underwater mod al

MODAL ANALYSIS OF SUBMERGED

STRUCTURE

Elaborated by: Fabiano N. Diesel

Date: 27/07/2010

Underwater structures have to

move the mass of surrounding

water when vibrating, so its

resonating frequency get low.

There are several researches

that determinate the added

mass from response frequency

function or experimental modal

analysis of dry structure.

Herein, it is shown a

methodology to get resonating

frequency from a coupled fluid-

structure system.

First of all, a slice of a dry plate

has its natural frequencies got

from a modal analysis.

Introduction

frequency Mode

275 Hz

754 Hz

1467 Hz

fixed support

fixed support

frictionless support

(keep it in XY Plane)

In-plane modes of a steel plate in vacuum

Modal Analysis of Dry Structure

A surrounding geometry was built to

perform the coupled modal analysis:

The geometry is a Multi-body part

composed by a structural part and a fluid

part, divided in three layer.

The mean layer is the fluid with no structure

interface. The closest layer from structure

is the fluid with structure interface where

the fluid elements must have pressure and

translation degree of freedom.

The last one (outer layer) is necessary just for harmonic or transient analysis, when

one would like to know acoustic pressure in far field, such as irradiating noise.

Whether structure has complex geometry, the FSI layer can be done by APDL, so the

fluid can be only one volume.

Submerged Structure Geometry

COMMAND branches are inserted into every FLUID part to specify Fluid

characteristics (element type and material properties).

mpdele,all,matid ! Delete all existing linear properties

mp,dens,matid,1000 ! Define mass density as 1000 kg/m³

mp,sonc,matid,1500 ! Define sonic velocity as 1500m/s

et,matid,30 ! Define Element Fluid 30

keyo,matid,2,0 ! Elements with FSI characteristics

mpdele,all,matid ! Delete all existing linear properties

mp,dens,matid,1000 ! Define mass density as 1000 kg/m³

mp,sonc,matid,1500 ! Define sonic velocity as 1500m/s

et,matid,30 ! Define Element Fluid 30

keyo,matid,2,1 ! Elements without FSI characteristics

FLUID 30 are first order elements, so everything must be meshed with that kind of element.

Second order acoustic fluid element FLUID220 and FLUID221 will be available in Ansys Realease 13.

Fluid Elements

Dropped Midside Nodes to generate a first order element mesh

Meshing

To couple fluid and structure, a FSI must be defined,

soon two NAMED SELECTION are created. First one,

selecting every face from interface, it will generate a

set of nodes. Last one, selecting body of the inner

fluid layer, resulting in a set of elements (just fluid

element in FSI).

A COMMAND branch is added into MODAL

environment.

FSI faces

Fluid Body cmsel,s,nfsi ! Select FSI nodes set

cmsel,s,efsi ! Select FSI elements set

sf,all,fsi ! Define FSI for selected nodes

allsel ! Reselecting everything again

Fluid – Structure Interface

Structure has equivalent supporting of dry structure. No one constrain is applied to

FLUID part, free face of fluid has behavior of rigid wall.

Because FSI, the matrices are unsymmetric, so an eigenvalue algorithm that can

handle those matrices must be selected, it is done by MODOPT APDL typed in same

Command Branch used to define FSI.

cmsel,s,nfsi ! Select FSI nodes set




modopt,unsym,10,1 ! Select UNSYMMETRIC algorithm for eigenvalue determination

! Determine 10 modes, beginning from 1Hz

structural support

Number of modes to determinate specified in MODOPT should be equal from “Analysis Settings > Max Modes to Find”

Modal Analysis of Submerged Structure

1st mode: 109Hz 2nd mode: 451Hz 4th mode: 961Hz 5th mode: 1369Hz

After solution is done, user should observe every mode identifying structural modes.

Another way to get resonating frequencies is to do an harmonic analysis with a

unitary force into a structure’s point and getting a displacement response frequency.

The peaks in the curve indicate the frequencies.

Modal Analysis Results

Performing an harmonic analysis

is similar to a modal analysis.

The same procedures regarding

the geometry, Named Selections

and Command branches must be

carried out.

Harmonic Analysis





specify range and intervals of frequency

only Full Method is available for unsymmetric matrices

specify damping

Harmonic Analysis of Submerged Structure

Harmonic Analysis – Forces and Results

A modal analysis with coupled field (Fluid and Structure) is fast way to get the

resonating frequencies and theirs vibrating modes, but user must observe every

mode and make distinction about structural and fluid mode.

Harmonic analysis is easier to distinguish structural modes and fluid modes,

although is more expensive. User must have previous knowledge of structure

dynamic behavior to place the exciting forces and to get response frequency

curves.

New advances will be available in Ansys Release 13, such as second order

acoustic elements and APDL Math feature to manipulate matrices.

Comments

When structure has a complex geometry, the surrounding fluid can be done by

just one volume. In such cases, the fluid FSI layer is done by APDL (below). User

must define a Named Selection with all faces in fluid-structure interface (NFSI).

/prep7 ! Go to Preprocessor environment

*get,ntype,etyp,,num,max ! Get the highest element type number

ntype = ntype + 1

et,ntype,30 ! Define Element Fluid 30

keyo,ntype,2,0 ! Elements with FSI characteristics


esln ! Select elements linked to active nodes

nsle ! Select nodes linked to active elements

esel,r,ename,,30 ! Select just element Fluid30 from active elements

emodif,all,type,ntype ! Change active elements to Fluid30 with FSI

cm,efsi,elem ! Create FSI elements set

allsel ! Select everything

/solu ! Go to Solution environment





Annex

3rd mode: 651Hz 5th mode: 1279Hz

Modal Analysis Results

← structural →

← fluid →

underwater mod al

Documents

fluid elements page

fsi characteristics

fluid structure interface

couple fluid

coupled fluidstructure

inner fluid layer

fsi elements

fsi layer