Effects of buoyancy on turbulent premixed V-flames by large-eddy simulation

Buoyancy effects on turbulent premixed V-flames are investigated under normal gravity (+g) and reversed gravity (–g). Numerical simulations employ large eddy simulation (LES) with a dynamic model for sub-grid scale stress. With the assumption of fast chemistry combustion, a progress variable c-equation is applied to describe the flame front propagation. The equations are solved using a projection-based fractional step method in two dimensions for low-Mach number flows. Computed LES results of buoyancy effects on flame angle and flame brush thickness are consistent with those obtained from experiments. In both +g and –g conditions, the effects of buoyancy become important with increase in Richardson number (Ri). Buoyancy force tends to close up the flame under +g, but has the opposite effect under –g. Buoyancy force also suppresses flame wrinkling in +g and enhances wrinkling in –g. While there is a lack of experimental data available, computed axial velocity is shown to be significantly affected by buoyancy downstream from the flame holder under moderate Reynolds number.     

Effects of variable specific heat on energy transfer in a high-temperature supersonic channel flow

An energy transfer mechanism in high-temperature supersonic turbulent flow for variable specific heat (VSH) condition through turbulent kinetic energy (TKE), mean kinetic energy (MKE), turbulent internal energy (TIE) and mean internal energy (MIE) is proposed. The similarities of energy budgets between VSH and constant specific heat (CSH) conditions are investigated by introducing a vibrational energy excited degree and considering the effects of fluctuating specific heat. Direct numerical simulation (DNS) of temporally evolving high-temperature supersonic turbulent channel flow is conducted at Mach number 3.0 and Reynolds number 4800 combined with a constant dimensional wall temperature 1192.60 K for VSH and CSH conditions to validate the proposed energy transfer mechanism. The differences between the terms in the two kinetic energy budgets for VSH and CSH conditions are small; however, the magnitude of molecular diffusion term for VSH condition is significantly smaller than that for CSH condition. The non-negligible energy transfer is obtained after neglecting several small terms of diffusion, dissipation and compressibility related. The non-negligible energy transfer involving TIE includes three processes, in which energy can be gained from TKE and MIE and lost to MIE. The same non-negligible energy transfer through TKE, MKE and MIE is observed for both the conditions.     

A Web services accessible database of turbulent channel flow and its use for testing a new integral wall model for LES

The output from a direct numerical simulation (DNS) of turbulent channel flow at Re_τ ≈ 1000 is used to construct a publicly and Web services accessible, spatio-temporal database for this flow. The simulated channel has a size of 8πh × 2h × 3πh, where h is the channel half-height. Data are stored at 2048 × 512 × 1536 spatial grid points for a total of 4000 time samples every 5 time steps of the DNS. These cover an entire channel flow-through time, i.e. the time it takes to traverse the entire channel length 8πh at the mean velocity of the bulk flow. Users can access the database through an interface that is based on the Web services model and perform numerical experiments on the slightly over 100 terabytes (TB) DNS data on their remote platforms, such as laptops or local desktops. Additional technical details about the pressure calculation, database interpolation, and differentiation tools are provided in several appendices. As a sample application of the channel flow database, we use it to conduct an a-priori test of a recently introduced integral wall model for large eddy simulation of wall-bounded turbulent flow. The results are compared with those of the equilibrium wall model, showing the strengths of the integral wall model as compared to the equilibrium model.     

A priori assessment of the subgrid scale viscous/scalar dissipation closures in compressible turbulence

An appraisal is made of several subgrid scale (SGS) viscous/scalar dissipation closures via a priori analysis of direct numerical simulation data in a temporally evolving compressible mixing layer. The effects of the filter width, the compressibility level and the Schmidt number are studied for several models. Based on the scaling of SGS kinetic energy, a new formulation for SGS viscous dissipation is proposed. This yields the best overall prediction of the SGS viscous dissipation within the inertial subrange. An SGS scalar dissipation model based on the proportionality of the turbulent time scale with the scalar mixing time scale also performs the best for the filter widths in the inertial subrange. Two dynamic methods are implemented for the determination of the model coefficients. The one based on the global equilibrium of dissipation and production is shown to be more satisfactory than the conventional dynamic model.     

Turbulent structures in cylindrical density currents in a rotating frame of reference

Gravity currents are flows generated by the action of gravity on fluids with different densities. In some geophysical applications, modeling such flows makes it necessary to account for rotating effects, modifying the dynamics of the flow. While previous works on rotating stratified flows focused on currents of large Coriolis number, the present work focuses on flows with small Coriolis numbers (i.e. moderate-to-large Rossby numbers). In this work, cylindrical rotating gravity currents are investigated by means of highly resolved simulations. A brief analysis of the mean flow evolution to the final state is presented to provide a complete picture of the flow dynamics. The numerical results, showing the well-known oscillatory behavior of the flow (inertial waves) and a final state lens shape (geostrophic adjustment), are in good agreement with experimental observations and theoretical models. The turbulent structures in the flow are visualized and described using, among others, a stereoscopic visualization and videos as supplementary material. In particular, the structure of the lobes and clefts at the front of the current is presented in association to local turbulent structures. In rotating gravity currents, the vortices observed at the lobes front are not of hairpin type but are rather of Kelvin-Helmholtz type.     

Surface tension effects on the behaviour of a rising bubble driven by buoyancy force

In the inviscid and incompressible fluid flow regime,surface tension effects on the behaviour of an initially spherical buoyancy-driven bubble rising in an infinite and initially stationary liquid are investigated numerically by a volume of fluid (VOF) method. The ratio of the gas density to the liquid density is 0.001, which is close to the case of an air bubble rising in water. It is found by numerical experiment that there exist four critical Weber numbers We₁,~We₂,~We₃ and We₄, which distinguish five different kinds of bubble behaviours. It is also found that when 1≤We2, the bubble will finally reach a steady shape, and in this case after it rises acceleratedly for a moment, it will rise with an almost constant speed, and the lower the Weber number is, the higher the speed is. When We >We₂, the bubble will not reach a steady shape, and in this case it will not rise with a constant speed. The mechanism of the above phenomena has been analysed theoretically and numerically.     

Evolution of the vortex structures and turbulent spots at the late-stage of transitional boundary layers

The nonlinear evolution process of new vortex structures at the late-stage of the transition, including the 3-D spatial structure of barrel-shaped vortex and dark spots structure observed by experiment research, has been confirmed by our computational results. The formation mechanisms of these structures have been explored. It is revealed that the new vortex structures, the ring-like vortex chain and induced disturbance velocities play a dominant role in the generation of turbulent spots.     

Assessment of subgrid-scale stress statistics in non-premixed turbulent wall-jet flames

We investigate the heat-release effects on the characteristics of the subgrid-scale (SGS) stress tensor and SGS dissipation of kinetic energy and enstrophy. Direct numerical simulation data of a non-premixed reacting turbulent wall-jet flow with and without substantial heat release is employed for the analysis. This study comprises, among others, an analysis of the eigenvalues of the resolved strain rate and SGS stress tensors, to identify the heat-release effects on their topology. An assessment of the alignment between the eigenvectors corresponding to the largest eigenvalues of these two tensors is also given to provide further information for modelling of the SGS stress tensor. To find out the heat-release effects on the dynamics of the turbulent kinetic energy and enstrophy dissipation, probability density functions (PDFs) and mean values are analysed. The mean SGS shear stress and turbulent kinetic energy both slightly increase in the buffer layer and substantially decrease further away from the wall, due to the heat-release effects. Contrary to the kinetic energy, heat release decreases the mean SGS dissipation of enstrophy in the near-wall region. Moreover, differences in the shapes of the PDFs between the isothermal and exothermic cases indicate changes in the intermittency level of both SGS dissipations. Heat release also increases the SGS stress anisotropy in the near-wall region. Although, the structure of the mean resolved strain-rate tensor only marginally differs between the isothermal and exothermic cases in the near-wall region, substantial differences are observed in the jet area, where compressibility effects are important and heat-release effects are found to promote compression states. The differences in the relative alignment between the SGS stress and resolved strain-rate tensors in the isothermal and exothermic cases are discussed in connection with the differences in the SGS dissipation of kinetic energy.     

Aspect ratio effects in turbulent duct flows studied through direct numerical simulation

Three-dimensional effects in turbulent duct flows, i.e., sidewall boundary layers and secondary motions, are studied by means of direct numerical simulation (DNS). The spectral element code Nek5000 is used to compute turbulent duct flows with aspect ratios 1–7 (at Re_{b, c} = 2800, Re_{τ, c} ? 180) and aspect ratio 1 (at Re_{b, c} = 5600, Re_{τ, c} ? 330), in streamwise-periodic boxes of length 25h. The total number of grid points ranges from 28 to 145 million, and the pressure gradient is adjusted iteratively in order to keep the same bulk Reynolds number in the centreplane with changing aspect ratio. Turbulence is initiated via a trip forcing active during the initial stages of the simulation, and the statistical convergence of the data is discussed both in terms of transient approach and averaging period. Spanwise variations in wall shear, mean-flow profiles, and turbulence statistics are analysed as a function of aspect ratio, and also compared with the spanwise-periodic channel (as idealisation of an infinite aspect ratio duct). The computations show good agreement with experimental measurements carried out in parallel at the Illinois Institute of Technology (IIT) in Chicago, and highlight the relevance of sidewall boundary layers and secondary vortices in the physics of the duct flow. The rich array of secondary vortices extending throughout the upper and lower walls of the duct, and their dependence on Reynolds number and aspect ratio, had not been reported in the literature before.     

Characteristic boundary conditions for simulations of compressible reacting flows with multi-dimensional,viscous and reaction effects

A generalized formulation of the characteristic boundary conditions for compressible reacting flows is proposed. The new and improved approach resolves a number of lingering issues of spurious solution behaviour encountered in turbulent reacting flow simulations in the past. This is accomplished (a) by accounting for all the relevant terms in the determination of the characteristic wave amplitudes and (b) by accommodating a relaxation treatment for the transverse gradient terms with the relaxation coefficient properly determined by the low Mach number asymptotic expansion. The new boundary conditions are applied to a comprehensive set of test problems including: vortex-convection; turbulent inflow; ignition front propagation; non-reacting and reacting Poiseuille flows; and counterflow cases. It is demonstrated that the improved boundary conditions perform consistently superior to existing approaches, and result in robust and accurate solutions with minimal acoustic wave interactions at the boundary in hostile turbulent combustion simulation conditions.     

溶解与热对流对固体颗粒运动影响的直接数值模拟

对牛顿流体内溶解与热对流对单颗粒在垂直管道中的沉降运动进行了直接数值模拟.流体运动由守恒方程计算,密度和黏性的变化考虑流场温度变化的影响,通过积分黏性应力和压力获得颗粒的受力跟踪颗粒运动,溶解引起的相变及其形状的变化由溶解潜热、溶解质量与分散相边界处的温度梯度的关系建立的方程决定.通过颗粒和流体间相互的作用力和力矩及边界条件的施加实现相间耦合.分别模拟了颗粒在等温流体、热流体、冷流体及颗粒溶解四种情况下的沉降过程.结果表明,在一定雷诺数内,热对流产生的颗粒尾迹处涡的脱落以及溶解引起的颗粒表面形态的变化引起了颗粒的横向摆动,并使颗粒沉降速度发生了变化. 关键词： 溶解 热对流 颗粒两相流 直接数值模拟     

漂移对聚焦高斯光束闪烁影响的数值模拟

采用非自适应坐标变换对聚焦高斯光束在湍流大气中的传输进行了数值模拟,结果显示轴闪烁指数并没有出现如Rytov理论所预言的随初始光束半径的增大而明显减小的现象,其原因在于Rytov近似理论未考虑大尺度湍涡产生的漂移效应对闪烁的贡献。对比数值模拟结果与漂移理论结果以及相关实验结果,三者相吻合,表明未考虑漂移效应的Rytov近似理论不能完全准确地描述聚焦光束的闪烁特征,在研究聚焦光束的闪烁时,应当考虑漂移的影响。     

燃烧管内悬浮铝粉燃烧爆炸过程的研究

 铝粉的燃烧与加速机理一直是相关部门研究的热点。为了深入研究其在受限空间内的燃烧与传播特性,基于双流体模型、湍流k-ε模型以及铝粉燃烧等模型,选用SIMPLE格式,对大型卧式燃烧管内铝粉颗粒与空气的两相悬浮流湍流燃烧加速转爆炸现象进行了数值研究,得到了铝粉火焰在管内的燃烧传播过程中管内各相关参数的详细变化情况,并与相关的实验结果吻合。     

A numerical investigation of the evolution of 2-D disturbances in hypersonic boundary layers and the effect on the flow structure due to the existence of shocklets

The evolution of 2-D disturbances in hypersonic boundary layer with Mach number 6,8, and 10 was investigated numerically by three different numerical schemes. At the entrance, second mode T-S waves with different amplitudes were introduced, and the relation between the Mach number and the amplitude of the disturbance when shocklets started to appear was investigated. By comparing the disturbance velocity profiles with those provided by linear stability theory, the effects of shocklets on flow structures were also investigated.     

热对流条件下固液密度比对颗粒沉降运动影响的直接数值模拟

本文应用任意拉格朗日-欧拉(ALE)算法对固液两相流流场中考虑热对流的非等温颗粒在竖直通道中的沉降运动进行了数值模拟.在牛顿流体中通过积分黏性应力和压力获得颗粒的受力跟踪颗粒运动,使用有限元方法数值求解流场的N-S方程和能量方程,模型不需经验假设.通过模拟来研究颗粒沉降的运动规律和热对流下固液密度比对固液两相流的影响作用.结果表明随着固液密度比的增加,颗粒经历了稳定沉降、周期性摆动,不规则摆动等过程;热对流使颗粒的摆动幅度和沉降速度发生变化;热对流对颗粒的影响作用随着固液密度比的增加而减小.     

Spatial resolution analysis of planar PIV measurements to characterise vortices in turbulent flows

The effects of spatial resolution of planar particle image velocimetry (PIV) on vortex size, swirling strength, circulation and population density characterisation are analysed using a series of experimental and numerical databases. The databases comprise a PIV database of an adverse-pressure-gradient turbulent boundary layer (APG TBL), a PIV database of a zero-pressure-gradient (ZPG) TBL in streamwise-wall-normal planes and streamwise-wall-normal slices of a direct numerical simulation (DNS) of a ZPG TBL. The effects of interrogation window and mesh sizes on the vortex parameters are analysed in the outer region of these flows using different qualitative and quantitative approaches. The quantitative analysis mainly capitalises on the possibility of mimicking the PIV data-sets with the DNS one. These approaches allow us to not only isolate the effects of mesh size and the interrogation window size but also to deduce the combined effects of other measurement errors in PIV. Typical values of mesh size and interrogation window size (0.01–0.03 of the boundary layer thickness) and typical levels of measurement uncertainties have significant effects on the vortex parameters. Moreover, each PIV error source affects the vortex parameters in different and frequently opposite manners. Hence, an optimal selection of measurement parameters such as the interrogation window size is indispensable in order to minimise the effects of spatial resolution and other measurement errors on the vortex parameters. Guidelines are presented in the Conclusions section of this paper. Finally, it is found that all the vortex parameters, when averaged across the outer region, are reasonably comparable in the ZPG and APG TBLs despite the fact that these are very different flows.     

Nonlinear evolution of turbulent spots in the near-wall shear flow

The initial model of turbulent spots with the wall disturbance using the pulse form was proposed. A group of three-dimensional coupling compact difference schemes with high accuracy and high resolution were developed, and implemented to simulate the formation and development of turbulent spots in the near-wall shear flow by means of direct numerical simulation of the Navier-Stokes equations. Growing and decaying modes were used to analyze nonlinear evolution characteristics of the turbulent spots.     

Coupling of turbulence on the ignition of multicomponent sprays

This study examines the effect of turbulence on the ignition of multicomponent surrogate fuels and its role in modifying preferential evaporation in multiphase turbulent spray environments. To this end, two zero-dimensional droplet models are considered that are representative of asymptotic conditions of diffusion limit and the distillation limit are considered. The coupling between diffusion, evaporation and combustion is first identified using a scale analysis of 0D homogeneous batch reactor simulations. Subsequently, direct numerical simulations of homogeneously dispersed multicomponent droplets are performed for both droplet models, in decaying isotropic turbulence and at quiescent conditions to examine competing time scale effects arising from evaporation, ignition and turbulence. Results related to intra-droplet transport and effects of turbulence on autoignition and overall combustion are studied using an aviation fuel surrogate. Depending on the characteristic scale, it is shown that turbulence can couple through modulation of evaporation time or defer the ignition phase as a result of droplet cooling or gas-phase homogenization. Both preferential evaporation and turbulence are found to modify the ignition delay time, up to a factor of two. More importantly, identical droplet ignition behavior in homogeneous gas phase can imply fundamentally different combustion modes in heterogeneous environments.