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首页> 外文期刊>International Journal of Heat and Fluid Flow >Instability and localized turbulence associated with flow through an axisymmetric sudden expansion
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Instability and localized turbulence associated with flow through an axisymmetric sudden expansion

机译：与通过轴对称突然膨胀的流动相关的不稳定性和局部湍流

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Numerical simulations of flow through a sudden expansion were performed for various Reynolds numbers. Planar channel flow and axisymmetric pipe flow configurations are simulated with expansion ratios of 2:1 and Reynolds numbers ranging from 15 to 2500. Above a critical Reynolds number, a symmetry-breaking bifurcation occurs in the planar cases leading to asymmetry in the reattachment zone and a corresponding deviation between the planar and pipe configurations. For the planar flows, the variation of the reattachment length with the Reynolds number in the laminar and transitional regimes achieves excellent agreement with experimental data from literature. As the Reynolds number increases in the axisymmetric pipe cases, unsteadiness of the flow occurs as vortical disturbances begin in the core region along the pipe centerline and in the separated shear layer that forms downstream of the expansion. The disturbances convect downstream and rapidly amplify until a disturbance bursting event occurs wherein strong turbulent fluctuations are generated along the periphery of the core jet. The interaction between the shear layer instability and disturbance bursting events is studied via the instantaneous vorticity transport equation. The analysis suggests that bursting events are due to the rapid production of fluctuating vorticity through radial tilting of streamwise vortical disturbances by the fluctuating radial strain rates that are induced by the coherent azimuthal vortices. Once saturation of the shear layer instability is achieved, redistribution of fluctuating vorticity into small scales diminishes the radial strain rates and halts this vorticity production mechanism. Viscous effects lead to a gradual relaminarization of the flow, yielding a localized region of turbulent flow that remains fixed in a time-averaged sense near the reattachment point.
机译：对各种雷诺数进行了通过突然膨胀的流动的数值模拟。模拟了平面通道流动和轴对称管道流动配置，膨胀比为2：1，雷诺数在15到2500之间。在临界雷诺数以上，在平面情况下会发生破坏对称的分叉，从而导致重新连接区和通道的不对称性。平面和管道配置之间的相应偏差。对于平面流，在层流和过渡状态下，重新连接长度随雷诺数的变化与文献中的实验数据非常吻合。随着轴对称管道中雷诺数的增加，流动不稳定，因为沿管道中心线的核心区域以及在膨胀下游形成的分离剪切层中开始出现涡旋扰动。扰动向下游对流，并迅速放大，直到发生扰动突发事件，其中沿核心射流的外围产生强烈的湍流波动。通过瞬时涡度输运方程研究了剪切层失稳与扰动爆发事件之间的相互作用。分析表明，爆裂事件是由于相干方位角涡旋引起的径向波动率波动引起径向涡流径向倾斜，从而迅速产生了涡旋波动。一旦达到剪切层不稳定性的饱和度，波动涡度重新分配为小尺度就将减小径向应变率，并停止这种涡度产生机制。粘性效应导致流体逐渐分层，从而产生湍流的局部区域，该区域在重新连接点附近以时间平均的方式保持固定。

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