FEM:MSC.Patran:gravity load simulation

[sneve]

Hi everybody,
I´d like to ask how to simulate a gravity load in MSC.Patran.

In my model, there is a simple container made of shell Quad 4 elements with composite properties. Container is full of water which is simulated by solid hex8 elements. These have an isotropic properties with a small E, and related density.
The structure is loaded just by the water mass. How can I simulate it?
(I can assign the gravity acceleration to these elements but how to make it related to the volume of elements?)
Thanks,
janek

 

[rb1957]

give it a density (ie mass) and apply an inertia acceleration

maybe post on the FE forum for more appropriate replies

 

[sneve]

thanks

 

[rb1957]

i'm not sure that this will model the pressure due to the head of liquid (actually i'm pretty darn sure it won't)

 

[RPstress]

I guess it might just work, depending on the properties given to the hexas and the way they're connected to the sides...however, it's much more usual to use pressure loads on the shells. As rb1957 says, using solids is to model fluid is basically wrong, even with highly modded properties. (Maybe almost zero shear and poissons and material non-linear modulus that is zero for tension? Gap elements for the interface to the tank?)

I suspect that using a weighted RBE3 from a point mass might be better than trying to simulate water with soild elements. You could have three point masses, one for each direction of acceleration with appropriate weighting for the relevant surfaces. (Actually, if the accelerations can be reversed you'd need six points, as the weigthing on surfaces paralell to the acceleration would need to ramp the other way.) A couple or three decades ago we'd never have wasted elements for this...you lot are spoiled these days!

This assumes you need the accelerations to apply to the structure as well. If it's just the fluid then apply unit pressure distributions to each surface and factor according to the accelerations. (I say this because putting unnecessary point masses is a pain, and weighted RBE3s can be hard to get right, and accelerations can be problematic - it's easy to wind up a factor of 32.2 or 386.4 out if you're not in metric! Even in metric you have to be careful if your length units are mm and your densities are g/cc or kg/m^3, and your accelerations are m/s^2.)

There may well be a more specialised method aimed at the specific problem. Also, it's the kind of thing that might be addressed by a special method in a preprocessor, which would produce the appropriate general FE loads automatically. It's a while since I used Patran and I don't know quite what's in there these days. From what I remember you could certainly do it with a field or two, and maybe someone has written a Patran Utility.

NB: getting any surface level changes must be tricky...can anybody suggest an automatic method?

 

[LewisRK]

Probably a bit late now but there is a tutorial on simulating hydrostatic pressure in Patran here:

http://www.mscsoftware.com/support/online_ex/previous_Nastran/Nas120/lesson021.pdf