You can use the inflow condition to control the flow of material into the
Eulerian domain.
Free Inflow
If no Eulerian boundary is defined, material can flow into the Eulerian
domain freely; and the material content and the state of each inflow material
are equal to that which presently exists within the element. If an Eulerian
boundary is defined, free inflow is the default inflow condition.
You can specify an Eulerian boundary where no inflow can occur—no material
or void can flow into the Eulerian domain through the specified boundary. The
normal component of the velocity is set to zero if the velocity is directed
inward at the boundary, while the tangential component of the velocity remains
unchanged.
You can also specify a boundary through which inflow can occur but the
influx volume contains only void. Due to the inflow of void, an Eulerian domain
that is initially completely full of material might become partially full
during the analysis.
The outflow condition can be used to simulate an unbounded domain by
reducing reflection at the outflow boundary or to prescribe a pressure field at
the boundary.
Free Outflow
If no Eulerian boundary condition is specified, material can flow out of the
Eulerian domain freely; and the material content and the state of each outflow
material are equal to that which presently exists within the element. If an
Eulerian boundary condition is defined, free outflow is the default behavior if
the void inflow condition is specified at the same surface.
A nonreflecting outflow condition can be used in boundary value problems
defined in unbounded domains or problems in which the region of interest is
small in size compared to the surrounding medium. Like the infinite element
formulation described in
Using Solid Medium Infinite Elements in Dynamic Analyses,
the nonreflecting outflow condition introduces additional normal and shear
tractions on the domain boundary that are proportional to the normal and shear
components of the velocity of the boundary. These boundary damping constants
are chosen to minimize the reflection of dilatational and shear wave energy
back into the finite element mesh. This condition does not provide perfect
transmission of energy out of the mesh except in the case of plane body waves
impinging orthogonally on the boundary in an isotropic medium. However, it
usually provides acceptable modeling for most practical cases. An exception is
the case when significant material transport occurs through the boundary, in
which case this condition is not suitable to be used.
Equilibrium outflow is another outflow condition that can effectively reduce
spurious reflection at artificial outflow boundaries in unbounded domains. It
is assumed that the stress is zero-order continuous across the element faces on
the boundary. Traction is applied to these element faces to balance the nodal
forces created by the stress in the boundary elements. This condition is
usually applied at the outflow boundary where the pressure distribution is
unknown.
It is common in flow problems to specify a zero pressure at the outlet of
the flow. Since the normal traction on the boundary contains the contribution
from both the pressure and the shear stress, the natural boundary condition,
also known as the “do-nothing condition,” is not sufficient to provide such a
condition if the shear behavior of the flow is also considered. The zero
pressure outflow condition applies a traction that counteracts the shear
contribution and, thus, generates a uniformly distributed pressure field on the
boundary. You can apply a distributed surface load (see
Surface Tractions and Pressure Loads)
on the same boundary to specify a nonzero pressure. This is the default outflow
condition if the inflow condition is not specified.
Applying Fixed Boundary Conditions in the Normal Direction
You can specify an Eulerian boundary where the flow direction is tangential
to the specified boundary surface. Eliminate the normal component of the flow
by combining the no inflow and no outflow conditions.
You can define a new Eulerian boundary in a restart analysis, but you cannot
specify a void inflow condition at this boundary. In addition, you cannot
change the inflow condition at an existing Eulerian boundary to the void inflow
condition in a restart analysis.