圆突扩两相流中气相脉动速度与速度梯度的关系

采用PDA对三种不同颗粒质量载荷的竖直向下圆管突扩流动进行了测量,发现在剪切较强的突扩发展区中气相脉动速度与速度梯度存在线性关系.采用强剪切流假设,进一步对管流中气相湍流的理论推导得到了与实验结果一致的结论.理论分析表明,颗粒浓度较小时,颗粒对气相湍流作用较弱,单相流中气相脉动速度与速度梯度的线性关系得以保持.同时推测,当增大颗粒浓度,颗粒对气相湍流的非线性作用增大,气相脉动速度与速度梯度的线性关系消失.对经典文献中的实验数据进行分析,验证了以上推论.     

磁场剪切对离子温度梯度模带状流产生的影响

对离子温度梯度模湍流非线性流体方程进行了解耦处理,得到包含磁场剪切效应的带状流与漂移波相互作用的非线性动力学方程.采用调制不稳定性的四波相互作用模型,研究了磁场剪切对带状流产生的影响.研究表明,在k_//值较小的范围内,当|k_//|增加时,带状流的增长率也呈增加的趋势. 关键词： 托卡马克等离子体 离子温度梯度模湍流 带状流 磁场剪切     

串列双圆柱时均流场特性分析

对Re=12 000,间距比L/D=1.167,2.333,3.500和4.667的串列双圆柱后方速度场进行了实验测量,分析串列双圆柱后方不同剖面处的速度分布规律和湍流度分布规律.并通过流函数理论模型对小间距比串列双圆柱后方流场进行了分析,得出如下结论:与单圆柱相比,当串列双圆柱间距比较小时,后方圆柱的自由剪切层明显向内偏移.随着间距比的变大,由于前方圆柱的尾流对后方圆柱的干扰,后方圆柱的自由剪切层变得越来越模糊.对于间距比较大的串列双圆柱,其对后方流场的扰动较强,致使后方流场湍流强度和最大速度衰减量较大.通过流函数理论模型分析发现,在小间距比条件下,串列双圆柱由于两个圆柱的相互干扰,使得圆柱后方涡相互靠近,并且后方涡向外倾斜的角度也减小,从而导致了自由剪切层向内侧偏移.      

库特（Couette）剪切流准确解的一般形式

 从N-S方程出发,得到了轴对称异型断面管纯剪切流动速度分布的一般解,进而推出了纯剪切流动有限宽度矩形断面管的速度分布和流量计算公式,证明了库特剪切流是有限宽度矩形断面管当宽度趋向无穷大时的一种特例。并对库特剪切流流量的计算误差进行了分析,给出了该公式的适用范围,同时对有限宽度矩形断面管剪切流公式进行了曲线拟合,得到了便于工程使用的流量计算公式。     

网格湍流对剪切层中预混火焰结构的影响

本文研究网格湍流对射流剪切层以及建立在其中的预混火焰的影响。利用热线风速仪测量射流的速度场,发现网格湍流使剪切层内湍流强度明显降低,抑制了低频速度脉动,同时增加了湍动能在小尺度脉动上的分配,使湍流更趋于各向同性,这表明网格湍流抑制了剪切层内的大涡和拟序结构。用细丝热电偶测量了火焰温度,结果显示网格湍流使火焰前峰的低频大幅摆动减少,小尺度皱褶增加,火焰区平均温度更高,说明网格湍流有利于剪切层中预混火焰的强化和稳定。     

应用GAO-YONG可压缩湍流模式数值模拟RAE2822翼型绕流

应用Gao-Yong可压缩湍流模式,数值模拟RAE2822二维翼型在两种不同来流情况下的跨音速粘性绕流问题.湍流模式的对流项用ROE格式离散,扩散项用中心差分格式离散,空间离散后的控制方程用多步Runge-Kutta显式时间推进格式求解.计算结果预测了翼型表面的压力系数的分布、平均速度剖面、激波的位置、马赫数等值线等情况.同时,对翼型表面激波与边界层相互干扰以及转捩问题进行分析计算,结果表明,Gao-Yong可压缩湍流模式结合适当的数值方法能够成功地模拟翼型跨音速粘性流动.最后,基于Gao-Yong可压缩湍流模式各项异性湍流粘性的机理,初步提出一种预测转捩起始位置的方法.     

三维边界层内诱导横流失稳模态的感受性机理

边界层感受性问题是层流向湍流转捩的初始阶段,在转捩过程中起关键性作用,尤其是三维边界层流动.因此,研究三维边界层感受性问题对进一步理解层流向湍流转捩机理以及湍流成因具有重要的理论意义.采用数值方法研究自由来流湍流与三维壁面局部粗糙相互作用下三维边界层的感受性问题,确定是否能在三维边界层内寻找一种新的横流失稳模态;确定在何种条件下三维边界层内能诱导出定常、非定常的横流失稳模态;探索自由来流湍流的强度、展向波数和法向波数以及三维壁面局部粗糙的大小和结构类型等因素在自由来流湍流与三维壁面局部粗糙作用下三维边界层内被激发出的感受性过程中有何影响,并确定何种横流失稳模态在三维边界层感受性过程中占据何种地位.对自由来流湍流与三维壁面局部粗糙作用激发三维边界层内感受性问题的深入研究,将有助于完善流动稳定性与湍流理论,为层流向湍流转捩过程的预测与控制提供合理的理论依据.     

高磁雷诺数下剪切流对双撕裂模中二级磁岛的影响

在高磁雷诺数下,当双撕裂模发展进入快速磁场重联阶段时,会发生二级磁岛不稳定性,加剧磁场能量的释放。本文基于扰动形式的守恒磁流体方程组发展高精度的数值模拟程序,在平板位形下研究反对称位形剪切流对双撕裂模中二级磁岛的影响。结果表明：随着剪切流强度和剪切梯度的增加,二级磁岛的数目以及电流片横纵比变小。此外,较强的极向剪切流能够抑制二级磁岛不稳定性的发生。     

下降管中稠密两相湍流的数值模拟

本文将统一二阶矩两相湍流模型和颗粒动力学理论结合,推导并封闭了稠密两相流考虑颗粒间碰撞的统一二阶矩两相湍流模型。该模型用颗粒动力学理论模拟颗粒之间的碰撞,用各向异性的统一二阶矩模型考虑气相和颗粒相的湍流脉动,并用输运方程描述气固两相湍流之间的相互作用。最后用该模型对下降管中的气粒两相流动进行了模拟,模拟所得的沿径向颗粒浓度分布和速度分布与实验吻合较好,预报结果也反应出了颗粒雷诺应力的各向异性。     

受限空间狭缝射流传热特性数值模拟研究

狭缝冲击射流是一种高效的换热形式,传统湍流模型无法准确预测其换热规律。采用常用的k-ε系列湍流模型和k-ω系列湍流模型,结合Kato-Launder模型、剪切流修正模型、间歇转捩模型和横流效应修正模型,对受限空间狭缝射流开展了数值模拟研究。结果表明,标准k-ω湍流模型结合Kato-Launder模型和剪切流修正模型可在两种典型冲击距离下都成功预测壁面传热特性,间歇转捩模型对SST k-ω剪切应力输运湍流模型预测能力的改善作用极其有限,而Kato-Launder模型可以极大改善SST k-ω湍流模型在较小冲击距离下的传热预测能力,但在较大冲击距离下,Kato-Launder模型会预测出更为显著的努塞尔数伪第二峰值。     

Role of the zonal flow in multi-scale multi-mode turbulence with small-scale shear flow in tokamak plasmas

The structural characteristics of zonal flows and their roles in the nonlinear interaction of multi-scale multi-mode turbulence are investigated numerically via a self-consistent Landau-fluid model. The multi-mode turbulence here is composed of a shorter wavelength electromagnetic (EM) ion temperature gradient (ITG) mode and a Kelvin-Helmholtz (KH) instability with long wavelengths excited by externally imposed small-scale shear flows. For strong shear flow, a prominent periodic intermittency of fluctuation intensity except for dominant ITG component is revealed in turbulence evolution, which onset time depends on the ion temperature gradient and the shear flow amplitudes corresponding to different KH instabilities. It is identified that the intermittency phenomenon results from the zonal flow dynamics, which is mainly generated by the KH mode and back-reacts on it. It is demonstrated that the odd symmetric components of zonal flow (same symmetry as the external flow) make the radial parity of the KH mode alteration through adjusting the drift velocities at two sides of the resonant surface so that the KH mode becomes bursty first. Afterwards, the ITG intermittency follows due to nonlinear mode coupling. Parametric dependences of the features of the intermittency are elaborated. Finally, associated turbulent heat transport is evaluated.     

Role of zonal flow predator-prey oscillations in triggering the transition to H-mode confinement

Direct evidence of zonal flow (ZF) predator-prey oscillations and the synergistic roles of ZF- and equilibrium E×B flow shear in triggering the low- to high-confinement (L- to H-mode) transition in the DIII-D tokamak is presented. Periodic turbulence suppression is first observed in a narrow layer at and just inside the separatrix when the shearing rate transiently exceeds the turbulence decorrelation rate. The final transition to H mode with sustained turbulence and transport reduction is controlled by equilibrium E×B shear due to the increasing ion pressure gradient.     

多组分颗粒稠密气固两相流动的数值模拟

基于气体分子运动理论和颗粒动理学方法，建立多组分颗粒气固两相流等温流动模型。模型考虑了颗粒相各组分颗粒温度的差异、气相与颗粒相以及颗粒相各组分之间的动量和能量的传递和耗散，以及相间作用。建立颗粒相粘性系数、颗粒相压力等物性参数计算模型。模拟计算颗粒相浓度、粒径分布等参数与实测值相吻合。     

Self-regulation of E x B flow shear via plasma turbulence

The momentum balance has been applied to the ExB flow in the edge region of a reversed field pinch (RFP) configuration. All terms, including those involving fluctuations, have been measured in stationary condition in the edge region of the Extrap-T2R RFP experiment. It is found that the component of the Reynolds stress driven by electrostatic fluctuations is the term playing the major role in driving the shear of the ExB flow to a value marginal for turbulent suppression, so that the results are in favor of a turbulence self-regulating mechanism underlying the momentum balance at the edge. Balancing the sheared flow driving and damping terms, the plasma viscosity is found anomalous and consistent with the diffusivity due to electrostatic turbulence.     

Research of the rapid pressure-strain correlation model in the rapid distortion limit

Even though a number of rapid pressure-strain models have been suggested and successfully tested for different flow situations by various authors,the model proposals still exhibit some apparent deficiencies when subjected to the flows with rapid distortion. From Mansour's relatively straightforward rapid distortion analysis,if an initially anisotropic flow undergoes a purely rapid rotation,the anisotropy measures will exhibit the behavior of the damped oscillations. Within the current framework of modeling the rapid pressure-strain correlation,i.e.,the models based on the assumption that the M-tensor for the rapid pressure-strain term is expand-able in the Reynolds-stress anisotropy tensor alone,all the model predictions fail to give the damped oscillations in the turbulence anisotropy. In the case of initially isotropic turbulence subjected to rapid distortion,Sj?gren and Johansson showed that all the existing rapid pressure-strain models would deliver the identical path in the anisotropy-invariant map for both homogeneous plane strain and shear flows. The rapid distortion analysis shows two distinct curves reflecting different flow physics. In this work,we try to present a possible way to create a system that can overcome these deficiencies with the aid of the rapid distortion theory (RDT).     

可压稠密气固两相流动过程的模拟计算

基于稠密气体分子运动论和颗粒动理学,建立可压稠密气固两相流动模型。采用梯度模拟来考虑气相可压缩性对气相湍流的影响。模拟计算表明气固两相射流速度沿轴向和径向减小,颗粒浓度下降。气固两相射流具有高的颗粒温度,呈现强烈的气固两相湍流流动特性。     

确定分布的展向Lorentz力调制下的槽道湍流涡结构

采用直接数值模拟方法,对槽道湍流中确定分布的Lorentz力的流动控制与减阻问题进行研究.讨论了Lorentz力作用于槽道湍流后,流场的特性和涡结构的特性,并对此类Lorentz力对槽道湍流的控制与减阻机理进行了讨论.研究发现:1)Lorentz力诱导的层流流场壁面附近存在梯度极大的展向速度剪切层,该剪切层容易形成流向涡结构;2)在给定合适参数的确定分布的Lorentz力作用下,湍流流场仅剩周期分布的准流向涡;3)与未控制流场相比,控制后的流场中,准流向涡的抬升高度大大降低,从而减小猝发强度,使壁面阻力下降.     

横向粗糙元壁面槽道湍流的大涡模拟研究

采用大涡模拟和浸没边界法相结合对不同高度和不同间距横向粗糙元壁面槽道湍流进行了模拟,得到了光滑壁面和粗糙壁面湍流的流向平均速度分布,雷诺剪切应力,脉动速度均方根和近壁区拟序结构。结果发现横向粗糙元降低了流向平均速度,增大了流动阻力,粗糙壁面湍流的雷诺剪切应力大于光滑壁面。粗糙元降低了流向脉动速度,增强了展向和法向脉动速度。粗糙元高度越高,对湍流流动影响越大,而粗糙元间距对湍流统计特性的影响不大。粗糙壁面仍然存在着和光滑壁面类似的条带结构。     

Temperature in ideal gas mixtures in Couette flow: A maximum-entropy approach

A binary mixture of ideal gases in Couette flow is studied in the framework of information theory. It is shown that if the total shear viscous pressure is imposed as a constraint, there is equipartition of energy between the different chemical species but not amongst the different spatial directions. In contrast, if a same shear rate is imposed on both species, there is no equipartition between species neither amongst spatial directions. In both situations, the thermodynamic temperature, defined from the Lagrange multiplier conjugated to the internal energy, is the same for both species, independently of the equality or not of their local-equilibrium temperature.     

Suppression of plasma turbulence during optimized shear configurations in JET

Correlation of density turbulence suppression and reduced plasma transport is observed in the internal transport barrier (ITB) region of JET tokamak discharges with optimized magnetic shear. The suppression occurs in two stages. First, low frequency turbulence and ion transport are reduced across the plasma core by a toroidal velocity shear generated by intense auxiliary heating. Then with the ITB formation, high frequency turbulence and electron transport are reduced locally within the steep pressure gradient region of the ITB.