Vector Operations in Ansys

A Quick Look at Some Vector Operations in

Mechanical APDL

Rick Fischer

Principal Engineer

Argonne National Laboratory

November 3, 2010

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Nuclear Engineering Division - Engineering Operations

2

What are vector commands?

A vector command operates on an

entire array of values. One command

completely fills

an array.

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Compare that to a scalar command, which

operates on a single-valued parameter.

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Why use vector operations?

Because they are FAST!!

•The cube below has 1,030,301 nodes.

•Let’s get the x and y coordinate for every node.

•The do loop makes a separate trip into the data

base and comes out with one value, so it must

enter the database 1,030,301 times.

•The vector command enters the database once

and comes out with 1,030,301 values.

•The savings in IO time is huge!

2.052 min.

0.000 min.

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Before I can use a vector command, I must create arrays.

This is done with the *DIM command. The array will be

initially filled with zeros.

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Where can I use vector commands?1. Mechanical (i.e. Workbench) in a command snippet.

2. Mechanical APDL (i.e. Ansys Classic) at the

command line.

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There are lots of

vector commands.

Some have many

options, some are

simple. Rather than

going through the

commands one at a

time, which is time

consuming and

boring, lets look at a

couple of typical

problems and see

how they can be

solved with vector

commands.

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PROBLEM #1: Find the total change in heat energy in a model with multiple

materials during a transient thermal analysis.

APPROACH: Find the change in heat per element and sum across the model.

eieie TcVqQ

multi-body part

applied temps

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First, lets get some raw data. We need to get nodal

geometry, elemental geometry, and material constants. We’ll

use *VGET to speed things up.

Just a simple do-loop for this. Only three materials, so a vector command

is not needed.

Note that we used seven *VGET commands. On a million

node model, that would take 14 minutes!

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At this point we have the material properties by material types,

but we don’t have them associated with an element.

mat #

density

specific

heat

element

centroid

coordinates

element

volumeelement #

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We will use the

*VOPER command

to “gather” the

material

properties to the

correct location.

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ParR(j)=Par1(Par2(j))

ParR(Par2(j))=Par1(j)

data orderPar2 ParR

Par1

This command is impossible to

describe verbally. Use the

diagrams below to decypher it.

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Next, we need to get temperatures. Temps are available only as

nodal temps, and we need an elemental temp. We could try and

do some boring, repetitive math and average the nodal temps

to get an elemental value, but here’s a slicker way.

Not a great description. It’s true, but

the MAP option is not limited to

mapping results. It can map any kind

of data from one set of reference

points to another set.

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An ncount x 3 matrix of nodal

x, y and z coordinates.

An ncount x 1 vector of nodal

temperatures.

An ecount x 3 matrix of

element centroid x, y and z

coordinates.

An ecount x 1 vector of

elemental temperatures. 3D

locations

data

We can use the *MOPER,,,MAP command to interpolate data (nodal

temps in TDAT) from a cloud of reference points (nodal coordinates in

NDATA) to a new cloud of points(element centroids in EDATA).

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All that’s left is the math.

We’ll use *VSCFUN (vector scalar function) to sum the individual

elemental heat losses across the entire model.

Note that in the first *voper, Par2 = tinit, a scalar parameter. Either

Par1 or Par2 or both can be a constant.

Note: four more

vector operations =

eight more minutes

saved!

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Back in Workbench Ansys Mechanical…

Create a command snippet

under Solution and insert

your APDL commands.

Note that these commands

will be executed after the

POST1 command.

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Next, run the solution.

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Then, click on Solution Information, and scroll down to find your results.

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CHECKING YOUR WORK: In Mechanical APDL, there are a couple

ways to see what kind of damage you are doing.

click here

To look at your Data array, click

parameters, Array Parameters,

Define/Edit, choose DATA, then click

on EDIT. The window below, right will

appear, displaying the DATA array

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Another method that allows dirrect access to a known locatiopn

is to use the *STATUS command. If you want to look at the

entries for element 567 in the DATA array, at the command line

type:

*status,data,567,567,1,10

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PROBLEM #2: Calculate high cycle fatigue damage at every node in a model of a

hydraulic pump and display it as a contour plot.

APPROACH:

1. Assign fatigue parameters for each material to each node.

2. Obtain minimum and maximum stress at each node. Calculate mean and

alternating stress.

3. Calculate the fatigue stress and damage at each node.

4. Plot the damage as a contour plot.

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First, lets get the material data we’ll

need for the fatigue calculations.

Estimate an S-N curve using the method given in Shigley, and fit it to

a straight line in log-log space. Then input the material data needed

for fatigue calculations.

Next, select the component element HEAD, and select all the nodes

associated with it.

Now, use *VGET to create a “mask” based on nodal selection status.

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What’ a mask? A mask is a column of 0s and 1s that

can be used to control the execution of the first vector

command that follows it.

Notice how the mask generated with the *vget

command controls the action of the *vfill command.

Last element

with MAT=1 (not

selected)

First element with

MAT=2 (selected)

-1 = defined, not

selected

1 = defined,

selected

Column 1 only gets filled when column 6 = 1

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Next, lets retrieve some stresses

and crunch the math.Modified Goodman line

adjust for a non-zero mean stress

find theoretical life

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To create our plot, we’ll use *vput

to put our calculated damage into

the database for plotting. Pick a

result that you are not using in

your report. Lets use sx, st, and

sz.

For this to work:

•No Powergraphics (/graphics,full).

•Can’t write over a derived result (e.g. s,eqv).

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Here’s a very useful undocumented *VPUT option.

Change the selection status to coincide with the mask specified by ParR

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At a glance, I can see every

potential fatigue hot-spot

everywhere on the entire

assembly.

The head has a hot-spot in the

fillet around the discharge port

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The real high damage on the

stainless steel housing is due the a

BC, (bonded contact on the

housing-head interface), so I can

ignore it if I want.

We specified an endurance limit for the

steel mounting flange, and it was above

Sf everywhere on the part. The macro

sets N to 108 cycles, so at 100,00 cycles,

the damage is 0.001.

Vector Operations in Ansys

Documents

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vector command

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