二次流和旋涡的产生,同时间隙内不稳定流动也会产生能量损失,造成扬程和效率下降。为低比转数小流量泵的设计与优化提供借鉴意义。

Numerical Investigation on Vortex Structure and Aerodynamic Noise Performance of Small Axial Flow Fan

The details of unsteady flow field in small axial fans are described and the relationship between the internal flow char- acteristics and aerodynamic noise of small axial flow fans are explored in the manuscript. Firstly, the broadband noise model is introduced to calculate the distributions of broadband noise sources in fan’s internal flow field, and further fan’s internal flow characteristics affecting broadband noise sources are analyzed by the main distributions of broad- band noise sources. Secondly, the unsteady characteristics of vortex structure in fan’s internal flow field are analyzed by large eddy simulation, and FH-W acoustic model is introduced to calculate aerodynamic noise affected by the unsteady characteristics of vortex structure. Finally, Monitoring points are set up near and far field of small axial fans, at which sound pressure level and spectral characteristics are analyzed. The results show that broadband noise sources are mainly distributed at the tip clearance close to blade trailing edge and one third of chord length of blade trailing edge of small axial fans. The maximum sound power of broadband noise sources at the tip clearance is greater than that at blade trail- ing edge. Sound power level of broadband noise near one third of blade chord length of blade trailing edge is first in- creasing and then decreasing when the distance between radial planes and the center of fan hub increases. Fan’s internal flow characteristics affecting broadband noise sources are the tip leakage vortex and the trailing edge vortex shedding. The tip leakage vortices at the leading edge of blade tip first integrate and then break down, while vortex cores of the trailing edge vortex shedding gradually move from blade hub to one-third of blade chord length and then move to far field within a one-seventh of the rotation cycle. Within a flow passage of fan rotation period, sound pressure level of the monitoring points is relevant to the unsteady feature of vortex structure and the main bands of aerodynamic noise of monitor points are irrelevant to the unsteady feature of vortex structure, relating to the distance between the monitor points and fan. With the above distance increasing, the main bands of aerodynamic noise are widened and moved from the low bands to high bands. All conclusions in the paper will provide significant references for reducing the noise of small axial flow fans.

暖通空调小流量风机机械振动信号自适应采集研究

当暖通空调小流量风机运行时,频繁产生的脉冲噪声增加了信号采集的复杂性并引入了干扰,使得采集到的振动信号含有大量不必要的噪声成分,降低了采集精度和准确性。为此,提出暖通空调小流量风机机械振动信号自适应采集方法。使用经验模态分解方法对振动信号进行处理,获得信号在时域和频域上的特征信息。根据振动信号的频率特征,设计自适应变采样算法在不同的时间段内动态地调整采样率,采集风机振动信号。使用数学形态滤波器调整信号的形状,去除脉冲干扰以提高采集信号的精度,并进一步优化信号的质量。实验结果表明,所提方法的振动信号自适应采集精度高,且采集时间短。

城镇燃气管道小孔泄漏流量系数的定量研究

流量系数是影响泄漏速率计算准确程度的一个重要参数。针对目前小孔泄漏模型中流量系数经验性取值易导致泄漏速率计算结果与实际泄漏速率间存在偏差的问题,通过理论计算模型、气相小孔泄漏试验、计算流体力学(Computational Fluid Dynamics,CFD)模拟仿真相结合的方法寻求偏差产生的原因,并利用试验测试数据的拟合分析结果得到圆孔和矩形孔流量系数的定量计算模型。研究显示:在试验测试与模拟仿真中,小孔泄漏的流量系数主要受泄漏压力和几何形状的影响,在相同的泄漏压力和泄漏面积下,矩形孔的泄漏速率高于圆孔的泄漏速率。理论计算模型忽略了相关因素对流量系数的影响,将流量系数设为定值是导致泄漏速率计算结果出现偏差的主要原因。通过非线性曲线拟合得到的圆孔和矩形孔流量系数的计算方程,能有效修正气相小孔泄漏的理论计算模型,进而得到更加准确的泄漏速率,为气体泄漏后的扩散和风险评估提供理论依据。

Performance and Internal Flow of Contra-Rotating Small-Sized Cooling Fan

High pressure and large flow rate small-sized cooling fans are used for servers in data centers and there is a strong demand to increase its performance because of increase of quantity of heat from servers. Contra-rotating rotors have been adopted for some of high pressure and large flow rate cooling fans to meet the demand. The performance curve of the contra-rotating small-sized cooling fan with 40 mm square casing was investigated by an experimental apparatus and its internal flow condition was clarified by the numerical analysis. The fan static pressure of the front rotor was extremely low and it increased significantly at the rear rotor. The uniform flow was achieved at the inlet of the rear rotor because of the special shape of the casing between the front and rear rotors. On the other hand, the tip leakage flow was large enough to influence on the main flow of the test cooling fan by the design specification of high pressure with compact rotor diameter.

正常胎儿脐静脉血流量Z评分模型的构建及其对小于胎龄儿围生期不良结局的预测价值

目的构建正常胎儿脐静脉血流量(Quv)Z评分模型,并探讨该模型对小于胎龄儿(SGA)围生期不良结局的预测价值。方法(1)纳入862例18~40周正常胎儿建立模型。以腹部超声测量的腹内段Quv作为因变量,孕周为自变量,采用回归分析获得Quv均数及标准差的最佳拟合方程后,根据公式[Z评分=(实际Quv测值-拟合Quv均值)/拟合Quv标准差]建立正常胎儿Quv Z评分模型。(2)另纳入53例正常胎儿(正常组)和109例SGA,并根据围生期有无不良结局将SGA分为不良结局组(n=50)、无不良结局组(n=59)。利用所建立的模型获得3组的胎儿Quv Z评分。通过绘制受试者工作特征曲线分析正常胎儿Quv Z评分预测SGA围生期不良结局的效能。结果(1)正常胎儿的Quv与孕周呈正相关(P<0.05),Quv的均数及标准差相对孕周的最佳拟合方程均为线性方程。根据公式获得正常胎儿Quv Z评分模型:Z评分=(Quv+346.069-20.754×孕周)(/-51.320+4.075×孕周)。正常组胎儿Quv的Z评分呈正态分布,且不随孕周而改变。(2)不良结局组、无不良结局组、正常组的胎儿Quv Z评分依次增加(P<0.05)。(3)正常胎儿Quv Z评分预测SGA围生期不良结局的曲线下面积为0.843,基于最佳截断值-2.06的预测敏感度、特异度、阳性预测值、阴性预测值分别为74.0%、86.4%、82.2%、76.4%。结论所建立的正常胎儿Quv Z评分模型可消除孕周的影响,具有较好的临床适用性。正常胎儿Quv Z评分是预测SGA围生期不良结局较敏感的指标。

高速轴配流中旋转径向小孔流量系数的实验研究

针对高转速轴配流中旋转径向小孔出流流量系数开展了实验研究。研究中涉及到两种流动方向,即自旋转缸体中心流过径向小孔向外离心运动称为正向流动,反之则为反向流动。结果表明:旋转小孔出流流量系数同时显著地受离心力和科氏力的影响;因离心力的赋能作用,正向流动时流量系数随转速的增加而变大,反向流动时离心力对小孔内流动有抑制作用,使流量系数随转速的增加而变小;两个方向的科氏力综合效果使正向流动流量系数随出流雷诺数增加而变小,反向流动流量系数随出流雷诺数的增加而变大;当出流雷诺数足够大时,两种流动方向、不同孔型的流量系数各自都有一个稳定值;各种孔型的正向流动流量系数稳定值分布均在0.65~0.80范围内。

基于开关流图法的Buck-Boost变换器小信号建模分析

阐述Buck-Boost变换器具有输出纹波低,电压可控等优点,广泛应用于新能源汽车电池发电、直流微电网。建立小信号模型有利于对电路系统主电路的控制和控制回路的设计。分析Buck-Boost变换器,利用开关流图法和信号支路的模型建立变换器的流图,从而推导出变换器的小型号模型。

利用局部特征匹配的运动小目标光流估计

基于深度光流估计的动态背景运动小目标检测,为了保证小目标的检测性能,一般采用较少的下采样次数以维持较高的分辨率,但由此带来了较大的计算耗时。特征匹配是深度光流估计的一个核心处理环节,其耗时在光流估计整体耗时中的占比较大,且对下采样次数非常敏感。据此,提出一种基于局部特征匹配的快速光流估计算法:引入目标运动信息,缩小特征匹配的空间范围,减少待处理的数据量;设计分块局部匹配策略,引入批处理机制,避免出现逐点局部匹配策略数据处理耗时过大问题,实现算法加速。在此基础上,在光流估计获取的光流场上,采用CenterNet网络检测运动目标对应的光流异常区域。从光流估计耗时、检测精度等方面开展了实验验证,结果表明:针对运动小目标检测,分块特征匹配光流估计比全局特征匹配光流估计耗时减少约25%,目标检测性能相当。