小型运动体高速倾斜入水空泡流动数值研究

为研究小型运动体高速倾斜入水时的空泡流动特性,采用求解雷诺时均Navier-Stokes方程的方法开展数值模拟。应用实验结果验证了数值方法的正确性。基于该方法对入水过程流体动力特性、流场结构特性与空泡发展进行分析,并研究了入水角度对入水流场的影响。研究结果表明:运动体在入水撞击阶段受到较大的阻力、升力和力矩,同时承受着极强的载荷;随着入水的深入,头部半球形高压区逐渐向头部中点移动,且空泡壁面的速度方向由指向水中向指向空泡内过渡;入水角度越小,撞击阶段阻力系数与砰击压力越小,入水后越容易发生弹道偏移,同时拉脱现象发生得越晚,入水空泡的最大尺寸越大。

超声速细长锥型射弹超空泡流动数值计算方法

基于超声速细长体运动理论,采用理想可压缩流体无旋定常流动以及超空泡尾部Riabushinsky闭合方式假定,建立了描述水下超声速条件下细长锥型射弹超空泡流动的非线性积分-微分方程.针对超声速流动特点,发展了该方程数值离散和迭代求解的新方法,采用一阶近似解作为超空泡流动数值计算的初始解,优化了初始迭代条件,提高了计算速度和精度.通过与超空泡细长比渐近解结果进行比较,验证了理论模型和计算方法的正确性及有效性.在超声速条件下,分析了流体压缩性效应以及不同马赫数对细长锥型射弹超空泡形态、表面压力系数和压差阻力系数的影响,为超空泡射弹的弹型优化和水中弹道预报提供了理论基础.

超空泡形态及其流动特性的数值模拟

对圆盘空化器分别采用CFD、"1/3法则"和空泡截面独立扩展原理三种不同方法数值模拟了水下航行体定常自然超空泡外形及其流动特性。应用CFD方法基于粘性多相流的空泡捕捉法,采用六面体网格,选择Singhal空化模型和SST湍流模式,数值求解均质超空泡流场RANS方程。研究表明:泡形态时变特性是一种行之有效的工程估算方法,应用空泡截面独立扩展原理其计算结果与CFD方法吻合较好,说明了CFD方法用于超空泡流动仿真计算的可行性和独立性原理快速估算超空泡形态的准确性;同时超空泡外形主要与头部空化器有关,空泡长度会由于航行体本身存在变长;可优先选择空泡截面独立扩展原理对水下航行体超空泡外形进行快速估算。

螺旋桨片状空泡的CFD分析

基于RANS求解器采用非结构化网格对螺旋桨敞水性能和空泡性能进行了数值模拟,主要研究了不可压缩性气体质量分数对螺旋桨片空泡预报结果的影响,空泡模型采用基于输运方程的完整空化模型,预报的敞水性能与试验值吻合较好,预报的空泡形态与文献结果及公开发表的试验结果相一致。这表明,用该方法预报螺旋桨空化性能是可行的,为以后进一步研究螺旋桨空泡打下了基础。

超空泡航行体楔形舵片流体动力学特性数值模拟

舵片是保证超空泡航行体运动稳定性和控制航行弹道的重要部分。文章基于均质平衡流模型和SST(Shear Stress Transport)湍流模型,计算了单独舵片的流体动力特性,并与试验数据进行了对比,结果符合较好,验证了计算模型的有效性。基于此方法,计算了单独舵片发生空化后在不同操舵状态下的非定常流体动力变化。结果表明,在攻角相同时,操舵状态下舵片的非定常升力系数和定常结果差别不大,而非定常阻力系数大于定常结果,并且操舵速度越快,阻力系数越大。另外计算了舵片发生空化后的流体动力系数,结果显示在攻角相同时,舵片的阻力系数和升力系数均小于其在全湿状态下的结果;在空化状态下,舵片升力系数的斜率小于全湿状态,并且舵片升力系数的斜率是变化的,存在某临界攻角,攻角大于此临界值时,升力系数的斜率减小,而此临界攻角恰好为舵片的吸力面刚刚出现空化时的攻角;操舵状态下舵片的阻力系数和升力系数的变化规律与定常结果一致,但是数值偏小。

DES模型在开放腔体圆柱壳入水空泡模拟计算中的应用及评价（英文）

基于混合介质雷诺平均N-S方程,对开放空腔圆柱壳垂直入水运动过程开展了数值研究,得到了压力场、速度场分布,空泡波动、闭合特征,空腔气体涨缩规律,以及流体动力变化规律,并分析了空腔结构在入水运动过程中对流场结构和流体动力的影响。结果表明:液体随气体涨缩同步进出开放端;开放端局部形成波动的压力源和周期性的压力场、速度场分布;入水空泡呈现波动形态,其扩展程度与开放端液体流速相关;空泡内形成气体漩涡,随空腔涨缩往返进出空泡,对空泡闭合具有抑制作用;流体动力呈波动变化规律,频率与气体涨缩频率一致,幅值与气体涨缩程度成正比。开放空腔结构在入水过程中空腔内气体发生涨缩运动,对流场结构和流体动力产生周期性扰动作用,在一定程度上可以减缓冲击、维持空泡及运动的稳定性。

二相流模式下JDC4-55可调桨粘性流场分析

运用CFD方法中的汽-液混合二相流模型模拟JDC4-55可调桨的水动力性能和空泡性能。通过改变进速和调节桨叶角度得到相应的水动力性能参数。分析空泡对螺旋桨水动力性能的影响,进行JDC4-55可调桨桨流场模拟分析。相对于单相流模型而言,JDC4-55可调桨汽-液混合二相流模型性能计算结果与实验值吻合得更好。

Numerical simulation of two-phase flow field in underwater sealing device based on dynamic mesh

In order to speed underwater launch of minor-caliber weapons,a sealing device can be set in front of underwater muzzle to separate water,preventing the muzzle from water immersion.By establishing and simplifying the model of underwater weapon sealing device and unstructured mesh computing domain model based on computational fluid dynamics（CFD）,dynamic mesh and user defined function（UDF）,the N-S equation is solved and the numerical analysis and calculation of the complex two-phase flow inside the sealing device are carried out.The results show that the gas discharged from the sealing device is conducive to the formation of the projectile supercavity.When the projectile is launched at 5munder water,the shock wave before and after the projectile has impact on the box body up to 100 MPa,therefore the sealing device must be strong enough.The research results have the vital significance to the design of underwater weapon sealing device and the formation of the projectile supercavitation.

Numerical investigation of cavitating flow behind the cone of a poppet valve in water hydraulic system

Computational Fluid Dynamics (CFD) simulations of cavitating flow through water hydraulic poppet valves were performed using advanced RNG k-epsilon turbulence model. The flow was turbulent, incompressible and unsteady, for Reynolds numbers greater than 43 000. The working fluid was water, and the structure of the valve was simplified as a two dimensional axisymmetric geometrical model. Flow field visualization was numerically achieved. The effects of inlet velocity, outlet pressure, opening size as well as poppet angle on cavitation intensity in the poppet valve were numerically investigated. Experimental flow visualization was conducted to capture cavitation images near the orifice in the poppet valve with 30° poppet angle using high speed video camera. The binary cavitating flow field distribution obtained from digital processing of the original cavitation image showed a good agreement with the numerical result.

Numerical Calculation of Supercavitating Flows over the Disk Cavitator of a Subsonic Underwater Projectile

To deal with the effect of compressible fluids on the supercavitating flow over the subsonic disk cavitator of a projectile, a finite volume method is formulated based on the ideal compressible potential theory. By using the continuity equation and Tait state equation as well as Riabouchinsky closure model, an“inverse problem”solution is presented for the supercavitating flow. According to the impenetrable condition on the surface of supercavity, a new iterative method for the supercavity shape is designed to deal with the effect of compressibility on the supercavity shape, pressure drag coefficient and density field. By this method, the very low cavitation number can be computed. The calculated results agree well with the experimental data and empirical formula. At the subsonic condition, the fluid compressibility will make supercavity length and radius increase. The supercavity expands, but remains spheroid. The effect on the first 1/3 part of supercavity is not obvious. The drag coefficient of projectile increases as the cavitation number or Mach number increases. With Mach number increasing, the compressibility is more and more significant. The compressibility must be considered as far as the accurate calculation of supercavitating flow is concerned.

湍流作用下可压缩液体中空化泡的动力学特性

以水为工作介质,在综合考虑了液体黏性、表面张力、液体压缩性以及湍流作用等因素的情况下,对孔板水力空化反应器中空化泡动力学行为特性进行了详细数值研究.详细分析了湍流作用、空化泡初始半径、孔板下游恢复压力、管道直径以及孔板直径与管道直径之比对空化泡运动特性及其所形成的压力脉冲的影响规律,发现了一些新现象,如空化泡半径二次生长现象.研究结果可为水力空化反应器的优化设计以及水力空化技术的实际应用提供一定的理论指导与技术支撑。

Validation of dynamic cavitation model for unsteady cavitating flow on NACA66

Unsteady cavitating flow is extremely complicated and brings more serious damages and unignorable problems compared with steady cavitating flow.CFD has become a practical way to model cavitation;however,the popularly used full cavitation model cannot reflect the pressure-change that the bubble experiences during its life path in the highly unsteady flow like cloud cavitating.Thus a dynamic cavitation model(DCM)is proposed and it has been considered to have not only the first-order pressure effects but also zero-order effect and can provide greater insight into the physical process of bubble producing,developing and collapsing compared to the traditional cavitation model.DCM has already been validated for steady cavitating flow,and the results were reported.Furthermore,DCM is designed and supposed to be more accurate and efficient in modeling unsteady cavitating flow,which is also the purpose of this paper.The basic characteristic of the unsteady cavitating flow,such as the vapor volume fraction distribution and the evolution of pressure amplitude and frequency at different locations of the hydrofoil,are carefully studied to validate DCM.It is found that not only these characteristics mentioned above accord well with the experimental results,but also some detailed transient flow information is depicted,including the re-entrant jet flow that caused the shedding of the cavity,and the phenomenon of two-peak pressure fluctuation in the vicinity of the cavity closure in a cycle.The numerical results validate the capability of DCM for the application of modeling the complicated unsteady cavitating flow.

A Rayleigh-Plesset based transport model for cryogenic fluid cavitating flow computations

The present article focuses on modeling issues to simulate cryogenic fluid cavitating flows.A revised cavitation model,in which the thermal effect is considered,is derivated and established based on Kubota model.Cavitating flow computations are conducted around an axisymmetric ogive and a 2D quarter caliber hydrofoil in liquid nitrogen implementing the revised model and Kubota model coupled with energy equation and dynamically updating the fluid physical properties,respecitively.The results show that the revised cavitation model can better describe the mass transport process in the cavitation process in cryogenic fluids.Compared with Kubota model,the revised model can reflect the observed"frosty"appearance within the cavity.The cavity length becomes shorter and it can capture the temperature and pressure depressions more consistently in the cavitating region,particularly at the rear of the cavity.The evaporation rate decreases,and while the magnitude of the condensation rate becomes larger because of the thermal effect terms in the revised model compared with the results obtained by the Kubota model.

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 sub- cavitating 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-co 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 vari- ous flow regimes including cavitating flow, Eur J Mech B/fluids, 2012, 36： 63-74）. Results are shown for a NACA66 hydro- foil subject to slow （quasi static, t2=6~/s, ＆＊ =0.18） and fast （dynamic, ＆=63~/s, dr＂ =1.89） pitching motions from a =0~ to a =15~. Both subcavitaing （or =8.0） and cavitating （cr=3.0） flows are considered. For subcavitating flow （or=8.0）, low frequency fluctuations have been observed when the leading edge vortex shedding occurs during stall, and delay of stall is ob- served with increasing pitching velocity. For cavitating flow （tr=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 or=3.0, large-scale sheet/cloud cavitation is observed, the cavity behavior is un- steady 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 visualiza- tions, 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.

Dynamic behaviors of cavitation bubble for the steady cavitating flow

In this paper, by introducing the flow velocity item into the classical Rayleigh-Plesset dynamic equation, a newequation, which does not involve the time term and can describe the motion of cavitation bubble in the steadycavitating flow, has been obtained. By solving the new motion equation using Runge-Kutta fourth order methodwith adaptive step size control, the dynamic behaviors of cavitation bubble driven by the varying pressure fielddownstream of a venturi cavitation reactor are numerically simulated. The effects of liquid temperature (correspondingto the saturated vapor pressure of liquid), cavitation number and inlet pressure of venturi on radial motionof bubble and pressure pulse due to the radial motion are analyzed and discussed in detail. Some dynamicbehaviors of bubble different from those in previous papers are displayed. In addition, the internal relationshipbetween bubble dynamics and process intensification is also discussed. The simulation results reported in thiswork reveal the variation laws of cavitation intensity with the flow conditions of liquid, and will lay a foundationfor the practical application of hydrodynamic cavitation technology.

Periodical shedding of cloud cavitation from a single hydrofoil in high-speed cryogenic channel flow

In order to explain criteria for periodical shedding of the cloud cavitation, flow patterns of cavitation around a piano-convex hydrofoil were observed using a cryogenic cavitation tunnel of a blowdown type. Two hydrofoils of similarity of 20 and 60 mm in chord length with two test sections of 20 and 60 mm in width were prepared. Working fluids were water at ambient temperature, hot water and liquid nitrogen. The parameter range was varied between 0.3 and 1.4 for cavitation number, 9 and 17 m/sec for inlet flow velocity, and -8° and 8° for the flow incidence angle, respectively. At incidence angle 8°, that is, the convex surface being suction surface, periodical shedding of the whole cloud cavitation was observed on the convex surface under the specific condition with cavitation number and inlet flow velocity, respectively, 0.5, 9 m/sec for liquid nitrogen at 192℃ and 1.4, 11 m/sec for water at 88℃, whereas under the supercavitation condition, it was not observable. Periodical shedding of cloud cavitation occurs only in the case that there are both the adverse pressure gradient and the slow flow region on the hydrofoil.