A new family of interface compression interpolation schemes based on
piecewise-linear interface calculation (PLIC). PLIC represents an interface by
surface-cuts which split each cell to match the volume fraction of the phase in
that cell. The surface-cuts are oriented according to the point field of the
local phase fraction. The phase fraction on each cell face — the interpolated
value — is then calculated from the amount submerged below the surface-cut.
The basic PLIC method generates a single cut so cannot handle cells in which
there are multiple interfaces or where the interface is not fully resolved. In
those cells, the interpolation reverts to an alternative scheme, typically
standard interface compression. PLIC, with a fallback to interface compression,
produces robust solutions for real engineering cases. It can run with large time
steps so can solve problems like hydrodynamics of a planing hull, with rigid
body motion of the hull (above). The user selects PLIC by the following setting
in fvSchemes:
div(phi,alpha) Gauss PLIC interfaceCompression vanLeer 1;
The multicut PLIC (MPLIC) scheme extends PLIC to handle multiple
surface-cuts. Where a single cut is insufficient, MPLIC performs a topological
face-edge-face walk to produce multiple splits of a cell. If that is still
insufficient, MPLIC decomposes the cell into tetrahedrons on which the cuts are
applied. The extra cutting carries an additional computational cost but requires
no fallback. The user selects MPLIC by the following setting in the fvSchemes
file:
div(phi,alpha) Gauss MPLIC;
Variants of the PLIC and MPLIC schemes are also available which use velocities
at the face points to calculate the face flux. These PLICU and MPLICU schemes
are likely to be more accurate in regions of interface under high shear.
More details can be found here:
https://cfd.direct/openfoam/free-software/multiphase-interface-capturing
Jakub Knir
CFD Direct Ltd.
A new run-time selectable interface compression scheme framework has been added
to the two-phase VoF solvers to provide greater flexibility, extensibility and
more consistent user-interface. The previously built-in interface compression
is now in the standard run-time selectable surfaceInterpolationScheme
interfaceCompression:
Class
Foam::interfaceCompression
Description
Interface compression corrected scheme, based on counter-gradient
transport, to maintain sharp interfaces during VoF simulations.
The interface compression is applied to the face interpolated field from a
suitable 2nd-order shape-preserving NVD or TVD scheme, e.g. vanLeer or
vanAlbada. A coefficient is supplied to control the degree of compression,
with a value of 1 suitable for most VoF cases to ensure interface integrity.
A value larger than 1 can be used but the additional compression can bias
the interface to follow the mesh more closely while a value smaller than 1
can lead to interface smearing.
Example:
\verbatim
divSchemes
{
.
.
div(phi,alpha) Gauss interfaceCompression vanLeer 1;
.
.
}
\endverbatim
The separate scheme for the interface compression term "div(phirb,alpha)" is no
longer required or used nor is the compression coefficient cAlpha in fvSolution
as this is now part of the "div(phi,alpha)" scheme specification as shown above.
Backward-compatibility is provided by checking the specified "div(phi,alpha)"
scheme against the known interface compression schemes and if it is not one of
those the new interfaceCompression scheme is used with the cAlpha value
specified in fvSolution.
More details can be found here:
https://cfd.direct/openfoam/free-software/multiphase-interface-capturing
Henry G. Weller
CFD Direct Ltd.
providing the shear-stress term in the momentum equation for incompressible and
compressible Newtonian, non-Newtonian and visco-elastic laminar flow as well as
Reynolds averaged and large-eddy simulation of turbulent flow.
The general deviatoric shear-stress term provided by the MomentumTransportModels
library is named divDevTau for compressible flow and divDevSigma (sigma =
tau/rho) for incompressible flow, the spherical part of the shear-stress is
assumed to be either included in the pressure or handled separately. The
corresponding stress function sigma is also provided which in the case of
Reynolds stress closure returns the effective Reynolds stress (including the
laminar contribution) or for other Reynolds averaged or large-eddy turbulence
closures returns the modelled Reynolds stress or sub-grid stress respectively.
For visco-elastic flow the sigma function returns the effective total stress
including the visco-elastic and Newtonian contributions.
For thermal flow the heat-flux generated by thermal diffusion is now handled by
the separate ThermophysicalTransportModels library allowing independent run-time
selection of the heat-flux model.
During the development of the MomentumTransportModels library significant effort
has been put into rationalising the components and supporting libraries,
removing redundant code, updating names to provide a more logical, consistent
and extensible interface and aid further development and maintenance. All
solvers and tutorials have been updated correspondingly and backward
compatibility of the input dictionaries provided.
Henry G. Weller
CFD Direct Ltd.
