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While solving structural problems, in a large number
of cases there are physical contacts between two
solid components. Numerically it is difficult to
handle this situation as the grid volume (on the fluid side) collapses
to zero at the contact surfaces. Also, due to
contact, a sudden impulsive force plays a role to
have a large non-linear deformation in the elastic
components. The following description will explain
how to set up a structural problem using the "contact
model". The contact model works for multiple pairs of
solid surfaces, both the components of a pair can be
elastic, or one elastic and other rigid.
On the top of the impulsive force, there are other
"adhesive" types of forces that arise at the two contact
surfaces, commonly known as "stiction force" in MEMS
industries. There is a recent development in CFD-ACE+ V2006
which allows users to specify that "additional" force
to the contacting pairs of solid surfaces. This tip
will also describe how to set up this stiction force.
At present, specification of stiction force is
limited to pairs having one elastic and other rigid
bodies.
Figure 1 shows a simulation result explaining the
"contact area" that the user might have in a structural
problem. The top plate is an elastic micro-mirror
which bends due to electrostatic actuation and hits
another elastic pad which bends due to stress
generated at the time of contact. To specify this
contact model, we need to consider the mirror bottom
plate and the pad top plate as one "contact pair" in CFD-ACE-GUI.
Following are the steps you need to follow to specify
a contact model.
Fig 1. Contact between two elastic surfaces
- In Model Option (MO) section, click on "Stress"
tab. You have to check the Contact Analysis
button. Please note that you have to use nonlinear
analysis to get this option available as contact
analysis is nonlinear. Refer to Fig 2.
Fig 2. Activating Contact Model
- Click BC tab and select the surface which is a
part of a contact pair. Since you have selected the
Contact Analysis in MO section, a Contact checkbox
appears in the Stress boundary condition panel.
Selecting this box will cause an option menu to
appear under Sub Type box. You can define the surface
to be either a "contact" or a "target" surface. In case
of contact between one elastic and other rigid
surface, conventionally the target surface is the
rigid one. Figure 3 shows the specification of a
contact surface, having a name "tip".
Fig 3. Specification of Contact Surface in
CFD-ACE-GUI
- To define target surface sets, select the surface
of interest as "Target Surface" and click on "Specify
Contact Target.." button. The Contact Target Pair
window pops up. In the Total Pair field, enter the
total number of pairs for the target surface and
click OK button. In the Pair Name box, enter the name
of each pair, each target pair name should have a
corresponding pair name for a contact surface. Click
the Close button to close the Contact Target Pair
dialog window. Refer to Fig 4 to see how a target
pair is specified as the same name "tip" as contact
surface.
Fig 4. Specification of Target Surface and
Contact Gap
- A Contact Gap is specified for each target
surface. This is the contact-target gap size at which
contact is assumed to occur. This is needed when
there is a fluid region between the contact and
target surfaces to avoid the fluid cells to be
squeezed to a zero value. You should specify a
small value depending on your geometric length scale.
Figure 4 shows that a value of 1E-008 m has been
specified.
To Remember: Contact and Target
surfaces are defined as pairs and are matched using a
Pair Name. All contact surfaces with a certain pair
name can potentially come into contact with all
target surfaces of the same name. A contact surface
group can be a member of only one pair, but a target
surface group may be a member of up to five different
pairs.
To Specify Stiction Force:
New in V2006 is the ability to model "Stiction" force.
On the contact surface, specify a value of Contact Force.
This is the total additional stiction force in that
pair. You can specify different values of stiction
force for different contact-target pairs. If the area
of contact increases during a transient simulation,
the total stiction force will be uniformly
distributed over the contact area. Figure 5 shows
that a stiction force of 1 micro-Newton is specified.
Fig 5. Specification of Additional Contact Force
(Stiction)
To Remember: The target surface
has to be rigid in order to activate stiction
force.
We welcome your discussion and comments about this
note on the ESI CFD
Community forum. A topic has already been started
and you can
find it here. [Access available only to
customers under a current support contract.]
Regards,
Abhra Roy
ESI CFD Customer Support
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ESI CFD Community Post of the Week!
roncresswell worked through some python scripting issues to
develop a CFD-GEOM python script that will programmatically create
the geometry for tapered pipe threads (something his CAD counterparts
could not do with their systems!).
See the post on the
CFD-GEOM Scripting board to learn more
about the problem and the solution.
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