非定常流动的快速拉格朗日涡方法数值模拟

采用快速拉格朗日涡方法数值模拟有复杂旋涡运动的非定常流动. 利用离散涡元模拟旋 涡的产生、聚集和输送过程. 拉格朗日描述法用来计算离散涡元的移动，而移动速度则利用 广义毕奥-萨伐尔公式结合快速多极子展开法计算，修正的涡半径扩散模型用来模拟离散涡 元的黏性扩散. 突然起动圆柱和大攻角下突然起动翼型的非定常有涡流动的数值模拟，及其 与试验结果的对比验证了方法的有效性. 另外，大攻角下突然起动翼型的计算结果给出了 翼型起动后吸力面旋涡的产生、发展，周期性非定常流动的形成，以及尾流旋涡结构等一些 重要的流动特征. [关键词] 非定常流有涡流动快速涡方法     

翼型空泡周期性流动的数值模拟及机理分析

采用基于正压关系的均质平衡流空化模型和低雷诺数修正的k-ε湍流模式,自行开发了空泡流数值模拟方法和计算软件,对绕翼型空泡的周期性流动现象进行了数值模拟.计算结果与实验数据的对比表明,空泡的宏观特征、流动特性、周期性脱落的斯坦顿数St等与试验结果接近,验证了计算结果的可靠性.空泡在大约一个周期的2/3时间段内成长,并在大约1/3周期时刻发生断裂脱落.这两个特征时间与高速摄像实验结果一致.所取工况对应的组合参数σ/2α=2.865,以翼弦长计算可得St=0.217,与文献的最新试验结果吻合.空泡周期性运动过程中升阻系数也周期性振荡,时均值C<,1>=0.41,C<,d>=0.097,振荡频率与空泡脱落频率一致.对空泡运动过程中流场结构的变化进行了分析,结果表明在大攻角条件下,空泡闭合区后的逆压梯度导致涡的形成及回射流的发展,沿壁面逆向流动的混合介质射流是引起空泡断裂的原因,回射流发展、涡结构变化与空泡非稳态演化过程存在密切的联系,探讨了翼型空泡发生周期性脱落的一些机理.     

可压缩流向涡与激波轴对称干扰的数值模拟1)

用NS方程数值模拟了可压缩流向涡和激波轴对称相互作用现象.数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较.结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征.提出了判断流向涡与运动激波相互作用中旋涡破碎的准则.     

可压缩流向涡与激波轴对称干扰的数值模拟

用ＮＳ方程数值模拟了可压缩流向涡和激波轴对称相互作用现象．数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较．结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征．提出了判断流向涡与运动激波相互作用中旋涡破碎的准则     

小Kc数振荡流中圆柱绕流流场结构的数值分析

本文综合Chorin的涡方法、网格涡方法和泊松方程快速解法的优点,采用固定离散涡的数目和位置,随时间变化调整离散涡强度的计算方法,对Kc数在1到5之间,β数在10到1665之间圆柱在振荡流动中的流场结构进行了数值模拟。并与U型振荡水槽中的流场涡结构显示照片比较,计算结果表明,数值解在稳定性和收敛性方面都是比较好的。与显示照片相比,其符合程度也是令人满意的。     

方形钝体受限绕流的三维数值模拟

采用一种具有二阶精度的分裂步有限元方法作为大涡模拟的空间离散格式,经过标准算例的验证后,对Re=1.0×104条件下的方形钝体三维受限湍流绕流流场进行了数值模拟.计算中,为消除初始效应,略去初始段的计算结果.数值分析表明在均匀来流条件下,该湍流场沿槽道轴面对称,并呈现出一定的拟周期特性.在流场特性分析的基础上,进行了湍流能耗场的分析,结果表明,方形钝体受限绕流的能耗主要集中在大涡丰富的流动区段内.计算过程反映出,采用该空间离散格式的大涡模拟方法,能够捕捉到非常丰富的涡系及涡动的时变过程,适用于方形钝体受限绕流的数值模拟.     

可压缩流向涡与激波轴对称干扰的数值模拟

用ＮＳ方程数值模拟了可压缩流向涡和激波轴对称相互作用现象。数值模拟包括定常和非定常两种情况,计算结果分别与相应的实验进行了比较,结果表明数值模拟成功地捕捉到了激波和旋涡相互作用过程中发生的激波波面变形,激波振荡,涡核变大以及激波波后出现驻点、回流区等流场特征。提出了判断流向涡与运动激波相互作用中旋涡破碎的准则。     

高超声速尾迹流场稳定性数值研究

通过数值模拟, 对高超声速尾迹流场进行了研究, 对其尾迹流动的失稳过程进行 了分析. 选取计算模型为圆球，Ma= 6.0, Re = 1.71\times 10^6(Re以球头半径为 参考长度). 通过数值模拟，首先得到的流动是稳定解，在底部发展出 一个主分离区和一个二次分离区，流动是轴对称状态. 不添加任何扰动继续进行计算，发现 底部流场缓慢发展出微弱的非定常流动. 随后，该现象继续发展，出现明显的结构失稳，得 到了无量纲周期为12.0的周期解. 给出了高超声速圆球绕流尾迹结构的周期性演化过 程，对其涡系结构的演化及奇点特征进行了分析. 研究表明该数值模拟方法可用于 底部流动稳定性问题的研究，同时证实了高超声速底部流动也存在流动不稳定性.     

后台阶流动的数值模拟

访述了大涡模拟的基本思想，指出大涡模拟的效率主要取决于四个因素，即流动中须有大尺度涡存在、合理的计算格式、合适的滤波器和亚格子应力模型。在深入考虑粘性不可压缩流Navier—Stokes方程各个子项作用的基础上，提出二阶全展开Euler—Taylor—Galerkin有限元方法作为大涡模拟的离散格式，并采用Gauss滤波器，对典型算例——后台阶处的流动进行大涡模拟，计算结果与相关文献符合的很好。从计算结果还可以看出大涡模拟与二阶全展开ETG有限元方法的结合在捕捉涡系及反映涡动时变过程方面具有明显的优势，说明大涡模拟适合于边界几何形状复杂区域流动的模拟。同时应用二阶全展开ETG有限元方法对低雷诺数粘性不可压缩后台阶流动进行了计算，得到与相关文献符合良好的计算结果，即该方法也可独立用于对低雷诺数粘性不可压缩流动的计算。     

混合层流动拟序结构的大涡模拟

采用大涡模拟方法对空间发展的二维平面混合层进行了数值模拟 ,动量方程采用分步投影法求解 ,亚格子项采用标准Smagorinsky亚格子模式模拟 ,压力泊松方程采用修正的循环消去法快速求解 ,同时求解了标志物输运方程以实现数值流场显示。模拟结果给出了混合层流动的瞬态发展过程以及流动中拟序结构的发展演变过程 ,成功地模拟了混合层发展中的各种瞬态细节过程 ,如涡的卷起、增长 ,涡与涡之间的配对、合并过程 ,以及大涡破碎为小涡的级联过程 ,为各种以混合层流动为原型流动的射流、尾流等工业流动的控制和优化提供了理论基础。     

充分发展圆管湍流的实验研究

采用粒子数字图像测速（digital particle image velocimetry,DPIV)和定量流动显示技术（quantitative flow visualization,QFA)对充分发展的圆管湍流进行了研究。测量结果和直接数值模拟（direct numerical simulation,DNS)结果进行了比较，结果表明作者开发的DPIV技术取得了满意的精度。在此基础上对圆管湍流的动力学机理进行了研究，分析了上抛和下扫在湍流生成中的贡献以及流动显示结构内的脉动速度分布，测量结果显示在圆管湍流的近壁区存在横向强脉冲现象和流动显示所能观察到的结构为上抛占主导地位的结构。     

Numerical simulation of an isolated vortex and shear flow interaction

A numerical solution for the Navier-Stokes equations in the unbounded region is considered for the interaction of an isolated vortex and shear flow. A Chebyshey collocation method in space and finite-difference method for temporal discretization are used. The results of the numerical experiments for the interaction are discussed. It is shown that shear flow can both increase and decrease the vortex dissipation rate.     

Numerical prediction of locally forced turbulent separated and reattaching flow

An unsteady numerical simulation was performed for locally forced separated and reattaching flow over a backward-facing step. The local forcing was given to the separated and reattaching flow by means of a sinusoidally oscillating jet from a separation line. A version of the k––f_μ model was employed, in which the near-wall behavior without reference to distance and the nonequilibrium effect in the recirculation region were incorporated. The Reynolds number based on the step height (H) was fixed at Re_H=33 000, and the forcing frequency was varied in the range 0St_H2. The predicted results were compared and validated with the experimental data of Chun and Chun. It was shown that the unsteady locally forced separated and reattaching flows are predicted reasonably well with the k––f_μ model. To characterize the large-scale vortex evolution due to the local forcing, numerical flow visualizations were carried out.     

Simulation of 2D external viscous flows by means of a domain decomposition method using an influence matrix technique

Two-dimensional external viscous flows are numerically approximated by means of a domain decomposition method which combines a vortex method and a finite difference method. The vortex method is used in the flow region which is dominated by convective effects, whereas the finite difference method is used in the flow region where viscous diffusion effects are dominant. An influence matrix technique combined with the uniformity condition of the pressure is used to enforce the tangential velocity boundary condition. Comparisons between numerical and experimental data show that the method is well adapted for simulating two-dimensional flows.     

水下发射航行体尾涡不稳定性分析

在水下连续发射过程中前一发航行体尾流会对后一发航行体运动姿态稳定性产生流动干扰现象. 因此, 研究尾流中涡旋结构演变机理对解决多弹体水下连续发射流动干扰难题具有重要的意义. 本文采用改进型分离涡模型与能量方程, VOF多相流模型与重叠网格技术相结合方法, 对航行体水下发射尾流演变过程开展精细化模拟研究, 其中模拟结果和实验吻合度较好, 验证了本文数值方法的有效性. 以航行体尾流区域为重点研究对象, 分析了尾流区瞬态流场分布, 讨论了横流强度和雷诺数对尾涡结构演变以及脉动压力分布特性的影响. 结果表明: 由于尾流区高速流体核心区与低速自由流相互作用导致Kelvin-Helmholtz不稳定现象出现, 可以清晰地发现涡旋结构在剪切力的作用下发生脱落. 在横流条件下, 航行体尾端脱落的涡环与涡腿形成发卡涡, 而多个发卡涡沿轴向间隔排列组成发卡涡包存在于尾流中. 随着横流强度增大, 形成多级发卡涡包结构, 而导致脉动压力二次峰值均出现的主要原因是尾流涡旋流场演变引起的. 随着雷诺数的增大, 尾流中由圆柱形涡和U型涡组成的二次涡结构逐渐明显, 不稳定性加强.      

Specified discharge velocity models for numerical simulations of laminar vortex rings

We numerically and theoretically investigate the flow generated at the exit section of a piston/cylinder arrangement that is generally used in experiments to produce vortex rings. Accurate models for the velocity profile in this section (also called specified discharge velocity, SDV models) are necessary in (i) numerical simulations of laminar vortex rings that do not compute the flow inside the cylinder and (ii) in slug-models that provide a formula for the total circulation of the flow. Based on the theoretical and numerical analysis of the flow evolution in the entrance region of a pipe, we derive two new and easy to implement SDV models. A first model takes into account the unsteady evolution of the centerline velocity, while the second model also includes the time variation of the characteristics of the boundary layer at the exit plane of the vortex generator. The models are tested in axisymmetric direct numerical simulations of vortex rings. As distinguished from classical SDV model, the new models allow to accurately reproduce the characteristics of the flow. In particular, the time evolution of the total circulation is in good agreement with experimental results and previous numerical simulations including the vortex generator. The second model also provides a more realistic time evolution of the vortex ring circulation. Using the classical slug-model and the new correction for the centerline velocity, we finally derive a new and accurate analytical expression for the total circulation of the flow.     

Pulsating flow past a spanwise rib in a channel at moderate Reynolds numbers

The results of visual investigations and direct numerical simulation of flow past a spanwise rib in a channel in a pulsating external flow at the Reynolds numbers corresponding to transition to turbulence in the separation region downstream of the rib in steady-state flow past the latter are represented. It is shown that the calculated and experimental data are in the adequate accordance. The effect of the forced unsteadiness parameters on the vortex flow structure downstream of the rib is analyzed. Some laws of the formation and evolution of the vortex structure downstream of the rib in a pulsating flow are obtained.     

圆柱空腔内涡破裂的LDA三维流动测量

针对旋转圆柱空腔内旋转流场泡状涡破裂现象,设计了实验装置,分析了实验系统中光路设置、折射率补偿、示踪粒子选择等关键问题的处理方法,采用激光多谱勒速度测量技术ＬＤＡ（ＬａｓｅｒＤｏｐｐｌｅｒＡｎｅｍｏｍ ｅｔｒｙ）进行测量,给出了圆柱空腔内子午面上二维速度全场和涡破裂区域的内部三维流动测量结果． 同时给出的数值计算模拟结果和实验测量结果非常吻合．     

Vortex flows of a viscous heat-conducting gas in a slightly divergent tube with volume energy supply

The results of a numerical investigation of viscous vortex flow in a slightly divergent tube with thermal energy supplied to the flow are presented. The initial stage of vortex flow development is considered for two different longitudinal velocity distributions simulating the velocity profiles in jet-like and wake-like vortex flows in the vicinity of the vortex axis. The first type of flow can be considered as a model for the near-axis region of the vortex formed in the flow around a delta wing at incidence. The second type can serve as a model for the near-axis region of the trailing vortex downstream of a high-aspect-ratio wing. The development of the two flows is studied for a constant area tube, a slightly divergent tube, and in the case of thermal energy supply from a volume energy source at a constant wall temperature.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 5, pp. 90–97, September–October, 1996.     

Experimental Investigation of Separated Shear Flow under Subharmonic Perturbations over a Backward-Facing Step

Subharmonic-perturbed shear flow downstream of a two-dimensional backward-facing step was experimentally investigated. The Reynolds number was Re_h = 2.0 ×10⁴, based on free-stream velocity and step height. Planar 2D-2C particle image velocimetry was employed to measure the separating and reattaching flow in the horizontal-vertical plane in the center position. The subharmonic perturbations were generated by an oscillating flap which was implemented over the step edge and driven by periodic Ampere force. The subharmonic frequency was 55 Hz as the half of the fundamental frequency of the turbulent shear layer. As a result of the subharmonic perturbations, the size of recirculation region behind the backward-facing step is reduced and the time-averaged reattachment length is 31.0% shorter than that of the natural flow. The evolution of vortices, including vortex roll-up, growth and breakdown process, is analyzed by using phase-averaging, cross-correlation function and proper orthogonal decomposition. It is found that Reynolds shear stress is considerably increased in which the vortices roll up and then break down further downstream. In particular, rapid growth of vortices based on the “step mode” occurs at approximate half of the recirculation region, caused by in interaction between the shear layer and the recirculation region. Furthermore, the coherent structures, which are represented by a phase-correlated POD mode pair, are reconstructed in phases in order to show regular patterns of the subharmonic-perturbed coherent structures.