Premchand C P, Krishnan Abin, Raghunathan Manikandan, Midhun P R, Reeja K V, Sujith R I, Nair Vineeth
Department of Aerospace Engineering, Indian Institute of Technology Bombay, 400076 Mumbai, India.
Department of Aerospace Engineering, Indian Institute of Technology Madras, 600036 Chennai, India.
Chaos. 2024 Aug 1;34(8). doi: 10.1063/5.0175991.
We propose a framework of Lagrangian Coherent Structures (LCSs) to enable passive open-loop control of tonal sound generated during thermoacoustic instability. Experiments were performed in a laboratory-scale bluff-body stabilized turbulent combustor in the state of thermoacoustic instability. We use dynamic mode decomposition on the flow-field to identify dynamical regions where the acoustic frequency is dominant. We find that the separating shear layer from the backward-facing step of the combustor envelops a cylindrical vortex in the outer recirculation zone, which eventually impinges on the top wall of the combustor during thermoacoustic instability. We track the saddle points in this shear layer emerging from the backward-facing step over several acoustic cycles. A passive control strategy is then developed by injecting a steady stream of secondary air targeting the identified optimal location where the saddle points spend a majority of their time in a statistical sense. After implementing the control action, the resultant flow-field is also analyzed using LCS to understand the key differences in flow dynamics. We find that the shear layer emerging from the dump plane is deflected in a direction almost parallel to the axis of the combustor after the control action. This deflection, in turn, prevents the shear layer from enveloping the vortex and impinging on the combustor walls, resulting in a drastic reduction in the amplitude of the sound produced.
我们提出了一个拉格朗日相干结构（LCSs）框架，以实现对热声不稳定性期间产生的音调声音的被动开环控制。实验在处于热声不稳定性状态的实验室规模钝体稳定湍流燃烧器中进行。我们对流场进行动态模态分解，以识别声学频率占主导的动态区域。我们发现，燃烧器后向台阶处的分离剪切层在外回流区域包围了一个圆柱形涡旋，在热声不稳定性期间，该涡旋最终撞击燃烧器的顶壁。我们在几个声学周期内跟踪从后向台阶出现的该剪切层中的鞍点。然后，通过向已识别出的鞍点在统计意义上花费大部分时间的最优位置注入稳定的二次空气流，制定了一种被动控制策略。实施控制措施后，还使用LCS对流场进行分析，以了解流动动力学中的关键差异。我们发现，控制措施实施后，从扩压平面出现的剪切层在几乎平行于燃烧器轴线的方向上发生偏转。这种偏转进而防止了剪切层包围涡旋并撞击燃烧器壁，从而使产生的声音幅度大幅降低。
