Cut cells, immersed boundaries and adaptation

Cut cells and immersed boundaries

A cut-cell method clips background cells at the body surface and applies fluxes or boundary conditions on the resulting faces. Very small fluid volumes can constrain the time step or require special stabilization. Other immersed-boundary formulations impose the wall through forcing, ghost values or related methods on a background grid. Inspect wall-stress and geometry handling in the particular solver before relying on aerodynamic drag or separation predictions.

Adaptive mesh refinement

Adaptation changes the grid in response to a chosen indicator. Gradient-based indicators can target shocks and shear layers. An adjoint-based method can target error in an output such as lift or drag. Isotropic refinement reduces spacing in every direction. Anisotropic adaptation can preferentially refine across strong gradients, for example normal to a shock.

Adaptation can occur between solves or during a run. For transient cases assess solution transfer, time stepping and the moving refinement pattern. In scale-resolving methods, grid changes also alter effective resolution and may affect the turbulence model.

Sources

• NASA Cart3D · Cartesian cut-cell, inviscid flow and adaptation
• Berger and Colella (1989) · local adaptive mesh refinement
• NASA · CFD Vision 2030, anisotropic and output-based adaptation