半圆头回转体自然空化流场分析

采用计算流体动力学软件FLUENT6．3对半圆头回转体进行流场分析,验证模型设置和计算方法的有效性,对不同空化数下的流场进行了求解．结果表明：在标准大气压下,空化数σ=0．096是该种头型的,晦界空化数;随着空化数的减小,超空泡在弹体上的长度逐渐增大,总阻力系数逐渐减小;超空泡具有很好的减阻特性,对于该种头型,减阻能力达59％～75％．     

绕空化器回转体通气空泡流态特征实验研究

为了研究通气空泡流态的转变机制,本文采用高速摄影技术对绕空化器回转体通气空泡的流场结构进行了观测,分析了通气率和空化器尺寸对通气空泡流态的影响.结果表明:随通气率的增大,空泡的发展过程整体可分为3个特征阶段,即快速增长阶段、缓慢增长阶段和相对稳定阶段.通气率的增大引起空泡不对称性加剧,空泡尾部泄气方式由回射流泄气逐渐转...     

头型对回转体非定常空化流动特性影响的实验研究

为了研究绕不同头型回转体的空穴发展特性,采用高速录像技术分别观察了绕平头和半锥角为45°的锥头回转体在不同空化数下的形态特征,应用粒子测速系统(PIV)测量了其速度、涡量及湍流脉动等分布情况。研究表明,相比于锥头回转体,同一空化数条件下,平头回转体的空穴最大尺度较大,且其随空化数的变化率亦较大;平头回转体空穴在发展的过程中表现为大尺度空泡团的融合、脱落及溃灭,表现较大的脉动,锥头回转体则为小尺度空穴的溃灭及再融合进而溃灭,空穴的脉动较小;空穴区域对应于低速高脉动区域;随着空化数的减小,速度分布趋于均匀,空穴内部流动脉动增强,高湍动能区域逐渐增大。     

回转体喷气减阻实验研究

采用避面喷气法在一回转体模型上进行了减阻实验，结果表明，使用喷气法降低水下物体阻力是可行的，并在一定条件下得到了8%的总阻力降，在对影响减阻的因素进行分析后，指出获得更大减阻效果是可能的。     

孔板空化器内流体空化特性研究

以孔板空化器为研究对象,推导了球形空泡的运动方程;将气液两相质量传输、流体流入与流出控制容积引起的气含率变化考虑在内,推导出孔板空化器内流体气含率变化方程。以空化模型为依据,通过数值模拟以10^-5 s时间步长获得孔板通道100 ms内瞬态空化中气泡分布、压力、速度特性。在孔板通道内建立径向截面利用气相体积流率分布进一步证明流体空化状态的稳定性。通过模拟和实验对两种不同通流截面积孔板结构在不同压差下对空化效果的影响进行验证。结果表明,可以以气相体积流率来衡量空化效果,增加孔板通道的数量比改变孔径更能影响空化效果。     

回转体零件CAPP系统的研究

结合ＦＺＣＡＰＰ系统的开发,研究了实用回转体零件计算机辅助工艺设计系统的设计方法,认为：以ＧＴ技术为指导,按零件结构和工艺的相似性,建立零件工序工步集,采用特征型面描述法,以人机交互的方式输入零件信息,采用派生与创成相结合的综合模式生成零件工艺内容的方法,既有较高的实用性,又有较好的先进性。     

回转体滑水航行流体动力特性研究

基于STAR-CCM+数值仿真软件,选用SST k-w湍流模型,采用VOF波构建运动体在静水面滑水数值计算模型.通过国外经典文献对构建模型进行校核,数值模拟结果与实验结果吻合性较好,流体动力数值误差均小于5％,在工程误差范围内,模型可用于回转体滑水航行工况的数值模拟计算.对回转体在不同速度、不同淹没深度滑水工况进行数值...     

回转体的标识量计算

针对一般回转体类零部件的标识量计算，引入数据封闭环的概念，应用函数拟合方法，直接标识量积分计算公式进行离散化求解，突出体现了计算过程的连续化采用本算法不仅易于编程，而且提高了计算精度与速度。     

绕过带凹槽的圆盘空化器的超空泡流的研究

采用VOF的方法,对带有不同凹槽数空化器的航行体诱导产生的三维超空泡,进行了数值模拟。分析了圆盘形空化器凹槽数对所形成的三维超空泡形状的影响;将数值模拟的结果与Logvinovich超空泡截面独立扩张原理的结果进行对比,发现数值计算结果与Logvinovich半经验公式计算结果具有较好的一致性;得出航行体的阻力系数随时间变化曲线。研究发现:随着空化器凹槽数的增加,航行体阻力系数以及超空泡的无量纲直径和长度均呈现出减小的趋势;带凹槽空化器产生的超空泡是典型的三维复合型空泡,其特征是:在来流方向上,不同位置处超空泡横截面的形状是不同的;但是超空泡具有很好的自修复特性,能够在较短时间内恢复成光滑椭球形。该研究的计算数据以及结论可为航行体空化器的设计提供参考。     

液氢和液氮绕水翼空化流动特性分析

为分析液氢和液氮两种低温流体介质的空化特性,通过对CFX软件二次开发,将Schnerr-Sauer空化模型和液氮、液氢随温度变化的物性参数嵌入到CFX求解代码中,同时耦合求解考虑汽化潜热影响的能量方程,从而在考虑热力学效应条件下,开展了液氢和液氮绕水翼空化流动的三维数值模拟研究,并将计算结果与试验数据进行对比,验证了数值方法的有效性.计算结果表明,热力学效应对液氢空化区域压力和温度参数变化影响更显著,在液氮空化核心区域内液相体积分数比液氢中的更小,在空泡尾部闭合区域从汽相向液相转化迅速.汽-液两相间质量传输特性可作为评估空化区域内温度、压力以及相体积分数分布的有效依据.     

Numerical simulation of unsteady cavitating flows around a transient pitching hydrofoil

The objective of this paper is to improve the understanding of the influence of multiphase flow on the turbulent closure model,the interplay between vorticity fields and cavity dynamics around a pitching hydrofoil.The effects of pitching rate on the subcavitating and cavitating response of the pitching hydrofoil are also investigated.In particular,we focus on the interactions between cavity inception,growth,and shedding and the vortex flow structures,and their impacts on the hydrofoil performance.The calculations are 2-D and performed by solving the incompressible,multiphase Unsteady Reynolds Averaged Navier Stokes(URANS)equations via the commercial CFD code CFX.The k-SST(Shear Stress Transport)turbulence model is used along with the transport equation-based cavitation models.The density correction function is considered to reduce the eddy viscosity according to the computed local fluid mixture density.The calculation results are validated with experiments conducted by Ducoin et al.(see Computational and experimental investigation of flow over a transient pitching hydrofoil,Eur J Mech/B Fluids,2009,28:728–743 and An experimental analysis of fluid structure interaction of a flexible hydrofoil in various flow regimes including cavitating flow,Eur J Mech B/fluids,2012,36:63–74).Results are shown for a NACA66 hydrofoil subject to slow(quasi static,6°/s,*0.18)and fast(dynamic,63°/s,*1.89)pitching motions from=0°to=15°.Both subcavitaing(=8.0)and cavitating(=3.0)flows are considered.For subcavitating flow(=8.0),low frequency fluctuations have been observed when the leading edge vortex shedding occurs during stall,and delay of stall is observed with increasing pitching velocity.For cavitating flow(=3.0),small leading edge cavities are observed with the slow pitching case,which significantly modified the vortex dynamics at high angles of attack,leading to high frequency fluctuations of the hydrodynamic coefficients and different stall behaviors compared to the subcavitating flow at the same pitching rate.On the other hand,for the fast pitching case at=3.0,large-scale sheet/cloud cavitation is observed,the cavity behavior is unsteady and has a strong impact on the hydrodynamic response,which leads to high amplitude fluctuations of the hydrodynamic coefficients,as well as significant changes in the stall and post-stall behavior.The numerical results also show that the local density modification helps to reduce turbulent eddy viscosity in the cavitating region,which significantly modifies the cavity lengths and shedding frequencies,particularly for the fast pitching case.In general,compared with the experimental visualizations,the numerical results with local density correction have been found to agree well with experimental measurements and observations for both slow and fast transient pitching cases.     

Physical and numerical investigation on transient cavitating flows

The objective of this paper is to investigate transient cavitating flows around a hydrofoil via combined physical and numerical studies. The aims are to 1) investigate the periodic formation, breakup, shedding, and collapse of the sheet/cloud cavities, 2) provide a better insight in the physical mechanism that governs the dynamics and structures of the sheet/cloud cavitation, 3) quantify the influence of cavitation on the surrounding flow structures. Results are presented for a Clark-Y hydrofoil fixed at an angle of attack of a=8° at a moderate Reynolds number, Re=7×105 , for sheet/cloud cavitating conditions. The experimental studies were conducted in a cavitation tunnel at Beijing Institute of Technology, China. The numerical simulations are performed by solving the incompressible, multiphase unsteady Reynolds-averaged Navier-Stokes (URANS) equations via the commercial code CFX using a transport equation-based cavitation model; a filter-based density corrected model (FBDCM) is used to regulate the turbulent eddy viscosity in both the cavitation regions near the foil and in the wake. The results show that numerical predictions are capable of capturing the initiation of the cavity, growth toward the trailing edge, and subsequent shedding in accordance with the quantitative features observed in the experiment. Regarding vapor shedding in the cavitating flow around the three-dimensional foil, it is primarily attributed to the effect of the re-entrant flow, which is formed due to the strong adverse pressure gradient. The results show strong correlation between the cavity and vorticity structures, demonstrating that the inception, growth, shedding, and collapse of sheet/cloud cavities are important mechanisms for vorticity production and modification.     

Computational modeling of cavitating flows in liquid nitrogen by an extended transport-based cavitation model

Developing a robust computational strategy to address the rich physical characteristic involved in the thermodynamic effects on the cryogenic cavitation remains a challenge in research. The objective of the present study is to focus on developing modelling strategy to simulate cavitating flows in liquid nitrogen. For this purpose, numerical simulation over a 2D quarter caliber hydrofoil is investigated by calibrating cavitation model parameters and implementing the thermodynamic effects to the Zwart cavitation model. Experimental measurements of pressure and temperature are utilized to validate the extensional Zwart cavitation model. The results show that the cavitation dynamics characteristic under the cryogenic environment are different from that under the isothermal conditions: the cryogenic case yields a substantially shorter cavity around the hydrofoil, and the predicted pressure and temperature inside the cavity are steeper under the cryogenic conditions. Compared with the experimental data, the computational predictions with the modified evaporation and condensation parameters display better results than the default parameters from the room temperature liquids. Based on a wide range of computations and comparisons, the extensional Zwart cavitation model may predict more accurately the quasi-steady cavitation over a hydrofoil in liquid nitrogen by primarily altering the evaporation rate near the leading edge and the condensation rate in the cavity closure region.     

支撑方式和模型尺度对轴对称体旋转导数的影响

旋臂水池是开展轴对称体水动力研究的常用试验设备之一,水池模型试验中的尺度效应和支撑方式一直是操纵性能试验研究的难点．在合理选取模型尺度、支杆与模型直径比、模型浸深以及模型速度的基础上,进行了轴对称体2种尺度模型和腹、尾支撑2种方式多工况下的旋转导数试验,并得出了线性和非线性旋转水动力导数．试验结果表明,在不考虑支杆干扰的情况下,模型试验中支撑方式对旋转导数的影响是可以忽略的,尺度效应要针对不同的试验对象分别考虑。     

TCP流带宽公平性研究

为了缓解TCP拥塞控制机制对于往返时间（RTT）较小流的偏向问题，现有方法在网络拥塞程
度严重时由于对RTT较大流的过分保护而不能有效地工作。为了解决这一问题，通过引进自适应的思想并
结合主动队列机制BLUE提出了改进方法。根据网络的拥塞状况动态调整对RTT较大流的保护程度，随着网
络拥塞程度的增大逐渐减小对RTT较大流的保护程度。仿真试验表明，在任何拥塞程度情况下，所提出的
机制均能有效地保障TCP流的带宽公平性。     

钝体绕流气动噪声源特性数值研究

为研究钝体绕流的气动噪声源特性,采用Realizablek-ε湍流模型与宽频带噪声源模型相结合的方法模拟钝体的声功率级和表面声功率级,比较并分析来流风速、钝体截面形式及尺寸对气动噪声源强度及其分布特性的影响规律,探讨气动噪声源的影响机制.结果表明:钝体绕流气动噪声源主要位于气流发生分离、湍流运动比较剧烈的地方,且钝体的外形越趋近于流线型,其气动噪声源强度越低;四极子噪声源对总噪声的贡献比偶极子噪声源的贡献小得多;柱体表面声功率级最大值与来流风速对数之间呈线性正相关,与截面尺寸之间呈线性负相关.最后提出了表面声功率级的数学预测模型,为工程结构的声环境设计及气动噪声控制提供参考.     

轴对称解析解对马蹄形隧道衬砌水压力及渗透量适用性研究

为简化马蹄形隧道衬砌水压力及渗透量的计算方法,基于轴对称解析解与等效周长法,求得不同衬砌渗透系数、注浆半径和衬砌厚度的衬砌水压力与渗透量,并与FLAC3D数值模拟同等条件下相应工况的衬砌水压力与渗透量进行对比.研究表明:轴对称解析解简化马蹄形断面计算隧道衬砌水压力及渗透量的结果与数值模拟结果误差为7.7%,可以将其应用于工程中;3种工况下,最不利位置均位于拱腰;衬砌渗透系数减小与衬砌厚度增大,衬砌安全系数均在增大,而注浆半径的增大对衬砌安全系数无明显影响;衬砌渗透系数减小,衬砌水压力增大,渗透量减小,注浆半径增大与衬砌厚度增大,衬砌水压力与渗透量均减小.     

桩周土阻尼系数的研究

通过对武汉某码头2根钢管桩动静载试验研究,探讨了桩周土阻尼系数对波动理论Smith法预测单桩静极限承载力和打桩时桩身最大锤击应力的影响规律.通过对打桩实测最大锤击应力的拟合,推荐了符合本地区地质条件的最佳土阻尼系数和吸着系数,其适用性在相同地质条件下的钢管桩中得到了证实.