Modelling bulk-viscotiy in NS mergers: hydrodynamic challenges

Abstract: In this talk I will discuss the challenges of modelling dissipative processes in binary neutron star merger simulations, with a focus on reaction-sourced bulk viscosity, likely the dominant dissipative effect. I will begin by reviewing the main hydrodynamic approaches and highlighting the few existing fully non-linear simulations that implement bulk viscosity. I will then explore the connection between reactions and bulk viscosity, from both a thermodynamic and a multi-scale perspective. Finally, I will address the additional complexity introduced by turbulence in mergers, showing how under-resolved turbulent dynamics can shift the effective notion of thermodynamic equilibrium and generate additional effective viscosities, reflecting the subtle ways in which turbulence interacts with microphysical dissipation.