Rayleigh-Taylor Unstable Flames -- Fast or Faster?

TL;DR

This study investigates the dynamics of Rayleigh-Taylor unstable flames in Type Ia supernovae, revealing that traditional models often fail to predict flame speeds accurately and proposing cusp formation as a key factor in flame acceleration.

Contribution

The paper provides a comprehensive analysis of RT unstable flames through 3D simulations, critically evaluates existing flame speed models, and introduces the idea that cusp formation influences flame propagation.

Findings

RT unstable flames are thinner with increased turbulence.

Existing turbulent flame speed models do not accurately predict measured speeds.

Cusp formation may cause flames to propagate faster than RT model predictions.

Abstract

Rayleigh-Taylor (RT) unstable flames play a key role in the explosions of Type Ia supernovae. However, the dynamics of these flames is still not well-understood. RT unstable flames are affected by both the RT instability of the flame front and by RT-generated turbulence. The coexistence of these factors complicates the choice of flame speed subgrid models for full-star Type Ia simulations. Both processes can stretch and wrinkle the flame surface, increasing its area and, therefore, the burning rate. In past research, subgrid models have been based on either the RT instability or turbulence setting the flame speed. We evaluate both models, checking their assumptions and their ability to correctly predict the turbulent flame speed. Specifically, we analyze a large parameter study of 3D direct numerical simulations of RT unstable model flames. This study varies both the simulation domain width and the gravity in order to probe a wide range of flame behaviors. We show that RT unstable flames are different from traditional turbulent flames: they are thinner, rather than thicker when turbulence is stronger. We also show that none of several different types of turbulent flame speed models accurately predicts measured flame speeds. In addition, we find that the RT flame speed model only correctly predicts the measured flame speed in a certain parameter regime. Finally, we propose that the formation of cusps may be the factor causing the flame to propagate more quickly than predicted by the RT model.