nearwall

Near-wall refers to the region of a fluid flow that lies immediately adjacent to a solid boundary. In this zone, viscous forces are strong, velocity changes are steep, and the flow characteristics differ markedly from those in the outer flow. The near-wall region governs wall shear stress, friction drag, and heat transfer, making accurate representation essential for predictions in many engineering and natural systems.

In computational fluid dynamics, resolving the near-wall region poses a major challenge. Direct numerical simulation resolves all scales of motion, including the viscous sublayer, but it is extremely costly. Large-eddy simulation resolves the larger turbulent structures while modeling the smaller scales, often with a near-wall treatment. Reynolds-averaged Navier–Stokes models commonly rely on turbulence closures and wall models to represent near-wall behavior without fully resolving the boundary layer.

Common near-wall modeling approaches include wall functions that approximate the relationship between wall shear stress and the outer flow, and low-Reynolds-number or enhanced wall treatment models that attempt a more detailed representation of the viscous sublayer. The law of the wall and various turbulence closures are used to bridge the region from the wall to the outer flow, balancing accuracy with computational cost.

Mesh design for near-wall regions emphasizes sufficient resolution normal to the boundary. Depending on the method, guidelines vary: DNS aims for very fine near-wall grids (y+ around 1), wall-resolved LES targets similarly fine spacing, and RANS with wall functions uses coarser spacing (higher y+ values). Practical considerations include drag prediction, heat transfer, and potential flow separation.

See also: wall-bounded turbulence, y-plus, law of the wall, shear stress, boundary layer.