A model problem comprising two subsytems (acoustic and hydrodynamic), both admitting unsteady flows, has been studied using numerical simulations. Simulations capture all key features observed in an experiment with a similar configuration, and reveal the differences in flow field evolution that manifest as an intermittent prelude to instability in this nonlinear system, coupling acoustic and hydrodynamic phenomena. The system consists of a uniform flow that enters a long duct and leaves through an orifice into the atmosphere. The duct supports acoustic modes, while the shear layer at the boundary of the jet emerging from the orifice can develop into a train of vortices. Over some range of inflow velocities that support a stable operation, duct acoustic modes and shear layer breakup result in aperiodic, small amplitude pressure fluctuations. As the flow rate is increased, fluctuations become essentially periodic and have much larger amplitudes. This transition from a quiescent to a (so-called) whistling state occurs over a range of flow rates and is characterized by intermittent, moderately large, aperiodic fluctuations. As in the experiment, the aperiodic time series from the quiescent state is found to be multifractal; multifractality is then lost as the flow transitions to whistling. Simulation flow fields reveal differences in the development of vortices in the bounding shear layer, and near the orifice walls, that give rise to the large changes in pressure fluctuations, as well as the intermittent behavior that precedes whistling.

中文翻译：

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表现出不稳定前奏的声-水动力系统的数值研究

已经使用数值模拟研究了包含两个子系统（声学和流体动力学）的模型问题，这两个子系统都允许非定常流动。仿真捕获了在具有类似配置的实验中观察到的所有关键特征，并揭示了流场演变的差异，这些差异表现为这种非线性系统中不稳定的间歇性前奏，耦合声学和流体动力学现象。该系统由进入长管道并通过孔口进入大气的均匀流组成。管道支持声学模式，而从孔口喷出的射流边界处的剪切层可以发展成一系列涡流。在支持稳定运行的某些流入速度范围内，管道声学模式和剪切层破裂会导致非周期性的小振幅压力波动。随着流速的增加，波动基本上变成周期性的并且具有更大的幅度。这种从静止状态到（所谓的）啸叫状态的转变发生在一定的流速范围内，其特征是间歇性的、中等大的、非周期性的波动。在实验中，发现静止状态的非周期时间序列是多重分形的；当流动过渡到啸叫时，多重分形就会消失。模拟流场揭示了边界剪切层和孔口壁附近涡旋发展的差异，这会导致压力波动的巨大变化，以及哨声之前的间歇行为。这种从静止状态到（所谓的）啸叫状态的转变发生在一定的流速范围内，其特征是间歇性的、中等大的、非周期性的波动。在实验中，发现静止状态的非周期时间序列是多重分形的；当流动过渡到啸叫时，多重分形就会消失。模拟流场揭示了边界剪切层和孔口壁附近涡旋发展的差异，这会导致压力波动的巨大变化，以及哨声之前的间歇行为。这种从静止状态到（所谓的）啸叫状态的转变发生在一定的流速范围内，其特征是间歇性的、中等大的、非周期性的波动。在实验中，发现静止状态的非周期时间序列是多重分形的；当流动过渡到啸叫时，多重分形就会消失。模拟流场揭示了边界剪切层和孔口壁附近涡旋发展的差异，这会导致压力波动的巨大变化，以及哨声之前的间歇行为。当流动过渡到啸叫时，多重分形就会消失。模拟流场揭示了边界剪切层和孔口壁附近涡旋发展的差异，这会导致压力波动的巨大变化，以及哨声之前的间歇行为。当流动过渡到啸叫时，多重分形就会消失。模拟流场揭示了边界剪切层和孔口壁附近涡旋发展的差异，这会导致压力波动的巨大变化，以及哨声之前的间歇行为。