极低湍流度下翼型近场尾流雷诺切应力的分布

简述了在西北工业大学低湍流度风洞中，用热线风速仪测量翼型边界层及近场尾流中速度型、湍流强度、雷诺正应力及切应力等的简况；文中着重分析了极低湍流度下（０．０２％）试验所得近场尾流中雷诺切应力的分布规律。结果表明，在尾流宽度方向及流动方向均有明显的规律性，且与常规风洞湍流度下结果相比，其有关特征值明显偏低。     

Numerical Modeling of the Development of a Streaky Structure in a Boundary Layer at a High Freestream Turbulence Level

The disturbances generated by external turbulence in the boundary layer on a flat plate set suddenly in motion are determined. A turbulent flow calculated by direct numerical simulation is taken as the initial conditions. The solution obtained simulates the initial stage of laminar-turbulent transition in the flat-plate boundary layer at a high turbulence level in the oncoming flow. The solution makes it possible to estimate the effects of different factors, such as nonstationarity, nonlinearity, and the parameters of the freestream velocity fluctuation spectrum, on disturbance enhancement in the boundary layer.     

粉尘等容燃烧容器内扬尘系统诱导湍流特性的实验研究

采用热线风速仪与系统平均法测定和研究了三种圆柱形封闭容器内的扬尘诱导湍流衰减特性,并与球形爆炸容器内已测定的扬尘诱导湍流衰减特性进行了比较。探讨了扬尘装置、容器体积、形状对扬尘诱导湍流瞬态特性的影响。实验结果显示,在粉尘等容燃烧容器内扬尘诱导湍流强度随时间的衰减特性具有一定相似性,它们均呈负指数关系。     

Turbulence properties in the cylinder wake at high Reynolds numbers

The present contribution analyses the turbulence properties in unsteady flows around bluff body wakes and provides a database for improvement and validation of turbulence models, concerning the present class of nonequilibrium flows. The flow around a circular cylinder with a low aspect ratio and a high blockage coefficient is investigated. This confined environment is used in order to allow direct comparisons with realisable 3-D Navier–Stokes computations avoiding ‘infinite’ conditions. The flow is investigated in the beginning of the critical regime at Reynolds number 140 000. The analysis is carried out by means of 2-D PIV, of 3-C PIV and of high-frequency 2-D PIV. The experimental analysis contributes to confirm the validity of advanced statistical turbulence modelling for unsteady flows around bodies.     

Experimental Investigation of the Effects of Turbulence and Mixing on Autoignition Chemistry

The autoignition of acetylene, released from a finite-sized circular nozzle into a turbulent coflow of hot air confined in a pipe, has been the subject of a recent experimental study to supplement previous work for hydrogen and n-heptane. As with hydrogen and n-heptane, autoignition appears in the form of well-defined localized spots. Quantitative information is presented concerning the effects of turbulence intensity, turbulent lengthscale and injector diameter on the location of autoignition. The effects of these parameters on inhomogeneous autoignition have not been investigated experimentally before. The present study establishes that increasing the bulk velocity increases the autoignition length, as was reported for hydrogen and n-heptane. For the same turbulence intensity, the autoignition length increases as the injector diameter increases and as the turbulent lengthscale decreases. A simultaneous decrease in turbulence intensity and increase in lengthscale causes a reduction in autoignition length. Further, the frequency of appearance of the autoignition spots has also been measured. It is found to increase when autoignition occurs closer to the injector, and also at higher velocities. The observed trends are consistent with expectations arising from the dependence of the mixture fraction and the scalar dissipation rate on the geometrical and flow parameters. The data can be used for the validation of turbulent combustion models.     

Investigation of Advanced Turbulence Models for the Flow in a Generic Wing-Body Junction

The predictive performance of several turbulence models, among them formulations based on non-linear stress-strain relationships and on stress-transport equations, is examined in a collaborative university-industry study directed towards a generic wing-body junction. The geometry consists of a variation of the symmetric NACA 0020 aerofoil mounted on a flat plate, with the oncoming stream aligned with the aerofoil's symmetry plane. The dominant feature of this flow is a pronounced horseshoe vortex evolving in the junction region following separation ahead of the aerofoil's leading edge. This case is one of 6 forming a broad programme of turbulence-model validation by UMIST, Loughborough University, BAE Systems, Aircraft Research Association, Rolls-Royce plc and DERA. Key aspects of this collaboration were a high level of interaction between the partners, the use of common grids and boundary conditions, and numerical verifications aimed at maximizing confidence in the validity of the computational solutions. In total, 12 turbulence models were studied by four partners. Model performance is judged by comparing solutions with experimental data for pressure fields on the plane wall and around the aerofoil; for velocity, turbulence energy, shear stress and streamwise normal stress in the upstream symmetry plane; and for velocity, turbulence energy and shear stress in cross-flow planes downstream of the aerofoil leading edge. The emphasis of the study is on the structure of the horseshoe vortex and its effects on the forward flow. The main finding of the study is that, for this particular 3D flow, second-moment closure offers predictive advantages over the other models examined, especially in terms of the far-field structure of the horse-shoe vortex, although no model achieves close agreement with the experimental data in respect of both mean flow and turbulence quantities. This revised version was published online in July 2006 with corrections to the Cover Date.     

低湍流度风洞中湍流度对平板边界层转捩影响的试验研究

本文报告了在西北工业大学壁低流度风洞中进行了平板边界层转捩试验研究的简况及初步结果，试验湍流度为０．０２％、０．１％及０．３３％，用恒温热线风速仪测量时均速度型，求得边界层沿流向的位移厚度分布，并用示波器观察速度脉动脉形变化，从而确定起始转捩点和完全转捩点位置。结果表明，转捩的规律性和国外经典结果极为吻合。     

Numerical Investigation of Turbulent Kinetic Energy Dynamics in Chemically-Reacting Homogeneous Turbulence

In this work, we investigate numerically the temporal evolution of Turbulent Kinetic Energy (TKE) of a chemically-reacting n-heptane and air mixture in statistically Homogeneous Isotropic Turbulence (HIT). Our specific focus is on the concurrent view of TKE evolution in both physical and scale (Fourier) spaces to identify the impact of reaction-induced heat release on turbulence. The simulation parameters are selected to represent the combustion characteristics of heavy hydrocarbon fuels under engine conditions. Results indicate that pressure dilatation work dominates the TKE evolution during the period of strong heat release and its dominance is attributed to the strong volumetric dilatation associated with the presence of reaction fronts in physical space. Viscous dissipation and viscous dilatation terms become much stronger with increasing heat release, primarily due to the increase in strain-rate and dilatation at the vicinity of the reaction fronts, but their magnitudes are still small compared to that of pressure dilatation work. In addition, the analysis in Fourier space shows that pressure dilatation work dominates the evolution of TKE not only in the mean, but also over a wide range of scales. The spectrum of pressure dilatation shows a power-law behavior, which is a direct consequence of the localized sheet-like reaction fronts in physical space. It is also shown that viscous dissipation spectrum initially removes kinetic energy at small scales when heat release is weak, but starts to remove kinetic energy at intermediate and later at large scales due to the presence of localized reaction fronts during the strong heat release period. More interestingly, it is observed that the inter-scale kinetic energy transfer spectrum moves energy from less dissipative scales (small scales) to scales where kinetic energy is more effectively removed by viscous dissipation work (large scales) during the period of strong heat release, which indicates possible up-scale kinetic energy transfer in Fourier space.     

高分子溶液壁湍流减阻机理的TRPIV实验研究

利用高时间分辨率粒子图像测速技术(TRPIV)对回流式水槽中低浓度高分子溶液壁湍流的减阻机理进行实验研究。通过对比分析高分子溶液和纯水平板湍流边界层在相同来流速度下的平均速度剖面、湍流强度和雷诺应力,发现高分子溶液的壁面摩擦阻力减小了21.77%,并且其缓冲层增厚,按对数律外移,雷诺应力减小;高分子聚合物主要在近壁区起到抑制湍流脉动的作用,而在主流区的作用不太明显。用流向局部平均多尺度速度结构函数和相干结构条件采样方法,检测并对比了高分子溶液和水的壁湍流相干结构“喷射”和“扫掠”事件中的脉动速度、展向涡量、雷诺应力等物理量的二维拓扑形态,发现高分子溶液近壁区相干结构在猝发时的脉动速度减小,涡量受到抑制,雷诺应力明显减小,说明高分子溶液湍流近壁区相干结构“喷射”和“扫掠”的强度变弱,猝发频率降低,动量和能量的输运减弱,揭示出高分子溶液减阻的重要机理。     

Investigation of Hydrodynamic Dispersion and Intra-pore Turbulence Effects in Porous Media

Transport in Porous Media - The aim of the present paper is to evaluate and compare the pore level hydrodynamic dispersion and effects of turbulence during flow in porous media. In order to compute...     

MHD Turbulence at the Laboratory Scale: Established Ideas and New Challenges

The properties of MHD turbulence in the electrically conducting fluids available in the laboratory (where the magnetic Reynolds number is significantly smaller than unity) may be summarised as follows:(1) The Alfven waves, even under their degenerated form at this scale, are responsible for a tendency to two-dimensionality. Eddies tend to become aligned with the applied magnetic field and inertia tends to restore isotropy. The competition between these mechanisms results in a spectral law t^-2k^-3.(2) When insulating walls, perpendicular to the magnetic field, are present and close enough to each other, two-dimensionality can be established with a good approximation within the large scales, and the predominant mechanism is the inverse energy cascade.(3) These columnar eddies are nevertheless submitted to a dissipation within the Hartmann boundary layers present at their ends, whose time scale is independent of the wave number. When this damping effect is negligible, ordinary 2D turbulence is observed with k^-5/3 spectra. On the contrary when this (ohmic and viscous) damping is significant this 2D turbulence exhibits k^-3 spectra.Besides these homogeneous (except within the Hartmann layers) conditions, for instance in shear flows such as mixing layers, almost nothing is known except that two-dimensionality may be well established. The first results of a recent experimental investigation (still in development) are presented. Some challenging questions are raised, such as the interpretation of a surprising difference between the transport of momentum and the transport of a scalar quantity (heat) across that layer. A video was shown during the oral presentation of this paper, illustrating the energy transfer toward the large scales and the weakness of the dissipation suffered by this 2D velocity field.     

New Molecular Transport Model for FDF/LES of Turbulence with Passive Scalar

The paper addresses the issue of modelling and computation of wall-bounded turbulent flows with passive scalars. In the present approach, the large eddy simulation (LES) method is used to compute the velocity field in the near-wall zone. The LES is coupled with the Lagrangian filtered density function (FDF) model for the transport of a passive scalar. In the paper, we propose two models to account for the molecular transport near the wall and investigate their behaviour in the limit case of small filter widths. One of the models is tested numerically, and computational results for a heated channel flow are compared with the available DNS data.     

An Experimental Investigation of the Turbulence Effect on the Combustion Propagation in a Rapid Compression Machine

The role of different combustion modes that govern the combustion propagation inside a rapid compression machine is discussed. Aiming at the control of the compression generated turbulence, the careful design of the UPMC-RCM is described. A methodology is then proposed to investigate the influence of the residual post-compression turbulence level on the visible combustion propagation process. Through the fuel type, the main parameter varied is the ratio of the ignition delay time to the characteristic decay time for the post-compression turbulence. A high-speed camera images the visible combustion. Particle Image Velocimetry provides two-dimensional velocity fields of the post-compression flow prior to ignition. The residual turbulence level is shown to influence the combustion propagation phenomenology, highlighting the coexistence of both volumetric and frontlike combustion modes for the shorter aforementioned ratios. This conclusion could contribute to an explanation of the experimental discrepancies among the ignition delays measured in different RCMs as the residual turbulence level is highly set-up dependent.     

网栅后湍流流动沿流程衰减特性的实验研究

应用ＬＤＶ测试技术对垂直方管内三种不同栅距的网所形成的湍流流动做了详细测量,网栅栅距Ｍ分别为１４ｍｍ,１０ｍｍ,６ｍｍ实验给出了管内不同位置湍流流动的各种参数沿流动方向的衰减规律,并就三种网栅进行比较分析,以探讨这种网栅流动所具有共同规律和在不同栅距对湍流结构及脉动大小的不同特点,从而对这种网栅所形成的湍流流结构较为清晰的了解。     

Near-Wake Turbulence Properties around a Circular Cylinder at High Reynolds Number

The present study investigates the turbulent properties of the flow around a circular cylinder in the near-wake and in the near-wall upstream region at the Reynolds number 140,000. A detailed cartography of the mean and turbulent velocity fields using a moderate blockage and aspect ratio is provided in order to use the present results for direct comparisons with realisable 3D Navier-Stokes computations. The flow structure is analysed by means of two experiments using respectively the LDV and the PIV techniques, both providing a refined grid of measurement points. The dynamics of the separation region, the growth and decay of turbulence in the near wake, as well as the spatial growth of the organised mode are analysed.     

An Attempt to Realize Experimental Isotropic Turbulence at Low Reynolds Number

This paper presents an attempt to realize experimental isotropicturbulence at low Reynolds number. For this aim an experimentalapparatus, a turbulence chamber “Box”, was designed and built togenerate a turbulent flow field in the center of the chamber. Theturbulent airflow field was generated by eight electrical fans placedsymmetrically at the eight internal corners of the externally cubicchamber. The turbulence intensity was controlled by the fans speed.Laser Doppler velocimeter (LDV) in single and two-point velocitymeasurements was used to fully characterize the turbulent field insidethe chamber. The main results indicate that the turbulence ishomogeneous and isotropic with a quasi-zero mean velocity within aspherical region of 20 mm radius from the center of the chamber. Themeasured turbulent integral length scale was found to be constant andindependent of the turbulence intensity (or fans speed). Furthermore, anoticeable spectral inertial subrange as prescribed by the Kolmogorovtheory has not been observed at the range of Reynolds number exploredhere, where Re_λ < 100. But rather a scaling region characterized by anexponent that is lower than the Kolmogorov value, ?5/3, has beenidentified. Moreover, the value of this exponent showed no definedtrend, while the width of the inertial scaling region expands as themicroscale Reynolds number increases.     

Experimental Investigation of the Effect of Coarse Particles on Decaying Grid-Generated Turbulence in a Two-Phase Flow

The results of an experimental investigation of the effect of particles on decaying grid-generated turbulence in a downward vertical turbulent gas-particle flow are presented. The dispersed particles were glass spheres with a mean size of 700 m. Titanium dioxide particles with a mean size of 2 m were used as the particle-markers modeling the carrier-medium flow. The turbulence was generated by grids with square cells of two sizes (4.8 and 10 mm) and an impenetrability parameter equal to 0.49 at a mean flow velocity equal to 9.5 m/s. The grid Reynolds numbers were 3000 and 6300. The damping of turbulence by the particles, manifested in an increase in the turbulence decay rate (viscous dissipation) and a decrease in the turbulence energy in the power-consuming spectral band, was detected.     

Investigation of Anisotropy-Resolving Turbulence Models by Reference to Highly-Resolved LES Data for Separated Flow

The predictive properties of several non-linear eddy-viscosity models are investigated by reference to highly-resolved LES data obtained by the authors for an internal flow featuring massive separation from a curved surface. The test geometry is a periodic segment of a channel constricted by two-dimensional (2D) `hills' on the lower wall. The mean-flow Reynolds number is 21560. Periodic boundary conditions are applied in the streamwise and spanwise directions. This makes the statistical properties of the simulated flow genuinely 2D and independent from boundary conditions, except at the walls. The simulation was performed on a high-quality, 5M-node grid. The focus of the study is on the exploitation of the LES data for the mean-flow, Reynolds stresses and macro-length-scale. Model solutions are first compared with the LES data, and selected models are then subjected to a-priori studies designed to elucidate the role of specific model fragments in the non-linear stress-strain/vorticity relation and their contribution to observed defects in the mean-flow and turbulence fields. The role of the equation governing the length-scale, via different surrogate variables, is also investigated. It is shown that, while most non-linear models overestimate the separation region, due mainly to model defects that result in insufficient shear stress in the separated shear layer, model forms can be derived which provide a satisfactory representation of the flow. One such model is identified. This combines a particular quadratic constitutive relation with a wall-anisotropy term, a high-normal-strain correction and a new form of the equation for the specific dissipation ω = ∈/k. This revised version was published online in July 2006 with corrections to the Cover Date.     

Turbulence Budgets in Buoyancy-affected Vertical Backward-facing Step Flow at Low Prandtl Number

The paper investigates buoyancy impact on the vertical flow over a backward-facing step at low Prandtl number by Direct Numerical Simulation. In particular, the very low Prandtl number of liquid sodium, 0.0088, is considered in the regime of mixed convection, i.e. for Richardson numbers below unity. The effects of buoyancy on mean flow, heat transfer and turbulence are assessed. Buoyancy is found to attenuate recirculation and, consequently, increase heat transfer. Turbulence is decreased in the attached boundary layer for moderate buoyancy impact but surpasses the levels found in forced convection at the largest Richardson number investigated. Beyond the mean flow and second moments, the budgets of turbulent kinetic energy, Reynolds shear stress, temperature variance, and turbulent heat flux components are studied and related to the alterations in the mean field quantities. Due to scale separation, production and dissipation nearly balance for temperature variance while this is not the case for turbulent kinetic energy. Similar findings for the turbulent heat fluxes show that the correlation between temperature and pressure gradient is the most important contribution to the budget aside from production and dissipation. In addition to the physical insight into this flow, the data presented may be used for the validation and improvement of turbulence models for liquid metal flows.     

Turbulence modelling of the flow past a pitching NACA0012 airfoil at and Reynolds numbers

This paper provides a study of the NACA0012 dynamic stall at Reynolds numbers 10⁵ and 10⁶ by means of two- and three-dimensional numerical simulations. The turbulence effect on the dynamic stall is studied by statistical modelling. The results are compared with experiments concerning each test case. Standard URANS turbulence modelling have shown a quite dissipative character that attenuates the instabilities and the vortex structures related to the dynamic stall. The URANS approach Organised Eddy Simulation (OES) has shown an improved behaviour at the high Reynolds number range. Emphasis is given to the physical analysis of the three-dimensional dynamic stall structure, for which there exist few numerical results in the literature, as far as the Reynolds number range is concerned. This study has shown that the downstroke phases of the pitching motion are subjected to strong three-dimensional turbulence effects along the span, whereas the flow is practically two-dimensional during the upstroke motion.