# Need Help Setting Constraints

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#### Guest

##### Guest
Howdy Hey!!

I'm trying to simulate (for fun and games during my lunch hour) tugging a rubber balloon down to the ocean floor... How do I set up my constraints such that pressure is exerted on all external surfaces (and of course some internal pressure) of my balloon??

#### Luis Aguirre

##### New member
Hello Hazmatt,

Here is the solution to your problem. Let's assume that you have a balloon with internal pressure of 3 atmospheres and of course the outside of the balloon sees 1 atmosphere. To run this analysis you will apply 2 atmospheres to the inside of the balloon an nothing to the outside this will create the same load condition as in the first case. In other words, use gage pressure to run your analysis. I Hope this helps.

Luis

G

#### Guest

##### Guest
I don't think you can get away without constraining anything. Otherwise, the model will fly off into space. Can you constrain the bottom of the balloon as if you were pulling it down from the knot?

#### bem

##### New member
An interesting problem, here's my 2 cents:

I created a hemisphere and a spherical coordinate system at the center of the hemisphere. I then constrained the inner surface fixed in theta & phi, but free in the radial direction. I then put a pressure on the outside. I'm getting a pretty uniform displacement in the radial direction.

Have you tried a spring-to-ground to constrain your model? Also, what (if anything) are you using to simulate the atmosphere inside the balloon.

#### Luis Aguirre

##### New member
Hazmatt,

Here is anothere way you may be able to solve your problem assuming that the ballon is a perfect sphere and pressure is applied equally in all surfaces. If this is the case you can use symmetry and analyze a quarter of the sphere applying symmetrical constraints. In this case the model will be fully contraint and it will not affect the results. The constraints are there only to enforce the symmetry of the material boundary condition. Hope this helps!

Luis