[ashtonJ]

Dear All,

I am using moving boundary method in ANSYS CFX to model fluid flow through a pipe, which is initially unimpeded and then narrows to form the constriction shown in attached image.


I created the below expression to produce that narrowing;
RampLength: 15 [mm]
RampeY: 0.0029296875 [mm^-2] *(min((abs((z-RampLength)^3) + 341.34[mm^3]), 512 [mm^3]))

I chose "Specified Displacement" for Mesh Motion option and defined the following displacements
x = 0.0
y = RampeY
z = 0.0

At the beginning of the run, the following error message is given:

A negative ELEMENT volume has been detected. This is a fatal |
| error and execution will be terminated. The location of the first |
| negative volume is reported below. |
| Volume : -0.2336E-12 |
| Location : ( 0.34043E-03, 0.15680E-02, 0.29921E-01)

Could anyone help me with this, please?

Regards,
AshtonJ

[ghorrocks]

FAQ: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F

[ashtonJ]

Dear ghorrocks,
Negative Volume problem is fixed by modifying the mesh, thank you.



The problem now is non-uniformity in deformation! As seen in the above image, the pipe deforms in a non-uniform way (image a); however, I expected the expression mentioned in my previous post, created a uniform deformation like image (b). do you know what the problem is?

Regards,
AshtonJ

[ghorrocks]

CFX V15 has much better defaults to get mesh motion working nicely. If you have not migrated to V15 this would be a very good reason to do so.

The way to fix this issue is by adjusting the mesh smoothing weighting factors. You can have it constant, or as a function of all sorts of things - element size and wall proximity are two parameters often used.

[ashtonJ]

Thank you. I actually could not find any useful information about about mesh smoothing weighting factors. Can you explain more or introduce a document about weighting factors.
I actually tried to fix my problem by multiplying the expression by each component of the wall normal vector to displace the nodes in the wall normal direction only like below

Displacement in X direction: Normal X * expression
Displacement in Y direction: Normal Y * expression

Then in Mesh Motion > Specified Displacement > Displacement > the followings were defined;

X Component = RampeX
Y Component = Rampe Y
Z Component = 0.0

Again, an error message is given straight away at the beginning of the run.
Do you know what is wrong with above settings.

[ghorrocks]

Have you done the debugging described in the FAQ: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F