Right Contact parameter for coupling SPH and FEM
Ignatius Ebo-Quansah
James M. Kennedy
Dear Ignatius,
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Interesting SPH particle spacing remarks:
Xu, J., Wang, J., and Souli, M., “SPH and ALE Formulations for Sloshing Tank Analysis”,
The International Journal of Multiphysics, Vol. 9, No. 3, pp. 209-224, September, 2015.
http://www.journal.multiphysics.org/index.php/IJM/article/view/9-3-209/251
Presentation can also be downloaded from the researchgate site,
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Some additional notes.
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Some recent comments I located.
Use contact_automatic_nodes_to_surface or contact_eroding_nodes_to surface,
depending on whether you'll ultimately be eroding solid structure or not. If
eroding_nodes_to_surface is necessary, it's advised NOT to use SMP, that is,
use MPP.
Use version R10.0 or later and set ITHK=1 on control_sph. This serves to
calculate the contact thickness automatically, based on the volume of each
SPH particle. In this case, the contact thickness of each particle is the cube
root of it's volume. (Think of the particle as a cube for purposes of contact
when ITHK=1.) SST on Card 3 of *CONTACT will override ITHK.
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Len Schwer has offered the following comments related to SPH mesh
refinement regarding a Lagrangian interface:
• I recommend a minimum of 4 (2x2) SPH particles per solid/shell segment,
with 9 (3x3) preferred for SPH to solid/shell contact, i.e. Nodes_to_Surface.
• This comment was offered with respect to tied contacts which also should
apply (see above comment) to other contacts.
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For your SPH contact, you can use either the contact_nodes_to_surface
or the contact_automatic_nodes_to_surface options.
To get the contact_automatic_nodes_to_surface to work properly, it
appears that you may need to specify a slave surface thickness, sst.
Some notes taken from Suri Bala's blog:
http://blog.d3view.com/2007/04/14/sph-contact-definitions/
http://ls-dyna-examples.blogspot.com/2009/12/sph-contact-definitions.html
Contact thickness for the SPH particles
When using automatic or non-automatic type one-way contacts, the
SPH particles have no associated contact thickness. The recommended
way to define the particle contact thickness is to use the 'SST'
parameter in the *CONTACT keyword or by using 'OPTT' parameter in
the *PART_CONTACT keyword. If the SPH particles are representing a
volume, then it may be suggested to use a resonable thickness to
avoid any nodal release due to excessive penetration. Typical values
of particle contact thickness when representing a volume is 0.8-1.0mm.
If the SPH particles are representing a shell type structure, then the
thickness of the shell can be used as the contact thickness.
It must be noted that there is no need for SINGLE_SURFACE type contacts
for defining the self interaction of SPH particles as they are accounted
for by the SPH formulation.
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Some further notes.
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Please make a simple hand check calculation of your SPH mass and make sure
that it agrees somewhat with the SPH mass calculated by LS-DYNA and given
in the "d3hsp" file.
Actually, this is advice to ALL who perform SPH simulations, especially
for 2D modeling. This should be checked to ensure correct mass modeling.
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Perhaps the following notes on particle meshing might be of some help.
The smoothing length (h) and the mesh characteristic length (Dx) are the
two spatial parameters to determine spatial resolutions (only one spatial
discretization parameter is used in finite elements).
In LS-DYNA, the initial smoothing length is constant for each part and is
calculated as maximum value of all the minimum distance for each particle.
For example, if we have three particle, A, B, C, then first the distance to
the nearest particle is computed for all three particles. Lets assume the
shortest distances for the particles A, B, and C are AD, BD, and CD
respectively. The default smoothing length DSML is then taken as
CLSH*maximum of (AD, BD, and CD) where CLSH is a scale factor to the
initially computed smoothing length DSML can vary between HMIN * DSML
and HMAX * DSML as the simulation progresses. The computed smoothing
length at time zero is based on per part basis such that each part will have
one smoothing length for all the particles that belongs to that part. After
determining the initial smoothing length, it is recomputed every cycle
for each particle based on the divergence of the velocity and can vary
with respect to time and space.
Because of the lack of a numerical grid, care must be employed in setting
the initial particle masses and coordinates. The particle mesh needs to be
enough regular. This means that all the particles of a given neighborhood
need to have the same mass. As a consequence, the particles of a same
material, which have the same initial density, need to have the same initial
volume. To preserve this, they need to be distributed on a uniform mesh.
When two or more SPH materials are defined in the model, it is not necessary
to define a contact between SPH parts. The contact between the SPH materials
is formed through the internal forces computed from the overlapping of
smoothing lengths and the associated constitutive equations. This is
different from the conventional contact mechanism defined by the friction
model. Non-slip condition is in general not available.
However, if both SPH parts and finite elements are defined in the model, the
user needs to define a contact between the particles and the other elements.
The particles are considered as nodes. Therefore any NODES_TO_SURFACE
contact can be used. The original developer recommends the soft constraint
formulation (SOFT=1) for the contact. With the soft constraint option, the
interface stiffness is based on the nodal mass and the global time step of
the calculation.
** NOTE
The above two paragraphs are basically taken from a rather old presentation
by Lacome [2001] which may be somewhat outdated. Please review old and
new presentations!
http://www.dynalook.com/fea-newsletters/fea-newsletters-2001/fea-newsletter-november-2001.pdf
*section_sph entries do not require a thickness similar to that requested
for *section_shell entries. The thickness of the sph part will be defined
by your sph particle meshing.
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Two other comments that may be of interest, which I have seen in various
SPH notes:
1. It is usually best to keep the particle spacing constant/regular
throughout the domain.
2. For two different SPH parts, if possible, it is best to have the same
particle mass.
Please see "Chapter VII - SPH Use" in the short note by Jean Luc Lacome,
Smoothed Particle Hydrodynamics - Part II.
http://www.feapublications.com/pages/pdfnews/2001oct.pdf
http://www.feapublications.com/ (November - 2001)
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Sincerely,
James M. Kennedy
KBS2 Inc.
June 16, 2022
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