Effect of Time Step Size and Turbulence Model on the Open Water Hydrodynamic Performance Prediction of Contra-Rotating Propellers

A growing interest has been devoted to the contra-rotating propellers （CRPs） due to their high propulsive efficiency, torque balance, low fuel consumption, low cavitations, low noise performance and low hull vibration. Compared with the single-screw system, it is more difficult for the open water performance prediction because forward and aft propellers interact with each other and generate a more complicated flow field around the CRPs system. The current work focuses on the open water performance prediction of contra-rotating propellers by RANS and sliding mesh method considering the effect of computational time step size and turbulence model. The validation study has been performed on two sets of contra-rotating propellers developed by David W Taylor Naval Ship R ＆ D center. Compared with the experimental data, it shows that RANS with sliding mesh method and SST k-ω turbulence model has a good precision in the open water performance prediction of contra-rotating propellers, and small time step size can improve the level of accuracy for CRPs with the same blade number of forward and aft propellers, while a relatively large time step size is a better choice for CRPs with different blade numbers.

Numerical Investigation of Contra Rotating Vertical-Axis Tidal-Current Turbine

In this study,the performance of a contra rotating vertical-axis tidal-current turbine was investigated.The incompressible unsteady Reynolds-averagedNavier-Stokes(U-RANS)equations were solved via two-dimensional(2D)numerical simulation using ANSYS Fluent computational fluid dynamics(CFD)code.An algorithm known as SIMPLE from the CFD code was used to calculate the pressure-velocity coupling and second-order finite-volume discretization for all the transport equations.The base turbine model was validated using the available experimental data.Three given scenarios for the contra rotating turbine were modeled.The contra rotating turbine performs better in a low tip speed ratio(TSR)than in a high TSR operation.In a high TSR operation,the contra rotating turbine inefficiently operates,surviving to rotate in the chaotic flow distribution.Thus,it is recommended to use contra rotating turbine as a part of new design to increase the performance of a vertical-axis tidal-current turbine with a lower TSR.

Unsteady Flow Condition between Front and Rear Rotor of Contra-Rotating Small Sized Axial Fan

Contra-rotating small-sized axial fans are used as cooling fans for electric equipment. In the case of the contra-rotating rotors, the blade row distance between front and rear rotors is a key parameter for the performance and stable operation. The wake and potential interference occur between the front and rear rotors and leakage flow from the front rotor tip influences on the flow condition of the rear rotor near the shroud when the blade row distance is small. Therefore, it is important to clarify the flow condition between front and rear rotors. The fan static pressure curves were obtained by the experimental apparatus and the numerical analysis was also conducted to investigate the internal flow between front and rear rotors. The leakage flow from the front rotor tip reaches the leading edge of the rear rotor when the blade row distance is small as L = 10 mm and the pressure fluctuations at the leading edge of the rear rotor tip becomes larger than those at other radial positions. In the present paper, the vorticity contour between front and rear rotors is shown and pressure fluctuations related to the leakage flow from the front rotor is investigated using the numerical analysis result. Then, suitable blade row distance for the contra-rotating small sized axial fan is discussed based on the internal flow condition.

Study on Contra-Rotating Small-Sized Axial Flow Hydro Turbine

It is thought that small hydropower generation is alternative energy, and the energy potential of small hydropower is large. The efficiency of small hydro turbines is lower than that of large one, and these small hydro turbine’s common problems are out of operation by foreign materials. Then, there are demands for small hydro turbines to keep high per- formance and wide flow passage. Therefore, we adopted contra-rotating rotors which can be expected to achieve high performance and low-solidity rotors with wide flow passage in order to accomplish high performance and stable opera- tion. Final goal on this study is development of an electric appliance type small hydro turbine which has high portability and makes an effective use of the unused small hydro power energy source. In the present paper, the performance and the internal flow conditions in detail of contra-rotating small-sized axial flow hydro turbine are shown as a first step of the research with the numerical flow analysis. Then, a capability adopting contra-rotating rotors to an electric appliance type small hydro turbine was discussed. Furthermore, the high performance design for it was considered by the numeri- cal analysis results.

Modeling and Numerical Simulation of Wings Effect on Turbulent Flow between two contra-rotating cylinders

Many industries in the world take part in the pollution of the environment. This pollution often comes from the reactions of combustion. To optimize these reactions and to minimize pollution, turbulence is a funda- mental tool. Several factors are at the origin of turbulence in the complex flows, among these factors, we can quote the effect of wings in the rotating flows. The interest of this work is to model and to simulate numeri- cally the effect of wings on the level of turbulence in the flow between two contra-rotating cylinders. We have fixed on these two cylinders eight wings uniformly distributed and we have varied the height of the wings to have six values from 2 mm to 20 mm by maintaining the same Reynolds number of rotation. The numerical tool is based on a statistical model in a point using the closing of the second order of the transport equations of the Reynolds stresses (Reynolds Stress Model: RSM). We have modelled wings effect on the flow by a source term added to the equation tangential speed. The results of the numerical simulation showed that all the average and fluctuating variables are affected the value of the kinetic energy of turbulence as those of Reynolds stresses increase with the height of the wings.

The Conceptual Design Experience of Sea-Going Ships with Alternative Propulsion System, Equipped with Dual-Mode Contra Propeller

The paper presents a technical solution which provides energy saving at full speed, as well as duplication of screw propeller and main propulsion system incase it＇s out of order. The description of the corresponding device, designed for transport refrigerator ship, is shown as well as its location on board. Value of energy saving from the use of contra propeller is estimated. The results of the assessment required power to move the ship at low speed by the action of the contra propeller operating in the reactive mode are demonstrated. It is shown, that the ship equipped with the proposed device will get the additional class notation related to the redundancy or duplication of the propulsion system of Russian Maritime Register of Shipping.

Internal Flow Condition between Front and Rear Rotor of Contra-Rotating Small-Sized Axial Fan at Low Flow Rate

Contra-rotating small-sized fans are used as cooling fans for electric equipment. The internal flow condition between the front and rear rotors of the contra-rotating small-sized fan is not known well especially at the low flow rate. Furthermore, the blade row distance between the front and rear rotors is an important parameter for the contra-rotating small-sized fan and its influence on the internal flow condition is not clarified at the low flow rate. Therefore, the internal flow condition of the contra-rotating small-sized fan at the low flow rate is investigated by the numerical analysis in this research. The numerical analysis results are validated by comparing the fan static pressure curves of the numerical results to the experimental results. The internal flow condition at the low flow rate is clarified using the numerical models of the different blade row distance L = 10 mm and 30 mm. In the present paper, pressure fluctuations phase locked each front and rear rotor’s rotation are shown and the influences of the wake and the potential interference are discussed by the unsteady numerical analysis results at the low flow rate.

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.

矿用对旋风机旋转失速信号特征识别与提取

针对旋风机旋转失速信号特征的识别与提取,提出了一种参数优化的变分模态分解(VMD)方法。首先,为了消除人为选择VMD参数的影响,提出一种以平均包络熵为目标函数的蛙跳优化算法,对VMD算法的模态个数K和惩罚因子α进行优化选取,实现了参数K和α的合理自动选取;然后,通过模拟信号验证了参数优化后的VMD方法的有效性;最后,利用参数优化的VMD方法对风机失速信号进行处理和分析,研究风机失速的频谱特性,并与经验模态分解(EMD)结果进行对比分析。研究结果表明:与EMD方法相比,参数优化的VMD方法在处理模态混叠方面更具有优势,其处理效果明显优于EMD,分解得到的IMF分量个数也少于EMD,易区分特征频率;另外,以优化获取的参数组合处理风机的失速信号时,能够很好地揭示失速特征频率在失速发展过程中的变化规律,对应于频率分量的波动,从大尺度的频率波动转变为小尺度的频率波动。

对旋轴流通风机叶轮内不可逆能量损失机理

随着节能降耗成为当今世界的迫切需要,提高通风机能量转换效率越来越受到重视,已成为通风领域内关键问题。掌握叶轮内不可逆能量损失演化机制是实现能量高效转化的前提与基础,但目前尚缺乏针对叶轮内不可逆能量损失机理方面的研究。为此,以对旋轴流通风机为研究对象,采用数值模拟和实验方法获得了不同流量工况下通风机内部流场。基于熵产理论,建立了通风机叶轮内不可逆能量损失理论模型,明确了叶轮内不可逆能量损失与空间流场参数的内在关系,实现了叶轮内不同类型能量损失的定量分析,结合叶轮内流动特征,明确了能量损失空间演化规律和产生原因。研究结果表明,熵产方法计算叶轮内不可逆能量损失是可靠的,直接黏性耗散损失和壁面摩擦损失是能量损失的重要组成部分,而湍流耗散则是引起能量损失的主要原因,达到总能量损失的60%~80%;对于前级叶轮,湍流耗散引起的能量损失在1.0Q_(BEP)流量工况(Q_(BEP)为最优工况流量)达到最小,而后级叶轮能量损失随流量的降低而增大。能量损失主要集中在Span=0.6~1.0(Span为叶展方向轮毂至机壳的无量纲距离)区域,在1.0Q_(BEP)工况达到总能量损失的70%;叶顶间隙泄漏流和叶片前缘溢流引起的螺旋旋涡、叶轮内回流、叶片压力面和吸力面流动分离以及叶片后缘尾迹都将导致能量损失产生,其中流动分离和尾迹引起的高能量损失区域较小,能量损失相对有限,而旋涡和回流显著影响叶轮内流体流动,最终导致叶顶附近区域出现大量能量损失。

考虑污染物排放的开式转子发动机总体性能建模及分析研究

开式转子发动机兼具涡桨发动机高推进效率和涡扇发动机高飞行速度的特点,是未来民用单通道客机理想动力装置之一。为了掌握开式转子发动机的性能变化规律,明确开式转子发动机相比于常规大涵道比涡扇发动机节油和降低污染物排放的优势,本文基于螺旋桨相似理论和动量理论,建立了考虑前后排桨扇相互影响的对转桨扇模型;与双轴燃气发生器进行匹配,建立了三轴齿轮传动开式转子发动机模型;同时建立了发动机污染物排放计算模型;对同技术水平的开式转子发动机和大涵道比涡扇发动机进行了性能对比。结果表明:所建立的对转桨扇模型与实验结果误差较小,最大误差不超过3%。飞行马赫数增大,桨扇功率系数增大,推力系数减小,耗油率增大;飞行高度增加,桨扇功率系数和推力系数均增大,耗油率呈减小的趋势。相比于同技术水平的大涵道比涡扇发动机,开式转子发动机在典型工况下的耗油率降低9%以上。在飞机起飞着陆循环内,开式转子发动机的UHC,CO和NOx三种污染物排放指数相比于大涵道比涡扇发动机降低10%以上,表明开式转子发动机可有效降低航空污染物的排放。

对旋风机新形叶片流致振动特性

为了优化对旋风机的流致振动特性,以某对旋风机叶片为研究对象,通过修改翼型的方式来提出一种性能良好且具有减振功能的新形叶片。比较了两种对旋风机流场性能,采用模态分析法进行了两种叶片的共振预测,以叶片表面非定常压力脉动作为叶片所受流体激振力,采用模态叠加法计算了两种叶片监控点强迫振动响应特性。结果表明:新形叶片具有良好的气动性能,正风性能与原始风机相差不大,反风性能优于原始风机;额定转速下,一级新形叶片相较原始叶片频率余量提高了8.9%,运行更安全;新形叶片监控点振动加速度响应最大峰值均低于原始叶片,新形叶片具有良好的减振效果;相较于一级叶片监控点,二级叶片监控点处两种叶片总振动加速度级差值最大,新形叶片减振效果更明显,且最大差值约为10.83 dB。

多层互剪搅拌桩工法的工艺因素模型试验研究

采用模型试验方法,对多层互剪搅拌桩工法(contra-rotational shear deep soil mixing,简称CS-DSM工法)的工艺因素进行了试验研究,探索了水泥掺量、单位桩长搅拌次数T、单位体积搅拌能量E以及内外钻杆转速比RN等工艺因素对搅拌桩均匀性与强度UCS的影响。模型试验研究发现,通过多层互剪搅拌能够根除地表冒浆、防止糊钻抱钻、提高固化材料利用率。18组模型试验结果阐明搅拌桩在T-E-UCS之间存在固有关联,并揭示出机械参数、输出能量与桩身强度之间的本质关系。提供的计算方法可以定性指导选取合理的工艺参数,实现桩身设计强度目标。作为重要工艺因素,内外钻杆转速比RN与桩身强度试验曲线存在极值点,建议在工程中将1.80~2.20作为获取桩身峰值强度的最优RN值域。CS-DSM工法应用的系列研究结果为高质量搅拌桩工艺控制原则和质量保障体系提供了试验依据。

Investigation of the Unsteady Disturbance in Tip Region of a Contra-Rotating Compressor near Stall

The present study investigated the spectrum characteristics of unsteady disturbance and the tip leakage vortex evolution during pre-stall process for a contra-rotating axial compressor(CRAC). Transient numerical simulation was carried out in a single passage of the CRAC. The original transient fluctuation and oscillation of the tip leakage vortex structure with varying flow capacity of the CRAC were revealed using circle-like pattern figure and phase-locked root mean square(PLRMS). Additionally, the tip leakage flow in terms of vortex structure evolution was visualized for the sake of revealing the flow mechanism during pre-stall process. Results show that the unsteady fluctuation first appears at φ=0.3622, and the fluctuation frequency is 2.86 BPF. Unsteady disturbance source is mainly located at the tip side of the downstream rotor leading edge. From the choking point to the near stall condition, tip leakage vortex is always found in the tip leading edge of the upstream rotor. In addition, the tip leakage vortex of upstream rotor remains in the same place over time, i.e., no fluctuation, even when the downstream rotor entered into stall state. Such a phenomenon indicates that the stall point of the contra-rotating compressor is determined by the downstream rotor. Moreover, the maximum fluctuation position is mainly concentrated on the interface between the mainstream and the tip leakage vortex of the downstream rotor. By throttling the compressor, the angle between the main leakage vortex and the circumferential direction decreases gradually. When the main leakage vortex touches and continuously impacts on the leading edge of the adjacent blade, the unsteady disturbance, which is different from that of BPF, appears firstly.

对转舵桨水动力性能研究

对转舵桨兼有操纵性能良好和推进效率高的特点,在科考船、海洋工程船等船型上有一定数量的应用。该文采用改进的延迟分离涡模拟方法模拟了对转舵桨的敞水性能,并与模型试验结果进行对比分析,结果表明:设计工况推进单元的推力系数与转矩系数数值模拟误差在3%以内,满足工程精度要求。通过不同舵角下的敞水模型试验发现,舵角对前桨敞水性能影响较小,而对后桨影响较大,案例中约2.5°舵角时推进单元横向力为0。通过空泡模型试验,观察立柱对前桨梢涡形态的影响,发现前桨梢涡与后桨桨叶及后桨梢涡相遇使该位置的空泡现象显著增强。基于对转舵桨各部分相互影响关系,引入诱导速度修正系数,将对转舵桨的桨叶设计问题转化为给定来流条件的桨叶设计问题。

对转式全回转舵桨的立柱优化分析

[目的]旨在研究立柱对对转式全回转舵桨敞水性能的影响。[方法]采用基于雷诺平均方法与k-ω湍流模型,对不同立柱型式的对转式全回转舵桨进行模型尺度和实尺度下的水动力性能预报,研究流场特性并监测后桨伴流,分析优化前后立柱尾部的流动分离现象。[结果]结果表明,立柱前缘削薄并整体前移的优化方案,使得整个舵桨的敞水推进效率提高1.01%;立柱采用扭曲设计且切面引入拱度的优化方案,使得敞水推进效率提高1.15%。另外,模型尺度的立柱尾部流动分离比实尺度的严重。[结论]研究表明,优化后的立柱可以改善立柱尾部的流动分离现象,提高后桨吸收前桨的尾涡能量,增加后桨流入量,对于提高对转式全回转舵桨的推进性能具有工程实用意义。

当代女性文学中的反性别暴力书写——以安赫莱斯·卡索的《逆风》为例

安赫莱斯·卡索(Angeles Caso)的小说《逆风》(Contra el viento)以欧洲中产阶级的视角生动展现了非洲移民女性群体在困境中坚韧生存的状态。这些移民面临着来自性别、阶级和种族三个方面的文化暴力和象征暴力,无数次经历身体暴力、精神暴力和性暴力的折磨。在探索移民边缘群体如何打破暴力链条的同时,该书作者安赫莱斯·卡索也对当前欧洲中心主义根深蒂固的现象以及多元文化实践所遇到的困境进行了反思。

基于同一车体结构模型的不同优化算法对比研究

高速列车工程优化设计中有多种优化算法,解决实际优化问题时选择合适的算法能提高优化设计效率。以某一高速列车铝合金车体为模型,通过有限元分析计算整备状态下的车体一阶垂弯频率较低。在灵敏度分析的基础上,应用正交试验对各个变量进行主效应分析,筛选出主要变量作为优化模型的设计变量。以提高一阶垂弯频率为目的,基于近似模型分别采用可行方向法和遗传算法进行优化求解,使用可行方向法优化后的车体一阶垂弯振动频率提升2.4%,二阶菱形振动提升1.9%,车体钢结构质量增重0.15%;使用遗传算法优化后车体一阶垂弯振动频率提高了4.2%,车体的二阶菱形振动频率提高2.5%,车体钢结构质量增重1.66%。对优化结果进行对比分析,两种不同算法的优化结果侧重点有所不同,可根据不同实际需求选择相应的优化算法,从而达到优化目的。

Pressure Measurement on Casing Wall and Blade Rows Interaction of Contra-Rotating Axial Flow Pump

An application of contra-rotating rotors has been proposed against a demand for developing higher specific speed axial flow pump. The blade rows interaction between front and rear rotors should be clarified for its stable operation and reduction of unsteady losses. In this paper, the static pressure distributions on casing wall are provided by measuring with the phase locked sampling method. The measurements are carried out for two types of the rear rotors with different blade number and chord length, and it is found that, for both types of rotors, the unsteady pressure fluctuations are more remarkable in the front rotor than in the rear rotor and they are caused by the rear rotor pressure field. The effects of pressure fluctuations will be discussed in more details toward understanding the blade rows interaction in the contra-rotating axial flow pump.