4气门DISI汽油机缸内湍流场POD分析

应用本征正交分解(POD)方法对一台4气门直喷式汽油机(DISI)缸内冷态湍流流场试验测量数据以及大涡模拟计算数据进行了分析,以深入考察该汽油机缸内湍流场拟序结构特性以及湍流场循环变动特性.结果表明,POD方法可以将流场湍流涡团结构按照含能数量进行分解,大尺度拟序结构和小尺度随机脉动涡团可以被有效分离,为研究其各自的形成与演化特征创造了条件.考察各模态的时间系数在不同周期间的统计变化规律,可以了解流场湍流结构的循环变动特性.相比较使用传统相平均方法来研究汽油机循环变动,POD方法可以单独研究不同能量涡团结构的循环变动特点,为进一步深入理解循环变动特性奠定基础.     

网格密度对内燃机冷态流场大涡模拟的影响

通过修改发动机多维CFD计算程序KIVA-3V,建立了内燃机压缩过程冷态流场的大涡模拟(LES)计算模型.利用此模型对内燃机压缩过程中缸内流场的水平速度及湍流动能进行分析,同时,分析了网格密度对内燃机缸内流场大涡模拟的影响.结果表明,当采用k-ε模型计算时,网格的精细程度对流场结构影响不大;在相同的计算网格下,与采用k-ε模型计算相比,采用LES计算显示了更为复杂的湍流结构,而且LES所能捕捉到的涡团结构范围要大于k-ε模型,计算得到的湍流动能也要低于k-ε模型;同时,网格越精细,这种效应越明显.     

四气门汽油机进气与压缩过程缸内流场的大涡模拟

通过修改发动机多维CFD计算程序KIVA-3V,建立了内燃机缸内冷态流场的大涡模拟(LES)计算模型.利用此模型对4气门汽油机进气与压缩过程中缸内流场进行了详细分析,并与k-ε模型进行了比较.研究结果表明:与采用k-ε模型计算时相比,采用LES计算时显示了更为复杂的湍流结构;采用LES计算时除了能够得到有序的大尺度涡团结构外,还能捕捉到大量不规则的小涡团结构,而采用k-ε模型计算时基本上捕捉不到不规则的小涡团结构.     

4气门直喷式汽油机缸内湍流场多周期循环变动的大涡模拟

应用大涡模拟方法对一台4气门直喷式汽油机(DISI)缸内冷态湍流流场进行了三维瞬态数值分析.通过连续13个工作循环的模拟计算,探索缸内湍流流动的循环变动特征与规律,并与PIV流场测试结果进行了对比.模拟相平均值及循环变动的均方值和试验值在总体上吻合得较好.计算结果表明:在进气过程前期缸内流场湍流脉动和循环变动都很强烈,两者强度为同一量级;但在后续过程中,湍流脉动不断衰减,其与循环变动的比值小于15%.大涡模拟方法不仅可以真实地反映内燃机循环过程中缸内气体流动的细节和规律,而且非常适合于研究内燃机的循环变动特性.     

湍流扩散火焰的大涡模拟及验证

对丙烷/空气射流扩散火焰进行了大涡模拟,并就截面流向速度无量纲分布、速度脉动无量纲分布、温度无量纲分布以及NOx生成与Sandia实验室测量数据进行了对比分析,得出大涡模拟方法可以较好地模拟湍流扩散火焰的结论.在此基础上对拟序结构与化学反应的相互影响以及雷诺数对各尺度的影响进行了研究,结果表明:在流场的起始阶段,燃烧对拟序结构的形成有促进作用;在发展阶段,燃烧对大涡拟序结构有削弱作用,却能使小涡的生成增加,从而增强了化学反应;大涡拟序结构的存在改变了各参数的分布;随着雷诺数的增大,各尺度横向脉动均变大,预测到的NOx减小,产生的最大值前移.     

稀疏化动态模态分解算法在发动机缸内流场研究中的应用

采用大涡数值模拟方法模拟了发动机缸内冷态流场,连续计算100个周期,获得了缸内多循环流场数据库,模拟结果通过粒子图像速度场测量技术(PIV)测量试验进行了验证.然后,采用动态模式分解(DMD)算法分析动态非线性系统流场数据库,以识别其流动特性.结果表明:DMD算法能够有效识别缸内涡团脉动频率,提取对应的流场结构,有利于发现在发动机整个工作过程中具有大衰变率的不稳定流场结构.此外,改进的"稀疏化"DMD算法可有效地对最重要的流场结构进行低维近似,这将有利于寻找影响和控制发动机缸内流场动态演化的方法.     

基于大涡模拟方法的二冲程发动机缸内冷态流场的模拟研究

应用大涡模拟方法对一台二冲程发动机缸内冷态湍流流场进行了三维瞬态数值分析。探索缸内流场的速度、压力及温度的变动情况,并与PIV流场测试结果进行了对比。三维模拟的缸压曲线与试验及一维模拟均吻合较好,但在速度概率密度函数方面与试验存在一定差异。模拟结果表明:本文建立的三维模型能够自然再现缸内冷态流动的随机大尺度涡流情况,模型可靠有效。模拟能够较好地反应火花塞位置附近速度随机波动情况,其结果对于后续研究缸内大尺度涡流对燃烧循环变动的影响机理具有一定的指导意义。     

内燃机压缩过程冷态流场的大涡模拟

通过修改发动机多维CFD计算程序KIVA-3V,建立了内燃机压缩过程冷态流场的大涡模拟（LES）计算模型。利用此模型对内燃机压缩过程中缸内流场的水平速度及湍流动能进行了详细分析,并与k-ε模型进行了比较。结果表明与采用k-ε模型计算时相比,采用LES计算时显示了更为复杂的湍流结构,而且LES所能捕捉到的涡团结构范围要大于k-ε模型。同时,采用LES计算时得到的湍流动能要远远低于k-ε模型。     

可变滚流对缸内流动循环变动影响的试验研究

利用PIV技术在一台基于直喷汽油机(gasoline direct injection,GDI)改造的光学发动机上测量了缸内滚流运动,通过进气道入口处翻板和进气道内挡板改变缸内滚流比,并利用本征正交分解(POD)方法将流场分解为平均流场、拟序流场、过渡流场和湍流流场,分析滚流运动对气流循环变动的影响。试验结果表明:翻板和挡板的组合能有效改变流场结构,使GDI汽油机缸内形成大尺度的单一滚流,滚流比提高近三倍。通过本征正交分解分析发现,拟序流场中拟序结构涡团的变动是缸内气流循环变动的主要来源。大尺度强滚流使平均流场占能比例大幅提升达30%,减少了能量向拟序流场的传递,使拟序流场循环变动降低近50%,从而抑制了缸内气流运动整体的循环变动。     

直喷式汽油机缸内涡量场的本征正交分解

直喷式汽油机的混合气形成质量依赖于缸内复杂湍流场.对缸内湍流场特性的全面认识和合理的控制有利于实现发动机的高效清洁燃烧.首先采用大涡模拟数值计算和粒子图像测速相结合的方式对缸内流场展开研究,获取了缸内瞬态涡量场信息,发现缸内大尺度涡团主要由进气射流和缸内原有流场的相互剪切作用产生,同时在不同周期之间,涡量场存在着明显的循环变动.然后应用本征正交分解方法对所获得的流场数据库进行深加工,更加科学和全面研究了缸内流场单一周期内的演变和多周期间的循环变动.同时还可以证明本征正交分解方法可以用来作为验证发动机缸内湍流场数值模拟结果准确性的有效工具.     

Large eddy simulation and preliminary modeling of the flow downstream a variable geometry swirler for gas turbine combustors

This work presents a novel swirler with variable blade configuration for gas turbine combustors and industrial burners. The flow dynamics downstream the swirler was explored using Large Eddy Simulation (LES). The resolved turbulence kinetic energy in the region where the flow exhibits the main flow phenomena was well above 80% of the total turbulent kinetic energy of the flow. It was evidently shown that the new swirler produces a central recirculation zone and a Rankine vortex structure which are necessary for swirl flame stabilization. Two Reynolds-averaged NavierStokes (RANS) simulation cases utilizing the standard and realizable k-ε turbulence models were also conducted for two objectives. The first is to demonstrate the validity of RANS/eddy-viscosity models in predicting the main characteristics of swirling flows with comparison to the LES results. The second objective is to comparatively investigate the flow features downstream the new swirler in both co-rotating and counter-rotating blade configurations. The results show that the counter-rotating configuration produces higher turbulence kinetic energy and more compact recirculation zone compared to the co-rotating configuration.     

基于RANS和LES的汽油、柴油及其掺混燃料压燃燃烧特性数值模拟研究

研究中首先采用RANS和LES两种湍流模型对汽油、柴油及汽柴油掺混燃料的喷雾进行了气液相贯穿距的标定。基于标定好的喷雾模型,采用RANS与LES对3种燃料在发动机中的燃烧过程开展了数值模拟研究。通过对比RANS与LES对部分预混燃烧数值模拟的差异,揭示了两种湍流模型对缸内流动、燃料输运及燃烧过程的影响机理。结果表明,RANS与LES都能够对柴油及掺混燃料的燃烧过程实现较好的预测,其中LES对汽油部分预混燃烧中滞燃期及放热规律的预测与试验更为接近。同时,LES对3种燃料NOx排放的计算结果都与试验更加接近,这与燃料在缸内放热的位置密切相关。     

Comparison of Reynolds averaged Navier-Stokes based simulation and large-eddy simulation for one isothermal swirling flow

The flow structure of one isothermal swirling case in the Sydney swirl flame database was studied using two numerical methods. Results from the Reynolds-averaged Navier-Stokes (RANS) approach and large eddy simulation (LES) were compared with experimental measurements. The simulations were applied in two different Cartesian grids which were investigated by a grid independence study for RANS and a post-estimator for LES. The RNG k-ε turbulence model was used in RANS and dynamic Smagorinsky-Lilly model was used as the sub-grid scale model in LES. A validation study and cross comparison of ensemble average and root mean square (RMS) results showed LES outperforms RANS statistic results. Flow field results indicated that both approaches could capture dominant flow structures, like vortex breakdown (VB), and precessing vortex core (PVC). Streamlines indicate that the formation mechanisms of VB deducted from the two methods were different. The vorticity field was also studied using a velocity gradient based method. This research gained in-depth understanding of isothermal swirling flow.     

Numerical simulation of flatback airfoil aerodynamic noise

The present paper presents a possible path for developing a large eddy simulation (LES) applicable to high Reynolds-number complex turbulent flows and the performance of the coupling of LES with statistical turbulence models around the flow over a blunt trailing edge configuration. The turbulent fluctuations in the boundary layers at the inflow region of the LES domain are generated by a synthesized turbulence method. The hybrid RANS-LES model showed considerable improvement in prediction accuracy even at a moderate grid resolution. The aerodynamic comparison with experimental data shows like results for the pressure distributions surrounding a flatback airfoil. To predict accurately the noise radiation from the blunt trailing edge and to save computational costs, the near-field region is computed by embedded LES while the surrounding region is simultaneously computed by RANS. The Brooks, Pope, and Marcolini (BPM) semi-empirical model is used for noise comparison with the hybrid RANS-LES result and experimental data. The present hybrid RANS-LES method is found to be adequate for predicting aerodynamic noise generation by vortical flow in the vicinity of a blunt trailing edge airfoil over a range of frequencies.     

基于分离涡方法的定日镜风效应研究

选用RANS湍流模型、大涡模拟与分离涡模型等计算流体动力学(CFD)方法,应用由新的湍流脉动流场产生方法DSRFG(discretizing and synthesizing random flow generation)模拟风场实际的湍流边界条件,对定日镜结构的风压分布及流场进行分析。将数值模拟结果与风洞试验数据进行对比,给出风向角为0°工况下定日镜结构的风压系数分布规律及特点,分析5种模型下定日镜周围的流场分布和涡量分布。结果表明,RANS模型虽然能够模拟出结构周围流场的基本规律,但却无法模拟出风场中的紊流和涡旋的分离扩散情况。大涡模拟及分离涡模型对风场中的紊流和涡旋的分离扩散有较好的模拟效果,在风压系数的分布上也与风洞试验数据拟合较好。在定日镜周围流场中脉动风压系数的分布上,分离涡模型的模拟结果较大涡模拟更为接近风洞试验值,且耗时更少。     

A FOUR-EQUATION MODEL FOR PREDICTION OF GAS-SOLID TURBULENT FLOWS

A four-equation model is proposed for prediction of dilute turbulent gas-solid flows where the ratio of the particle and the gas densities is large. The model is based on explicit algebraic relations for Reynolds stresses and turbulent fluxes of the void fraction, which were derived in an earlier work within the context of Reynolds-averaged Navier-Stokes (RANS) methodology. These relations are manipulated here to derive nonlinear eddy-viscosity-type models for thin-shear flows. Further, new models are proposed for third-order correlations which are also simplified for thin-shear flows. These models are used to propose four transport equations for the turbulence kinetic energy of the carrier phase and its rate of dissipation, the turbulence kinetic energy of the dispersed phase, and the velocity covariance of the two phases. The final four-equation model is implemented for prediction of a particle-laden turbulent jet, and encouraging agreements with available laboratory data are observed.     

Study on flame-vortex interaction in a spark ignition engine fueled with methane/carbon dioxide gases

A numerical study on the in-cylinder flame-vortex interaction of gaseous spark ignited engine fueled with methane/carbon dioxide is carried out by means of large-eddy method. Evolution of in-cylinder turbulence in charge phase and flame-vortex interaction during combustion process is analyzed in great detail. It's found out that the large scale coherent structures are transformed into homogeneous small scale vortexes during the intake and compression stroke. The strong vortex cores are generated by interaction between flame and in-cylinder background turbulence. Those generated vortex cores wrinkle flame surface and augment turbulent flame speed. The contra-rotation between the two vortexes of vortex-pair in the unburned area results in the appearance of large scale flame wrinkles, which is because the vortex-pair movement leads to the local entrainment and hence stretchs of the flame surface. With the increase of volume fraction of carbon dioxide in the gases, the turbulent flame speed is decreased, the effect of vortex pair on the flame structure is weakened, and the level of the flame wrinkling is decreased correspondingly.     

Effects of free-stream turbulence on high pressure turbine blade heat transfer predicted by structured and unstructured LES

Recent developments and demonstrations for the prediction of turbulent flows around blades point to Large Eddy Simulations (LES) as a very promising tool. Indeed and despite the fact that this numerical method still requires modeling and intense computing effort compared to Reynolds Average Navier–Stokes (RANS), this fully unsteady simulation technique provides valuable information on the turbulent flow otherwise inaccessible. Theoretical limits and scales of wall bounded flows are now well mastered in simple cases but complex industrial applications usually introduce unknowns and mechanisms that are difficult to apprehend beforehand especially with LES which is usually computationally intensive and bounded to code scalability, mesh quality, modeling performances and computer power. In this specific context, few studies directly address the use of fully structured versus unstructured, implicit versus explicit flow solvers and their respective impact for LES modeling of complex wall bounded flows. To partly address these important issues, two dedicated structured and unstructured computational solvers are applied and assessed by comparing the predictions of the heat transfer around the experimental high pressure turbine blade profile cascade of Arts et al. [6]. First, both LES predictions are compared to RANS modeling with a particular interest for the accuracy/cost ratio and improvement of the physical phenomena around the blade. LES’s are then detailed and further investigated to assess their ability to reproduce the inlet turbulence effect on heat transfer and the development of the transitioning boundary layer around the blade. Quantitative comparisons against experimental findings show excellent agreement especially on the pressure side of the profile. Detailed analysis of the flow predictions provided by both the structured and unstructured solvers underline the importance of long stream-wise streaky structures responsible for the augmentation of the heat transfer and leading to the transition of the suction-side boundary layer.