2D Pressure Vessel analysis 1

[vtmike]

Hi,

When I apply an external pressure load on a 2D model of a cylinder (plane strain analysis), I start to get rigid body motions.

Why would ANSYS WB create rigid body motions for an elliptical cylinder with equal pressure applied on the outside circumference?

 

[vtmike]

Attached is an image showing the loading condition.

 

[TGS4]

Do you have any boundary conditions that prevent the annulus from moving in the up-down or left-right directions? If not, then even small issues with rounding error (machine-level rounding) will cause the rigid-body motion you describe.

Apply appropriate restraints in the X and y directions and you'll be fine.

 

[vtmike]

It is plain strain analysis of an oval cylinder which is perfectly symmetric with constant thickness. If it was unsymmetric I would understand the rigid body motions, but why am I getting rigid body motions for a perfectly symmetric body? and what boundary conditions can you apply in such a case?

 

[Micpage]

Hi,

Even if you have a perfectly symmetric body, you must constrain your displacements. It is necessary because that's the way that the finite element technology works. To solve a linear problem, the software create a matric system K*U=F. If you don't specify any restraint, U = 0 and the system cannot be solved. The software add weak spring to stabilize the model.

In your cas, I see two options of constraint
- Use Weak spring and with probe option verify the force in each direction. If it is negligeable, your solution is near the reality.

- Create a line in the symetri plane of your model. Fix vertically a dot of the end of the line and fix horizontally the line. So the model will be stable.

Regards,

Michael Page
IDÉA Innovation PME