Creating a HEX only Mesh

Altair Forum User

I have been trying to create a solid mesh comprised of only HEX8 elements. I have been using the 3D solid mesh tab. In this tab I choose the ONE VOLUME option. I pick a solid to mesh, and subsequently mesh it. But I noticed that there is a SOURCE SHELL tab where I can choose quads, trias, or mixed. I am able to mesh the entire construct, but in so doing a layer of shell elements is created. This layer of shells is unwanted because I feel that a mixture of shell and solid elements in a single part will yield an error when I run the simulation.

Is there a way to avoid this creation of shell elements and attain a mesh that is composed solely of HEX elements?

Additionally, this HEX mesh will be treated as a fluid. To do so are there other meshing parameters that must be set? Or is this done in the ALE_REF_SYS_NODE and GROUP panels only?

Thank you, any help would be greatly appreciated.

Brad

mvass

Source shell tab does exactly as it says in ONE VOLUME option. If one or more surfaces of your volume have been shell meshed, you can use this mesh as a source to 'drive' your volumetric mesh. If you are able to mesh your volume with hexas then you should not see any shells created there unless something is done wrong. 'Mixing' shells with solid elements is not always erroneous: When I'm doing volume meshes for external flow studies (CFD), I use the option of faces creation (after completion of the volume mesh) to create my boundary zones. Same principle applies when creating the FE model of a sandwich structure and use 3D elements to simulate the honeycomb and 2D to simulate the faces. In addition the 'faces' option acts also as a validator of your volumetric mesh: If shell elements appear between your solid elements it means that there is a mesh discontinuity there and your volume is not 'watertight'.

Regarding your question about the group panels I am sorry but I am not using RADIOSS as a solver (except for simple analyses) and I haven't done any analyses involving fluid-structure interaction (so far).

Altair Forum User

Thank you for you initial response. I want to make sure I understand a few of the things you wrote and perhaps added details of what I am doing will prove useful to answering this question.

I should mention that I use LS DYNA as the solver. The model itself is still very basic at this point. I am trying to include two parts in the model. One, the inner part, that is comprised of MAT NULL to represent fluid. And second, a part that fully encases the MAT NULL component. This second part will be elastic or viscoelastic in nature. These two parts will subsequently undergo a compression test.

In reviewing some of the online demos on the LS DYNA website, it appears that the components used in demos where ALE (arbitrary lagrangian eulerian) interactions are incorporated utilize HEX elements exclusively. When I attempt to 3D volume mesh my components I always end up with one face of the component that has QUAD elements and the rest of the model that is composed of HEX elements. How is it that I can prevent the creation of these QUAD elements?

I have noted that shell elements can be used from the 2D automesh option to 'drive' a solid HEX mesh through out the rest of a model. But the same issue of having a mixture of shell and solid elements persists. I have resorted to deleting all of the QUAD elements using the TOOLS_DELETE panel after creating my HEX mesh. But I would like to know if there is a better more simple way of creating a HEX-only mesh.

Another point about trying to simulate Fluid with Hypermesh/LS DYNA. The demos online that use ALE seem to use rigid materials only when interacting with a fluid. Is it even possible to use material cards that aren't RIGID in these simulations?

Another question about 'contact interactions'. In the two demos that are most pertinent to my work, one uses a AUTO_NODES_TO_SURFACE contact algorithm; while the other uses a CONSTRAINED LAGRANGE_IN_SOLID card and does not incorporate any contact algorithm. What, if any, advantages are gained by using one of these options. And are there any particular situations in which one option is more desirable than the other?

I know this is a lengthy post, but any information on the subject is greatly appreciated.

Brad