Direct numerical simulation of a puff and a slug in transitional cylindrical pipe flow
Direct numerical simulation of a puff and a slug in transitional cylindrical pipe flow
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DOI：
10.1017/s0022112099004681
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发表时间：
1999-05
影响因子：
3.7
通讯作者：
H. Shan;B. Ma;Zhao-shun Zhang;F. Nieuwstadt
中科院分区：
文献类型：
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作者：
H. Shan;B. Ma;Zhao-shun Zhang;F. Nieuwstadt
过渡管流的直接数值模拟进行了光谱元素方法的帮助下，并用于调查在实验中观察到的“湍流”流动的局部区域。这种区域可以分为两种类型：喷烟区和段塞区。在雷诺数Re = 2200时模拟了通常在雷诺数低值下发现的喷流，其中雷诺数Re基于平均速度UB和管道直径D。段塞发生在较高的雷诺数，它是模拟为Re = 5000。计算从层流管流开始，在给定的轴向位置和有限的时间内，在层流管流中加入规定的速度扰动。然后，扰动进一步发展成喷流或段塞结构。模拟结果证实了实验观察到的事实，即对于喷烟，前缘附近的速度变化比段塞更缓慢，在段塞中观察到几乎不连续的变化。喷流和段塞流的前沿和后沿的位置是根据模拟计算的，作为时间的函数。前缘的传播速度是恒定的，对于喷烟和段塞，分别等于1.56UB和1.69UB。对于后缘的速度分别为0.73UB和0.52UB。通过将在不同时间获得的模拟结果重新缩放到固定长度，我们定义了系综平均值。该方法用于计算烟团和段塞的平均特性，例如平均速度的空间分布、湍流速度波动以及壁面剪切应力。通过计算颗粒轨迹，我们研究了喷烟和段塞对流体的卷吸和分离。我们发现，烟流通过其后缘离开，并通过其前缘夹带。段塞夹带流体通过其前缘和后缘的大部分。因此，喷烟内部的流体不断地与外部的流体交换，而段塞内部的流体保持在那里。这些卷吸/分离特性与Wygnanski和香槟（1973）的测量结果相一致，这意味着喷烟具有波动现象的特征，而段塞则更多地表现为随气流运动的物质特性。最后，我们更详细地研究了喷烟内的速度场。在以平均速度运动的坐标系中，我们发现后缘和前缘附近都有回流区，这至少在定性上与实验数据一致。我们还发现流向涡，主要是在后缘区域，也已观察到在实验中，并被认为在过渡过程中的动态中发挥重要作用。
A direct numerical simulation of transitional pipe flow is carried out with the help of a spectral element method and used to investigate the localized regions of ‘turbulent’ flow that are observed in experiments. Two types of such regions can be distinguished: the puff and the slug. The puff, which is generally found at low values of the Reynolds numbers, is simulated for Re = 2200 where the Reynolds number Re is based on the mean velocity UB and pipe diameter D. The slug occurs at a higher Reynolds number and it is simulated for Re = 5000. The computations start with a laminar pipe flow to which is added a prescribed velocity disturbance at a given axial position and for a finite time. The disturbance then evolves further into a puff or slug structure. The simulations confirm the experimentally observed fact that for a puff the velocity near the leading edge changes more gradually than for a slug where an almost discontinuous change is observed. The positions of the leading and trailing edges of the puff and slug are computed from the simulations as a function of time. The propagation velocity of the leading edge is found to be constant and equal to 1.56UB and 1.69UB for the puff and slug, respectively. For the trailing edge the velocity is found to be 0.73UB and 0.52UB, respectively. By rescaling the simulation results obtained at various times to a fixed length, we define an ensemble average. This method is used to compute the average characteristics of the puff and slug such as the spatial distribution of the mean velocity, the turbulent velocity fluctuations and also the wall shear stress. By computing particle trajectories we have investigated the entrainment and detrainment of fluid by a puff and slug. We find that the puff detrains through its trailing edge and entrains through its leading edge. The slug entrains fluid through its leading and through most of its trailing edge. As a consequence the fluid inside the puff is constantly exchanged with fluid outside whereas the fluid inside a slug remains there. These entrainment/detrainment properties which are in agreement with the measurements of Wygnanski & Champagne (1973) imply that the puff has the characteristics of a wave phenomenon while the slug can be characterized more as a material property which travels with the flow. Finally, we have investigated in more detail the velocity field within the puff. In a coordinate system that travels with the mean velocity we find recirculation regions both near the trailing and leading edges which agrees at least qualitatively with experimental data. We also find streamwise vortices, predominantly in the trailing-edge region which have been also observed in experiments and which are believed to play an important role in the dynamics of the transition process.