Ansys

cozzie55

Hey
I'm just doing some modelling using Ansys.
I was advised to apply a Hexahedral mesh to my models. Can anyone tell me why this type of mesh should be used?

I think it may have something to do with spare/box shape elements being easier and more accurate to predict strain with. But i want to be definite about this

Darren1o1

cozzie55 wrote: »

Hey
I'm just doing some modelling using Ansys.
I was advised to apply a Hexahedral mesh to my models. Can anyone tell me why this type of mesh should be used?

I think it may have something to do with spare/box shape elements being easier and more accurate to predict strain with. But i want to be definite about this

Ansys help and theory guide is very helpful at explaining this.

CatFromHue

I think triangles/tetrahedrals are less accurate than quads/hexs.

Another reason is that quads/hexs can distort more (better aspect ratio) than triangles/tetras.

Darren1o1

CatFromHue wrote: »

I think triangles/tetrahedrals are less accurate than quads/hexs.

Another reason is that quads/hexs can distort more than triangles/tetras.

This is not true. The application of tri/tets is typically advised for certain geometries and can be more accurate. If one was simply less accurate than another, why would you not simply obsolete the less accurate element. It is specific to geometry and boundary conditions. For instance, implementing a mesh on a circle (say on a 2d crossflow) I would apply triangular mesh as I have have had better results in this case. Aspect ratio can be also controlled in the mesher for both types of element.

CatFromHue

Darren1o1 wrote: »

This is not true. The application of tri/tets is typically advised for certain geometries and can be more accurate. If one was simply less accurate than another, why would you not simply obsolete the less accurate element. It is specific to geometry and boundary conditions. For instance, implementing a mesh on a circle (say on a 2d crossflow) I would apply triangular mesh as I have have had better results in this case. Aspect ratio can be also controlled in the mesher for both types of element.

I presumed the OP was talking about FEA and was reading from Pg 203 of the book "Practical Stress Analysis with Finite Elements" which I think you're familiar with. Bizarrely enough in class today the lecturer brought up this very topic in specific to the Constant Strain Triangle Element (a linear three node element).

Darren1o1

CatFromHue wrote: »

I presumed the OP was talking about FEA and was reading from Pg 203 of the book "Practical Stress Analysis with Finite Elements" which I think you're familiar with. Bizarrely enough in class today the lecturer brought up this very topic in specific to the Constant Strain Triangle Element (a linear three node element).

Sorry had heat transfer on the brain. I have found in practice that using a triangular mesh proximal to a complex shape, going, to quad gives the best result. Without knowing anything about the problem there is simply no way to argue for any case. I find the use of the help functions steers you well and gives good examples. If the OP would care to clarify the problem then I am sure we could be of help to them.

cozzie55

Darren1o1 wrote: »

Sorry had heat transfer on the brain. I have found in practice that using a triangular mesh proximal to a complex shape, going, to quad gives the best result. Without knowing anything about the problem there is simply no way to argue for any case. I find the use of the help functions steers you well and gives good examples. If the OP would care to clarify the problem then I am sure we could be of help to them.

Rite will i'm trying to model simply supported 2m laminated timber beams, that have dowels inserted at angles through the laminates to joint them. So its a static structural problem. As it is a beam ie rectangular geometry, then i'm assuming a "square" mesh is more applicable than say a triangular one for most testing conditions.

The main reason i'm questioning the advise, is that largest mesh that the software applies is one with 48000 elements which takes the computer ages to solve(which is a pain when you have to run the solution a number of times changing parameters to get the rite solution). If i could use another mesh type the number of elements can be reduced ie an automatic method reduces it to 14000 elements.

Turbulent Bill

cozzie55 wrote: »

Rite will i'm trying to model simply supported 2m laminated timber beams, that have dowels inserted at angles through the laminates to joint them. So its a static structural problem. As it is a beam ie rectangular geometry, then i'm assuming a "square" mesh is more applicable than say a triangular one for most testing conditions.

The main reason i'm questioning the advise, is that largest mesh that the software applies is one with 48000 elements which takes the computer ages to solve(which is a pain when you have to run the solution a number of times changing parameters to get the rite solution). If i could use another mesh type the number of elements can be reduced ie an automatic method reduces it to 14000 elements.

Have you implemented any available symmetries or model reduction methods to reduce the number of elements required? You might only be interested in the beam's behaviour around the dowels (not the whole 2m length), so simulating the whole thing is a waste of time. You could run a really basic beam model just to ensure the boundary conditions etc. are right, then focus on an area.

In general, you need a really clear idea of what you want out of an analysis before starting one.

cozzie55

Turbulent Bill wrote: »

Have you implemented any available symmetries or model reduction methods to reduce the number of elements required? You might only be interested in the beam's behaviour around the dowels (not the whole 2m length), so simulating the whole thing is a waste of time. You could run a really basic beam model just to ensure the boundary conditions etc. are right, then focus on an area.

In general, you need a really clear idea of what you want out of an analysis before starting one.

Yep i've already quartered the beam and am using zero displacement supports along the divided faces to reduce the size of my model and it still has 48000 nodes. This is due to the inclination angle of the dowels.

Darren1o1

cozzie55 wrote: »

Yep i've already quartered the beam and am using zero displacement supports along the divided faces to reduce the size of my model and it still has 48000 nodes. This is due to the inclination angle of the dowels.

What about mapping your mesh focusing the mesh concentrations on the points of concern?

Darren1o1

Turbulent Bill wrote: »

Have you implemented any available symmetries or model reduction methods to reduce the number of elements required? You might only be interested in the beam's behaviour around the dowels (not the whole 2m length), so simulating the whole thing is a waste of time. You could run a really basic beam model just to ensure the boundary conditions etc. are right, then focus on an area.

In general, you need a really clear idea of what you want out of an analysis before starting one.

+1

ZRelation

If you throw up a picture of your part/mesh it would help in getting a better answer. As has been said, think carefully about how the mesh size can be reduced.

Which elements are better will depend a lot on your problem. It may even be a good idea to look at both tets and hex elements to see the difference in solution/ solution convergence (if even just to convince yourself of which is most suitable).

In my experience in general tet elements are usually discouraged except when necessitated by difficult to mesh geometries. Quadratic hex elements are typically recommended for elastic beams in bending.

Darren1o1

ZRelation wrote: »

In my experience in general tet elements are usually discouraged except when necessitated by difficult to mesh geometries. Quadratic hex elements are typically recommended for elastic beams in bending.

Agreed.

danthefan

Could anyone suggest what might be the best Ansys element type to use for a thin sheet of metal (1mm thick) that's to undergo a large deformation?

CatFromHue

Are you talking about for example a deep drawing process?

If so an axisymmetric shell element might be a good idea ;-)

danthefan

CatFromHue wrote: »

Are you talking about for example a deep drawing process?

If so an axisymmetric shell element might be a good idea ;-)

I am indeed.

I was thinking that alright and I tried it but it wouldn't mesh for me I think, gave me an error that the mesh command wasn't valid with that element or somesuch. I'll try again tomorrow. I have the whole thing modelled as axisymmetric but with the blank as a plane element I couldn't even get it to nearly converge. Cheers for the help.

danthefan

Yeah this is my problem. I try use one of the axisymmetric shell elements, Shell 208 or 209, and when I go to mesh I get the error "Element type 1 is Shell209, which cannot be used with the AMES command. Meshing of area 1 aborted".

Would anyone know what's going on here, or how I can get it meshed?

I'm using Ansys 12.1, Mechanical APDL.