cfx-fsi_130_lect-06_6dof.pdf

7/27/2019 CFX-FSI_130_lect-06_6DOF.pdf

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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



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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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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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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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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December 2010

Customer Training MaterialCreating a Rigid Body in CFX-Pre

Insert a Rig id Bodyinto the Flow Analysis


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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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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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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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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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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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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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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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December 2010


Hull wall boundary mesh motion defined by the Rigid Body

Solution


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December 2010


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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December 2010


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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December 2010


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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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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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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December 2010


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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December 2010


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



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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



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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



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December 2010

Customer Training MaterialRigid Body Solution



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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

cfx-fsi_130_lect-06_6dof.pdf

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