MOOSE Newsletter (May 2023)

Framework Improvements

Positions system

A new system was implemented in MOOSE. Positions are used to keep track of the locations of objects during MOOSE-based simulations. By default, they are updated when the mesh changes and on every execution. They can be used to spawn MultiApps distributed at the positions stored in the Positions object. Several Positions were implemented to help kickstart the use of the new system:

Positions can now also be used to match source and target locations during transfers. By specifying the "use_nearest_position", target locations in the target application will only be matched with source locations that are nearest to the same Position compared to any other Position in the Positions object.

Possibility to combine TimeSteppers

The time stepper system in MOOSE has been improved. A new [TimeSteppers] block has been introduced to replace the functionality of the current [TimeStepper] block. The new system keeps the features of [TimeStepper] and also supports composing multiple time steppers to compute the time step size. Users can still use [TimeStepper] for now, but in the future this block will be deprecated.

When more than one time stepper is provided, the new time stepper system will automatically compute the minimum time step among all the input time steppers as the time step size to use for that application. The new time stepper system also provides a "lower_bound" parameter to further control the time stepper size (e.g. enable growth driven by a single time stepper). This system is controllable via the Controls system. The user can turn on/off the provided time steppers to control the usage of time steppers ; for example, one could make time stepper(s) only active during a certain time period.

Modules improvements

Fluid properties for the NaK eutectic

Fluid properties were added for the NaK eutectic in the fluid properties module. These are extracted from the public Sodium handbook chapter on fluid properties (Bomelburg et al., 1972). The fluid properties implemented should suffice for use with the Navier Stokes module.

Functor solid thermal properties

A functor version of the solid thermal properties material has been added to the solid properties module. Functor materials are more flexible, enabling all types of arbitrary evaluations on faces or points within an element, for example. This addition is leveraged by the finite volume heat conduction solver.

Dimensionality reduction in the stochastic tools module

A Proper Orthogonal Decomposition (POD) aided dimensionality reduction algorithm has been added to the stochastic tools module. It supports gathering solution fields in parallel and extracting dominant feature vectors. The feature vectors can then be used in the new PODMapping object to reduce future solution fields into a low-dimensional latent space. The new features are fully compatible with the surrogate system which means that surrogates can be trained for reduced solution fields. An InverseMapping user object has been added to reconstruct approximates of the solution fields with given coordinates in the latent space.

Bug Fixes and Minor Enhancements

  • Moving source points in Dirac Kernels, such as the ReporterPointSource, are now supported.

  • A ParsedVectorAux has been added to MOOSE. It enables forming vector auxiliary variables in a very flexible manner, and it can be used for initialization of these variables, a long standing limitation of using vector variables.

  • Added error checking for calling variable APIs that would result in use of Assembly neighbor AD data which has not yet been implemented. Consequently, the neighbor test gradient in ADInterfaceKernel no longer pretends to have derivative data.

  • Advection schemes other than upwind and average can now be used in FVAdvection, e.g. TVD schemes such as min_mod and vanLeer. This is useful for users who want to model scalar advection in a constant background velocity field.

  • Dirichlet finite volume boundary conditions can now be controlled using the Controls system.

  • Undisplaced neighbor reference points are now always used when reinitializing displaced mesh neighbor finite element data. This is both an optimization and a bug fix for contexts like cohesive zones, in which attempting to invert the physical points from the element side into reference points on the neighbor side would fail because the element and neighbor sides are no longer coincident.

  • The ElementValueSampler now supports mixes of finite volume and finite element variables for postprocessing all element values to a CSV file.

  • The StitchedMeshGenerator will no longer merge boundaries that have the same id in both stitched meshes by default. This was causing confusion and necessitated re-numbering ahead of stitching. The option to merge is controlled by a boolean input parameter.

  • The ParsedGenerateSideset can now select sides with a boundary filter, in addition to the pre-existing subdomain and neighbor subdomain filters and selection through the parsed expression.

Always ensure the mesh is prepared for use

Systems like finite volume rely on links being formed between elements and their neighbors. Modules like thermal_hydraulics rely on knowing the correct spatial dimension of a mesh if a mesh has been rotated or translated in space. This kind of data relies on the libMesh mesh being properly prepared for use. Prior to this month, it was possible for a simulation to begin without the mesh being prepared, resulting in anything from segmentation faults to poor solution convergence to weird results. This buggy behavior has been fixed by going through a lot of mesh generators which don't prepared the mesh and adding lines like mesh->set_isnt_prepared(). This fix should "silently" benefit all users.

Add capability to abort nonlinear/linear solves early using the Terminator

Using an "execute_on" of LINEAR or NONLINEAR, the Terminator can now cut short almost immediately a solve, in order to cut the iterative solve and try again with a smaller time step.

Added function average post-processors

The post-processors FunctionElementAverage and FunctionSideAverage were added, which compute the average of a function over blocks and boundaries, respectively.

Added documentation for the Functor system

Functors offer a powerful interface that allows using interchangeable Functions, Variables and functor material properties. Specific Functor documentation was created, and all classes that can leverage it link to this page.

In addition, to facilitate transition to using Functor material properties a converter from functor material properties to regular material properties was added, for both scalar and vector material properties.

References

  1. H.J. Bomelburg, C.R.F. Smith, H.N. Royden, V.A. Swanson, A.W. Thiele, and R.J. Tuttle. Sodium-nak engineering handbook volume i sodium chemistry and physical properties, chapter 1: physical properties. Technical Report ISBN 0 677 03020 4, Under the auspices of the United States Atomic Energy Commission, 1972.[BibTeX]