Modeling rotational effects in eddy-viscosity closures

A new approach to sensitize turbulence closures based on the linear eddy-viscosity hypothesis to rotational effects is proposed. The principal idea is to ‘mimic' the behavior of a second moment closure (SMC) in rotating homogeneous shear flow; depending on the ratio of the mean flow to the imposed rotational time scales, the model should be able to bifurcate between two stable equilibrium solutions. These solutions correspond to exponential or algebraic time dependent growth or decay of turbulent kinetic energy. This fundamental behavior of SMCs is believed to be of importance also in the prediction of non-equilibrium turbulence. A near-wall turbulence model which is based on the linear eddy-viscosity hypothesis is modified in the present study. Wall proximity effects are modeled by the elliptic relaxation approach. This closure has been successfully applied in the computation of complex, non-equilibrium flows in inertial frames of reference. The objective of the present study is to extend the predictive capability of the model to include flows dominated by rotational effects. The new model is calibrated in rotating homogeneous turbulent shear flow and subsequently tested in three different cases characterized by profound effects of system rotation or streamline curvature. It is able to capture many of the effects due to imposed body forces that the original closure is incapable of. Good agreement is obtained between the present predictions and available experimental and DNS data.     

Recent progress in the development of the Elliptic Blending Reynolds-stress model

The Elliptic Blending Reynolds Stress Model (EB-RSM), originally proposed by Manceau and Hanjalić (2002) to extend standard, weakly inhomogeneous Reynolds stress models to the near-wall region, has been subject to various modifications by several authors during the last decade, mainly for numerical robustness reasons. The present work revisits all these modifications from the theoretical standpoint and investigates in detail their influence on the reproduction of the physical mechanisms at the origin of the influence of the wall on turbulence. The analysis exploits recent DNS databases for high-Reynolds number channel flows, spanwise rotating channel flows with strong rotation rates, up to complete laminarization, and the separated flow after a sudden expansion without and with system rotation. Theoretical arguments and comparison with DNS results lead to the selection of a recommended formulation for the EB-RSM model. This formulation shows satisfactory predictions for the configurations described above, in particular as regards the modification of the mean flow and turbulent anisotropy on the anticyclonic or pressure side.     

Some Results Based on Near-wall Turbulence

In this paper some results based on the near-wall mean characteristics of a bounded turbulent flow are presented. In the study empirical polynomials and experimental data for an attached wall-bounded flow are used with the objective of studying the time-scales similarities in the very-near-wall region. As a result of this analysis a new parameter to characterize the high to low-Reynolds turbulence transition in the context of turbulence models is proposed. A relation for the Reynolds stress in the buffer region is also proposed, which allows a mean velocity profile through the buffer region to be obtained. This mean velocity profile joins the logarithmic ones at the beginning of the inertial sub-layer and fits appropriately to experimental data. Another result derived from the previous analysis is an expression for the eddy viscosity through the very-near-wall region. Comparison of this expression with those relations used by four known low-Reynolds models reveals that it has a very good performance.     

A Quasi-Realizable Cubic Low-Reynolds Eddy-Viscosity Turbulence Model with a New Dissipation Rate Equation

A non-linear relationship of the Reynolds stresses in function of the strain rate and vorticity tensors, with terms up to third order, is developed. Anisotropies in the normal stresses, influence from streamline curvature or rotation of the reference frame, and swirl effects are accounted for. The relationship is linked to ak–ε model with a modified transport equation for the dissipation rate. A new low-Reynolds source term is introduced and a model parameter is written in terms of dimensionless rate-of-strain and vorticity. The model is checked on different realizability constraints. It is shown that practically all constraints are fulfilled. The model is numerically tested on a fully developed channel and pipe flow, both stationary and rotating. The plane jet–round jet anomaly is addressed. Finally, the model is applied to the flow over a backward-facing step. Results are compared with a linear low-Reynolds k–ε model and the shear stress transport model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modelling of Progressive Strain Tests by a Non-linear Relaxation Approach

A model based on the thermodynamics of relaxations of continuous media (called DNLR as Distribution of nonlinear relaxations) is used to describe the tension/torsion progressive strain phenomenon. The proposed model is compared to the experimental data obtained by Benham on mild steel at room temperature with satisfactory results. We discuss the capacities and the relevance of the model obtained.     

Boger流体的松弛谱对非线性粘弹性表征的影响

分析了一种Boger流体的松弛时间谱和该流体的粘弹性表征问题。文中采用带正约束的非线性规划方法得到了包含4个、5个、6个松弛时间的描述此Boger流体线性粘弹特性的时间谱。这些新时间谱比前文报道的含3个松弛时间的谱能更好地描述该流体的储能模量G′。用PSM模型结合这些新松弛时间谱都不能很准确地描述这个Boger流体的第一法向应力差N1的实验数据。作者提出的3r模型描述此Boger流体的N1的结果比PSM模型的计算结果更接近实验值。     

拉格朗日松弛算法在平台罗经多故障诊断技术中的应用研究

最优多故障诊断问题是一个NP-hard问题。针对平台罗经这一复杂系统，采用有向图描述元件与测试点间的因果依赖关系，并建立系统的多信号模型。在考虑元件发生故障的先验概率的前提下，提出一种基于拉格朗日算法（LRA）和子梯度优化算法（SOA）近最优多故障诊断算法，并在某型平台罗经的方位稳定系统多故障诊断中得到应用。结果表明：该方法相对于传统的单故障诊断方法诊断速度快，适用于平台罗经这类大型复杂系统的多故障诊断。     

Effect of coarse particles on the heat transfer in a particle-laden turbulent boundary layer

The temperature distribution in particle-laden turbulent flow, in a flume, was investigated both by DNS and experimentally. Simulations were performed at Re=171 and Pr=5.4 in order to study the interaction between the particle motion and flow turbulence. Two-way coupling was used to obtain various turbulence statistics, the grid resolution was sufficiently fine to resolve all essential turbulent scales. The effect of particle diameter on momentum, heat transfer and particle deposition was considered. The details of particle-turbulence interaction depend on the particle Stokes number and the particle Reynolds number.

The spatial structures of instantaneous flow and temperature fields were visualized. Low frequency small oscillations of deposited particles were observed. It was found that these small deviations from the initial position, caused strong changes in the instantaneous temperature field near the particle.

The experiments provided details of the temperature field on the heated wall close to the particle. In the front of the particle, a sharp increase in heat transfer coefficient was observed. The experimental results agree well with the computational predictions.     

用正弦波加载和超声实验研究饱和砂岩弛豫特征和衰减峰

采用Metravib分析仪在-50℃~100℃温度条件及300~400Hz的正弦波作用下进行泵油饱和砂岩应力应变实验,获得了虚模量温度峰与对应实模量的剧烈亏损规律,显示出饱和砂岩有别于由衰减峰引起的典型粘弹性行为。利用超声法对饱和大理岩与花岗岩样品进行波速和衰减特性测试,获得了大理岩的Biot衰减峰变化规律。该研究对岩石物理理论模型的发展具有重要意义,可以利用岩层的衰减和波速频散特性进行油气藏的预测,降低勘探风险。     

Turbulence modelling and role of compressibility on oil spilling from a damaged double hull tank

The viscosity plays an important role, and a multiphase solver is necessary to numerically simulate the oil spilling from a damaged double hull tank (DHT). However, it is uncertain whether turbulence modelling is necessary, which turbulence model is suitable; and what the role of compressibility of the fluids is. This paper presents experimental and numerical investigations to address these issues for various cases representing different scenarios of the oil spilling, including grounding and collision. In the numerical investigations, various approaches to model the turbulence, including the large eddy simulation (LES), direct numerical simulation and the Reynolds average Navier–Stokes equation (RANS) with different turbulence models, are employed. Based on the investigations, it is suggested that the effective Reynolds numbers corresponding to both oil outflow and water inflow shall be considered when classifying the significance of the turbulence and selecting the appropriate turbulence models. This is confirmed by new lab tests considering the axial offset between the internal and the external holes on two hulls of the DHT. The investigations conclude for numerically simulating oil spilling from a damaged DHT that when the effective Re is smaller the RANS approaches should not be used and LES modelling should be employed; while when the effective Reynolds numbers is large, the RANS models may be used as they can give similar results to LES in terms of the height of the mixture in the ballast tank and discharge but costing much less CPU time. The investigation on the role of the compressibility of the fluid reveals that the compressibility of the fluid may be considerable in a small temporal‐spatial scale but plays an insignificant role on macroscopic process of the oil spilling. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermomechanical material modelling based on a hybrid free energy density depending on pressure,isochoric deformation and temperature

In order to represent temperature-dependent mechanical material properties in a thermomechanical consistent manner it is common practice to start with the definition of a model for the specific Helmholtz free energy. Its canonical independent variables are the Green strain tensor and the temperature. But to represent calorimetric material properties under isobaric conditions, for example the exothermal behaviour of a curing process or the dependence of the specific heat on the temperature history, the temperature and the pressure should be taken as independent variables. Thus, in the field of calorimetry the Gibbs free energy is usually used as thermodynamic potential whereas in continuum mechanics the Helmholtz free energy is normally applied. In order to simplify the representation of calorimetric phenomena in continuum mechanics a hybrid free energy density is introduced. Its canonical independent variables are the isochoric Green strain tensor, the pressure and the temperature. It is related to the Helmholtz free energy density by a Legendre transformation. In combination with the additive split of the stress power into the sum of isochoric and volumetric terms this approach leads to thermomechanical consistent constitutive models for large deformations. The article closes with applications of this approach to finite thermoelasticity, curing adhesives and the glass transition.     

后掠机翼低速流动转捩位置的升华法测量

报导了在西北工业大学低湍流度风洞中采用升华法测量后掠翼上表面流动转捩位置及其随攻角变化而改变情况,并参照后掠翼压力分布的实验结果进行分析.本文涉及的升华法,是根据边界层的层流区和湍流区流态差别来判断边界层转捩的一种测量方法.升华法的测量结果表明:在小攻角范围内,转捩位置逐渐提前,当达到某一攻角时,转捩快速接近前缘.这一结果和压力分布的实验结果相吻合,说明升华法能够较准确地显示转捩线位置.由于影响后掠翼转捩位置的因素较多,诸如边界层内横流的不稳定性、壁面的干扰、后掠翼的三维流动效应等,致使出现了攻角为4°时转捩位置呈现出一条斜线的现象.     

Dynamic Systems Analysis and Control Based on a Configuration Manifold Model

This paper presents a configuration manifold model for the analysis of dynamic systems and the development of control algorithms from both geometrical and topological points of view. The fundamental theory of surfaces and differential manifolds endowed with Riemannian metrics is overviewed. The concepts of configuration manifolds (C-manifolds) and their immersions and embeddings are then introduced and applied to dynamic systems modeling. An explicit form of the smooth embedding for a given dynamic system with its C-manifold is derived. In an open serial-chain robotic system, a topological equivalence, i.e. a homeomorphism, is found and shown to be useful for dynamic model reduction. With topology being viewed as the structure of geometry, we discover and prove that the kinematics of a dynamic system determines its topology so that the kinematics is virtually a structure of the system's dynamics. This key point of view is further extended to the development of an adaptive control strategy. A computer simulation study is finally performed to verify the proposed model and adaptive control scheme.     

Algorithms for a Vehicle Dynamics Monitoring System,Based on Model Reference Structure

Vehicle control depends heavily on the knowledge of the vehicle operatingconditions. One of the most important parameters for its control is thetyre–road friction coefficient (µ). An appropriate way toestimate the vehicle operating conditions is the Model Reference Approach.This technique requires a model that provides estimated states which can becompared with the measured states, the difference is used to determine thereal operating conditions. This paper presents two different applications ofthe Model Reference Techniques to estimate tyre–road frictioncoefficient; these are based on the relation between tyre forces and slip,and on the vehicle lateral behaviour using an extended Kalman filter.Experimental data from the test vehicle confirms the good results obtainedin the friction estimation based on the tyre slip–force relation. Theestimation using an extended Kalman filter on lateral behaviour showsaccurate tracking. The next step to be taken is to integrate all thealgorithms in the test vehicle and to validate them for a wide range ofoperating conditions, in order to have reliable information for the activesystem control.     

求解非线性规划的序列有理规划SRP方法

基于分子、分母皆线性的两点有理逼近，本文对于非线性规划提出了序列有理规划ＳＲＰ方法，按两种情况进行了研究。第一种为ＳＲＰ－Ｌ方法，将原问题化为等效的ＬＰ问题求解；第二种为ＳＲＰ－Ｑ方法，将原问题化为等效的ＱＰ问题求解。本文的工作说明，两点有理逼近函数对于改进优化方法是有益的。     

基于软弹流润滑模型的液压格莱圈密封性能分析

基于软弹流理论建立了液压往复格莱圈密封的数值分析模型,该耦合模型包括流体力学、接触力学、变形和热分析.数值求解获得了密封区域的膜厚、流体压力和接触压力分布,以及单个行程的流量和活塞杆表面的摩擦力,揭示了液压往复格莱圈密封的密封机理.通过参数化进一步分析了不同密封表面均方根粗糙度、密封压力和往复速度对密封性能的影响.结果表明:在本研究中,密封区域表现为混合润滑状态,且以微凸体接触摩擦为主;较小的密封表面均方根粗糙度值具有较好的密封性能,增大密封压力会导致泄漏量和摩擦力均增加,而增大往复速度有利于减小泄漏量.     

Model of Nonlinear Evolution of Long-Wave Perturbations on an Ideally Conducting Jet with Current in a Longitudinal Magnetic Field. Collision of Magnetized Jets

Within the framework of the magnetohydrodynamic approach, a system of equations is derived for nonlinear evolution of long-wave axisymmetric perturbations on a conducting fluid jet with surface electric current, located along the axis of a conducting solid cylinder in a longitudinal magnetic field. The fluid is assumed to be inviscid, incompressible, and ideally conducting, like the cylinder walls. It is shown that, if the longitudinal field is uniform and the axial flow is shear-free, this system can be either hyperbolic or elliptic-hyperbolic, depending on problem parameters. The boundaries of hyperbolicity and ellipticity regions in the space of solutions are determined. In the hyperbolicity region, equations of characteristics and conditions on them are obtained. The problem of the decay of velocity discontinuity on the jet is considered. Conditions are found for the existence of a continuous self-similar solution in the hyperbolicity region, corresponding to collision of jets.     

一个基于恢复时间的新剪稀模型及其对聚合油PAO 650流变特性的模拟

用球-杆比拟流体层上的分子,通过分析两流体层上球-杆的接触、通过和恢复的过程,基于球-杆恢复到原状态的时间,建立了一个新剪稀模型.该模型虽然在形式上与Carreau模型有相似之处,但其公式仅含有两个流变参数,且是经由严格的理论推导得到的.通过与Carreau模型得到的广义黏度曲线的比较,确定了新流变公式的待定参数.在点接触流变热弹流润滑的数学模型中使用新流变公式,数值模拟了大黏度聚合油PAO 650的流变特性,并与该油在文献中的试验曲线进行了比对.结果表明新流变模型能够较好地模拟大黏度聚合油的流变特性,从而证实了其正确性.进一步,使用新公式计算得到了更多工况下的流变特性曲线,光滑的、合理的曲线分布说明新流变模型具有广泛的适用性.     

对基于恢复时间的剪稀流变模型的进一步探讨及其对squalane油品流变特性的模拟

本文作者前期基于球-杆变形后恢复到原状态的时间,建立了新的形式较简单的流变模型,并模拟了黏度较高的聚合油PAO 650的摩擦系数曲线.本文中将该模型的应用范围进行了推广,模拟了黏度较低的squalane油品的流变特性.把该流变公式应用到点接触热流变弹流润滑的数学模型中,通过与试验测得的摩擦系数的比较确定了使用该模型时squalane油品的待定参数值,进而得到了点接触热流变弹流润滑的完全数值解.结果表明:解得的压力、膜厚和温度的变化规律均符合预期,且摩擦系数曲线与试验结果整体吻合性较好.新流变模型对高、低黏度的油品均能得到合理的流变特性曲线,说明作者的基于恢复时间的流变模型具有一定的正确性和可应用性.另外,由新模型计算得到的squalane油品的剪应力曲线呈现出一近似水平段,这也在一定程度上解释了流变试验文献中多次提到的极限剪应力现象.