Flow-Sound Interaction Mechanisms with Application to Bluff Body Wakes and Separated Shear Flows

自由剪切流，如钝体后的尾迹，空穴和侧枝上的分离剪切层，以及通过孔口和阀门的喷流，都是非常不稳定的。自由剪切流的不稳定性产生周期性的涡量脱落，其频率随流速线性变化。在许多工业应用中，如换热器、锅炉、透平机械、管道系统和反应堆容器中的管束，湍流中的涡量脱落很弱，并且高度无序/不连贯。然而，当涡度脱落与声学模式耦合时，出现了反馈机制，涡度脱落变得更强和更连贯。这会导致经常与过度振动相关的严重噪音问题的产生，这可能会导致运行中断，在某些情况下可能会迅速升级为灾难性故障。由于声学共振现象尚未完全了解，它可能是危险的不可预测的。这项研究的目的是促进对流-声相互作用机制的认识，以及流诱导声学共振的实现条件。在未来五年的研究计划中，重点将集中在两种主要流型上，即钝体尾迹和分离剪切流，以阐明管束中流声耦合(焦点区域I)和翅片圆柱群共振激励(焦点区域II)的基本机制。此外，还将制定可用于预测管阵中声共振发生的声学衰减标准(焦点区域III)。拟议的研究结果将有助于解决与几个重要工业应用中突然激发的流动引起的噪音和振动问题有关的挑战，以及缺乏在设计阶段预测其发生的指导方针。这不仅将减少由于潜在灾难性故障而导致的这些工程应用的计划外停工，而且还将允许建造更大、更安全和更高效的设备。

Free shear flows, such as wakes behind bluff bodies, separated shear layers over cavities and side branches, and jet flows through orifices and valves, are highly unstable. The instabilities of the free shear flows generate periodic vorticity shedding with frequencies that vary linearly with the flow velocity. In turbulent flow, which is typical in many industrial applications, such as tube bundles in heat exchangers, boiler plants, turbomachines, piping systems, and reactor vessels, the vorticity shedding is weak and highly unorganized/incoherent. However, when the vorticity shedding couples with an acoustic mode a feedback mechanism occurs and the vorticity shedding becomes stronger and much more coherent. This leads to the generation of acute noise problems that are often associated with excessive vibration, which could result in operation interruption and in some cases may escalate rapidly to catastrophic failures. Since the acoustic resonance phenomenon is not yet fully understood, it can be dangerously unpredictable. The objective of the proposed research is to advance the state of knowledge of the flow-sound interaction mechanisms and the conditions under which flow-induced acoustic resonances materialize. In the next five years of the research program the focus will be on two main flow configurations where bluff body wakes and separated shear flows are encountered to elucidate the fundamental mechanisms of flow-acoustic coupling in tube bundles (FOCUS AREA I) and resonance excitation from cluster of finned cylinders (FOCUS AREA II). Moreover, an acoustic damping criterion that can be used to predict the occurrence of acoustic resonance in tube arrays will be developed (FOCUS AREA III). The findings of the proposed research will help to tackle challenges associated with the sudden excitation of flow-induced noise and vibration problems in several important industrial applications and the lack of guidelines to predict their occurrence during the design stage. This will not only reduce unplanned shutdowns of these engineering applications due to potential catastrophic failures but also will allow for the construction of larger, safer, and more efficient equipment.