要約

Characteristics of large-scale flows obtained by numerically filtering the Direct Numerical Simulation (DNS) results of flows over flat and wavy surfaces with and without roughness are studied. A normalized kernel filter with a top-hat weight function of a few different sizes and shapes are used. The large structures of instantaneous flows are found to be represented fairly well by the filtered flows over both smooth and rough surfaces if the appropriate shape of filter is used. However, significant fractions of the turbulent stresses, the turbulent shear stress in particular, are removed by filters of commonly assumed shape and sizes in many Large-Eddy Simulations (LES). In addition near solid walls, the boundary stresses are lost and they need to be augmented to balance the momentum in LES that do not resolve near-wall flows. A method using the formally-filtered equations of motion is suggested.