有旋和无旋气固两相湍流的不同结构

了解有旋和无旋突扩气粒两相详细湍流结构对控制燃料-空气混合、火焰稳定以及燃烧污染物生成很重要.周力行等曾经对其中的两相时平均流场和湍流特性进行过雷诺平均的RANS模拟和测量的研究,但是BANS模拟不能给出详细的两相湍流的瞬态结构.本文建立了二阶矩两相亚网格尺度应力模型,对有旋和无旋同轴突扩气粒两相流动进行了双流体大涡模...     

气粒流动两相亚网格尺度动能模型的大涡模拟

目前多数两相流动的大涡模拟采用单相流动的亚网格尺度应力模型,没有考虑或者没有完整地考虑两相亚网格尺度应力间的相互作用。本文提出一种两相亚网格尺度动能模型(LES-kkp),可以完整地考虑其相互作用。将该模型用于突扩气粒两相流动的大涡模拟,同时进行统一二阶矩(USM)两相湍流模型的雷诺平均模拟(RANS)。瞬态结果显示出两相流动中的气体形成了典型的拟序结构,但颗粒流动结构完全不同于气体的,没有形成涡结构。LES-kkp和RANS-USM模拟结果实验检验表明,对时间平均速度两种模拟给出结果与实验结果吻合良好,即RANS-USM得到了LES-kkp的验证。对两相均方根脉动速度和两相脉动速度关联量,LES-k-kp的模拟结果优于RANS-USM的模拟结果。     

圆形射流湍流场的大涡模拟研究

使用发展的大涡模拟并行程序,数值模拟了圆形射流湍流场,与实验值进行详细比较,对大涡模拟程序进行检验,并分析网格尺度对计算结果的影响。结果表明,大涡模拟计算的平均速度场和雷诺应力场与实验值符合得非常好,验证了该大涡模拟计算程序的可靠性和精确度。同时发现一阶统计矩受网格尺度影响不大,二阶及高阶矩与网格尺度有较大的关系,因此使用大涡模拟方法计算湍流及湍流燃烧问题应对网格依赖性进行必要分析,至少保证二阶统计矩具有较好的精度。     

乙醇-空气液雾两相流动和燃烧的大涡模拟

采用动态Smagorinsky亚网格应力模型,EBU(ED,eddy dissipation)燃烧模型和欧拉-拉氏两相流模型,对乙醇-空气湍流两相流动和燃烧进行了大涡模拟(LES).瞬态模拟结果与实验结果定性一致.在火焰瞬态结构的高温区域,旋涡强度较大,在高温区边缘附近存在的拟序结构有脱落的趋势.乙醇液滴通过喷雾器喷出...     

钝体燃烧室热态流场的大涡模拟

对圆盘凸台钝体燃烧室的热态流场进行了大涡模拟,利用PIV实验数据,显示了大涡模拟对平均速度及均方根速度脉动量的预测与实验中的测量值基本吻合,模拟获得的钝体后回流区、钝体边缘存在的切应力层等湍流特征信息也得到了实验验证。采用两种不同亚网格模型的大涡模拟结果与实验对比表明,在流动情况特别复杂的区域,Germano动态亚网格模型对流场,尤其是湍流脉动的预测准确性要高于标准Smagorinsky亚网格模型。     

一种改进后的亚格子特征尺度表达式

针对大涡模拟涡粘性亚格子模式中使用网格尺度为特征尺度存在的不足,提出一种改进的亚格子特征尺度表达式,利用结合Sagaut混合尺度模式获得的改进模式与Smagorinsky模式、Germano模式和Sagaut模式等进行对比研究.时间发展混合层和中性大气边界层的大涡模拟结果表明,改进的有效亚格子特征尺度能合理反映亚格子脉动的时空分布特性,同时扩展了适用范围.在对耗散程度、流动演化过程、可解湍流强度和可解雷诺应力等方面的模拟,改进模式优于基准模式,表明亚格子脉动有效特征尺度在大涡模拟中具有一定的理论基础和广泛的应用价值.     

颗粒增强湍流的新模型和气粒流动的湍流变动

用雷诺应力方程模型和极细的网格系对单个颗粒受湍流气体绕流进行了数值模拟,研究了改变颗粒直径和气体相对速度时颗粒增强气体湍流的规律.据此构造了颗粒尾涡增强气体湍流的新模型.将此子模型加入到两相流动模型中,对竖直和水平通道内气粒两相流动进行了数值模拟,和实验结果的对照表明,考虑颗粒尾涡增强气体湍流效应得到的气体湍流脉动速度的模拟结果比不考虑此效应的模拟结果好得多.     

旋流两相流动的DSM—PDF两相湍流模型

本文提出了气相雷诺应力方程模型和颗粒概率密度输运方程模型相结合而构成的ＤＳＭ—ＰＤＦ两相湍流模型,并用该模型和ｋ－ε－Ｋｐ模型模拟了文献中测量的旋流数为０．４７的旋流突扩气粒两相流动．和测量结果的对照表明,两模型都能较好地模拟族流数不高的两相流动平均速度场,但ＤＳＭ—ＰＤＦ模型可以揭示出两相湍流的各向异性,因此有预报强旋两相流动的潜力．     

旋风分离器上部空间各种二次涡的数值模拟

采用改进的雷诺应力模型对旋风分离器上部环形空间内的强旋湍流流动进行了数值模拟,重点分析了顶板和升气管附近的局部二次涡。数值模拟结果与实验对比表明雷诺应力模型对环形空间内的强旋流动有良好的预报精度。旋风分离器上部环形空间不仅在顶板下存在有二次涡,而且在升气管外壁附近还存在有局部二次涡,局部二次涡的尺度比较小,而且沿周向有变化,呈现非轴对称分布。     

DSM-LPDF两相湍流模型及旋流两相流动的模拟

本文由流体－颗粒速度的拉氏联合概率密度函数（ＰＤＦ）输运方程出发,用Ｓｉｍｏｎｉｎ建议的Ｌａｎｇｅｖｉｎ模型封闭颗粒所遇到流体瞬时速度的条件期望项,并用Ｍｏｎｔｅ Ｃａｒｌｏ方法直接求解 ＰＤＦ输运方程,将其和求解流体雷诺应力方程模型的有限差分方法结合,建立了雷诺应力－拉氏ＰＤＦ（ＤＳＭ－ＬＰＤＦ,简称ＤＬ）两相湍流模型．用此模型模拟了旋流数为０．４７的突扩旋流气粒两相流动,并与文献中ＰＤＰＡ实验和用类似于单相流动湍流模型封闭方法的时平均统一二阶矩（ＵＳＭ）模型的预报进行了对比．     

Constrained large-eddy simulation of separated flow in a channel with streamwise-periodic constrictions

Constrained large-eddy simulation (CLES) method has been recently developed by Chen and his colleagues for simulating attached and detached wall-bounded turbulent flows. In CLES, the whole domain is simulated using large-eddy simulation (LES) while a Reynolds stress constraint is enforced on the subgrid-scale (SGS) stress model for near wall regions. In this paper, CLES is used to simulate the separated flow in a channel with streamwise-periodic constrictions at Re = 10,595. The results of CLES are compared with those of Reynolds-averaged Navier-Stokes (RANS) method, LES, detached eddy simulation (DES) and previous LES results by Breuer et al. and Ziefle et al. Although a coarse grid is used, our results from the present LES, DES and CLES do not show large deviations from the reference results using much finer grid resolution. The comparison also shows that CLES performs the best among different turbulence models tested, demonstrating that the CLES provides an excellent alternative model for separated flows. Furthermore, the cross-comparisons among different CLES implementations have been carried out. Our simulation results are in favor of using the constraint from algebraic RANS model or solving the RANS model equations in the whole domain with a length scale modification according to the idea from DES.     

Investigation of turbulence models for computation of swirling flows

Different turbulence models were studied in application to calculation of swirling flows. The differential models of turbulent viscosity considering streamline curvature and the method of detached eddy simulation were used. Weakly and strongly swirling flows were considered at the example of concentrated vortex in a tube, swirling flow in a diffuser, and vortex breakdown through an abrupt expansion. The RANS models with correction to flow swirling represented well the experimental data for the weakly swirling flows. In case of strongly swirling flows, it was more correct to use the method of detached eddy simulation.     

A new mixed subgrid-scale model for large eddy simulation of turbulent drag-reducing flows of viscoelastic fluids

A mixed subgrid-scale(SGS) model based on coherent structures and temporal approximate deconvolution(MCT) is proposed for turbulent drag-reducing flows of viscoelastic fluids. The main idea of the MCT SGS model is to perform spatial filtering for the momentum equation and temporal filtering for the conformation tensor transport equation of turbulent flow of viscoelastic fluid, respectively. The MCT model is suitable for large eddy simulation(LES) of turbulent dragreducing flows of viscoelastic fluids in engineering applications since the model parameters can be easily obtained. The LES of forced homogeneous isotropic turbulence(FHIT) with polymer additives and turbulent channel flow with surfactant additives based on MCT SGS model shows excellent agreements with direct numerical simulation(DNS) results. Compared with the LES results using the temporal approximate deconvolution model(TADM) for FHIT with polymer additives, this mixed SGS model MCT behaves better, regarding the enhancement of calculating parameters such as the Reynolds number.For scientific and engineering research, turbulent flows at high Reynolds numbers are expected, so the MCT model can be a more suitable model for the LES of turbulent drag-reducing flows of viscoelastic fluid with polymer or surfactant additives.     

A comparative study of turbulence models for non-premixed swirl-stabilized flames

ABSTRACT

The accuracy of turbulent swirl-stabilized flame simulation strongly depends on the choice of turbulence model. In this study, four 3D unsteady turbulence closures, including large eddy simulation, scale-adaptive simulation, and two detached eddy simulation variants, along with four RANS models, including RNG k??, SST k?ω, transition SST, and RSM, are examined for moderate- and high-swirl case studies. It is observed that the scale-adaptive simulation provides the most accurate results for almost all variables and both swirl conditions in the reactive flow. Only the 3D unsteady models predict the vortex breakdown bubble and flame attachment state correctly. However, based on our error analysis, the flow and composition fields predicted by the RANS models are in acceptable agreement with the experimental fields, especially the ones of transition SST when higher swirl number cases or minor species concentration are of interest. Moreover, it is concluded that the viscosity ratio criterion is a better measure of the local LES quality than the turbulent kinetic energy ratio, and the accuracy of a hybrid simulation may be much more dependent on the ability of the model to operate close to the RANS mode where the grid resolution is not sufficient for a resolving simulation than the fraction of the resolved kinetic energy. Finally, the propriety of the base (RANS) model of a DES for the application of interest is important, such that DES with realizable k?? outperforms the commonly used DES with SST k?ω model.     

Visualisation and analysis of large-scale vortex structures in three-dimensional turbulent lid-driven cavity flow

In this paper, large eddy simulation (LES) of a three-dimensional turbulent lid-driven cavity (LDC) flow at Re = 10,000 has been performed using the multiple relaxation time lattice Boltzmann method. A Smagorinsky eddy viscosity model was used to represent the sub-grid scale stresses with appropriate wall damping. The prediction for the flow field was first validated by comparing the velocity profiles with previous experimental and LES studies, and then subsequently used to investigate the large-scale three-dimensional vortical structures in the LDC flow. The instantaneous three-dimensional coherent structures inside the cavity were visualised using the second invariant (Q), Δ criterion, λ₂ criterion, swirling strength (λ_ci) and streamwise vorticity. The vortex structures obtained using the different criteria in general agree well with each other. However, a cleaner visualisation of the large vortex structures was achieved with the λ_ci criterion and also when the visualisation is based on the vortex identification criteria expressed in terms of the swirling strength parameters. A major objective of the study was to perform a three-dimensional proper orthogonal decomposition (POD) on the fluctuating velocity fields. The higher energy POD modes efficiently extracted the large-scale vortical structures within the flow which were then visualised with the swirling strength criterion. Reconstruction of the instantaneous fluctuating velocity field using a finite number of POD modes indicated that the large-scale vortex structures did effectively approximate the large-scale motion. However, such a reduced order reconstruction of the flow based on the large-scale vortical structures was clearly not as effective in predicting the small-scale details of the fluctuating velocity field which relate to the turbulent transport.     

Simulation of spray combustion in a lean-direct injection combustor

Large-eddy simulation (LES) of a liquid-fueled lean-direct injection (LDI) combustor is carried out by resolving the entire inlet flow path through the swirl vanes and the combustor. A localized dynamic subgrid closure is combined with a subgrid mixing and combustion model so that no adjustable parameters are required for both non-reacting and reacting LES. Time-averaged velocity predictions compare well with the measured data. The unsteady flow features that play a major role in spray dispersion, fuel–air mixing and flame stabilization are identified from the simulation data. It is shown that the vortex breakdown bubble (VBB) is smaller with more intense reverse flow when there is heat release. The swirling shear layer plays a major role in spray dispersion and the VBB provides an efficient flameholding mechanism to stabilize the flame.     

Study of vortex core precession in combustion chambers

The article presents the results of experimental investigation of swirling flow of lean propane/air flame in a model combustion chamber at atmospheric pressure. To study the unsteady turbulent flow, the particle image velocimetry technique was used. It was concluded that dynamics of high swirl flows with and without combustion was determined by a global helical mode, complying with a precessing double-spiral coherent vortex structure. The studied low swirl flame had similar size and stability characteristics, but amplitude of the coherent helical structure substantially oscillated in time. The oscillations were associated with intermittently appearing central recirculation zone that was absent in the nonreacting flow. It is expected that the low swirl flow without the permanent central recirculation zone should be more sensitive to an external active control. In particular, this result may be useful for suppression of thermoacoustic resonance in combustion chambers.     

旋流燃烧室内同向和反向旋转射流湍流流动的数值模拟

针对发展高效低污染旋流燃烧技术的需要,对旋流燃烧室内两股同轴旋转射流相互作用的湍流旋流流动进行了数值模拟.计算中采用了一种新的代数Reynolds应力模型和QUICK离散格式.在两股射流同向旋转和反向旋转两种条件下,将模拟得到的燃烧室内湍流旋流流动的时均气体轴向速度场、切向速度场和静压场与实验数据进行了比较.     

Reynolds数22 000的孤立方柱绕流的大涡模拟

方柱绕流是典型的钝体绕流问题,蕴含了丰富的流体力学现象,对这类流动的准确预测面临着诸多挑战.采用自主发展的大涡模拟程序,对来流Mach数M=0.3,Reynolds数Re_D=22 000的绕孤立方柱流动进行了细致模拟,亚格子模型使用动力涡黏模型.对计算结果的分析表明,大涡模拟所得的平均流场及Reynolds应力分布与已有实验数据和直接数值模拟结果均吻合较好,验证了预测结果的可靠性;在此基础上对瞬态流场进行了研究,展示了计算条件下方柱绕流分离转捩及尾迹区旋涡交替脱落形成Karman涡街的全过程,为更细致的流动机理探索奠定了基础.