  Nonlinear simulations of neutron star mergers are complicated by the need to
represent turbulent dynamics. As we cannot (yet) perform simulations that
resolve accurately both the gravitational-wave scale and the smallest scales at
which magneto/hydrodynamic turbulence plays a role, we need to rely on
approximations. Addressing this problem in the context of large-eddy models, we
outline a coherent Lagrangian filtering framework that allows us to explore the
many issues that arise, linking conceptual problems to practical
implementations and the interpretation of the results. We develop understanding
crucial for quantifying unavoidable uncertainties in current and future
numerical relativity simulations and consider the implications for neutron-star
parameter estimation and constraints on the equation of state of matter under
extreme conditions.
