Identifying optimal location for control of thermoacoustic instability through statistical analysis of saddle point trajectories

TL;DR

This paper introduces a Lagrangian Coherent Structures-based framework to passively control thermoacoustic instability by targeting saddle point trajectories in the shear layer, significantly reducing sound amplitude.

Contribution

It develops a novel passive control strategy using saddle point analysis and LCS to identify optimal injection locations for suppressing thermoacoustic instability.

Findings

Passive control reduces sound amplitude drastically.

Shear layer deflection prevents vortex impingement.

Flow dynamics analyzed with LCS confirm control effectiveness.

Abstract

We propose a framework of Lagrangian Coherent Structures (LCS) 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 (DMD) 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 (ORZ), which eventually impinging 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 analysed 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.