Learning Forum

[leoack]

Hello,

I'm having some issues with a modal analysis of a full ship model in APDL. The model has all panels and stiffeners in it, when I run the modal analysis I get the first six modes equal to zero which makes sense as it is a free-free model but then the following modes are not the global modes of the entire ship but just of singular panels. Is there a way to eliminate the local modes? I know in FEMAP/Nastran this would be done with the ASET constraint using master nodes but I don't know how to do it in APDL?

Secondly, if I want to use FLUID30 elements to simulate the water around the ship is there a way to just overlay them onto the elements of the ship using something like the ESURF command? My idea was something along the line of: 

et,5,30 
mp,dens,5,1250
mat,5
esel,s,cent,z,0,3800
esurf 

But ESURF does not work here as it only works for line and area elements, is there another way? Any help is much appreciated, thanks in advance! 

[Erik Kostson]

Make sure all your elements are connected - for a large ship frequencies of global modes (say 1st bending mode) should be below the local panel modes.

Acoustic elements are way to complicated and comp. intensive for a global analysis of ships (vibration).

There used to be a Guyan reduction to do as you mentioned, but since the mass (matrix - depends of choice of masters which is tricky) is approximate I do not think it is supported anymore.

All the best

Erik

 

[leoack]

Hello Erik, 

thanks for the fast response! If I only get 6 zero modes and then values significantly above zero (3. Hz) shoudn't that indicate that all elements are connected as I have a fully unconstrained model? 

[Erik Kostson]

Hi - it sounds so yes - there are some other techniques that are common and appropriate for calculating wet modes of ships (so taking into account the fluid added water effect) using panel methods - we do not have that currently.

One example of such methods can be found here 

https://www.atlantis-press.com/article/22234.pdf

[leoack]

Hello Erik,

at the moment I am more concerned with why I get these high deformations of singular panels and not the global modes. Without the dry modes it doesn't make sense to look into wet modes. Since I have the functioning FEMAP model I am sure that I have all masses and since they are all connected it should give me the same result if I don't need to implement a Guyan reduction like in FEMAP (Nastran ASET constraint). If I have the list of Master nodes from FEMAP can I somehow implement the Guyan reduction in APDL? I looked into substructuring but that seems to be for different applications, would I have to assign the slave nodes to each master node manually? 

[Erik Kostson]

 

Hi

Yes, wet modes are the important ones in the end of the day, and you can not do that (vibroacoustics is not an option) -

the paper I sent if you looked is not from us, but it is to show how you calculate (and what tools you use) wet modes using Nastran that has a speical method to do that and is the common tool for doing this calculation. So if you have that tool it is very easy to do, see the paper where they actually explain the theory and the commands to be used in detail.

We said that there used to be a Guyan reduction in Ansys APDL, but it is not available anymore.

Thank you

Erik