Volume Fraction Reinitialization

The Volume Fraction Reinitialization model allows for an on demand modification of the free surface by sharpening or smearing the interface, based on user-defined criteria for reinitializing the volume fraction field.

In some VOF simulations, smearing can occur during remeshing or mesh replacement. This smearing occurs due to the interpolation of the volume fraction field, causing the interpolated solution to lose its sharpness and become diffusive when transferred from the old mesh to the refined mesh. The Volume Fraction Reinitialization model can be used to re-sharpen the interface by specifying a target interface width for the reinitialization of the volume fraction field. The Volume Fraction Reinitialization model can use the same triggering mechanism as remeshing.

The following image shows the volume fraction distribution of a simulation where the interface is re-sharpened with volume fraction reinitialization after smearing due to remeshing.

Before remeshing	After remeshing	After remeshing with reinitialization

Interface smearing can occur in VOF simulations due to large local CFL numbers resulting from excessively large time steps, small cells, or poorly converged solutions. When the CFL constraints of the HRIC convection scheme are violated, the HRIC scheme reverts to an unconditionally stable, but more diffusive first-order scheme. To address this issue, the Volume Fraction Reinitialization model can be used to accelerate the recovery of sharp interfaces. User-defined metrics based on volume fraction gradients are introduced to monitor and identify smeared interfaces. Once a certain level of smearing is exceeded, reinitialization is initiated.

The Volume Fraction Reinitialization model can also be applied in conjunction Adaptive Mesh Refinement (AMR). AMR generally preserves sharp interfaces well by preventing the refinement of interface cells or by ensuring that sharpness is maintained even when those cells are refined. In cases where such interface sharpness is not necessary or desired, the Volume Fraction Reinitialization model can be used to reduce the sharpness in those areas, which also reduces the computational cost of the simulation.

The Volume Fraction Reinitialization model can also be used for intentional smearing of the free surface to stabilize simulations where the discontinuous nature of the volume fraction profile can lead to instabilities. These instabilities can occur, for example, in surface-tension dominated simulations or during the initial stages of a simulation.

As an alternative in many of these situations, the Modified HRIC scheme with a reduced or increased level of interface sharpness can be used (see Interface Sharpness).