The solid to SPH option (Sol2SPH) enables you to turn a solid element into particles in order to increase the time
step/robustness of a Lagrangian calculation, while not significantly changing the physics.
Optimization in Radioss was introduced in version 13.0. It is implemented by invoking the optimization capabilities of
OptiStruct and simultaneously using the Radioss solver for analysis.
An axi-symmetry condition can be modelized through the use of two conditions with
respect to two planes intersecting at the axis of symmetry. A spheric symmetry
condition can be modelized through the use of three conditions with respect to three
planes intersecting at the center of symmetry.
Nevertheless, these kinds of symmetries are not treated the same way.
For instance, in case of an axi-symmetry condition, not all ghost particles are built
around the axis of symmetry. The only symmetric particles of real particles with
respect to the two symmetry planes are built.
Therefore, some characteristics of axi-symmetry (respectively, spheric symmetry)
conditions can be closed to the axis of symmetry (respectively, the center of
symmetry).
Nodes closed to the axis of symmetry (respectively, the center of symmetry) and lying
on a symmetry plane (P) can get a normal to (P) velocity which is non-zero, since
their neighborhood is not symmetric with respect to plane (P).
Kinematic Boundary Condition
With respect to the previous discussion: adding the kinematic boundary condition an
explicit way allows to enforce it.
A kinematic boundary condition will be added to the nodes belonging to the nodes
group specified into the /SPHBCS option, so that:
If "Slide" type, the velocity of the node in direction
"Dir" is set to zero
If "Tied" type, the velocity of the node in all directions
is set to zero
In case of several kinematic boundary conditions applied to the same node through
different SPH symmetry conditions, the kinematic boundary conditions are composed
automatically, even if the kinematic boundary conditions are applied into
non-orthogonal directions.
Figure 3 indicates that if two kinematic boundary conditions are
applied to N through two symmetry conditions with respect to planes (P1) and (P2), the two boundary conditions are modified so that the
velocity in the plane normal to common axis of (P1) and (P2) will remain zero.
Note: If one of the two symmetry
conditions is a type "Tied" condition, the velocity of
N in all directions is set to zero.
It also allows application to the same node, a kinematic boundary condition through a
SPH symmetry condition (/SPHBCS) and a standard boundary
condition (/BCS) at the same time, as long as the standard
boundary condition is not given in a moving skew system, but a fix skew system or
the global skew system. The two conditions are then composed the same way.
Part Mass
You must be advised that when a particle lies on a symmetry plane at time t=0, the
mass and the initial volume considered for the particles are
respectively:(1)
Where, mp is the mass specified into property
set.
When a particle lies on symmetry planes at time t=0,(2)
Ghost particles built from this particle will get the same initial volume and
mass.
When , the previous equation may provide an error on mass
and energies output for the part the particles belong to, with respect to the
physical model.
Formulation Level
When a symmetry plane is defined, and even if a kinematic condition is set for all
particles lying on the symmetry plane, particles lying at time zero inside the
domain are theoretically able to cross the symmetry plane.
This is specific to SPH for which stiffness between particles does not increase to an
infinite value when particles collapse. So it can occur when the particles, which
lie on the symmetry plane let the particles which were inside the domain go through
the symmetry plane.
If Ilev=0, particles crossing the symmetry plane
will not be (progressively) taken into account anymore in the computation, neither
than their symmetric particles which then lie inside the domain.
If Ilev=1, particles which have crossed the
symmetry plane rebound an elastic way upon the symmetry plane: their velocity in the
normal direction to the plane is set the opposite.
Note: When Ilev=1, it is strongly recommended to
associate kinematic condition to all particles lying on symmetry plane at time
zero, for computational time reasons.
Maximum Created Number of Ghost Particles
Ghost particles are created at each search for neighbors time within the security
distance, and then destroyed when a new search occurs (a new set of ghost particles
is then created).
At any search time, all ghost particles which are inside the security distance of any
real particle are created.
In practice, some more particles, strictly necessary, are created: a symmetric
particle Gi to particle Ni is created, with respect to symmetry plane , if neighbor of :(3)
Where, and are the smoothing lengths related to particle and .
As long as no real particles cross the symmetry plane (all real particles lie on the
same side of the symmetry plane), this criteria is sufficient to get all ghost
neighbors of all real particles inside the security distance, since:
for
And,
Particles, which one can expect to remain far from the symmetry plane all along the
simulation, will never be symmetrized. This provides a way to over-estimate the
number of particles which will be symmetrized at one time.
When a particle Ni has to be symmetrized with respect to
n conditions, the particle Ni creates n ghost particles. The following quantity must
remain less than Maxsph (since v14.0.220,
Maxsph is ignored and the memory is dynamically
allocated).(4)
Where,
Number of conditions
Number of particles to be symmetrized with respect to condition
The default value which is the number of SPH symmetry conditions multiplied by the
number of particles will be enough to treat any problem.
Solid to SPH Options (Sol2SPH)
The solid to SPH option (Sol2SPH) enables you to turn a solid element into particles
either in order to increase the time step/robustness of a Lagrangian calculation,
while not changing the physics.
Time Step
Two SPH time step methods are available in Radioss:
Particle time step (/DT/SPHCEL)
Nodal time step (/DT/NODA)
In particle time step, stable time step is computed as:(5)
Where,
Smoothing length related to particle
Sound speed at location
(6)
It is recommended to set time step scale factor to 0.3.
In nodal time step, stable time step is computed as:(7)
Where,
Mass for particles
Stiffness based on SPH interaction
For time step scale factor , it is recommended to set it to 0.67.
Thermal Analysis
Heat transfer is now available between SPH particles and finite elements with Ithe=1 in
/INTER/TYPE7 and /INTER/TYPE21; and with
/THERM_STRESS/MAT, thermal expansion in SPH is also
possible.