Rayleigh-Taylor unstable flames at higher Reynolds number

TL;DR

This study investigates Rayleigh-Taylor unstable flames at higher Reynolds numbers, confirming previous findings that RT instability dominates flame dynamics and challenges existing turbulent flame speed models.

Contribution

It extends prior research by analyzing RT unstable flames at higher Reynolds numbers, reinforcing the dominance of RT instability over turbulence in flame behavior.

Findings

RT instability remains dominant at higher Reynolds numbers.

RT flames are thinner with stronger turbulence.

Turbulent flame speed models still do not predict flame speed accurately.

Abstract

Rayleigh-Taylor (RT) unstable flames are a key component of Type Ia and Iax supernovae explosions, but their complex hydrodynamics is still not well understood. These flames are affected not only by the RT instability, but also by the turbulence it generates. Both processes can increase the flame speed by stretching and wrinkling the flame. This makes it hard to choose a subgrid model for the flame speed in full star Type Ia or Iax simulations. Commonly used subgrid models get around this difficulty by assuming that either the RT instability or turbulence is dominant and sets the flame speed. In previous work, we evaluated the physical assumptions and predictive abilities of these two types of models by analysing a large parameter study of 3D direct numerical simulations of RT unstable flames. Surprisingly, we found that the flame dynamics is dominated by the RT instability and that RT unstable flames are very different from turbulent flames. In particular, RT unstable flames are thinner rather than thicker when turbulence is strong. In addition, none of the turbulent flame speed models adequately predicted the flame speed. We also showed that the RT flame speed model failed when the RT instability was strong, suggesting that geometrical burning effects also influence the flame speed. However, these results depended on simulations with $Re\lesssim720$. In this paper, we extend the parameter study to higher Reynolds number and show that the basic conclusions of our previous study still hold when the RT-generated turbulence is stronger.