Split DEM and PIC out of the solids page; document solids.model and dem.lsd

Jordan Musserrequested to merge
jmusser/exa-docs:released-key-renames into main

The solids page keeps the types and their properties. New DEM model and PIC model pages (separate panes in the GUI) hold the model inputs: dem.contact_model (lsd), the linear spring-dashpot parameters under the prefix dem.lsd, the shared friction and tangential history inputs, the restitution coefficients for every solids type, and the PIC inputs. solids.dem and solids.pic are gone; solids.model, set in the particle options, applies to every type.

See https://mfix.netl.doe.gov/gitlab/exa/mfix/-/merge_requests/1957

Merge request reports

docs/source_docs/tutorials/fluid/index.rst

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@@ -4,7 +4,7 @@ Fluid 
=======



Tutorials in this category use MFIX-Exa's fluid solver with no particle

phase present (``solids.dem = none``, ``solids.pic = none``). This includes

phase present (``solids = none``). This includes

ordinary single-fluid flow as well as any case that tracks two immiscible

fluids by density within the same solver, such as a Rayleigh-Taylor case.

docs/source_docs/user_guide/inputs/dem_model.rst

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DEM model

=========



.. _InputsDEM:



Inputs for the discrete element method (DEM), used when ``solids.model = dem``

(see :ref:`particle options<InputsParticleOptions>`). Every type listed in

``solids`` is a DEM type.



Contact model

-------------



The following input is defined with the prefix ``dem``:



+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

|                               | Description                                                             |   Type   | Default   |

+===============================+=========================================================================+==========+===========+

| contact_model                 | Contact model for particle-particle and particle-wall collisions.       | String   | lsd       |

|                               |                                                                         |          |           |

|                               | Options:                                                                |          |           |

|                               |                                                                         |          |           |

|                               | * ``lsd`` : Linear spring-dashpot.                                      |          |           |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+



The linear spring-dashpot model takes the following inputs, defined with the

prefix ``dem.lsd``:



+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

|                               | Description                                                             |   Type   | Default   |

+===============================+=========================================================================+==========+===========+

| spring_const.pp               | Normal spring constant :: particle to particle collisions [required]    | Real     |  0        |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| spring_const.pw               | Normal spring constant :: particle to wall collisions [required]        | Real     |  0        |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| spring_tang_fac.pp            | Tangential-to-normal spring constant factor :: particle to particle     | Real     | 0.2857    |

|                               | collisions                                                              |          |           |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| spring_tang_fac.pw            | Tangential-to-normal spring constant factor :: particle to wall         | Real     | 0.2857    |

|                               | collisions                                                              |          |           |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| damping_tang_fac.pp           | Factor relating the tangential damping coefficient to the normal        | Real     |  0.5      |

|                               | damping coefficient :: particle to particle collisions                  |          |           |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| damping_tang_fac.pw           | Factor relating the tangential damping coefficient to the normal        | Real     |  0.5      |

|                               | damping coefficient :: particle to wall collisions                      |          |           |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+



Restitution coefficients

------------------------



The following inputs use the solids type names from ``solids`` to define restitution

coefficients and are defined with the prefix ``dem.restitution_coeff``. They must be

defined for every solid-solid and solid-wall combination.



+--------------------------+-------------------------------------------------------------------------+----------+-----------+

|                          | Description                                                             |   Type   | Default   |

+==========================+=========================================================================+==========+===========+

| [dem_solid].[dem_solid2] | Specifies the restitution coefficient between DEM solids.               | Real     |  0        |

|                          | Order is not important and this could be defined as                     |          |           |

|                          | ``[dem_solid2].[dem_solid]``.  If both are defined they must be equal.  |          |           |

+--------------------------+-------------------------------------------------------------------------+----------+-----------+

| [dem_solid].wall         | Specifies the restitution coefficient between solid and the wall.       | Real     |  0        |

|                          | Order is not important and this could be defined as                     |          |           |

|                          | ``wall.[dem_solid]``.  If both are defined they must be equal.          |          |           |

+--------------------------+-------------------------------------------------------------------------+----------+-----------+



Friction and tangential history

-------------------------------



.. |RollingFricModelA| replace:: :math:`\boldsymbol{\tau}_{ij}^{(r)} = -\mu_{r}\hat{r}_{ij} \left| f_{ij}^{(n)} \right| \frac{\omega_{ij}}{\left|\omega_{ij}\right|}`

.. |RollingFricModelB| replace:: :math:`\boldsymbol{\tau}_{ij}^{(r)} = -\mu_{r}\hat{r}_{ij} \left| f_{ij}^{(n)} \right| \boldsymbol{u}_{ij}^{\omega}`



The following inputs apply to every contact model and are defined with the prefix ``dem``:



+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

|                               | Description                                                             |   Type   | Default   |

+===============================+=========================================================================+==========+===========+

| friction_coeff.pp             | Friction coefficient :: particle to particle collisions [required]      | Real     |  0        |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| friction_coeff.pw             | Friction coefficient :: particle to wall collisions [required]          | Real     |  0        |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| tan_history                   | Include tangential history force in the collision model.                | Bool     | false     |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| tan_history.max_contacts      | Maximum number of contacts per particle tracked at any point when       | Int      | 10        |

|                               | tangential history is enabled.                                          |          |           |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| rolling_friction              | Rolling friction model :cite:p:`zhou1999,ai2011,wensrich2012,blais2019` | String   | None      |

|                               |                                                                         |          |           |

|                               | Options:                                                                |          |           |

|                               |                                                                         |          |           |

|                               | * ``None``                                                              |          |           |

|                               |                                                                         |          |           |

|                               | * ``ModelA``                                                            |          |           |

|                               |                                                                         |          |           |

|                               |      |RollingFricModelA|                                                |          |           |

|                               |                                                                         |          |           |

|                               | * ``ModelB``                                                            |          |           |

|                               |                                                                         |          |           |

|                               |      |RollingFricModelB|                                                |          |           |

|                               |                                                                         |          |           |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+

| rolling_friction.coefficient  | Rolling friction coefficient when using a rolling friction model        | Real     | 0         |

+-------------------------------+-------------------------------------------------------------------------+----------+-----------+





Example

-------



Below is an example for specifying the inputs for two DEM solids.



.. code-block:: none



   solids       = sand  char

   solids.model = dem



   dem.contact_model = lsd



   dem.lsd.spring_const.pp     =   100.0

   dem.lsd.spring_const.pw     =   100.0



   dem.lsd.spring_tang_fac.pp  =     0.2857

   dem.lsd.spring_tang_fac.pw  =     0.2857



   dem.lsd.damping_tang_fac.pp =     0.5

   dem.lsd.damping_tang_fac.pw =     0.5



   dem.friction_coeff.pp       =     0.25

   dem.friction_coeff.pw       =     0.15



   dem.restitution_coeff.sand.sand =  0.85

   dem.restitution_coeff.sand.char =  0.88

   dem.restitution_coeff.char.char =  0.90



   dem.restitution_coeff.sand.wall =  0.85

   dem.restitution_coeff.char.wall =  0.89



.. _InputsDEMImplicitDrag:



Implicit drag

-------------



By default (``solids.momentum.update = explicit``), the fluid-particle drag force is applied

explicitly to particles,



.. math::



   u_p^{n^{\prime}+1} = u_p^{n^\prime} + dt_{\mathrm{DEM}}\left(\beta^n (u_f^{n+1} - u_p^{n}) + \mathcal{F}_p^{n^\prime}\right)/m_p



where

:math:`u_p` is the `p`-th particle velocity at the indicated time level,

:math:`\beta^n` is the drag coefficient,

:math:`u_f^{n+1}` is the fluid velocity,

:math:`\mathcal{F}_p` are all non-drag forces (e.g., gravitational, buoyancy, collision etc.), and

:math:`m_p` is particle mass.

Terms exist at different time levels due in part to how the fluid and particle models are coupled

and partly because particles typically sub-step in time. Specifically, a time step advances the fluid from

:math:`t^n` to :math:`t^{n+1}`, then particles take multiple smaller time steps to traverse the same

total time. In the above equation, prime markers differentiate quantities updated at each DEM sub-step.



The updated particle velocity, :math:`u_p^{n^{\prime}+1}`, is substituted into the drag expression when

``solids.momentum.update = implicit``.



.. math::



   u_p^{n^{\prime}+1} = \frac{u_p^{n^\prime} + dt_{\mathrm{DEM}}\left( \beta^n u_f^{n+1} + \mathcal{F}_p^{n^\prime} \right)/m_p}{1 + dt_{\mathrm{DEM}} \beta^n / m_p}



By using the updated velocity, the drag force computed for the fluid and particles is inconsistent,

and interphase momentum is not conserved. However, implicit drag may be needed for numerical stability

when the particle density is much less than the fluid (such as bubbles). The implicit update is

required when virtual mass is included, and the virtual mass coefficient is added to the particle

mass in the denominator.

docs/source_docs/user_guide/inputs/model_options.rst

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   The thermodynamic pressure is a scalar value, not a scalar field. This is a consequence of

   the low Mach number assumption.



.. _InputsParticleOptions:



Particle options

----------------
@@ -145,7 +147,8 @@ The following inputs are defined using the prefix ``solids``: 
+---------------------+---------------------------------------------------------------------------+-------------+-------------+

|                     | Description                                                               |   Type      | Default     |

+=====================+===========================================================================+=============+=============+

| model               | Solids model used by the solids solver.                                   | String      | None        |

| model               | Solids model used for every type in ``solids``. Required when             | String      | None        |

|                     | solids types are defined.                                                 |             |             |

|                     |                                                                           |             |             |

|                     | Options:                                                                  |             |             |

|                     |                                                                           |             |             |

docs/source_docs/user_guide/inputs/pic_model.rst

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PIC model

=========



.. _InputsPIC:



Inputs for the particle-in-cell (PIC) method, used when ``solids.model = pic``

(see :ref:`particle options<InputsParticleOptions>`). Every type listed in

``solids`` is a PIC type.



The following inputs define the PIC solids model and its associated frictional stress parameters:



.. math::



   \tau_p = \frac{P_s \varepsilon_p^{\beta}}{\max \left[ (\varepsilon_{cp} - \varepsilon_p), \epsilon (1-\varepsilon_p)\right]}





The following inputs are defined with the prefix  ``pic``:



+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

|                                    | Description                                                             |   Type   | Default   |

+====================================+=========================================================================+==========+===========+

| close_pack                         | Solids volume fraction at close pack, :math:`\varepsilon_{cp}`.         | Real     |   0.55    |

|                                    | Below this threshold, the solid stress tensor has minimal impact.       |          |           |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| pressure_coefficient               | Pressure linear scale factor in frictional stress model, :math:`P_s`.   | Real     | 100.0     |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| beta                               | Volume fraction exponential scale factor in frictional stress model,    | Real     |   3.0     |

|                                    | :math:`\beta`.                                                          |          |           |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| small_number                       | Non-singularity term in frictional stress model, :math:`\epsilon`.      | Real     |   1.0e-7  |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| damping_factor                     | An empirical dampening factor for the frictional stress model.          | Real     |   0.4     |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| damping_factor_wall_normal         | Normal coefficient of restitution for parcel-wall collisions.           | Real     |   0.3     |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| damping_factor_wall_tangent        | Tangential coefficient of restitution for parcel-wall collisions.       | Real     |   0.99    |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| parcels_per_cell_at_pack           | Specifies the number of particles that can occupy a regular             | Real     |  32.0     |

|                                    | computational cell near close packing.                                  |          |           |

|                                    |                                                                         |          |           |

|                                    | Larger values reduce sensitivity to small changes in parcel positions,  |          |           |

|                                    | which helps stabilize the computed solids volume fraction and, by       |          |           |

|                                    | extension, the frictional stress model. However, increasing this value  |          |           |

|                                    | also results in simulations tracking a greater number of parcels.       |          |           |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| verbose                            | Verbosity level for PIC-related output.                                 | Int      |   0       |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+





The solids volume fraction is required to evaluate the frictional stress model when advancing parcels from :math:`t^n` to

:math:`t^{n+1}`. Ideally, this evaluation would use either the volume fraction at the midpoint of the time step,

:math:`\varepsilon_p^{n+1/2}`, or at the end of the step, :math:`\varepsilon_p^{n+1}`.



However, since neither of these values is available a priori, an iterative approach is used. An initial approximation of

the volume fraction is used to advance parcels to a tentative position at :math:`t^{n+1}`. These new positions are then

used to compute an updated volume fraction field, which is subsequently used in the next iteration to recompute the

tentative parcel positions.



The following keywords control the iterative PIC algorithm and are defined with the prefix  ``pic``:



+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

|                                    | Description                                                             |   Type   | Default   |

+====================================+=========================================================================+==========+===========+

| initial_step_type                  | Specifies the initial solids volume fraction used in PIC iterations to  | String   | "nth_eps" |

|                                    | approximate the volume fraction at the end of the time step,            |          |           |

|                                    | :math:`t^{n+1}`                                                         |          |           |

|                                    |                                                                         |          |           |

|                                    | Options (case-insensitive):                                             |          |           |

|                                    |                                                                         |          |           |

|                                    | * ``nth_eps`` - use the volume fraction from the previous time step,    |          |           |

|                                    |   :math:`t^{n}`                                                         |          |           |

|                                    | * ``zero_eps`` - set the initial volume fraction to zero                |          |           |

|                                    | * ``taylor_approx`` - use a Taylor series expansion about parcel        |          |           |

|                                    |   positions to approximate the volume fraction at :math:`t^{n+1}`       |          |           |

|                                    |                                                                         |          |           |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| max_iter                           | Maximum number of iterations used to approximate the end state          | Int      |   3       |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| advance_vel_p                      | Specifies the weighting factor used to compute the averaged particle    | Real     |   0.5     |

|                                    | velocity field during parcel advancement. The field-averaged velocity   |          |           |

|                                    | is used when applying forces to parcels.                                |          |           |

|                                    |                                                                         |          |           |

|                                    | This parameter takes values between 0 and 1, blending the particle      |          |           |

|                                    | velocity at the beginning and end of the time step. A value of ``0.5``  |          |           |

|                                    | (default) yields a time-centered average.                               |          |           |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

| velocity_reference_frame           | The velocity reference frame scales the field-averaged velocity         | Real     |   0.5     |

|                                    | used to compute the slip velocity between a parcel and the bulk when    |          |           |

|                                    | when the parcels is moving towards an area of higher solids             |          |           |

|                                    | concentration. This parameter takes values between 0 and 1.             |          |           |

+------------------------------------+-------------------------------------------------------------------------+----------+-----------+

docs/source_docs/user_guide/inputs/solids_model.rst

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