Dear friends
I am trying to use CODE_ASTER , a finite element software to simulate a fish cage‘s movement. I am using HHT method to solve the problem with alpha=-1/3. My force is added with Morison equation like this : fn=0.5ρCdLDV|V| and it is added at each selected time step(defined before calculation).
When the velocity is under 0.5m/s, my job seems well ,but when I change a higher velocity, the deformation become pretty bad, the cage is oscillating . In my job, there is no structural damping ,only the numerical damping is added. I am wondering what will cause this problem? If it is the problem of implicit solver or the way I add the force?
There is a GIF of the problem. I hope to get some ideas.
see
Article Nonlinear hydroelastic analysis of an aquaculture fish cage ...
Maged G. Bin-Saad Thank you for your help , I think his job is done with ABAQUS AQUA which is done by the software itself . I will try to change my Morison into theirs which has a inertial part to try.
Dear friend,
This problem you describe has been known for a long time and can be due to several causes.
Firstly, the use of FEM for the calculation of mesh cages is not always recommended. This is due to the fact that the shape changes of the netlike structure can be relatively large. This is due to the fact that the hydrodynamic loads are relatively large compared to the tensile forces in the mesh bars.
A second reason may be the discretisation of the netlike structure. A discretisation that is too fine-meshed can lead to numerical problems, a discretisation that is too coarse may be physically impermissible. Here you could try a little - also in conjunction with the hydrodynamic loads.
And last but not least, you should allow a slight stretching of the net material. You will see that this has a positive effect on the peak loads in the mesh bars (they become smaller), which are ultimately responsible for the structural vibrations.
If all this does not help, you need to test what frequencies of the waves are allowed, that means also in the hydrodynamic loads (you mentioned the Morison equation, which is known to consist of the Froude-Krylov force, the hydrodynamic inertia force and the viscous load).
Good luck, Mathias Paschen
Mathias Paschen Really thank you for your help ! I will try your ideas , I recently find that may be caused by a too large time step of 0.01s. Maybe 1e-6 will be better but will cost a year to calculate 2 hours' test. And I hope to know a slight stretching means what? I am using cable elements which will be stretched like a bar in the calculation.
Of course, in this case, you may have a computing time problem. I remeber very well another numerical problem I had many years ago when calculating oscillating, hydrodynamically loaded ropes. I found that the differences between large tensile forces on the left and right sides of the bar elements were very small. They were in the order of magnitude of numerical accuracy. So it became a problem of numerical (in-)stability. I solved the problem by increasing the necessary number of significant digits.
Mathias Paschen Thank you ! Maybe I should not just use software but to write some own code to try.
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