Learning Forum

[mmiah]

Hi all,

I am working on this LS-DYNA FSI model with a honeycomb like aluminum structure (red) floating on water (blue) as seen in the attached figure. The structure is made of sixteen chambers - eight on top and eight on bottom, separated by a thin horizontal aluminum layer. The bottom eight chambers are supposed to be filled with air and some water so that the buoyant force keeps the specimen floating on water without the top chambers getting immersed below water level. Both ambient air and trapped air are modeled as an AMMG with a continuous layer right above the water AMMG. I have defined coupling among various parts with Constrained_Lagrange_in_Solid. I am having issues with S-ALE approach so just using traditional ALE approach right now. I cannot use the volume filling keyword for coupling.

When I gravity initialize the model as shown in the third figure, the code shows error messages saying 'invalid pointer'. But when I lower the specimen into the water so that the horizontal aluminum partition is below water level then the code progresses without any error message. We have experimental results demonstrating that the specimen should float on water below the partition level so that the bottom eight chambers are partially air filled. The idea is to apply earthquake motions so that those trapped air (between water and the horizontal aluminum layer) provides seismic input attenuation by compressing and expanding during the rocking of the specimen.

I would like to get your suggestions on how I can -
1) gravity initialize this model with the initial specimen position right above the water level? In the attachments, third configuration does not converge but the fourth one does. I wonder if the model is not constrained enough with the third configuration since there is a very small contact between the specimen and the water only by the specimen's thin edges. But the fourth configuration has more contact with water with the horizontal partition.
2) define the coupling between the bottom chambers and air and (b) the bottom chambers and water. In the current model, I have created a LAG seg-set by selecting the interior surfaces of all the bottom eight air chambers and creating two coupling cards - 1. water and seg-set and 2. trapped air and seg-set. Can these coupling cards simulate the compression and expansion of the trapped air?

Thank you,

Mamun

[Alex R.]

Hello Mamun,

Here are some feedback points on how I would tackle this problem. My starting position will be having the aluminum part right above the water. 

Create 10 AMMGs: 1 for water, 1 for vacuum or general air, 8 for air, one for each compartment underwater.
It’s important to ensure each compartment is filled with a different AMMG so the coupling can be tracked correctly.

Now, proceed with filling the ALE/S-ALE mesh. First, create eight mesh cubes (dummy parts, can be elastic), one for each underwater compartment, they will be used in the filling operation. Use *INITIAL_VOLUME_FRACTION_GEOMETRY for this. Take advantage of the container filling options: PART (CNTTYP = 1) and Plane (CNTTYP = 3).

Start by filling the entire ALE mesh with vacuum. Then, fill the water using the PLANE specification. Finally, fill each compartment with air by specifying the filling geometry for PART. Refer to *INITIAL_VOLUME_FRACTION_GEOMETRY for more details.

Coupling can be defined using 9 *CONSTRAINED_LAGRANGE_IN_SOLID: Part Lagrange for wate, 8 Part Lagrange for trapped air. Be sure to use MCOUP < 0. This will require wrapping the AMMGs into *SET_MULTI-MATERIAL_GROUP. Each SET_MULTI-MATERIAL_GROUP should contain only one AMMG.

Gradually ramp up the gravity curve from 0 to apply a gentler boundary condition and reduce oscillations in the model.

You may also consider using the S-ALE solver instead of the ALE solver. See *ALE_STRUCTURED_MESH and *ALE_STRUCTURED_MESH_CONTROL_POINTS for further details.

Please let me know how it goes, 
Thank you,
Alex