Colloquia

Summary

Entropy patches, localized regions of non-uniform thermodynamic properties, are key contributors to indirect combustion noise in propulsion systems. As these patches pass through choked nozzles, they generate upstream-traveling acoustic waves that can trigger thermoacoustic instabilities in gas turbines and rocket engines. Such instabilities pose serious design challenges, as they may lead to degraded performance or even catastrophic failure.

This research uses two-dimensional Euler-based numerical simulations to investigate entropy patch interactions with choked nozzles. The primary objective was to determine under which conditions the acoustic response can be accurately captured using simplified, linear reduced-order models.

Two modeling approaches were examined: the quasi-steady model and the inertial model. These models differ fundamentally in their treatment of the sound-generation mechanism. A key outcome of the study is the identification of a critical dimensionless length scales that determine the applicability of each modeling regime.

A second dimensionless parameter, the entropy patch strength, was introduced to evaluate the limits of linearization. Results show that for sufficiently strong disturbances, nonlinear effects dominate, rendering linear models inadequate.

The findings offer a clear framework for selecting appropriate modeling strategies for entropy-induced noise, supporting the development of quieter, more stable, and more reliable combustion systems.