Propagation of a Thermohaline Mixing Front in 3D DNS and 1D Models

Surface abundances of red-giant branch stars cannot be explained by standard mixing models and require "anomalous mixing" that is believed to be thermohaline mixing. In these stars, a destabilizing compositional source is provided by nuclear burning, and this source causes a thermohaline-unstable zone to grow in size. Here, we consider a system where a thermohaline-unstable layer with a continuous compositional source gradually propagates into a thermohaline-stable layer. We present a suite of 3D simulations across different molecular diffusivities (parameterised by Pr and diffusivity ratio, τ) and initial gradients (parameterised by the density ratio, R₀). We also present 1D simulations where temperature and composition are evolved according to a diffusion equation with a turbulent diffusivity given by local models from Brown et al (2013). We compare the propagation of the thermohaline front predicted by 1D models to those found in 3D DNS and see how well these match across parameter space.