Detached Eddy Simulation方法模拟不同类型翼型的失速特性

应用DES(DetachedEddySimulation)方法数值模拟了3种不同失速类型的翼型的升力特性。DES方法结合了RANS(ReynoldsaveragedNavierStokes)和LES(LargeEddySimulationapproaches)的优点。基于SpalartAllmaras湍流模型,在近壁面DES体现为RANS模型的特点而在远离物面处又具有LES的亚格子模型的特性。对此模型使用了LUSGS隐式格式求解。通过和实验结果对比,显示这种方法可以有效地预测翼型的失速特性。

Analysis of the Pump-turbine S Characteristics Using the Detached Eddy Simulation Method

Current research on pump-turbine units is focused on the unstable operation at off-design conditions, with the characteristic curves in generating mode being S-shaped. Unlike in the traditional water turbines, pump-turbine operation along the S-shaped curve can lead to difficulties during load rejection with unusual increases in the water pressure, which leads to machine vibrations. This paper describes both model tests and numerical simulations. A reduced scale model of a low specific speed pump-turbine was used for the performance tests, with comparisons to computational fluid dynamics（CFD） results. Predictions using the detached eddy simulation（DES） turbulence model, which is a combined Reynolds averaged Naviers-Stokes（RANS） and large eddy simulation（LES） model, are compared with the two-equation turbulence mode results. The external characteristics as well as the internal flow are for various guide vane openings to understand the unsteady flow along the so called S characteristics of a pump-turbine. Comparison of the experimental data with the CFD results for various conditions and times shows that DES model gives better agreement with experimental data than the two-equation turbulence model. For low flow conditions, the centrifugal forces and the large incident angle create large vortices between the guide vanes and the runner inlet in the runner passage, which is the main factor leading to the S-shaped characteristics. The turbulence model used here gives more accurate simulations of the internal flow characteristics of the pump-turbine and a more detailed force analysis which shows the mechanisms controlling of the S characteristics.

Detached-eddy Simulation for Time-dependent Turbulent Cavitating Flows

The Reynolds-averaged Navier-Stokes（RANS）,such as the original k-ω two-equation closures,have been very popular in providing good prediction for a wide variety of flows with presently available computational resource.But for cavitating flows,the above equations noticeably over-predict turbulent production and hence effective viscosity.In this paper,the detached eddy simulation（DES） method for time-dependent turbulent cavitating flows is investigated.To assess the state-of-the-art of computational capabilities,different turbulence models including the widely used RANS model and DES model are conducted.Firstly,in order to investigate the grid dependency in computations,different grid sizes are adopted in the computation.Furthermore,the credibility of DES model is supported by the unsteady cavitating flows over a 2D hydrofoil.The results show that the DES model can effectively reduce the eddy viscosities.From the experimental validations regarding the force analysis,frequency and the unsteady cavity visualizations,more favorable agreement with experimental visualizations and measurements are obtained by DES model.DES model is better able to capture unsteady phenomena including cavity length and the resulting hydrodynamic characteristics,reproduces the time-averaged velocity quantitatively around the hydrofoil,and yields more acceptable and unsteady dynamics features.The DES model has shown to be effective in improving the overall predictive capability of unsteady cavitating flows.

Detached-eddy simulation of flow around high-speed train on a bridge under cross winds

In order to describe an investigation of the flow around high-speed train on a bridge under cross winds using detached-eddy simulation(DES), a 1/8th scale model of a three-car high-speed train and a typical bridge model are employed, Numerical wind tunnel technology based on computational fluid dynamics(CFD) is used, and the CFD models are set as stationary models. The Reynolds number of the flow, based on the inflow velocity and the height of the vehicle, is 1.9×10~6. The computations are conducted under three cases, train on the windward track on the bridge(WWC), train on the leeward track on the bridge(LWC) and train on the flat ground(FGC). Commercial software FLUENT is used and the mesh sensitivity research is carried out by three different grids: coarse, medium and fine. Results show that compared with FGC case, the side force coefficients of the head cars for the WWC and LWC cases increases by 14% and 29%, respectively; the coefficients of middle cars for the WWC and LWC increase by 32% and 10%, respectively; and that of the tail car increases by 45% for the WWC whereas decreases by 2% for the LWC case. The most notable thing is that the side force and the rolling moment of the head car are greater for the LWC, while the side force and the rolling moment of the middle car and the tail car are greater for the WWC. Comparing the velocity profiles at different locations, the flow is significantly influenced by the bridge-train system when the air is close to it. For the three cases(WWC, LWC and FGC), the pressure on the windward side of train is mostly positive while that of the leeward side is negative. The discrepancy of train's aerodynamic force is due to the different surface area of positive pressure and negative pressure zone. Many vortices are born on the leeward edge of the roofs. Theses vortices develop downstream, detach and dissipate into the wake region. The eddies develop irregularly, leading to a noticeably turbulent flow at leeward side of train.

Comparison of Passive Control Methods on Cavity Aeroacoustic Using Delayed Detached Eddy Simulation

Delayed detached eddy simulation(DDES)is performed to investigate an open cavity at Ma0.85.Clean cavity and cavity with leading-edge saw tooth spoiler and flattop spoiler,are modeled.The results obtained from clean cavity prediction are compared with experimental sound pressure level(SPL)data from QinetiQ,UK.DDES results agree well with the experimental data.Furthermore,comparisons are made with the predicted SPL between the three configurations to find out the effect of different passive control methods.Both the spoilers can suppress the over-all SPL up to 8dB.The main focuses of this investigation are to exam the DDES method on cavity aeroacoustic analysis and test the noise suppression effect by saw tooth spoiler and flattop spoiler.

The Influence of Inflow Condition on the Generation of Tumbling Flow Using Detached Eddy Simulation

One of the objectives of car manufacturers is to improve engine performance, reduce consumption reduce emissions. To achieve this objective, it is important to understand the phenomena involved in the combustion chambers of engines. These phenomena are numerous and complex in nature such: the aerodynamics, fuel-air mixture, turbulence, combustion and the cycle to cycle instabilities that cause more problems. One of the factors responsible for the phenomenon of cycle to cycle variations is the instability of the characteristics of the vortex flow Tumble. This instability may be due to changes in initial conditions. This study is achieved in order to contribute in a better understanding of engine flow by using a Detached Eddy Simulation Shear-Stress Transport (DES SST) model, which is a hybrid RANS/LES model. These simulations have been performed with the commercial CFD (computational fluid dynamic) code (FLUENT) coupled with our own development based on UDF facilities given by FLUENT. To explore the suitability of the 3D DES STT to simulate the internal flow, the calculation is performed for a model tumbling flow at constant volume. This flow has been measured in an experimental set up and measurements are used to initiate and to validate simulations. For this case study, we consider simplified engine geometry. To generate tumbling motion, we use non-reacting DES with a single cycle (SC) strategy. Also, with this strategy we study the effect of initial conditions on the instabilities that accompany a vortex type tumble.

Detached-eddy simulation of supersonic turbulent flow over rearward facing step

Detailed formulation of the detached-eddy simulation （DES） based on shear stress transport model （SST） was deduced. Based on finite volume method, detached-eddy simulation of super- sonic turbulent flow over rearward facing step was conducted. Multi-block structured non-conform grid was designed. The obtained distribution of wall pressure coefficient along the downstream of the step was in good agreement with experiment. The structure of supersonic flow over rearward facing step is complicated and contains boundary layer-shock interaction, shock-shock interaction and sec- ondary flow separation. The predictions of the corner pressure and location of the reattachment shock are more exact than the Reynolds-averaged method. Results show that detached-eddy simulation can simulate the supersonic separated flows exactly and has advantages over Reynolds-averaged method. The partition of the detached-eddy simulation can be easily achieved by using multi-block non-conform grid, which reduces the requirement of the computational resources as well.

基于DDES耦合FW-H方法的吊舱推进器水动力噪声特性

为研究吊舱推进器非定常流场和诱发的声场结构,采用DDES耦合FW-H方法,对其开展了高精度非定常数值计算.结果表明:由于吊柱诱发的强烈动静干涉作用,使螺旋桨至吊柱区域内的压力脉动幅值上升;吊舱推进器的涡量分布较为复杂,由于吊舱体的扰流作用,除螺旋桨本身产生的主、副涡外,还出现干涉尾涡结构,且主涡区向上偏移.通过计算盘面上各个声接收点处的谐波频率标准差发现,声场在浆盘面上无明显声指向性.通过设置空间曲面内的声接收点阵列,获得了吊舱推进器的声场空间分布特性.发现轴频和叶频始终是其主要频率;在螺旋桨桨盘面附近的声压脉动无明显指向性,但在空间曲面上,轴频和叶频的分布特征明显,存在有明显形状的高低值区;在吊舱推进器后方,声压逐渐衰减,但叶频处能量衰减趋势小于轴频处.

CFD Simulation of Local and Global Mixing Time in an Agitated Tank

The Issue of mixing efficiency in agitated tanks has drawn serious concern in many industrial processes. The turbulence model is very critical to predicting mixing process in agitated tanks. On the basis of computational fluid dynamics（CFD） software package Fluent 6.2, the mixing characteristics in a tank agitated by dual six-blade-Rushton-turbines（6-DT） are predicted using the detached eddy simulation（DES） method. A sliding mesh（SM） approach is adopted to solve the rotation of the impeller. The simulated flow patterns and liquid velocities in the agitated tank are verified by experimental data in the literature. The simulation results indicate that the DES method can obtain more flow details than Reynolds-averaged Navier-Stokes（RANS） model. Local and global mixing time in the agitated tank is predicted by solving a tracer concentration scalar transport equation. The simulated results show that feeding points have great influence on mixing process and mixing time. Mixing efficiency is the highest for the feeding point at location of midway of the two impellers. Two methods are used to determine global mixing time and get close result. Dimensionless global mixing time remains unchanged with increasing of impeller speed. Parallel, merging and diverging flow pattern form in the agitated tank, respectively, by changing the impeller spacing and clearance of lower impeller from the bottom of the tank. The global mixing time is the shortest for the merging flow, followed by diverging flow, and the longest for parallel flow. The research presents helpful references for design, optimization and scale-up of agitated tanks with multi-impeller.

Applications of scale-adaptive simulation technique based on one-equation turbulence model

A modified scale-adaptive simulation （SAS） technique based on the Spalart- Allmaras （SA） model is proposed. To clarify its capability in prediction of the complex turbulent flow, two typical cases are carried out, i.e., the subcritical flow past a circular cylinder and the transonic flow over a hemisphere cylinder. For comparison, the same cases are calculated by the detached-eddy simulation （DES）, the delayed-detached eddy simulation （DDES）, and the XY-SAS approaches. Some typical results including the mean pressure coefficient, velocity, and Reynolds stress profiles are obtained and compared with the experiments. Extensive calculations show that the proposed SAS technique can give better prediction of the massively separated flow and shock/turbulent-boundary-layer interaction than the DES and DDES methods. Furthermore, by the comparison of the XY-SAS model with the present SAS model, some improvements can be obtained.

DETACHED EDDY SIMULATION OF HYDRAULIC CHARACTERISTICS ALONG THE SIDE-WALL AFTER A NEW ARRANGEMENT-SCHEME OF THE SUDDEN LATERAL ENLARGEMENT AND THE VERTICAL DROP

In order to study the cavitation damage in a side-wall when a sudden lateral enlargement and a vertical drop are imposed at the radial gate, a new arrangement-scheme is proposed, where the sudden lateral enlargement and the vertical drop can be imposed at the outlet of the gate chamber. The hydraulic characteristics along the side-wall are simulated by the detached eddy simulation and the Volume Of Fluid （VOF） method. The numerical results agree well with those of experiment. The experimental and numerical results show that the flow condition is smooth with only a weak water-wing appearing behind the lateral cavity, and the length of the lateral cavity becomes longer and is mainly affected by the size of the lateral enlargement and the zone of negative pressure after the water impacts the side-wall would disappear. The hydraulic characteristics of the new arrangement-scheme are beneficial to the prevention of the cavitation damage in the side-wall and the Detached Eddy Simulation （DES） with the VOF method can well predict the hydraulic characteristics after the new arrangement-scheme of the sudden lateral enlargement and the vertical drop.

Detached-eddy simulation of wing-tip vortex in the near field of NACA 0015 airfoil

In the present study, the formation of the wing-tip vortex from a rectangular NACA0015 wing with a square tip at the Reynolds number of 1.8× 105 and the angles of attack （AOA） α = 8° and 10° were simulated with an incompressible detached eddy simulation （DES） method and the Reynolds averaged Navier-Stokes （RANS） equations with the SA model respectively. Numerical results were compared with experimental results to validate the capability of the employed methods in resolving tip vortex flows. The results show that DES model could capture the complicated three-dimensional structures in the vortex, and the streamwise vorticity and the cross-flow velocity agree with the experiment results quite well, but RANS-SA model with the same grid as that of DES failed to capture the correct structures and under-predicted the streamwise vorticity in the vortex by 40%. The present study suggests that under the same calculation cost, DES but not RANS-SA could be used to effectively predict the flow characteristics in tip vortex.

基于DDES方法的汽油机进气流动湍流特性

采用基于切应力输运(SST)耦合延迟分离涡模拟(DDES)方法,对某汽油机进行了稳流试验条件下的数值模拟,并采用光学发动机上3D-粒子图像测速(PIV)试验数据对结果进行了验证,在此基础上研究了缸内湍流场的空间域及频域特征.通过与试验及雷诺平均(RANS)数值模拟结果的对比发现,DDES方法对发动机缸内复杂流动模拟有明显优势.结果表明:滚流中心面内高气门升程下缸内流动对称趋势较低气门升程工况更好,三维流动区域主要集中在燃烧室及进气门下方区域.平均流雷诺数决定了缸内流场湍流能谱的整体特征,平均流雷诺数越高的区域,湍流能量进入惯性输运的涡团尺度越小.湍流雷诺数主要影响惯性子区长度,湍流雷诺数越高,湍流发展越充分,惯性子区越明显.湍流脉动强度的平方代表湍流能谱所包围的面积;相对湍流脉动强度则影响低频湍流能量密度的高低.

DES与DDES在湍流分离中的原理与性能研究

随着工程上流动结构的日益复杂,兼具雷诺平均Navier-Stokes(RANS)方法高效率与大涡模拟(LES)高精度的分离涡模拟(DES)类混合方法成为现阶段工程中最有效的湍流模拟方法之一。围绕DES类混合方法中的DES与延迟分离涡模拟(DDES)方法开展工作,分析二者开关函数构造上的不同,研究延迟因子作用机理,并考察DES与DDES方法的求解能力。研究表明:DES与DDES方法在模拟表现上存在一定差异,DDES方法通过引入延迟因子,保护RANS求解区域,改善模化应力不足,降低了DDES方法对交界面系数CDES敏感程度;DDES方法在计算过程中容易出现过度保护,导致求解瞬时涡结构能力不如DES方法,分析与延迟因子引入比重及开关函数构造形式有关。

吹风比和肋板对叶片尾缘开缝气膜冷却特性的影响

为了阐明吹风比和肋板对燃气透平叶片尾缘开缝区域气膜冷却性能的影响,采用延迟-分离涡模拟方法求解了尾缘开缝模型的流量系数、非定常流场结构和气膜冷却效率,采用实验数据考核了延迟-分离涡模拟方法对流量系数和气膜冷却效率预测的有效性,获得了使尾缘开缝壁面气膜冷却效率最佳吹风比。结果表明:流量系数随吹风比增加而增大,但吹风比大于0.65后,流量系数几乎不受吹风比影响;在吹风比0.20~0.65范围内,尾缘开缝壁面气膜冷却效率随吹风比增加而增加;在吹风比0.80~1.25范围内,冷热气掺混剧烈,冷却效率略微下降;肋板结构增加了冷气通道的阻塞效应,并限制了开缝区域冷气旋涡的发展,导致展向涡提前扭曲、变形和分解;相对于无肋板结构,带肋尾缘开缝结构的流量系数下降了约5%,气膜冷却性能提升了约10.8%。开缝下游的旋涡脱落和冷热气流间的掺混是影响开缝壁面冷却效率的主要原因,综合考虑冷气消耗和气膜冷却效率,无肋板时最佳吹风比为0.65,带肋板时最佳吹风比为0.5。

Delayed Detached Eddy Simulation of Subcritical Flow past Generic Side Mirror

In the present study, the subcritical flow past a generic side mirror on a base plane is investigated at the Reynolds number of 5.2 × 10~5 using delayed detached eddy simulation(DDES) turbulence model. Asides from the capability of capturing main features of the large recirculation vortex in the wake of the side mirror and the front horseshoe vortex, the accuracy of DDES estimation of recirculation length is significantly increased by over20%, compared to the detached eddy simulation(DES) estimation using the same grid. And DDES prediction of pressure coefficient at the trailing edge of the mirror is in good agreement with the experiments, which is more accurate than both DES and large eddy simulation(LES) results. The results verify the capacity of DDES turbulence model to solve the turbulent flow around the side mirror. This is a key foundation for possible future study of full simulation of external flow field of vehicle.

Constrained large-eddy simulation and detached eddy simulation of flow past a commercial aircraft at 14 degrees angle of attack

With the development of computational power and numerical algorithms,computational fluid dynamics(CFD) has become an important strategy for the design of aircraft,which significantly reduces the reliance on wind-tunnel and flight tests.In this paper,we conducted a numerical investigation on the flow past a full commercial aircraft at Mach number 0.2 and 14 degrees angle of attack by means of Reynolds-averaged Navier-Stokes(RANS),detached-eddy simulation(DES) and our newly developed constrained large-eddy simulation(CLES).The objective of this paper is to study the capability of these models in simulating turbulent flows.To our knowledge,this is the first large-eddy simulation method for full commercial aircraft simulation.The results show that the CLES can predict the mean statistical quantities well,qualitatively consistent with traditional methods,and can capture more small-scale structures near the surface of the aircraft with massive separations.Our study demonstrates that CLES is a promising alternative for simulating real engineering turbulent flows.

Detached-eddy simulation of turbulent coherent structures around groynes in a trapezoidal open channel

The hydrodynamics in a straight open channel with a multiple-embayment groyne field was investigated using the detached-eddy simulation(DES).A series of short groynes were included on a 1:3 side slope of the channel.This work focuses on the turbulent coherent structures around groynes on an uneven bottom.Flows around groyne fields are characterized by massive separation and highly unsteady vortices.DES can capture a wide spectrum of eddies at a lower computational cost than the large eddy simulation(LES)or direct numerical simulation(DNS).In the present work,a zonal DES model(ZDES)was used to simulate the flow around groynes.The ZDES model is a modified version of the DES designed to overcome the model-stress depletion(MSD)of the RANS/LES hybrid model.The vortex system consists of the horseshoe vortex(HV)formed at the base of the obstructions,the necklace vortex(NV)that wrapped the groyne tips near the free surface,and the shedding vortex(SV)underneath the free surface.The effects of the incident flow and local topography on the vortex evolution were investigated by analyzing the mean flow structures and the instantaneous turbulent flow fields.Some important vortices cannot be captured because of the averaging process,while some flow structures cannot be observed in the instantaneous flow.The mean flow is only a reflection of the averaging process when complex vortices are present.

LARGE-EDDY AND DETACHED-EDDY SIMULATIONS OF THE SEPARATED FLOW AROUND A CIRCULAR CYLINDER

The separated turbulent flow around a circular cylinder is investigated using Large-Eddy Simulation （LES）, Detached-Eddy Simulation （DES, or hybrid RANS/LES methods）, and Unsteady Reynolds-Averaged Navier-Stokes （URANS）. The purpose of this study is to examine some typical simulation approaches for the prediction of complex separated turbulent flow and to clarify the capability of applying these approaches to a typical case of the separated turbulent flow around a circular cylinder. Several turbulence models, i.e. dynamic Sub-grid Scale （SGS） model in LES, the DES-based Spalart-Allmaras （S-A） and κ-ω Shear-Stress- Transport （SST） models in DES, and the S-A and SST models in URANS, are used in the calculations. Some typical results, e.g., the mean pressure and drag coefficients, velocity profiles, Strouhal number, and Reynolds stresses, are obtained and compared with previous computational and experimental data. Based on our extensive calculations, we assess the capability and performance of these simulation approaches coupled with the relevant turbulence models to predict the separated turbulent flow.

Towards an entropy-based detached-eddy simulation

A concept of entropy increment ratio()is introduced for compressible turbulence simulation through a series of direct numerical simulations(DNS). represents the dissipation rate per unit mechanical energy with the benefit of independence of freestream Mach numbers.Based on this feature,we construct the shielding function fs to describe the boundary layer region and propose an entropy-based detached-eddy simulation method(SDES).This approach follows the spirit of delayed detached-eddy simulation(DDES)proposed by Spalart et al.in 2005,but it exhibits much better behavior after their performances are compared in the following flows,namely,pure attached flow with thick boundary layer(a supersonic flat-plate flow with high Reynolds number),fully separated flow(the supersonic base flow),and separated-reattached flow(the supersonic cavity-ramp flow).The Reynolds-averaged Navier-Stokes(RANS)resolved region is reliably preserved and the modeled stress depletion(MSD)phenomenon which is inherent in DES and DDES is partly alleviated.Moreover,this new hybrid strategy is simple and general,making it applicable to other models related to the boundary layer predictions.