[Music]
welcome to the santa's how-to series
video
in mechanical engineering simulations
having parts interact with another is
extremely common
and for this contact is often utilized
take for example this ball bouncing on
the ground
or these gear teeth interacting without
contact or other types of connections
the parts would not interact worse yet
with incorrect contact setup
the parts may not behave as expected so
what is necessary to ensure
the contact is detected correctly the
contact definition has a distinction
between the contact side and the target
side
in this video we will focus on the
proper designation of the contact and
target side
in order to achieve proper behavior and
numerical accuracy
ready let's go
the final element representation of a
body's deformation
is governed by this well-known equation
f
equals k u where f is the force k is the
stiffness
and u is the displacement now if we have
two bodies
that don't share a common node then a
force acting on one body
will not be transmitted to the other
body since there is no connection
between them
this is a key point as bodies that are
meshed and do not share topology
do not have shared nodes if we wish for
them to interact
we need contact and target elements to
establish the interaction
contact and target elements are assigned
to the two surfaces
to allow the parts to interact with
contact elements
there is a concept of a contact
detection point
and those contact detection points
cannot pass through the target phase
the contact detection point is either
the integration point
or the node and can be controlled by the
user
so the finer the mesh the more detection
points and less likely penetration will
happen
depending on the mesh density of the
parts it may govern which body should be
the contact
and which should be the target okay
let's start up ansys workbench and have
a closer look
first let's discuss a simple case of a
ball bouncing on the ground
in order to accurately capture the
interaction between the ball and the
ground
we need to define the contact correctly
let's begin by opening the archive
project in ansys workbench
and then double click on the model to
open ansys mechanical
here we have a spherical ball and a flat
body representing the ground
when we double click on the mesh we see
the body sizing has been applied to the
ball
to get a finer mesh than the mesh on the
ground
next let's take a look at the contact
definitions
when the geometry is first imported in
ansys mechanical
the software automatically generates
bonded contact between
surfaces that are close to each other it
is important to review these
auto-generated contacts
and make sure they are defined correctly
before proceeding with the rest of the
model setup
go to connections contacts and choose
the contact
when you click on any contact pair two
additional window panels open up
which show the contact side in red and
the target side in blue
here we can see that the entire surface
of the ball is defined as the target
side
while the top surface of the flat body
is defined as the contact side
we will know in a minute whether or not
this selection was correct
also note that if the scoped regions of
the contactor target
is not correct you can easily add remove
or replace the scoping
to do this you can pick the geometry and
click the respective icon
and then the contact or target and that
scoping will be updated
and indicated by the red or blue
coloring of the selection
we have defined a frictionless contact
now the behavior is defined to be
asymmetric
we'll discuss about the behavior option
in a few minutes
we've applied an initial velocity of
1000 millimeters per second to the ball
a boundary condition of fixed support is
applied to
the side faces of the body
we saw this system here we see that the
ball penetrates
considerably through the top surface of
the flat body
so this choice of contact and target
side selection
is incorrect let's revisit the concept
of detection points
to understand why this happened
since the flat plate is the contact side
the detection points are on the flat
body
since the flat body has a coarse mesh we
have fewer number of detection points
according to the contact algorithm a
contact detection point cannot penetrate
through a target surface
in this case by the time the detection
points on the flat body
detect the presence of the target
surface some portions of the ball
have already penetrated into the flat
body
to avoid this we need to increase the
number of detection points
which can be achieved when we have a
finer mesh
so the first rule of thumb is to assign
the contact side
to the surface with the finer mesh
coming back to our example
let's look at the mesh again
we see that the ball has a finer mesh as
compared to the flat body
so we need to switch the contact and
target sides
there's an easy way to do this simply
right click on the given contact pair
and choose
flip contact target let's solve the
system now
when we look at the results again we see
that the ball does not penetrate through
the flat body this time
let's take a moment to discuss the
behavior option that we used
in the example we just saw we specified
the behavior
of the contact pair to be asymmetric
that means that one surface is
designated to be the contact side
and the other surface is designated to
be the target side
another type of behavior that we can
specify is symmetric
in case of symmetric behavior each
surface is both a contact side
and a target side and we generate two
contact pairs
from the same two surfaces symmetric
behavior may be recommended in cases
where the contact and target sides are
not
easily distinguished or in cases where
both surfaces
have very coarse meshes in this way
the symmetric contact algorithm enforces
the contact constraint conditions
at more surface locations than the
asymmetric contact algorithm
symmetric behavior can be a bit more
computationally expensive
now compared to the overall solution
time this increase is typically small
contact results are only available on
the contact side
as in our asymmetric case so while using
symmetric behavior
will allow us to obtain contact results
on both the contacting surfaces
this can make the results interpretation
more challenging
as the actual value will need to be the
average value from the two surfaces
now you'll also notice that there are
two other options for behavior
program controlled and auto asymmetric
when we specify program controlled the
default is auto asymmetric
where the solver tries to pick the
better choice for the contact behavior
but if we now understand contact versus
target behavior
we are in a better position to specify
it manually
and we can thus choose asymmetric and
again
check our designation of contact and
target
and flip if necessary finally
while i mentioned that program control
defaults to auto asymmetric
there are some cases where it will
default to other behaviors
such as asymmetric for flexible to rigid
body contact
and symmetric for flexible to flexible
body contact
where nonlinear adaptive remeshing also
known as
nlad is specified
apart from mesh density there are a few
more points to be considered
while assigning the contact and target
sites another consideration
is the geometry of the contacting bodies
generally a convex shaped body such as
the ball in our example
is defined to be the contact side while
the flat
our concave shaped body is defined to be
the target side
certain exceptions may exist for example
when an edge or a corner comes in
contact with a surface
irrespective of the shape of the surface
the edge
or the corner is always defined to be
the contact side
while the surface is defined to be the
target side
this ensures that the contact detection
points lie
on the sharp edge or corner and prevent
penetration
another common rule is if we have two
contacting surfaces made of different
materials
the body composed to the softer material
should typically be assigned the contact
side while the body composed to the
stiffer material
is typically assigned the target side
this allows the contact detection points
to follow the deformed surface of the
softer body
now let's consider the case where the
ball in the previous example is made of
a very soft material
while the flat ground is made of a much
stiffer material
again the surface of the ball is
designated to be the contact surface
while the flat ground is the target
surface
we can see that the ball deforms
considerably and along with that
deformation the associated detection
points
and those points do not penetrate the
ground so let's summarize
a contact creates contact and target
elements and it prevents the contact
detection points on the contact elements
from going through the target elements
a contact size should typically have the
finer mesh of the two bodies
so it will have enough contact detection
points to avoid penetration
if a convex surface is expected to come
into contact with a flat
or concave surface the flat or concave
surface should be the target side
it is also recommended that the contact
side is on the body of the softer
material
so contact detection points can follow
the deformed
surface lastly when it's hard to
distinguish the contact and target sides
or self-interaction is involved
symmetric contact behavior can be used
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