雷诺数对圆形渐缩喷嘴湍流射流的影响

本文研究雷诺数(Re)对圆形渐缩喷嘴湍流射流的影响.实验在射流出口雷诺数为 Re = 4050—20100 的范围内进行,分别测量了射流出口、中心线的平均及湍流流场以及部分径向剖面速度分布.所有测量均采用单热线恒温热线风速仪进行高频采样,所测流场范围在轴向上为 0—30d(这里d为射流出口直径).虽然出口速度分布均为"平顶帽"形,但测量结果依然反映出Re对射流出口以及下游流场有强烈的影响.当Re小于临界值(～10000)时 关键词： 雷诺数 圆形射流 热线风速仪     

圆管潜射流生成偶极子涡特性实验

利用溢流恒压装置产生具有稳定出流速度的圆管潜射流, 结合染色液流态显示方法, 在多种射流无量纲潜深d/H、雷诺数Re以及限制数C的组合下, 实验研究了该潜射流动量在有限深密度均匀流体中的演化特性, 其中d为射流潜深, H为水深. 研究表明, 当C<1时潜射流表现为深水特征, 而当1≤ C<2时潜射流表现为过渡特征, 在这两种情况下均不产生任何形式的大尺度相干结构; 当2≤ C<10时潜射流表现为浅水特征, 而C≥10时潜射流表现为极浅水特征, 在这两种情况下均产生大尺度的偶极子涡结构. 对极浅水特征潜射流, 在各种无量纲潜深下, 偶极子涡结构的无量纲形成时间t_f^*与无量纲射流时间T_inj^*均满足相同的正比例关系; 对浅水特征潜射流, 当d/H=0.5时, t_f^*与T_inj^*满足某种线性关系, 但对其他无量纲潜深, t_f^*与T_inj^*之间无明显规律. 关键词： 圆管潜射流 限制数 偶极子涡结构 形成时间     

层流圆管潜射流生成蘑菇形涡结构特性数值研究

基于不可压Navier-Stokes方程, 采用计算流体力学方法, 数值模拟与分析了层流圆管潜射流在密度均匀黏性流体中的演化机理及其表现特征, 定量研究了蘑菇形涡结构无量纲射流长度L^*、螺旋型涡环半径R^*及其包络外形长度d^*等几 何特征参数随无量纲时间t^*的变化规律. 数值结果表明, 蘑菇形涡结构的形成与演化过程可分为三个不同的阶段: 启动阶段、发展阶段和衰退阶段. 在启动阶段, L^*和d^*随t^* 线性变化, 而R^*则近似为一个常数; 在发展阶段, 蘑菇形涡结构的演化具有自相似性, L^*, R^*和d^*与t^*1/2均为同一正比关系, 而且雷诺数和无量纲射流时间不影响该正比关系; 在衰退阶段, L^* 和R^* 正比于t^*1/5, 而d^*则近似为一个常数. 此外, 还对蘑菇形涡结构二次回流点、 动量源作用中心及其几何中心的速度变化规律、垂向涡量分布特征和 涡量-流函数关系进行了分析. 关键词： 圆管潜射流 蘑菇形涡结构 演化机理     

A large eddy lattice Boltzmann simulation of magnetohydrodynamic turbulence

Large eddy simulations (LES) of a lattice Boltzmann magnetohydrodynamic (LB-MHD) model are performed for the unstable magnetized Kelvin–Helmholtz jet instability. This algorithm is an extension of Ansumali et al. [1] to MHD in which one performs first an expansion in the filter width on the kinetic equations followed by the usual low Knudsen number expansion. These two perturbation operations do not commute. Closure is achieved by invoking the physical constraint that subgrid effects occur at transport time scales. The simulations are in very good agreement with direct numerical simulations.     

Simulation of Kelvin-Helmholtz Instability with Flux-Corrected Transport Method

The sixth-order accurate phase error flux-corrected transport numerical algorithm is introduced, and used to simulate Kelvin-Helmholtz instability. Linear growth rates of the simulation agree with the linear theories of Kelvin Helmholtz instability. It indicates the validity and accuracy of this simulation method. The method also has good capturing ability of the instability interface deformation.     

Self-sustained oscillations in a large magneto-optical trap

We have observed self-sustained radial oscillations in a large magneto-optical trap, containing up to 10(10) Rb85 atoms. This instability is due to the competition between the confining force of the magneto-optical trap and the repulsive interaction associated with multiple scattering of light inside the cold atomic cloud. A simple analytical model allows us to formulate a criterion for the instability threshold, in fair agreement with our observations. This criterion shows that large numbers of trapped atoms N>10(9) are required to observe this unstable behavior.     

Nonlinear Effects Related to the Simultaneous Excitation of Three Instabiities in Magnetized Plasma

Experimental results are presented on the nonlinear effects related to the simultaneous excitation of three low‐frequency instabilities in the magnetized plasma column of a Q‐machine, namely the potential relaxation instability, the electrostatic ion‐cyclotron instability and the Kelvin‐Helmholtz instability. The influence of electron drift and magnetic field intensity on appearance, dynamics and mutual interaction of these instabilities was investigated (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self-sustained oscillations in a closed side branch system

Self-sustained oscillations of the flow in a closed side branch system due to a coupling of vortex shedding with acoustical resonances are considered. The configuration consists of two closed side branches of same length placed opposite to each other along a main pipe. This is called a cross-junction. Numerical simulations, based on the Euler equations for two-dimensional inviscid and compressible flows, are performed. As the radiation into the main pipe is negligible at the resonance frequency, this acoustically closed system is a good test-case of such Euler numerical calculations. The numerical results are compared to acoustical measurements and flow visualization obtained in a previous study. Depending on the flow conditions, the predicted pulsation amplitudes are about 30-40% higher than the measured amplitudes. This is partially due to the absence of visco-thermal dissipation in the numerical model but also to the effect of wall vibrations in experiments. A simple analytical model is proposed for the prediction of the pulsation amplitudes. This model is based on Nelson's representation of the shear layer as a row of discrete vortices convected at constant velocity from the upstream edge towards the downstream edge. When the downstream edge is sharp, this results in a spurious interaction between the singularity of the vortices and of the edge flow. This artefact is partially compensated by suppressing the singularity of the acoustical flow at the edge, or when a junction with rounded edges, as found in engineering practice, is considered. In spite of its crudeness, the analytical model provides a fair prediction (within 30%) which makes it useful for engineering applications.     

Large eddy simulations of a circular orifice jet with and without a cross-sectional exit plate

The effect of a cross-sectional exit plane on the downstream mixing characteristics of a circular turbulent jet is in- vestigated using large eddy simulation （LES）. The turbulent jet is issued from an orifice-type nozzle at an exit Reynolds number of 5 ×104. Both instantaneous and statistical velocity fields of the jet are provided. Results show that the rates of the mean velocity decay and jet spread are both higher in the case with the exit plate than without it. The existence of the plate is found to increase the downstream entrainment rate by about 10% on average over the axial range of 8-30de （exit diameter）. Also, the presence of the plate enables the formation of vortex rings to occur further downstream by 0.5-1 .Ode. A physical insight into the near-field jet is provided to explain the importance of the boundary conditions in the evolution of a turbulent jet. In addition, a method of using the decay of the centreline velocity and the half-width of the jet to calculate the entrainment rate is proposed.     

Spatial structure of the flow at round jet outflow into a narrow channel

Results on visual studies of the flow structure in case of the round submerged jet in a narrow channel are presented. These studies were carried out for the laminar and turbulent flows. The typical large-scale structures and zones of intensive turbulent mixing were identified in the flow. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 08-08-00417-a).     

Experimental study and large Eddy simulation of a coaxial jet with perforated obstacles to control thermal mixing characteristics

Thermal mixing phenomena of a coaxial jet with perforated obstacles are analyzed both experimentally and numerically. Perforated obstacles are used in front of a coaxial jet that has different temperatures of the fluid to control the thermal mixing behavior. An experimental set-up was constructed to perform several cases and all cases are simulated by using large eddy simulation (LES) turbulence model. Results of the study presented that inserting perforated obstacle affects the mixing performance positively and the best mixing performance is obtained in the case that the obstacle has the highest permeability. Experimental and computational results were compared and a good agreement was obtained.     

On the Kelvin–Helmholtz and von Kármán vortices in the near-wake of semicircular cylinders with flaps

The signatures of the Kelvin–Helmoltz (K–H) and von Kármán (VK) vortices shed from a semicircular cylinder with flaps of length L/d = 0, 1/3, 1, 2, and 3 were investigated using hotwire anemometry. Here, L and d denote the flap length and diameter of the semi-circular cylinder, respectively. Experiments were performed at Reynolds numbers spanning one order of magnitude, Re ∈ [8.4 × 10³,?6.7 × 10⁴]. The results highlight the impact of the flow modulation through rigid flaps on the wake characteristics and dominant vortex shedding. The increase of flap length resulted in reduced mean shear in the near-wake, which influenced the onset and coherence of the K-H instability. Indeed, these motions are less likely to be present in the wake of the L/d = 3 case. The flaps also impacted the frequency of the VK shedding; the associated Strouhal number increased from 0.2 to 0.3 for flaps L/d ? 1. Only the cases without with the shortest flaps (L/d = 1/3) followed St = 0.2. There is a distinctive dependence of the f_{K ? H}/f_VK on Reynolds number and flap length. This ratio followed the well-known power-law relationship of circular cylinders in the case without flaps. However, the Reynolds number exponent decreased with increased flap length.     

The effect of subgrid-scale models on the entrainment of a passive scalar in a turbulent planar jet

Classical large-eddy simulation (LES) modelling assumes that the passive subgrid-scale (SGS) models do not influence large-scale quantities, even though there is now ample evidence of this in many flows. In this work, direct numerical simulation (DNS) and large-eddy simulations of turbulent planar jets at Reynolds number Re_H = 6000 including a passive scalar with Schmidt number Sc = 0.7 are used to study the effect of several SGS models on the flow integral quantities e.g. velocity and scalar jet spreading rates. The models analysed are theSmagorinsky, dynamic Smagorinsky, shear-improved Smagorinsky and the Vreman. Detailed analysis of the thin layer bounding the turbulent and non-turbulent regions – the so-called turbulent/non-turbulent interface (TNTI) – shows that this region raises new challenges for classical SGS models. The small scales are far from equilibrium and contain a high fraction of the total kinetic energy and scalar variance, but the situation is worse for the scalar than for the velocity field. Both a-priori and a-posteriori (LES) tests show that the dynamic Smagorinsky and shear-improved models give the best results because they are able to accurately capture the correct statistics of the velocity and passive scalar fluctuations near the TNTI. The results also suggest the existence of a critical resolution Δ_x, of the order of the Taylor scale λ, which is needed for the scalar field. Coarser passive scalar LES i.e. Δ_x ≥ λ results in dramatic changes in the integral quantities. This fact is explained by the dynamics of the small scales near the jet interface.     

超燃火焰稳定凹腔自激振荡特性的数值研究

对超声速冷流条件下用于超燃冲压发动机的凹腔火焰稳定器的自激振荡特性进行研究.采用混合RANS/LES方法对非定常流场进行数值模拟,考虑了凹腔的长深比和后缘角度两个关键参数.混合RANS/LES方法很好的捕捉到流场非定常大尺度结构并揭示了凹腔自由剪切层的演化过程.对凹腔压力振荡历程进行幅频分析,所得到的频率和理论分析结果与文献计算结果符合的很好.结果表明,凹腔的长深比和后缘角度对凹腔自激振荡特性都有很大的影响.随着凹腔长深比的减小,振荡能量趋于集中到某些频率对应的振荡模式上.随着凹腔后缘倾角的减小,大部分频率对应的振荡很快的被削弱;相对于陡后缘凹腔,小角度后缘凹腔只有较高频率对应的振荡模式存在.     

Self-sustained oscillations in a plasma-wall system with strongly inhomogeneous diffusion of charged particles

A simple system with a hydrogen plasma confined by a magnetic field parallel to the bounding material wall is considered. The charged particles diffuse out of the plasma, recombine on the wall and return into the plasma volume as neutrals, which are ionized by electrons. It is demonstrated that macroscopic self-sustained oscillations are an intrinsic feature of such a system if the diffusion coefficient of charged particles is strongly inhomogeneous in the plasma.     

Large Eddy Simulations of a piloted lean premix jet flame using finite-rate chemistry

A Large Eddy Simulation (LES) model capable of accurately representing finite-rate chemistry effects in turbulent premixed combustion is presented. The LES computations use finite-rate chemistry and implicit LES combustion modelling to simulate an experimentally well-documented lean-premixed jet flame stabilized by a stoichiometric pilot. The validity of the implicit LES assumption is discussed and criteria are expressed in terms of subgrid scale Damköhler and Karlovitz numbers. Simulation results are compared to experimental data for velocity, temperature and species mass fractions of CH₄, CO and OH. The simulation results highlight the validity and capability of the present approach for the flame and in general the combustion regime examined. A sensitivity analysis to the choice of the finite-rate chemistry mechanism is reported, this analysis indicates that the one and two-step global reaction mechanisms evaluated fail to capture the reaction layer with sufficient accuracy, while a 20-species skeletal mechanism reproduces the experimental observations accurately including the key finite-rate chemistry indicators CO and OH. The LES results are shown to be grid insensitive and that the grid resolution within the bounds examined is far less important compared to the sensitivity of the finite-rate chemistry representation. The results are analyzed in terms of the flame dynamics and it is shown that intense small scale mixing (high Karlovitz number) between the pilot and the jet is an important mechanism for the stabilization of the flame.     

Large-scale flow structures in a planar offset attaching jet with a co-flowing wall jet

Conditional averaging techniques were used to examine the periodic motions that were observed in flows consisting of an offset planar jet and a co-flowing planar wall jet. The offset jet is one jet height (H_j) away from the wall and has a Reynolds number of approximately 40, 000, based on H_j and flow-rate averaged velocity U_o; for the co-flowing jets, different heights (i.e., 0.18H_j and 0.5H_j) and velocities (i.e., 0.56U_o and 0.36U_o) were considered. The flows had periodic motions with frequencies fH_j/U_o = 0.28 and 0.49 or fH_c/U_o = 0.23 and 0.25, where H_c is the distance between the jets. The periodic motions were present in both the inner shear layer of the offset jet above the re-circulation region and the outer shear layer of the wall jet below the re-circulation region. The motions from the inner shear layer of the offset jet persisted in the shear layer that formed downstream of the re-circulating region. There were periodic motions in the outer shear layer of the offset jet particularly in the flow with the smaller wall jet. The relative contribution of the motions to the total fluctuations increased as the flow evolved downstream reaching a maximum approximately 4H_c downstream of the flow exit. The relative contribution of the periodic motions to the turbulent fluctuations was similar in the two flows but the periodic motions had a much larger impact on the near-wall velocity and pressure fluctuations in the flow with the smaller wall jet due to the trajectory of the periodic structures, the distance of the periodic structures to the wall and the size of these structures.     

Investigation of a turbulent wall jet in forced convection issuing into a directed coflow stream

In the present paper, we have studied numerically the directed coflow stream effects on mean and turbulent flow properties of a turbulent plane wall jet in forced convection emerging into a directed coflow stream. The system of equations governing the studied configuration is solved with a finite difference scheme using a staggered grid for numerical stability, not uniform in the two directions of the flow. The modified version of the first-order low Reynolds number k–? turbulence model is used and compared to existing experimental findings. It is found that predicted results are in satisfactory agreement with the experimental data and that the wall jet fluid decays faster in presence of a directed coflow stream. Results show also that the increase of coflow deviation angles causes an increase of the growth rates of the dynamic and thermal half-width of the jet and enhances the turbulent mixing. It is found that the longitudinal development of normalised forms of the jet characteristics parameters at different directed coflow velocity ratios can be reasonably well collapsed onto universal trends through the use of momentum length scale.