二维水翼型空化流的数值计算(英文)

In order to predict the effects of cavitation on a hydrofoil, the state equations of the cavitation model were combined with a linear viscous turbulent method for mixed fluids in the computational fluid dynamics (CFD) software FLUENT to simulate steady cavitating flow. At a fixed attack angle, pressure distributions and volume fractions of vapor at different cavitation numbers were simulated, and the results on foil sections agreed well with experimental data. In addition, at the various cavitation numbers, the vapor fractions at different attack angles were also predicted. The vapor region moved towards the front of the airfoil and the length of the cavity grew with increased attack angle. The results show that this method of applying FLUENT to simulate cavitation is reliable.     

Effect of a drag-reducing polymer solution ejection on tip vortex cavitation

Experiments regarding the modification of the foil geometry and/or active or passive mass injection in the vortex core have been performed to investigate the possibility of inhibiting tip vortex cavitation. The ejection at very low flow rates of drag-reducing polymer solutions at the tip of hydrofoils and propeller blades has demonstrated effectiveness as a tip vortex cavitation inhibitor. This paper reports the results obtained with an elliptical hydrofoil, of 8cm maximum chord and 12cm haif-span, operating at Reynolds numbers, of =10⁶, much larger than those previously reported in the literature. Lift coefficients and critical cavitation numbers were determined for a variety of flow and polymer solution ejection conditions. Tangential and axial components of the mean velocity as well as velocity fluctuations along the vortex path were also measured. At 12.5 m/s free stream velocity and a variety of angles of attack, the ejection of a 500 ppm aqueous solution of a drag-reducing polymer at a flow rate of about 5 cm³/s leads to a decrease of up to 30% in the cavitation number. This occurs without modification of the lift coefficient and, hence, of the midspan bound circulation of the foil. Moreover, water injection does not cause any appreciable change in the cavitation numbers. The tangential velocity profiles along the vortex path during polymer ejection indicate that the potential region remains the same, while the viscous core dimension increases, and the maximum tangential velocity decreases substantially as compared to the no ejection or water ejection experiments. Thus, the pressure coefficients at the vortex axis are smaller than for the no ejection or water ejection cases and cause the reduction of the critical cavitation numbers. It is speculated that this inhibition effect is due only to swelling of the polymer solution when exiting the ejection orifice.List of symbols a core radius (distance to the vortex axis for maximum tangential velocity) - C ₁ lift coefficient - c _max maximum chord - Cp pressure coefficient at the vortex axis - Cp _min minimum pressure coefficient at the vortex axis - d _e diameter of the ejection port - m ejection flow rate - P reference pressure - P _v vapor pressure - V free stream velocity - V _a axial velocity - V _t tangential velocity - v _r radial component of the velocity resulting from jet swelling - x downstream distance from the tip of the foil - y, r distance to the vortex axis - angle of attack - r difference between the swollen jet and the ejection port radii - boundary layer thickness - tip vortex intensity - _d ( _de ) desinent cavitation number (with ejection) - _i ( _ie ) inception cavitation number (with ejection) - _ii normal stresses - viscosity - v kinematic viscosity - p specific mass     

新型抗空泡翼型剖面设计研究

基于Eppler翼型设计方法提出一种新的抗空泡翼型剖面设计方法。新的设计方法利用预先给定的最大厚度和设计升力系数进行翼型剖面设计,并将厚度及拱度的分布与攻角的设置相结合,将所需的厚度与拱度分布转换为合理的攻角分布作为输入,以便用Eppler方法进行翼型剖面设计,并通过对攻角分布进行修改来调整空泡斗的位置。计算表明,所提出的方法对控制翼型剖面的厚度和拱度分布有效,利用该法设计的翼型剖面具有较好的空泡性能。     

系列头体的空泡试验研究：初生空泡与发展空泡形态

学了掌握空泡形态及空泡水动力特征，为工程设计提供依据，本文对五个典型头体在不同空泡数，不同攻角条件下肩空泡形态特征进行了水洞试验研究，建立了平头系列轴对称体在不同攻角条件下的初生空泡数和发展空泡特征几何参数的工程计算公式。     

Study on the cavitation characteristics of cupped foils

The effects of cupping on a 0.7 radius foil section with a maximum thickness ratio of 3.5% of the Gawn-Burrill propeller series were studied both numerically and experimentally. A cupped foil increases its lift as a result of the induced camber by the cup, as the numerical results demonstrate. Also, the minimum pressure location of cupped foil moves from midchord to the cupped position near the trailing edge when the foil is in shock-free entrance. The calculated results and the tests confirm that cupped foil increases lift and may improve the cavitation performance of an underpitched propeller.     

Prediction of tip vortex cavitation inception using coupled spherical and nonspherical bubble models and Navier–Stokes computations

A spherical and a nonspherical bubble dynamics models were developed to study cavitation inception, scaling, and dynamics in a vortex flow. The spherical model is a modified Rayleigh–Plesset model to account for bubble slip velocity and for nonuniform pressures around the bubble. The nonspherical model is embedded in an unsteady Reynolds-averaged Navier–Stokes code with appropriate free-surface boundary conditions and a moving chimera grid scheme around the bubble. The effect of nonspherical deformation and bubble/flow interaction on bubble dynamics is illustrated by comparing spherical and nonspherical models. It is shown that nonspherical deformations and bubble/flow interactions are important for an accurate prediction of cavitation inception. The surface-averaged pressure-modified Rayleigh–Plesset scheme is a significant improvement over the conventional spherical model, and is able to capture the volume changes of a bubble during its capture. It is also a fast scheme for studying scaling. In a preliminary study, the scaling effects on cavitation inception were examined using two different Reynolds numbers owing to two different chord lengths. The nuclei-size effect on the prediction of cavitation inception was also studied, and its important effects are highlighted.     

PIV velocity field measurements of flow around a KRISO 3600TEU container ship model

The main purpose of this investigation was to demonstrate a useful application of the particle image velocimetry (PIV) method to analyze the complex flow characteristics around a ship. For a sample illustration, the KRISO 3600TEU container ship model was chosen. The flow structure in the stern and near-wake region of the model has been investigated experimentally in a circulating water channel. Instantaneous velocity fields measured by the PIV velocity field measurement technique have been ensemble-averaged to give details of flow structures such as the spatial distributions of the local mean velocity, vorticity, and turbulent kinetic energy. The free-stream velocity was fixed at U _o = 0.6m/s, and the corresponding Reynolds number based on the length between perpendiculars was about 9.0 × 10⁵. The container ship model shows a complicated three-dimensional flow structure in the stern and near-wake regions. The PIV results clearly revealed the formation of large-scale bilge vortices in the stern region and their effect on the flow in the near-wake. The results shown here provide valuable information for hull form design and the validation of viscous ship flow codes and of turbulence models.     

Numerical simulation of the flow around two-dimensional partially cavitating hydrofoils

In the present study, a new approach is applied to the cavity prediction for two-dimensional （2D） hydrofoils by the potential based boundary element method （BEM）. The boundary element method is treated with the source and doublet distributions on the panel surface and cavity surface by usethe of the Dirichlet type boundary conditions. An iterative solution approach is used to determine the cavity shape on partially cavitating hydrofoils. In the case of a specified cavitation number and cavity length, the iterative solution method proceeds by addition or subtraction of a displacement thickness on the cavity surface of the hydrofoil. The appropriate cavity shape is obtained by the dynamic boundary condition of the cavity surface and the kinematic boundary condition of the whole foil surface including the cavity. For a given cavitation number the cavity length of the 2D hydrofoil is determined according to the minimum error criterion among different cavity lengths, which satisfies the dynamic boundary condition on the cavity surface. The NACA 16006, NACA 16012 and NACA 16015 hydrofoil sections are investigated for two angles of attack. The results are compared with other potential based boundary element codes, the PCPAN and a commercial CFD code （FLUENT）. Consequently, it has been shown that the results obtained from the two dimensional approach are consistent with those obtained from the others.     

The numerical friction line

This paper presents a study on the numerical calculation of the friction resistance coefficient of an infinitely thin plate as a function of the Reynolds number. Seven eddy-viscosity models have been selected: the one-equation turbulence models of Menter and Spalart–Allmaras; the k-ω two-equation model proposed by Wilcox and its TNT, BSL and SST variants and the two-equation model. The flow has been computed at 14 Reynolds numbers in sets of seven geometrically similar Cartesian grids to allow a reliable estimation of the numerical uncertainty. The effect of the computational domain size has been reduced to negligible levels (below the numerical uncertainty). And the same holds for the iterative and round-off errors. In the finest grids of each set, the numerical uncertainty of the friction resistance coefficient is always below 1%. Special attention has further been given to the solution behaviour in the laminar-to-turbulent transition region. Curve fits have been applied to the data obtained at the 14 Reynolds numbers and the numerical friction lines are compared with four proposals from the open literature: the 1957 ITTC line, the Schoenherr line and the lines suggested by Grigson and Katsui et al. The differences between the numerical friction lines obtained with the seven turbulence models are smaller than the differences between the four lines proposed in the open literature.     

水翼部分空化流动二维数值仿真研究（英文）

In the present study, a new approach is applied to the cavity prediction for two-dimensional(2D) hydrofoils by the potential based boundary element method(BEM). The boundary element method is treated with the source and doublet distributions on the panel surface and cavity surface by the use of the Dirichlet type boundary conditions. An iterative solution approach is used to determine the cavity shape on partially cavitating hydrofoils. In the case of a specified cavitation number and cavity length, the iterative solution method proceeds by addition or subtraction of a displacement thickness on the cavity surface of the hydrofoil. The appropriate cavity shape is obtained by the dynamic boundary condition of the cavity surface and the kinematic boundary condition of the whole foil surface including the cavity. For a given cavitation number the cavity length of the 2D hydrofoil is determined according to the minimum error criterion among different cavity lengths, which satisfies the dynamic boundary condition on the cavity surface. The NACA 16006, NACA 16012 and NACA 16015 hydrofoil sections are investigated for two angles of attack. The results are compared with other potential based boundary element codes, the PCPAN and a commercial CFD code(FLUENT). Consequently, it has been shown that the results obtained from the two dimensional approach are consistent with those obtained from the others.     

Study on the design of propeller blade sections using the optimization algorithm

A new method for designing propeller blade sections is presented. A vortex lattice method is used to evaluate the performance and the time-dependent pressure distribution on the blade surface in a non-uniform flow, while efficient optimization algorithms are used to modify the blade sections. Two different designs were carried out in this study. The first was a design to realize a target pressure distribution in a rotating three-dimensional flow. A two-dimensional wing theory was used to obtain the target pressure distribution. The predicted increase in efficiency and the reduction in the cavity volume were confirmed by model experiments. The second was a design to maximize the propeller efficiency. By this method, the propeller efficiency was improved by 1.2% under the constrains of constant thrust and a prescribed margin for face cavitation.     

基于混合迭代法的二维水翼空泡性能分析(英文)

In order to study cavitation characteristics of a 2-D hydrofoil, the method that combines nonlinear cavitation model and mixed-iteration is used to predict and analyze the cavitation performance of hydrofoils. The cavitation elements are nonlinearly disposed based on the Green formula and perturbation potential panel method. At the same time, the method that combines cavity shape for fixed cavity length (CSCL) iteration and cavity shape for fixed cavitation number (CSCN) iteration is used to work out the thickness and length of hydrofoil cavitations. Through analysis of calculation results, it can be concluded that the jump of pressure and velocity potentially exist between cavitation end area and non-cavitations area on suction surface when cavitation occurs on hydrofoil. In certain angles of attack, the cavitation number has a negative impact on the length of cavitations. And under the same angle of attack and cavitation number, the bigger the thickness of the hydrofoil, the shorter the cavitations length.     

中低速船底部断阶对流场和阻力的影响

为分析断阶对船舶流场及阻力性能的影响,针对一条中低速船舶在断阶位置、断阶高度不同,不计自由面影响且船舶无攻角条件下,采用有限体积法和SIMPLEC算法,耦合求解Navier-Stokes方程和连续性方程。采用SSTk-ω两方程模型,压力采用标准格式;动量方程、雷诺应力封闭方程均采用二阶迎风格式。计算结果与原型船模的对比性分析显示:与原型船模相比,断阶导致船模总粘性阻力均略有增加,断阶位置下游一定范围内的船体壁面切应力以及压力系数均有所增加;断阶高度为船舶吃水的0.037 5%与0.025%相比,断阶高度小阻力增幅小;与14站处断阶相比,位于13站处的断阶方案的船模总阻力系数较大。     

Hydrodynamic loads on ice-class propellers during propeller-ice interaction

Hydrodynamic loads on a propeller blocked with simulated ice were studied using a cavitation tunnel. Comparative predictions were made using a panel method. The propeller was a model of the Canadian Coast Guard's R-class icebreake propeller, and the ice block was simulated using a solid blockage. Experimental results show the open water performance of the propeller, its performance behind a blockage, and the effects of cavitation in these conditions, as well as the loading on the simulated ice block. Panel method predictions were made of the time series propeller performance in the blocked flow. Cavitation during propellerice interaction resulted in a reduction of mean suction load on the ice block. Block load measurements indicated an increase in the oscillation about the mean value of the loads, with a variation in the phase of the loading with respect to blade position as compared with the non-cavitating results. Comparisons of panel method results with the measured block loads support the reliability of the dynamic measurements.List of symbols D propeller diameter - F block drag load - K _T thrust coefficient,T/(n ² D ⁴) - K _B block load coefficient,F/(n ² D ⁴) - K _Q torque coefficient,Q/(n ² D ⁵) - Q propeller torque - T propeller thrust - n propeller rotational speed - J propeller advance coefficientV _A/(nD) - P _A ambient pressure at propeller - P _ATM atmospheric pressure - P _V vapour pressure of water - V _A propeller advance speed - dissolved gas content - _s saturated dissolved gas content at atmospheric pressure - _o open water propeller efficiency - cavitation number, (P _A –P _V )/(0.5(nD)²) - density of water     

大翼展混合驱动无人水下航行器水动力特性研究

为提高无人水下航行器的操纵性和运动控制算法的高效性,对航行器水的动力特性进行分析尤为重要。基于黏性流体理论,采用高效的计算域分区法提高航行器变攻角网格的生成效率,开展不同攻角和航速下的航行器水动力数值预报与分析,定量给出航行器的升阻力、升阻比和俯仰力矩系数随攻角的变化规律,得到航行器上浮和下潜时对应的最优攻角和动压力中心点偏移量,为航行器的运动的姿态控制提供设计和指导依据。分析攻角对航行器压力场、速度场和涡量场的影响规律,为后续优化航行器构型、提高航行器的水动力性能奠定基础。     

基于求解速度势通量的指定压力分布二维翼型设计方法

为了提升翼型的水动力和空泡等性能,指定压力分布的翼型剖面设计的方法多数集中在给定攻角下的翼型剖面的设计,该方法存在计算量较大,收敛性不理想,特别是推广到三维问题时,上述问题尤为突出,限制了翼型设计的进一步发展。文章以势流理论面元方法为基础,通过求解指定压力分布条件下翼型表面的速度势通量,获得翼型表面形状的修正量,并将修正量分解为攻角的变化以及剖面自身的变化两部分,从而得到了翼型唯一的设计攻角和翼型剖面几何（厚度分布、拱度分布）。文中采用上述方法对二维翼型问题进行了设计验证,表明该方法可以设计任意指定压力分布的翼型剖面,理论上该方法可以用于全三维翼型的设计问题。     

Influence of turbulence closure models on the vortical flow field around a submarine body undergoing steady drift

Manoeuvring underwater vehicles experience complex three-dimensional flow. Features include stagnation and boundary layer separation along a convex surface. The resulting free vortex sheet rolls up to form a pair of streamwise body vortices. The track and strength of the body vortex pair results in a nonlinear increase in lift as body incidence increases. Consequently, accurate capture of the body vortex pair is essential if the flow field around a manoeuvring submarine and the resulting hydrodynamic loading is to be correctly found. This work highlights the importance of both grid convergence and turbulence closure models (TCMs) to the strength and path of the crossflow-induced body vortices experienced by the DOR submarine model at an incidence angle of 15°. Five TCMs are considered; Spalart–Allmaras, k-ε, k-ω, shear stress transport, and the SSG Reynolds stress model. The SSG Reynolds stress model shows potential improvements in predicting both the path and strength of the body vortex over standard one- and two-equation TCMs based on an eddy viscosity approach.     

导管螺旋桨叶梢泄漏涡机理研究及一种推迟梢涡空化的方法

文章运用数值模拟研究了导管螺旋桨叶梢泄漏涡机理,并提供了一种推迟泄漏涡空化的方法。通过分析不同间隙时的泄漏涡的空泡数发现,随着间隙增大,最小空泡数发生了位置向下游移动,靠近桨叶吸力面,其数值减小。间隙尺寸影响梢涡空泡数的机理很复杂,可以从三个方面来解释：第一、间隙大小改变了泄漏流动速度,从而影响了泄漏涡水动力参数;第二、间隙大小影响叶梢区域压强分布,泄漏涡压强随之改变;第三、间隙大小改变了间隙内黏流的产生和发展,影响泄漏涡黏性分布。而且叶梢泄漏涡核是导管螺旋桨空泡初生位置。文中研究一种叶梢喷射流方法降低叶梢涡核压降,推迟空化初生,探究了不同喷射速度和喷射间隙高度、喷射角度对泄漏涡空泡数的影响。从模拟结果看,叶梢喷射可以降低叶梢翼型载荷,改善泄漏涡核压强分布,推迟泄漏涡空化。比较不同喷射速度,发现速度越高,越能有效提高泄漏涡核压强,速度足够高时甚至可以“吹掉”涡核;研究不同喷射间隙高度发现,间隙越高,越能有效提高泄漏涡核压强,但由于会降低了叶梢更大区域的载荷,降低了桨叶推力;喷射角度的研究表明,轻微的正向预旋对推迟叶梢空化有利。     

小攻角下水下高速航行体超空泡流特性研究

为研究小攻角下水下高速航行体超空泡形态及水动力特性,利用商业CFD软件Fluent6.2,对小攻角下高速航行体超空泡流进行数值模拟,分析了空化数、攻角对水下高速航行体空泡形态以及水动力特性的影响规律.研究表明,攻角为能够明显改变空泡形态的轴对称性;攻角越大,航行体的阻力系数也增大,不利于超空泡的减阻,甚至会导致航行体的失稳.研究结果将为开展水下高速航行体超空泡实验研究提供理论参考.     

Large-eddy simulation of cross-flow around ship sections

The present work is motivated by phenomena occurring in the flow field around ship-like bodies with an incoming lateral flow (cross-flow, 90 ° drift angle). Three-dimensional unsteady flows around different ship sections are investigated by using computational fluid dynamic (CFD) tools with large-eddy simulation (LES) subgrid-scale turbulence model. The simulation results are compared to measurements at several Reynolds numbers in the 90–200,000 range. Focus in our investigation is on the characterization of the motion of vortex structures generated by the separated flow. Another target in the study is to obtain better knowledge of the hydrodynamic forces acting on the sections. Computed pressure and drag coefficients are compared with experimental measurements. The comparison between simulations and measurements shows that an LES model can predict the flow field around ship sections in detail and the hydrodynamic forces acting on the sections.