If divU is valid the velocity divergence is included in the pcorr equation.
This simplifies the logic in multiphase moveMesh functions supporting
incompressible (with or without mass sources) and compressible fluids.
The cell-base momentum/pressure algorithm in the multiphaseEuler solver module
has been substantially updated to improve consistency, conservation and reduce
drag generated staggering patterns at sharp interfaces and the boundaries with
stationary phases. For most if not all cases this new algorithm can be used to
provide well resolved and reliable solutions where the faceMomentum algorithm
would have been chosen previously in order to obtain sufficiently smooth
solutions but at the expense of a noticeable loss in accuracy and resolution.
The first significant change in the momentum/pressure algorithm is in the
interpolation practice used to construct the flux predictor equation from the
cell momentum equation: rather than interpolating the H/A ratio to the faces
i.e. (H/A)_f the terms in the momentum equation are interpolated separately so
that H_f/A_f is used. The same approach is used for the drag i.e. (D_f/A_f) and
virtual mass contributions. The advantage of this change is that the phase
forces are now consistent in both the momentum and flux equations, i.e. sum to
zero for each pair of phases.
The second significant change is in the handling of ddtCorr which converts the
old-time time-derivative contributions in H from velocity to flux which is now
consistent due to the change to H/A interpolation and also generalised to use
the fvc::ddtCorr function which has been updated for multiphase. Additionally
ddtCorr may optionally be applied to the time-derivative in the virtual mass
term in a consistent manner so that the contributions to the flux equation sum
to zero for each pair of phases.
The third significant change is the addition of an optional drag correction term
to the momentum corrector to reduce the staggering patters generated in the
velocity field due to sudden changes in drag force between phase, e.g. at sharp
interfaces between phases or at the boundaries with stationary phases. This is
particularly beneficial for fluidised bed simulations. However this correction
is not and cannot be phase consistent, i.e. the correction does not sum to zero
for pairs of phases it is applied to so a small drag error is introduced, but
tests so far have shown that the error is small and outweighed by the benefit in
the reduction in numerical artefacts in the solution.
The final significant change is in the handling of residualAlpha for drag and
virtual mass to provide stable and physical phase velocities in the limit of the
phase-fraction -> 0. The new approach is phase asymmetric such that the
residual drag is applied only to the phase with a phase-fraction less than
residualAlpha and not to the carrier phase. This change ensures that the flow
of a pure phase is unaffected by the residualAlpha and residual drag of the
other phases that are stabilised in pure phase region.
There are now four options in the PIMPLE section of the fvSolutions dictionary
relating to the multiphase momentum/pressure algorithm:
PIMPLE
{
faceMomentum no;
VmDdtCorrection yes;
dragCorrection yes;
partialElimination no;
}
faceMomentum:
Switches between the cell and face momentum equation algorithms.
Provides much smoother and reliable solutions for even the most challenging
multiphase cases at the expense of a noticeable loss in accuracy and resolution.
Defaults to 'no'.
VmDdtCorrection:
Includes the ddtCorr correction term to the time-derivative part of the
virtual-mass term in the flux equation which ensures consistency between the
phase virtual mass force on the faces but generates solutions which are
slightly less smooth and more likely to contain numerical artefacts.
Defaults to 'no'.
Testing so far has shown that the loss in smoothness is small and there is
some noticeable improvement is some cases so in the future the default may
be changed to 'yes'.
dragCorrection:
Includes the momentum corrector drag correction term to reduce the
staggering patters generated in the velocity field due to sudden changes in
drag force at the expense of a small error in drag consistency.
Defaults to 'no'
partialElimination:
Switches the partial-elimination momentum corrector which inverts the drag
matrix for both the momentum equations and/or flux equations to provide a
drag implicit correction to the phase velocity and flux fields. The
algorithm is the same as previously but updated for the new consistent drag
interpolation.
All the tutorials/modules/multiphaseEuler tutorial cases have been updated and
tested with the above developments and the four options set appropriately for
each.
The mixture compressibility/density is now included in CorrectPhi for
compressible mixtures, consistent with the compressibility handling in the
pressure equation. This improves consistency, robustness and convergence of the
pcorr equation.
The computation of the current and total accumulated mass is now shared
across all zeroDimensionalMassSource models. So, multiple models can be
used simultaneously.
If a negative mass flow rate is specified, the mass source fvModel will
now remove mass by adding implicit sources to the transport equations.
Properties are thereby removed at their current value. This is stable,
and is analogous to a zero-gradient outlet boundary condition.
so that the residualAlpha applied to stabilised the dispersed phase does not
affect the continuous phase in the limit of it becoming pure with or without
partial-elimination.
Flux correction is now applied if either the topology changed or the mesh is
moving and correctPhi is true. This strategy allows moving mesh cases without
topology change to be run without any change to the fluxes which is appropriate
for solid-body motion of the entire domain or a rotating sub-domain with NCC.
Class
Foam::solvers::functions
Description
Solver module to execute the \c functionObjects for a specified solver
The solver specified by either the \c subSolver or if not present the \c
solver entry in the \c controlDict is instantiated to provide the physical
fields needed by the \c functionObjects. The \c functionObjects are then
instantiated from the specifications are read from the \c functions entry in
the \c controlDict and executed in a time-loop also controlled by entries in
\c controlDict and the \c maxDeltaT() returned by the sub-solver.
The fields and other objects registered by the sub-solver are set to
NO_WRITE as they are not changed by the execution of the functionObjects and
should not be written out each write-time. Fields and other objects created
and changed by the execution of the functionObjects are written out.
When restarting from a time directory which does contain the \c subSolver
fields the optional \c controlDict entry \c subSolverTime may be provided to
specify which time the \c subSolver should be instantiated for, after which
time is reset to \c startTime for the restart.
to provide the old-time absolute flux. This avoids possible
pressure-velocity-flux decoupling (staggering) within the MRF region using
ddtCorr to better couple the velocity and flux fields.
The current mesh is now swapped with the new mesh prior to the mapping
of fields and other properties. Previously the new mesh was copied into
the current mesh.
This change means that both meshes are valid during the mapping
operation, and properties of either can be used. It should also now be
be more efficient as a swap operation just exchanges list pointers and
sizes, whilst a copy requires duplicating all the primitive mesh data.
It is now possible to calculate field values of VolInternalFields, e.g. the
cached kEpsilon:G field in the
tutorials/modules/incompressibleFluid/pitzDailySteady case:
#includeFunc cellMax(kEpsilon:G)
These two new fvModels operate between a film and a VoF region to transfer film
to the corresponding VoF phase when the film is thick enough to be resolved by
the VoF solver or from the VoF phase to the film when the near-wall resolution
is too low and it is better to treat it as a wall film.
This functionality is equivalent to the VoFPatchTransfer fvModel developed for
the old film implementation but coded in a much more general manner with
implicit treatment of the mass loss from the film or VoF region for better
numerical stability and robustness.
The simple tutorials/modules/CHT/VoFToFilm case is provided to demonstrate a
film being deposited on a surface as the plate is withdrawn from a liquid. It
is an updated version of the tutorials/modules/compressibleVoF/plateFilm case.
