Hydrodynamics

[mathias_peron]

Hi everyone,

As I'm modelling a water tower in a seismical region I neet to take into account the ovement of water during the seismic analysis.

I have found very interesting topics on the forum and very interesting articles before to post here.

I would like to use the Housner's model for elevated tank and I'm following what have been done before here: http://forums.autodesk.com/t5/robot-structural-analysis-forum/how-can-model-hydrodynamic-pressure-on...

I still have few questions:

- When setting the convective mass (mass on node in Robot), I understand that it needs to be only in X and in Y. But when setting the impulsive mass (that is the mass that doesn't move), don't we need to set it in Z as well?

- Can the impulsive mass be set exactly like the convective mass but with bars with infinite stiffness?

- Do I need to check the box "Apply to all cases" when setting the masses?

Thank you in advane and have a nice day!

Mathias.

[mathias_peron]

Please find my model attached for better understanding.

[Artur.Kosakowski]

Please check:

http://forums.autodesk.com/t5/robot-structural-analysis-forum/how-can-model-hydrodynamic-pressure-on...

http://forums.autodesk.com/t5/robot-structural-analysis-forum/how-to-model-liquid-load-by-earthquake...

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[mathias_peron]

I have seen those topics before.
I'm asking questions that haven't been posted in those.

If someone vould answer them and check my model i woukd be most grateful.

Have a nice day,

Mathias.

[Artur.Kosakowski]

- When setting the convective mass (mass on node in Robot), I understand that it needs to be only in X and in Y. But when setting the impulsive mass (that is the mass that doesn't move), don't we need to set it in Z as well?

I think you should include Z for it when you assume you will have 'vertical' shapes of vibrations.

- Can the impulsive mass be set exactly like the convective mass but with bars with infinite stiffness?

You may consider converting variable surface load into mass (the conversion cannot be done for the hydrostatic one).

- Do I need to check the box "Apply to all cases" when setting the masses?

Only when you want to include them in generation of forces of inertia.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[mathias_peron]

Thanks for your answer it helps a lot.

"Only when you want to include them in generation of forces of inertia."

I don't understand what it means, could you explain me please? Do you think I should check this box?

Thanks again.

[Artur.Kosakowski]

Let me write it again. You may keep it marked or unmark it in your case but when you want to use this mass for body force load you should mark it.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[mathias_peron]

Sorry to bother you again.

It i the first time that I see this body force window, I didn't use that before. What is it for?

What I did to take into account the convective mass was to **** a mass to a node (with the window "added masses - nodes") and then to link it with truss bars to my walls using truss bars having a material and a section that represent my stiffness. Is that correct?

By the way, if I set a mass in Z to the nodes, do i need to link it with additional bars to the top and bottom of the tank?

Thanks again.

[mathias_peron]

I think I understand what you mean.

To make the convective mass work, I need first to assign a mass to the node PLUS the mass should be assign to all load cases in order to have a body force assigned to the mass.

Is that correct?

But still, what is this "body force" window for? Do I need to use it?

Thank you in advance.

Mathias

[Artur.Kosakowski]

I would try the following:

1. Convective mass assumed as 'small' part of water that moves sideways following the horizontal movements of a tower - modeled as in the topic you mentioned in your first message

2. Impulsive mass - 'all water weight' converted into mass and assigned to walls of the tank at the top of the tower

Body force:

Imagine that you want to design a mast of a boat and you have a piece of equipment of its top. When the boat swings sideways on waves the weight of the equipment being moved sideways generates a force of inertia which you need to consider in the design of the mast. This is what the body force stands for.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[mathias_peron]

Thanks a lot for the answer.

1 - Its exactly what I did.

2 - I have set a line load on the tank walls representing the impulsive mass at the level of the impulsive mass should be and then used the load to mass conversion on this particular case. I believe it should be the same. Correct?

Thanks for the explanation of the body force. I understand perfectly. So of course I should use this body mass command on the convective mass. And because I'm using the body mass command, I should assign the mass to all load cases. Correct?

Have a nice day,

Mathias.

[Artur.Kosakowski · Accepted solution]

2 - I have set a line load on the tank walls representing the impulsive mass at the level of the impulsive mass should be and then used the load to mass conversion on this particular case. I believe it should be the same. Correct?

I think it should be OK 🙂

Thanks for the explanation of the body force. I understand perfectly. So of course I should use this body mass command on the convective mass. And because I'm using the body mass command, I should assign the mass to all load cases. Correct?

To modal one(s) but if you mark apply to all it is fine too but in such case you'll see them in the loads table as well.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[mathias_peron]

Thanks Artur for your patience and for answering my questions so many times and not only today 🙂

Have a nice day!

[Artur.Kosakowski]

My pleasure. Good luck with your project :-).

[mathias_peron]

Hi Artur, quick question here:

When using body forces on added masses, do I put the values in all directions or only in Z?

X = 9.81 m/s2
Y = 9.81 m/s2
Z = 9.81 m/s2 (what about the sign here?)


Thanks again.

[mathias_peron]

Sorry for the stupid question. Body force should be only set in the direction of Z.

[mathias_peron]

However, I still have trouble to understand to which case the body force should be assigned to.

If I follow your logic, the convective mass should be assigned to the modal case in order to be taken into account, but problem is that the body force cannot be assigned to any modal cases in Robot. So, is it really taken into account in the end?

Have a nice day,

Mathias.

[mathias_peron]

After experiments, I believe that body force doesn't have an influence on the modal analysis.

Body force should be applied to "activate" the mass in static cases. As I'm using it only in the modal case, I don't need it.0

Correct?

Have a nice day,

Mathias.

[Artur.Kosakowski]

Please check:

http://forums.autodesk.com/t5/robot-structural-analysis-forum/body-forces-and-added-mass/m-p/3377955

http://forums.autodesk.com/t5/robot-structural-analysis-forum/body-forces-on-added-masses/m-p/337757...

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[mathias_peron]

Hi Artur and thanks for the useful links but they don't answer exactly my question.

I understand that body force is to "activate" the mass in static cases.

I understand as well that it is important to have a body force assigned to the mass for the modal analysis to represent forces of inertia of the mass when the structure swings.

However, applying a body mass to a modal case is impossible with robot (the box is grey). So how do I do that?

Have a nice day,

Mathias.