共查询到20条相似文献,搜索用时 156 毫秒
1.
2.
在大型重力式密度分层水槽中, 对内孤立波与圆柱型结构的相互作用特性开展了系列实验. 基于两层流体中 内孤立波的KdV,eKdV和MCC理论, 建立了圆柱型结构内孤立波载荷的理论预报模型, 给出了该载荷理论预报模型中3类内孤立波理论的适用性条件.研究表明, 圆柱型结构内孤立波水平载荷包括水平Froude-Krylov力、附加质量力和拖曳力3个部分, 可以由Morison公式计算, 而内孤立波垂向载荷主要为垂向Froude-Krylov力, 可以由内孤立波诱导动压力计算.系列实验结果表明, 附加质量系数可以取为常数1.0, 拖曳力系数与内孤立波诱导速度场的雷诺数之间为指数函数关系, 而且基于理论预报模型的数值结果与系列实验结果吻合. 相似文献
3.
在改进传统的传递矩阵法中,Riccati 传递矩阵法具有普遍的意义,国内已开始试用,是一种值得推广的方法.本文应用传统的传递矩阵法和改进后的Riccati 传递矩阵法,对16m~3大型挖掘机的1∶5动臂模型,进行了扭转振动和弯曲振动的分析,得到频率在10~3Hz 以下的11阶振动模态.并与实验结果进行了比较.结果说明,传递矩阵法用得恰当,能以较少的计算工作量得到动臂一类结构振动的主要特性.分柝结果已提交科研任务委托单位. 相似文献
4.
?????? ????? 《力学与实践》1988,10(4):43-21
在改进传统的传递矩阵法中,Riccati 传递矩阵法具有普遍的意义,国内已开始试用,是一种值得推广的方法.本文应用传统的传递矩阵法和改进后的Riccati 传递矩阵法,对16m~3大型挖掘机的1∶5动臂模型,进行了扭转振动和弯曲振动的分析,得到频率在10~3Hz 以下的11阶振动模态.并与实验结果进行了比较.结果说明,传递矩阵法用得恰当,能以较少的计算工作量得到动臂一类结构振动的主要特性.分柝结果已提交科研任务委托单位. 相似文献
5.
高阶矩模型是湍流模式理论研究中的难点和前沿. 自周培源先生首次建立一般湍流的雷诺应力输运方程起, 为了更精确的预测复杂流动, 人们从未间断过对高阶矩模型的研究. 尤其进入新世纪以来, 随着计算机硬件水平的飞跃和高精度数值算法的突破, 湍流模拟方法正由RANS向LES转变. 而无论对于RANS框架、LES框架还是两者混合, 高阶矩模式都是其中先进封闭模式的代表. 基于此, 本文对高阶矩模型的发展情况进行了总结, 重点包括高阶矩模型中各项的建模方式、尺度提供方程的演化历程和数值求解技术的关键需求. 然后, 通过几类典型湍流问题展示了其相对于传统涡黏模型的优势, 并且给出了部分CFD软件对高阶矩模型的集成情况. 最后对高阶矩湍流模型未来面临的挑战和发展的方向进行了展望. 相似文献
6.
无耦合条件下的多子系统静动态统一本构关系(下)--铝的双子系统静动态统一本构模型 总被引:2,自引:0,他引:2
构造了标量形式的无耦合条件下双子系统静动态统一本构模型.推导出第1和第2子系统中加载应变速率临界值ε·c1和ε·c2,当应变速率ε·分别低于和高于某个临界值时,相应子系统中的不可逆行为分别是与时间无关的和与时间相关的.由于当ε·跨越ε·c1和ε·c2时,内变量的求解公式发生变化,所以动态强度随ε·变化的规律发生变化.经与铝的实验结果比较确认,本构模型能够描述材料的多种静动态力学行为. 相似文献
7.
8.
风场模拟中AR模型的若干问题 总被引:1,自引:0,他引:1
自回归(AR)模型具有计算量小,模拟速度快等优良特性,在风场模拟中得到了广泛应用.本文对AR模型进行了系统的研究,将脉动风场模拟中广泛应用的AR模型归为两大类,对模型中的参数从理论上进行了合理的解释.对两种模型模拟脉动风场时涉及到的Wiener-KJaintchine公武的交换形式,通过分析对其进行了修正,指出算法上可以采用FFT技术来计算互相关矩阵的元素以提高计算效率.提出了AR模型编程中偶然发现的自回归顺序问题,算例表明两种不同方法的风速时程样本及其无偏自相关估计和自功率谱估计均有较大的影响,希望能引起更多同行对该问题的注意.尽管标量过程AR模型简单且易于掌握,但不能考虑时滞问题.相比之下,理论分析和数值实验都证明,向量过程的AR模型在精度总体要高于标量过程的AR模型,但其运算时间也相应增多. 相似文献
9.
10.
11.
Conditional Moment Closure for Large Eddy Simulations 总被引:1,自引:0,他引:1
A conditional moment closure (CMC) based combustion model for large-eddy simulations (LES) of turbulent reacting flow is proposed
and evaluated. Transport equations for the conditionally filtered species are derived that are consistent with the LES formulation
and closures are suggested for the modelling of the conditional velocity, conditional scalar dissipation and the fluctuations
around the conditional mean. A conventional β-probability density distribution of the scalar is used together with dynamic
modelling for the sub-grid fluxes. The model is validated by comparison of simulations with measurements of a piloted, turbulent
methane-air jet diffusion flame. 相似文献
12.
The mapping closure of Chen et al. [Phys. Rev. Lett., 63, 1989] is a transported probability density function (PDF) method that has proven very efficient for modelling of turbulent mixing in homogeneous turbulence. By utilizing a Gaussian reference field, the solution to the mapping function (in homogeneous turbulence) can be found analytically for a range of initial conditions common for turbulent combustion applications, e.g. for binary or trinary mixing. The purpose of this paper is to investigate the possibility of making this solution a presumed mapping function (PMF) for inhomogeneous flows. The PMF in turn will imply a presumed mixture fraction PDF that can be used for a wide range of models in turbulent combustion, e.g. flamelet models, the conditional moment closure (CMC) or large eddy simulations. The true novelty of the paper, though, is in the derivation of highly efficient, closed algebraic expressions for several existing models of conditional statistics, e.g. for the conditional scalar dissipation/diffusion rate or the conditional mean velocity. The closed form expressions nearly eliminates the overhead computational cost that usually is associated with nonlinear models for conditional statistics. In this respect it is argued that the PMF is particularly well suited for CMC that relies heavily on manipulations of the PDF for consistency. The accuracy of the PMF approach is shown with comparison to DNS of a single scalar mixing layer to be better than for the β-PDF. Not only in the shape of the PDF itself, but also for all conditional statistics models computed from the PDF. 相似文献
13.
We present a method of direct quadrature conditional moment closure (DQCMC) for the treatment of realistic turbulence-chemistry interaction in computational fluid dynamics (CFD) software. The method which is based on the direct quadrature method of moments (DQMOM) coupled with the conditional moment closure (CMC) equations is in simplified form and easily implementable in existing CMC formulation for CFD. The observed fluctuations of scalar dissipation around the conditional mean values are captured by the treatment of a set of mixing environments, each with its pre-defined weight. Unlike the early versions of the DQCMC method the resulting equations are similar to that of the first-order CMC, and the ??diffusion?? term is strictly positive and no correction factors are used. We present results for two mixing environments where the resulting matrices of the DQCMC can be inverted analytically. We have performed this analysis for a simple hydrogen flame using a multi species chemical scheme containing nine species. The effects of the fluctuations around the conditional means are captured accurately and the predicted results are in very good agreement with observed trends from direct numerical simulations. Furthermore, the differences between the first order CMC and DQCMC are discussed. 相似文献
14.
We consider the chemical reaction in a turbulent flow for the case that the time scale of turbulence and the time scale of
the reaction are comparable. This process is complicated by the fact that the reaction takes place intermittently at those
locations where the species are adequately mixed. This is known as spatial segregation. Several turbulence models have been
proposed to take the effect of spatial segregation into account. Examples are the probability density function (PDF) and the
conditional moment closure (CMC) models. The main advantage of these models is that they are able to parameterize the effects
of turbulent mixing on the chemical reaction rate. As a price several new unknown terms appear in these models for which closure
hypothesis must be supplied. Examples are the conditional dissipation 〈 χ ∣ φ 〉, the conditional diffusion 〈 κ ∇2 φ ∣ u, φ 〉 and the conditional velocity 〈 u ∣ φ 〉. In the present study we investigate these unknown terms that appear in the PDF and CMC model by means of a direct
numerical simulation (DNS) of a fully developed turbulent flow in a channel geometry. We present the results of two simulations
in which a scalar is released from a continuous line source. In the first we consider turbulent mixing without chemical reaction
and in the second we add a binary reaction. The results of our simulations agree very well with experimental data for the
quantities on which information is available. Several closure hypotheses that have been proposed in the literature, are considered
and validated with help of our simulation results.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
Computation of Conditional Average Scalar Dissipation in Turbulent Jet Diffusion Flames 总被引:2,自引:0,他引:2
The modelling of conditional scalar dissipation in locally self-similar turbulent reacting jets is considered. The streamwise
dependence in the transport equation of the conserved scalar pdf is represented by a function solely dependent on centreline
mixture fraction. This procedure provides a simple model suitable for non-homogeneous flows and ensures positive values for
conditional scalar dissipation. It has been tested in pure hydrogen-air jet diffusion flames using a Conditional Moment Closure
method with detailed 12species, 23 reactions chemistry. The calculations show good agreement of the averaged scalar dissipation
with reference values and the model proves to be superior to previous models based on homogeneous flows if the distribution
of the conditional scalar dissipation in mixture fraction space is compared with experimental results. A dependence of NO
predictions on the model of conditional scalar dissipation can be observed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
The performance of a variety of scale similarity (SS) type models for closure of sub-grid scalar flux in the context of Large Eddy Simulations (LES) of premixed turbulent combustion has been assessed. In addition to the well-known SS models, a more recent development by Anderson and Domaradzki (2012) is included in the analysis and also further model extensions and improvements are discussed. The work is based on a priori analysis of two Direct Numerical Simulation (DNS) databases of freely propagating turbulent premixed flames with a range of different Lewis and turbulent Reynolds numbers. Depending on the balance between the effects of flame normal acceleration due to heat release and the effects of turbulent velocity fluctuations, as well as the filter size, the subgrid-scalar flux exhibits both local gradient and counter-gradient transport which presents a considerable modelling challenge. The assessment is based on a correlation analysis and on the magnitude of the model expressions conditional on the Favre averaged reaction progress variable in comparison to the value obtained from DNS. Despite the fact that most of the models have been developed in the context of momentum transport in non-reactive flows they show either comparable or better performance in comparison to more conventional models used for reactive scalar flux closure. It is found that some models are sensitive to the test filter width and recommendations are provided in this regard. Further it is observed that the use of a Favre test filter substantially increases the correlation strength in direction of mean flame propagation where effects of heat release are most pronounced. 相似文献
17.
K.R. Sreenivasan 《Flow, Turbulence and Combustion》2004,72(2-4):115-131
It is now well established that quantities such as energy dissipation, scalar dissipation and enstrophy possess huge fluctuations in turbulent flows, and that the fluctuations become increasingly stronger with increasing Reynolds number of the flow. The effects of this small-scale “intermittenc” on various aspects of reacting flows have not been addressed fully. This paper draws brief attention to a few possible effects on reaction rates, flame extinction, flamelet approximation, conditional moment closure methods, and so forth, besides commenting on possible effects on the resolution requirements of direct numerical simulations of turbulence. We also discuss the likelihood that large-amplitude events in a given class of shear flows are characteristic of that class, and that, plausible estimates of such quantities cannot be made, in general, on the hypothesis that large and small scales are independent. Finally, we briefly describe some ideas from multifractals as a potentially useful tool for an economical handling of a few of the problems touched upon here. 相似文献
18.
This paper follows the evolution in understanding of the multiple mapping conditioning (MMC) approach for turbulent combustion
and reviews different implementations of MMC models. As the MMC name suggests, the original version represents a consistent
combination of CMC-type conditional equations (conditional moment closure) and generalised mapping closure. It seems that
the strength of the MMC model, and especially that of its stochastic version, lies in a more general (and much more transparent)
interpretation. In this new generalised interpretation, we can replace complicated derivations by physical reasoning and the
model appears to be a natural extension of modelling approaches developed in recent decades. MMC can be seen as a methodology
for enforcing certain known characteristics of turbulence on a conventional mixing model. This is achieved by localising the
mixing operation in a reference space. The reference space variables are selected to emulate the properties of a turbulent
flow which have a strong effect on reactive quantities. The best and simplest example is an MMC model which has a single reference
variable emulating the mixture fraction. In diffusion flames turbulent fluctuations of reacting quantities are strongly correlated
with fluctuations of the mixture fraction. By making mixing local in the reference mixture fraction space a CMC-type mixing
closure is enforced. In the original interpretation of MMC the reference variables are modelled as Markov processes. Since
the reference variables should emulate properties of turbulent flows as realistically as possible the next step, and the basis
of generalised MMC, is to remove the Markovian restriction and set reference variables equal to traced Lagrangian quantities
within DNS or LES flow fields. Indeed, no Markov value can emulate the mixture fraction better than the mixture fraction itself.
(Using a Markov vector process of dimension higher than the number of conditioning variables represents a more economical
alternative for producing reference variables in generalised MMC.) The generalised MMC approach effectively incorporates the
mixture fraction-based models, the PDF methods and LES/DNS techniques into a single methodology with possibility of blending
useful features developed previously for conventional models. The generalised approach to MMC stimulates a more flexible understanding
of simulations using sparsely placed Lagrangian particles as tools that may provide accurate joint distributions of reactive
scalars at relatively low computational cost. The physical reasoning behind the new interpretation of MMC is supported by
example computations for a partially premixed methane/air diffusion flame (Sandia Flame D). The scheme utilises LES for the
dynamic field and a sparse-Lagrangian filtered density function method with MMC mixing for the scalar field. Two different
particle mixing schemes are tested. Simulations are performed using only 35,000 Lagrangian particles (of these only 10,000
are chemically active) on a single workstation. The relatively low computational cost allows the use of realistic chemical
kinetics containing 34 reactive species and 219 reactions.
Intended for publication in the special issue of Flow, Turbulence and Combustion arising from the 2nd ECCOMAS Thematic Conference
on Computational Combustion held at Delft in mid-2007. 相似文献
19.
Large-eddy simulations (LES) have been coupled with a conditional moment closure (CMC) method for the computation of a series of turbulent spray flames. An earlier study by Ukai et al. (Proc. Combust. Inst. 34(1),1643–1650, 2013) gave reasonable results for the prediction of temperature and velocity profiles, but some limitations of the method became apparent. These limitations are primarily related to the upper limit in mixture fraction space. In order to enhance the applicability of the LES-CMC model, this paper proposes a two-conditional moment approach to account for the existence of pre-evaporated fuel by introducing two sets of conditional moments based on different mixture fractions. The two-conditional moment approach is first tested for a non-reacting test case. The results indicate that the spray evaporation induces relatively large conditional fluctuations within a CMC cell, and one set of conditional moments might not be sufficient. The upper limit of the mixture fraction space is dynamically selected for the solution of the second set of conditional moments, and the corresponding CMC solution in a CFD cell is estimated by interpolation between the two conditional moments weighted by the amount of vapour emitted within the domain. The cell-filtered value is given by integration of the conditional moment across mixture fraction space using a bounded β-FDF for the distribution of the scalar. As a result, the fuel concentration profiles given by LES and the two-conditional moment approach are shown to agree well. Then, the two-conditional moment approach is applied to four different flame configurations. The comparison of LES cell quantities and conditionally averaged moments indicates that the two sets of conditional moments are necessary for accurate predictions in zones where gas phase mixture fraction is significantly increased by droplet evaporation within the computational domain. The unconditional temperature profiles clearly show that the new approach improves the predictions of mean temperature especially along the centerline. Also, the better predictions of the temperature field improve the accuracy of the predicted mean axial droplet velocities. Overall, good agreement with the experimental results is found for all four cases, and the methodology is shown to be applicable to flames with a relatively wide range of fuel vapour concentrations. 相似文献
20.
R.W. Bilger 《Flow, Turbulence and Combustion》2004,72(2-4):93-114
Scalar dissipation is of great importance in the theory and modelling of combustion and other reacting turbulent flows. Measurements of scalar dissipation are found to lack the quality assurance of checks available from the conservation equations. Conditional averages of the scalar dissipation, so important in turbulent reacting flow theory and modelling, have qualitative and quantitative dependences that are very dependent on the details of the flow and mixing conditions. Accordingly, effort needs to focus on viable means of modelling it. Fluctuations of the scalar dissipation about the conditional mean are also important. Research results in this area need to be made more accessible to the combustion scientist. Heat release effects, so important in turbulent premixed combustion, are found to be much less important in non-premixed combustion. 相似文献