Learning Forum

[Caleb Wood]

Hello,

I am simulating a FSI problem where water waves are impacting a rigid wharf structure using the S-ALE method. The water is modelled using MAT_NULL, and the air is modelled using MAT_VACUUM. I am able to use ALE_STRUCTURED_MESH_VOLUME_FILLING to get the overall problem defined, however I am not sure how to remove the water from where the piles are supposed to be and set it to a void material, which causes the water to move at the beginning of the problem. If anyone knows how to be this I would greatly appreciate it. Note that the water surface is not shown in the attached images but it is just below the deck level. Also, I have tried using the GEOM_PART option but have not had any luck. Thank you.

[Jim Day]

It may be helpful to the audience to know how the piles are modeled. Are these beam elements or shell elements? Confirm the cross-section shape is an I-section/H-section? Also describe/show how you've defined the FSI coupling between water and piles.

[Caleb Wood]

I have two models which I am experimenting with, one where the piles are made of shell elements and one where they are made of solid elements. The cross-section is W360x174. The coupling is defined using ALE_STRUCTURED_FSI.

You mentioned beam elements, would it be possible to use beam elements in an FSI problem where waves are impacting piles? It is my understanding that beam elements would not capture the water flow around the pile accurately, and would influence the results downstream, do you know if this is the case? My simulation runs for 15 seconds so each pile should be subjected to 7-8 waves. 

Thank you.

[ido]

Hello Caleb,

[Q1] How to remove the water from where the piles are supposed to be and set it to a void material

[A1] If your piles are solid LAG parts, you can define a segment-set representing the pile’s surface. Make sure all the segment normals point in the same dir. Then you can use that segment-set as a “container” for doing vol-filling to fill in that space. 

Is there any reason you are not using ICFD for this? It is implicit and can deal with longer run time. You need “My simulation runs for 15 seconds so each pile should be subjected to 7-8 waves. “

ALE is an explicit hydrcode. So its

dt_max ~ [min_elm_width/sound_speed] 

It depends on how thick your piles are and how many ALE elms you have spanning that thickness. To do proper penalty coupling you can try to account for the overlapped space under the piles as questioned in [Q1] by doing the vol-filling as discussed. 

If the ALE elms are big and there are not too may ALE elms across the piles’ thickness, then you may try by simple treat the piles as being submerged in the water space (even in the overlapped space). We’ll get some interaction forces, but we cannot be sure how well the piles-water coupling may be. You can try to validate this by simple/small examples of submerged FSI and compare to data.

In any case, 15s is an eternity for hydrocodes! Of course it depends on how many elms you have, which determines the elm width, which determines the dt ==> how long to run 15s! etc.

Given the fact that LAG:ALE relative mesh resolution should be as close to 1:1 as possible as a guideline, 

Regards,

Ian Do

 

[Caleb Wood]

This is just a proof of concept model, the run time will likely not stay at 15 seconds, and it is my understanding that ALE is better suited for FSI problems considering high dynamic loadings than ICFD, however I could be incorrect. My end goal is to simulate part of a real world wharf against high-impact wave loadings. 

As for the volume filling, it seems very dependent on the mesh resolution. Since I am using an H-pile, my ALE elements need to be very fine in order for the area which the H-pile occupys to be filled with a void material, to the point where the model run time approaches 1000+ hours. Are you aware of any other methods to fill the area with a void material which arent as dependent on the mesh density? Also if not, do you think ICFD would be better for this application even though I need to consider high dynamic wave impact loadings (think of wave loads resulting from hurricane winds)?

Thank you.

[ido]

Hello Caleb,

If it is a conceptual run, then I would simply model 1 leg of the piles with its FSI with water. Then evaluate if the FSI is active and makes sense.

From this you will be able to extrapolate for the amount of resource you will need for whichever modeling approach or design you will choose. 

You will also learn what resolutions are needed and how to construct the rest of the FSI mechanisms, etc. 

Regards,

 

Ian Do