Mitigation of thermoacoustic oscillations by open-loop acoustic forcing: Physical mechanisms and an a priori control-design framework

Open-loop acoustic forcing effectively mitigates thermoacoustic oscillations, yet its underlying physical mechanisms are not fully understood. This study characterizes these mechanisms by examining the suppression of natural modes and the concurrent amplification of forced modes. By investigating the forced response of a lean-premixed turbulent combustor under two distinct self-excited states, we identify two critical concurrent processes. First, suppression of the natural mode is primarily determined by the attenuation of the flame response at the natural frequency, which disrupts the thermoacoustic feedback loop and leads to a corresponding reduction in pressure oscillations. Second, amplification of the forced mode is governed mainly by the system’s acoustic response, specifically the proximity of the forcing frequency to the natural frequency and its harmonics. Notably, the peak of the forced-mode pressure response does not coincide with that of the flame response. Furthermore, the optimal forcing frequency for achieving the maximum overall pressure reduction does not coincide with the frequency of FDF gain peak. To quantify the suppression of the natural mode across various forcing frequencies, dual-frequency flame-response measurements are performed, with the primary frequency corresponding to the natural mode and the secondary to the external forcing. The optimal frequency bands for suppressing the primary flame response coincide with those for mitigating self-excited mode under unstable conditions. These dynamics are qualitatively reproduced using a low-order model. Based on these findings, an a priori control-design framework is proposed, in which instability-prone frequencies are first identified (e.g., using a low-order thermoacoustic network model incorporating a flame model), and the effective forcing-frequency range is then determined via dual-frequency flame-response measurements. This approach enables prediction of optimal forcing parameters without triggering self-excited oscillations, thereby avoiding extensive parameter sweeps under unstable conditions and reducing the risk of hardware damage.Novelty and significance statementPrevious studies have mainly focused on demonstrating the effectiveness of open-loop acoustic forcing in mitigating thermoacoustic oscillations, with the underlying control mechanisms typically interpreted holistically. However, detailed investigations of energy accumulation in the forced mode remain largely unexplored. In this study, we reinterpret the control mechanism in terms of the combined contributions of the natural and forced modes and, for the first time, demonstrate that the forced mode exhibits a much stronger frequency dependence from an acoustic perspective than from the flame-response perspective. Furthermore, the optimal frequency bands for suppressing the primary flame response (determined via dual-frequency forcing) closely align with those for suppressing the self-excited flame response. This correlation enables us to propose an a priori control-design framework for determining forcing parameters under stable conditions, allowing safe and cost-efficient design of an open-loop control strategy.