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5-1 ANSYS, Inc. Proprietary

© 2009 ANSYS, Inc. All rights reserve.!"ly 2009

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Chapter 5

6-DOF Rigid Body Solver

Solving FSI Applicationsusing Wor!ench" A#S$S%echanical and CF&

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Training Manual Rigid Body FSI

' 6-DOF solutions availa!le as '-(eature in version )*

' Fluid (orces+tor,ues on a !ody auto-calculated' Body response included in (lo solution

( )ither via *esh *otion or via i**erse soli

' Si.pli(ied FSI case here !ody does not change shape

under (luid load

( +an *ae ass"*ptions ao"t its ehavio"r

(oes not nee the e/pense o a "ll str"ct"ral si*"lation

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Training Manual Rigid Body Dyna.ics

' Forces and tor,ues acting on a rigid !ody can !e

su..ed and assu.ed to act on+a!out the centre o( .ass

' Chasles/ 0heore.1 0he general displace.ent o( a rigid

!ody is a linear .otion o( a origin point plus a rotation

around the origin point

( +an separate translation an rotation

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Training Manual

' 0ranslational e,uation o( .otion" applied to Centre o(

%ass

' Discreti2ed using i.plicit #e.ar integration sche.e

– De(ault integration para.eters give *nd order accuracy

– Advantage over previous e3plicit C4 i.ple.entation

' Can add in(luence o( e3ternal spring or e3ternal (orce to

ΣF

Rigid 0ranslation

F x P

Σ== Gmdt

d

G x 3 Acceleration a!out centre o( .ass

ExtSpringAero )]([ F x x F F Σ+−Σ−+=Σ sok mg

3 inear %o.entu. x P m=

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Training Manual

' Rotational e,uation o( .otion a!out Centre o( %ass

' 0o .ethods o( discreti2ation availa!le

– Si*o-4ong 16 7ea"lt. Secon orer, iteratively conservative8

– irst :rer ;ac

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Training Manual Creating a Rigid Body in CF&-7re

' 4na!le Beta Features

( Case Options > General

' Insert a Rigid Body into the

Flow Analysis

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Training Manual Rigid Body Basic Settings

' Mass

(@igi oy *ass

' Location

( Che 2 o"nary region o the rigi oy

' Coord Frame

( "st create a +oor ra*e at the centre

o *ass an select here

( +annot constrain a oy to rotate ao"t

a point

' Mass Moment of Inertia

( )nter co*ponents or the ass o*ento Inertia tensor

' See ne/t slies

( As calc"late

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Training Manual %ass %o.ent o( Inertia 0ensor

' 0his tensor descri!es an o!8ects resistance to changes in

its rotation rate

' It/s a sy..etric tensor" so I xy = I yx ( Dence only $ co*ponents are entere on the ;asic Settings panel

• I xx descri!es the .o.ent o( inertia around the 3-a3is hen

the o!8ects are rotated around the 3-a3is

( Non-Eero

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Training Manual %ass %o.ent o( Inertia 0ensor

' For rotation a!out only the y-a3is" the tensor si.pli(ies

to1

' For rotation a!out the 3 and y a3es e have1

' See http1++en9iipedia9org+ii+%o.ent:o(:inertia (or

detailed !acground on .ass .o.ent o( inertia

=

000

00

000

yy I I

=

000

0

0

yy yx

xy xx

I I

I I

I

http://en.wikipedia.org/wiki/Moment_of_inertiahttp://en.wikipedia.org/wiki/Moment_of_inertia

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Training Manual

' ./ternal Forces 0 1or2+es

- ;se S#ring or 3al+e option- Spring1- Set :rigin coors an Spring +onstant

-

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Training Manual

' All state varia!les de(ining

rigid !ody can !e initiali2ed in

ter.s o( the rigid !ody

coordinate (ra.e

' De(ault !ehaviour is to use

A+tomatic

– Assu.es ,uiescent conditions

unless a previous solution is

provided to restart (ro.

Rigid Body Initiali2ation

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Training Manual

' A(ter creating the rigid !ody" set

.esh .otion para.eters on!oundaries" su!do.ains and+or

inter(aces

' O#tion 4 Rigid Body Sol+tion

' Rigid Body 4 5Location

' Motion Constraints

– Ignore 1ranslations

– Ignore Rotations

Rigid Body %esh %otion

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Training Manual Rigid Body %esh %otion 43a.ple

' Ship hull e3a.ple

' *-DOF

( Rotation a!out y-a3is

( 0ranslation along the 2-

a3is

' A su!do.ain .oves ith

the rigid !ody so that

near-all .esh ,uality

can !e .aintained

' See 4&> in the e/am#les

(older

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' ?ull all !oundary .esh .otion de(ined !y the Rigid Body

Solution

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' Su!do.ain .esh .otion also de(ined !y the Rigid Body Solution

( ?ull and su!do.ain rotate and translate together as a rigid !ody

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' A Do.ain Inter(ace is used !eteen the su!do.ain and the rest o(

the do.ain

' 0he su!do.ain side o( the inter(ace uses the sa.e .esh .otion

setting as the su!do.ain and hull

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' 0he other side o( the inter(ace uses the Rigid Body Solution to set

the .esh .otion" !ut Ignore Rotations is selected

' 0he .esh slides at the do.ain inter(ace so rotational .otion is not

trans.itted to the outer do.ain

' 0ranslational .otion is passed and a!sor!ed !y the outer do.ain

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' 0his e3a.ple de.onstrates

the pre(erred topology henrotation a!out a single a3is

is included

' For rotation a!out .ultiplea3es surround the rigid !ody

ith a sphere hen

signi(icant rotation occurs

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Training Manual

' Solver Control Advanced O#tions

' "#date Fre2+ency – .very 1ime Ste# @ 43plicit

– .very Coefficient Loo# @ I.plicit

' "st co*ine

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Training Manual Rigid Body Solution

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5 21 ANSYS, Inc. Proprietary !"ly 2009

Training Manual i.itations

' i.itations ill !e addressed hen availa!le as a (ully

released (eature' C4 access o( rigid !ody varia!les

( Cannot access displace.ents + rotations o( the centre o( .ass

in C4

' Availa!le in ;ser FOR0RA#

( ?oever" can access .esh displace.ent in(or.ation on the

!oundaries

' Attach .onitor point to a node

' ;se call!acs to ,uery .esh displace.ents+position on !oundaries

' ;se o( 6-DOF in .oving re(erence (ra.es

( #ot availa!le in rotating (ra.es

( Can use stationary (ra.es ith rotating .esh" as in

previous e3a.ple

Documents

CFX-FSI_120_lect-05_6DOF