  The subgrid-scale modelling of a low Mach number strongly anisothermal
turbulent flow is investigated using direct numerical simulations. The study is
based on the filtering of the low Mach number equations, suited to low Mach
number flows with highly variable fluid properties. The results are relevant to
formulations of the filtered low Mach number equations established with the
classical filter or the Favre filter. The two most significant subgrid terms of
the filtered low Mach number equations are considered. They are associated with
the momentum convection and the density-velocity correlation. We focus on
eddy-viscosity and eddy-diffusivity models. Subgrid-scale models from the
literature are analysed and two new models are proposed. The subgrid-scale
models are compared to the exact subgrid term using the instantaneous flow
field of the direct numerical simulation of a strongly anisothermal fully
developed turbulent channel flow. There is no significant differences between
the use of the classical and Favre filter regarding the performance of the
models. We suggest that the models should take into account the asymptotic
near-wall behaviour of the filter length. Eddy-viscosity and eddy-diffusivity
models are able to represent the energetic contribution of the subgrid term but
not its effect in the flow governing equations. The AMD and scalar AMD models
are found to be in better agreement with the exact subgrid terms than the other
investigated models in the a priori tests.
