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L6-1ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training Material
Solving FSIApplications UsingANSYS Mechanical andCFX
Lecture 6
6-DOF Rigid Body Solver
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FSI with ANSYS Mechanical and CFX
L6-2ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body FSI
CFX includes a 6-DOF rigid body solver
Fluid forces/torques on a body auto-calculated Body response included in flow solution
Either via mesh motion or via immersed solid
Simplified FSI case where body does not change shape
under fluid load
Can make assumptions about its behaviour
Does not need the expense of a full structural simulation
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FSI with ANSYS Mechanical and CFX
L6-3ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Dynamics
Forces and torques acting on a rigid body can be
summed and assumed to act on/about the centre of mass
Chasles Theorem: The general displacement of a rigid
body is a linear motion of a origin point plus a rotation
around the origin point
Can separate translation and rotation
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FSI with ANSYS Mechanical and CFX
L6-4ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Translation
Translational equation of motion, applied to Centre of
Mass
Discretized using implicit Newmark integration scheme
Default integration parameters give 2nd order accuracy
Advantage over previous explicit CEL implementation
Can add influence of external spring or external force to
F
FxP
G
mdt
d
Gx = Acceleration about centre of mass
ExtSpringAero )]([ FxxFF
sokmg
= Linear MomentumxP m
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L6-5ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Rotation
Rotational equation of motion about Centre of Mass
Two methods of discretization available
Simo-Wong [1] (Default. Second order, iteratively conservative)
First Order Backward Euler
Can add influence of external torsion spring or external
torque to MExt
MI
dt
d
dt
dB)(
BBB
B
dt
d
II
I
)()(1
BBBB IMI
= Angular Momentum
ExtsoSpringAero )]([ MMM kB
= Moment of Inertia tensorI
[1] Simo, J.C., Wong, K.K., Unconditionally Stable Algorithms for Rigid Body Dynamics that exactly Preserves
Energy and Momentum, Int. J. Num. Methods in Eng., vol. 31, 19-52 (1991)
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FSI with ANSYS Mechanical and CFX
L6-6ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialCreating a Rigid Body in CFX-Pre
Insert a Rig id Bodyinto the Flow Analysis
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FSI with ANSYS Mechanical and CFX
L6-7ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Basic Settings
Mass
Rigid body mass
Locat ion
The 2D boundary region of the rigid body
Coord Frame
Must create a Coord Frame at the centre
of mass (based on the initial rigid bodyposition) and select here
Cannot constrain a body to rotate about
a point
Mass Moment of Inert ia
Enter components for the Mass Moment
of Inertia tensor
See next slides
As calculated with respect to the rigid
body coordinate frame
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L6-8ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialMass Moment of Inertia Tensor
This tensor describes an objects resistance to changes in
its rotation rate
Its a symmetric tensor, soIxy=Iyx
Hence only 6 components are entered on the Basic Settings panel
Ixx describes the moment of inertia around the x-axis
when the objects are rotated around the x-axis
Non-zero when you have rotation about the x-axis
Ixy describes the moment of inertia around the y-axis
when the objects are rotated around the x-axis, etc
Non-zero when you have rotation about the x and y axis
zzzyzx
yzyyyx
xzxyxx
III
III
III
I
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L6-9ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialMass Moment of Inertia Tensor
For rotation about only the y-axis, the tensor simplifies to:
For rotation about the x and y axes we have:
See http://en.wikipedia.org/wiki/Moment_of_inertia for
detailed background on mass moment of inertia
000
00
000
yyII
000
0
0
yyyx
xyxx
II
II
I
http://en.wikipedia.org/wiki/Moment_of_inertiahttp://en.wikipedia.org/wiki/Moment_of_inertia -
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L6-10ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Dynamics
External Forces / Torques- Use SpringorValueoption
- Spring:- Set Origin coords and Spring Constant
- Value- Enter Cartesian components (can use CEL
expressions)
Degrees o f Freedom Select Translational / Rotational DOF
Default is None need to set at leastone DOF
Enter Gravity Vector
Acts at the centre of mass as set byCoord Frame
Should be consistent with Domaingravity (if specified in the Domain)
Everything specified in Rigid Body
Coord Frame
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FSI with ANSYS Mechanical and CFX
L6-11ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Initialization
All state variables defining rigid
body can be initialized in termsof the rigid body coordinate
frame
Default behaviour is to useAutomat ic
Assumes quiescent conditions
unless a previous solution is
provided to restart from
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FSI with ANSYS Mechanical and CFX
L6-12ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Mesh Motion
After creating the rigid body, set
mesh motion parameters onboundaries, subdomains and/or
interfaces
Option = Rig id Body Solut ion
Rigid Bod y =
Motion Con strain ts
Can ignore Translations or
Rotations
The boundary that corresponds to the rigid body should clearly
move with the rigid body, without ignoring any motion
To maintain mesh quality, you may want other boundaries/interfaces
to move using only the translations/rotations from the RB solution
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L6-13ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Mesh Motion Example
Ship hull example
2-DOF
Rotation about y-axis
Translation along the z-
axis
A subdomain moves with
the rigid body so that
near-wall mesh quality
can be maintained
See EX4 in the examples
folder
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L6-14ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Mesh Motion Example
Hull wall boundary mesh motion defined by the Rigid Body
Solution
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L6-15ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Mesh Motion Example
Subdomain mesh motion also defined by the Rigid Body
Solution Hull and subdomain rotate and translate together as a rigid body
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L6-16ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Mesh Motion Example
A Domain Interface is used between the subdomain and the
rest of the domain The subdomain side of the interface uses the same mesh
motion setting as the subdomain and hull
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L6-18ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Mesh Motion Example
This example demonstrates
the preferred topology whenrotation about a single axis
is included
For rotation about multipleaxes surround the rigid body
with a sphere when
significant rotation occurs
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L6-19ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialCEL Access of Rigid Body Variables
Use the rbstate() CEL function to access rigid body variables
E.g. rbstate(Linear Velocity X)@RigidBodyObject The returned values are with respect to the Global Coord Frame
Variables that can be accessed are:
Position X/Y/Z, Linear Velocity X/Y/Z, Linear Acceleration X/Y/Z,
Euler Angle X/Y/Z, Angular Velocity X/Y/Z,Angular Acceleration X/Y/Z
If a component (X/Y/Z) is not provided the magnitude is returned,
except for Euler Angle which requires a component
A beta feature allows values to be returned in the rigid body
coordinate frame
E.g. rbstate(linacc x_Coord Name)@RigidBodyObjectwhere linacc x is the short form variable name. See theVARIABLESfile in .../ANSYS Inc/v130/CFX/etc to find the short formnames
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L6-20ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Solver Control
Solver Contro l > Rig id Body
Contro l
Update Frequency
Every Time Step
Explicit coupling between the rigid body
solution and the flow field. Lowest
computational cost, but weakestcoupling. Suitable for loosely coupled
cases; will be unstable for more tightly
coupled cases
Every Coeff ic ient Loop / Iteration
Tighter coupling that is iteratively-implicit. Higher computational cost, butmore stable for large timestep use and cases with high virtual-mass (body-
mass ratio). May still failthe forces from the flow field dont get a chance
to stabilize after receiving the new rigid body position. Can use under-
relaxation (see later).
FSI i h ANSYS M h i l d CFX
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L6-21ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Solver Control
Update Frequency (con t.)
General Coup l ing Con trol
The most robust approach; same
approach as stagger/coupling
iterations in 2-way FSI. Set the
number of Rigid Body updates to
perform per timestep. After each RB
update within a timestep, the flowsolver will perform the number of
coefficient loops set under Basic
Settings.
UnderInternal Coup l ing Data
Trans fer Contro lcan set UnderRelaxation Facto rsand
Convergence Control
Available forUpdate Frequency
other than Every Timestep
FSI ith ANSYS M h i l d CFX
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FSI with ANSYS Mechanical and CFX
L6-22ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Solver Control
Can adjust under relaxation for
forces & torques sent to the RB
solver and for mesh motion
received from the RB solver
External Force set via a Linear Spring
is not under-relaxed
Under relaxation is usually the first
choice to improve robustness andis easy to use
Default under relaxation is 0.75
The default Simo Wong IntegrationMethod for Angular Momentum is
recommended
FSI ith ANSYS M h i l d CFX
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L6-23ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Monitor Plots
Default monitor plots are
created
Rigid Body Convergence,
Euler Angles& Posit ion
Select underMonito rs > Rigid
Body
Motion convergence is basedon the distance moved
compared to the last time the
RB solver was called
Force/Torque convergence is
based on the change inforce/torque divided by the
force/torque magnitude
See CFX-Pre Solver Control
doc for further details
FSI ith ANSYS M h i l d CFX
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L6-24ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Monitor Plots
Can also access additional plots;
create a new monitor or right-click
to access Monito r Propert ies
Angu lar/L inear Accelerat ionand
Angu lar/L inear Veloci tyare
available in addition to the default
Posit ion, Euler AngleandForce/MotionConvergenceplots
FSI ith ANSYS M h i l d CFX
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FSI with ANSYS Mechanical and CFX
L6-25ANSYS, Inc. Proprietary
2010 ANSYS, Inc. All rights reserved.Release 13.0
December 2010
Customer Training MaterialRigid Body Solution
FSI with ANSYS Mechanical and CFX
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ANSYS Inc Proprietary Release 13 0
Customer Training MaterialLimitations
Cant be combined with MFX 2-way FSI
No contact/collision modelling with walls or other rigidbodies
Practically, this only matters for the Immersed Solid approach
since the mesh would fold prior to a collision
An immersed solid driven by 6-DOF has no problems movingthrough a wall and outside the flow domain
Cant be used in rotating domains
General constraints cant be applied
Cant make a rigid body rotate about a point, other than itscenter of mass