ANSYS Meshing is a general-purpose, intelligent, automated high-performance product that helps engineers to produce the most appropriate mesh for accurate, efficient multi-physics solutions.
With the release of ANSYS version 18 earlier this year, engineers were introduced to a variety of new and innovative enhancements that help improve the quality of their meshing, and speed up the simulation process.
Join PADT’s Simulation Support Manager Ted Harris, for an in depth look at new mechanical meshing capabilities made available in ANSYS 18.0, 18.1 and 18.2!
This free webinar will cover a variety of new and improved capabilities within the latest version of ANSYS, including:
- Improved diagnostics/mesh metrics
- More flexible mesh controls
- New physics preference for Hydrodynamics
- and much more!
Don’t miss this informative presentation – Secure your spot today!
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One of the tough challenges in creating meshes for CFD simulations is the requirement to create a mesh that works with very different geometry. With Overset meshing you can create the ideal mesh for each piece of geometry in your model, and let them overlap where they touch and the program handles the calculations at those boundaries. All of this is handled simply in the ANSYS Workbench interface and then combined in ANSYS FLUENT.PADT-ANSYS-Fluent-Overset-Meshing-2017_07_05-1
If you do CFD simulations then you know the struggle that is involved in meshing. It is a fine balance of accuracy, speed, and ease of set up. If you have complex geometry, large assemblies, or any difficulty meshing then this blog article is for you.
Why should I spend time making a good mesh?
The mesh is arguably one of the most important parts of any simulation set up. A good mesh can solve significantly faster and provide more accurate results. Conversely, a poor mesh can make the simulation have inaccurate results and be slow to converge or not converge at all. If you have done any simulation then you know that hitting the solve button can feel like rolling the dice if you don’t have a robust meshing tool.
When is it going to matter?
A good mesh is going to matter on a Friday afternoon when you need to get the simulation started before you leave for the weekend because it takes two days to run and you need to deliver results on Monday but you are up against the clock because you have to get to your kid’s soccer game by 5pm and the mesh keeps crashing.
A poor mesh can do more than just reorganizing you’re social agenda. A poor mesh can drastically change results like pressure drop in an internal flow passage or drag over a body. If you go into that meeting on Monday and tell your boss that the new design is going to perform 10% better than the previous design – you need to be confident that the design is 10% better not 10% worse.
What should I do when I need to create a good mesh?
If you’re the poor soul reading this on a Friday afternoon because you are trying to frantically fix you’re mesh so you can get your simulation running before the weekend – I pity you. Continue reading for my proprietary step by step approach titled “How to get you’re CFD mesh back on track!” (Patent pending).
Step 1) Know your tools
ANSYS has been developing its meshing technology since the beginning of time (not really but almost) – it’s no surprise that its meshing algorithms are the best in the business. In ANSYS you have a large number of tools at your disposal, know how to use them.
The first tool in your toolbox is the ANSYS automatic meshing technology. It is able to predictively apply settings for your part to get the most accurate automatic mesh possible. It has gotten so good that the automatic mesh is a great place to start for any preliminary simulations. If you want to get into the details, ANSYS meshing has two main groups of mesh settings – Global Meshing Parameters and Local Meshing Parameters. Global mesh parameters are great for getting a good mesh on the entire model without going into detailed mesh settings for each part.
But when you do have to add detailed meshing settings on a part by part basis then local mesh settings won’t let you down.
Step 2) Know your physics
What is your primary result of interest? Drag? Pressure drop? Max velocity? Stagnation? If you can quantify what you are most interested in then you can work to refine the mesh in that region so as to capture the physics accurately. ANSYS allows you set local sizing parameters on bodies, faces, lines, and regions which allow you to get the most accurate mesh possible but without having to use a fine mesh on the entire part.
Step 3) Know your mesh quality statistics
Mesh quality statistics can be a good way to gauge the health of your mesh. They are not a foolproof method for creating a mesh that will be accurate but you will be able to get an idea of how well it will converge. In ANSYS meshing there is a number of mesh quality statistics at your fingertips. A quick and easy way to check your mesh is to look at the Minimum Orthogonal Quality statistic and make sure it is greater than 0.1 and Maximum Skewness is less than 0.95.
Step 4) Know your uncertainty
Every test, simulation, design, process etc… has uncertainty. The goal of engineering is to reduce that uncertainty. In simulation meshing is always a source of uncertainty but it can be minimized by creating high quality meshes that accurately model the actual physical process. To reduce the uncertainty in meshing we can perform what is called a mesh refinement study. Using the concept of limits we can say that in the limit of the mesh elements getting infinitely small than the results will asymptotically approach the exact solution. In the graph below it can be seen that as the number of elements in the model are increased from 500 – 1.5million the result of interest approached the dotted line which we can assume is close to the exact solution.
By completing a mesh refinement study as shown above you can be confident that the mesh you have created is accurately capturing the physics you are modeling because you can quantify the uncertainty.
If you currently just skip over the meshing part of your CFD analysis thinking that it’s good enough or if your current meshing tool doesn’t give you any more details than just a green check mark or a red X then it’s time dig into the details of meshing and start creating high quality meshes that you can count on.
For more info about advanced meshing techniques in ANSYS – see this PDF presentation that is a compilation of ANSYS training material on the meshing subject.Advanced Techniques in ANSYS Meshing_Blog
If you still haven’t figured out how to get your mesh to solve and its 5pm on Friday see below*
*Common pitfalls and mistakes for CFD meshing:
- Choose your turbulence model wisely and make sure your mesh meets the quality metrics for that model.
- Make sure you don’t have boundary conditions near an area of flow recirculation. If you are getting reverse flows at the boundary then you need to move your boundary conditions further away from the feature that’s causing the flow to swirl in and out of the boundary.
Sometimes you want to take two parts and and prepare them for meshing so that they either share a surface between them, or have identical but distinct surfaces on each part where they touch. In this simple How-To, we share the steps for creating both of these situations so you can get a continuous mesh or create a matching contact surface in ANSYS Mechanical.PADT-Presentations-Grey_White-Wide
Occasionally when solid geometry is imported from CAD into ANSYS SpaceClaim the geometry will come in as solids, but when a mesh is generated on the solids the mesh will appear to “leak” into the surrounding space. Below is an assembly that was imported from CAD into SpaceClaim. In the SpaceClaim Structure Window all of the parts can be seen to be solid components.
When the mesh is generated in ANSYS Mechanical it appears like the assembly has been successfully meshed.
However, when you look at the mesh a little closer, the mesh can be missing from some of the surfaces and not displayed correctly on others.
Additionally, if you create a cross-section through the mesh, the mesh on some of the parts will “leak” outside of the part boundaries and will look like the image below.
Based on the mesh color, the mesh of the part in the center of the assembly has grown outside of the surfaces of the part.
To repair the part you need to go back to SpaceClaim and rebuild it. First you need to hide the rest of the parts.
Next, create a sketch plane that passes through the problem part.
In the sketch mode create a rectangle that surrounds the part. When you return to 3D mode in SpaceClaim, that rectangle will become a surface that passes through the part.
Now use the Pull tool in SpaceClaim to turn that surface into a part that completely surrounds the part to be repaired, making sure to turn on the “No Merge” option for the pull before you begin.
After you have pulled the surface into a solid, it should like the image below where the original part is completely buried inside the new part.
Now you will use the Combine tool to divide the box with the original part. Select Combine from the Tool Bar, then select the box that you created in the previous step. The cutter will be activated and you will move the cursor around until the original part is highlighted inside the box. Select it with the left mouse button. The Combine tool will then give you the option to select the part of the box that you want to remove. Select the part that surrounds the original part. After it is finished, close the combine tool and the Structure Tree and 3D window will now look like the following:
Now move the new solid that was created with the Combine tool into the location of the original part and turn off the original one and re-activate the other parts of the assembly. The assembly and Structure Tree should now look like the pictures below.
Now save the project, re-open the meshing tool, and re-generate the mesh. The mesh should now be correct and not “leaking” beyond the part boundaries.
Overcoming convergence difficulties in nonlinear structural problems can be a challenge. I’ve written a couple of times previously about tools that can help us overcome those difficulties:
- Overcoming Convergence Difficulties in ANSYS Workbench Mechanical, Part I: Using Newton-Raphson Residual Information
- Overcoming Convergence Difficulties in ANSYS Workbench Mechanical, Part II: Quick Usage of Mechanical APDL to Plot Distorted Elements
I’m pleased to announce a new tool in the ANSYS Mechanical tool belt in version 17.0.
With version 17.0 of ANSYS we get a new meshing option for structural simulations: Nonlinear Mechanical Shape Checking. This option has been added to the previously available Standard Mechanical Shape Checking and Aggressive Mechanical Shape Checking. For a nonlinear solution in which elements can become significantly distorted, if we start with better-shaped elements they can undergo larger deformations without encountering errors in element formulation we may encounter fewer difficulties as the nodes deflect and the elements become distorted. The nonlinear mechanical setting is more restrictive on the element shapes than the other two settings.
We’ve been recommending the aggressive mechanical setting for nonlinear solutions for quite a while. The new nonlinear mechanical setting is looking even better. Anecdotally, I have one highly nonlinear customer model that reached 95% of the applied load before a convergence failure in version 16.2. That was with the aggressive mechanical shape checking. With 17.0, it reached 99% simply by remeshing with the same aggressive setting and solving. That tells you that work has been going on under the hood with the ANSYS meshing and nonlinear technology. By switching to the new nonlinear mechanical shape checking and solving again, the solution now converges for the full 100% of the applied load.
Here are some statistics using just one measure of the ‘goodness’ of our mesh, element quality. You can read about the definition of element quality in the ANSYS Help, but in summary better shaped elements have a quality value close to 1.0, while poorly shaped elements have a value closer to zero. The following stats are for tetrahedral meshes of a simple turbomachinery blade/rotor sector model (this is not a real part, just something made up) comparing two of the options for element shape checking. The table shows that the new nonlinear mechanical setting produces significantly fewer elements with a quality value of 0.5 or less. Keep in mind this is just one way to look at element quality – other methods or a different cutoff might put things in a somewhat different perspective. However, we can conclude that the Nonlinear Mechanical setting is giving us fewer ‘lower quality’ elements in this case.
|Shape Checking Setting||Total Elements||Elements w/Quality <0.5||% of elements w/Quality <0.5|
Here are images of a portion of the two meshes mentioned above. This is the mesh with the Aggressive Mechanical Shape Checking option set:
The eyeball test on these two meshes confirms fewer elements at the lower quality contour levels.
And this is the mesh with the Nonlinear Mechanical Shape Checking option set:
So, if you are running nonlinear structural models, we urge you to test out the new Nonlinear Mechanical mesh setting. Since it is more restrictive on element shapes, you may see longer meshing times or encounter some difficulties in meshing complex geometry. You may see a benefit in easier to converge nonlinear solutions, however. Give it a try!
It’s not a series of articles until there’s at least 3, so here’s the second article in my series of ‘what not to do’ in ANSYS…
Just in case you’re not familiar with thin sweep meshing, here’s an older article that goes over the basics. Long story short, the thing sweep mesher allows you to use multiple source faces to generate a hex mesh. It does this by essentially ‘destroying’ the backside topology. Here’s a dummy board with imprints on the top and bottom surface:
If I use the automatic thin sweep mesher, I let the mesher pick which topology to use as the source mesh, and which topology to ‘destroy’. A picture might make this easier to understand…
As you can see, the bottom (right picture) topology now lines up with the mesh, but when I look at the top (left picture) the topology does not line up with the mesh. If I want to apply boundary conditions to the top of the board (left picture), I will get some very odd behavior:
I’ve fixed three sides of the board (why 3? because I meant to do 4 but missed one and was too lazy to go back and re-run the analysis to explain for some of future deflection plots…sorry, that’s what you get in a free publication) and then applied a pressure to all of those faces. When I look at the results:
Only one spot on the surface has been loaded. If you go back to the mesh-with-lines picture, you’ll see that there is only a single element face fully contained in the outline of the red lines. That is the face that gets loaded. Looking at the input deck, we can see that the only surface effect element (how pressure loads are applied to the underlying solid) is on the one fully-contained element face:
If I go back and change my thin sweep to use the top surface topology, things make sense:
The top left image shows the thin sweep source definition. Top right shows the new mesh where the top topology is kept. Bottom left shows the same boundary conditions. Bottom right shows the deformation contour.
The same problem occurs if you have contact between the top and bottom of a thin-meshed part. I’ll switch the model above to a modal analysis and include parts on the top and bottom, with contact regions already imprinted.
I’ll leave the thin sweeping meshing control in place and fix three sides of the board (see previous laziness disclosure). I hit solve and nothing happens:
Ah, the dreaded empty contact message. I’ll set the variable to run just to see what’s going on. Pro Tip: If you don’t want to use that variable then you would have to write out the input deck, it will stop writing once it gets to the empty contact set. Then go back and correlate the contact pair ID with the naming convection in the Connections branch.
The model solves and I get a bunch of 0-Hz (or near-0) modes, indicating rigid body motion:
Looking at some of those modes, I can see that the components on one side of my board are not connected:
The missing contacts are on the bottom of the board, where there are three surface mounted components (makes sense…I get 18 rigid body modes, or 6 modes per body). The first ‘correct’ mode is in the bottom right image above, where it’s a flapping motion of a top-mounted component.
So…why don’t we get any contact defined on the bottom surface? It’s because of the thin meshing. The faces that were used to define the contact pair were ‘destroyed’ by the meshing:
Great…so what’s the take-away from this? Thin sweep meshing is great, but if you need to apply loads, constraints, define contact…basically interact with ANYTHING on both sides of the part, you may want to use a different meshing technique. You’ve got several different options…
- Use the tet mesher. Hey, 2001 called and wants its model size limits back. The HPC capabilities of ANSYS make it pretty painless to create larger models and use additional cores and GPUs (if you have a solve-capable GPU). I used to be worried if my model size was above 200k nodes when I first started using ANSYS…now I don’t flinch until it’s over 1.5M
Look ma, no 0-Hz modes!
- Use the multi-zone mesher. With each release the mutli-zone mesher has gotten better, but for most practical applications you need to manually specify the source faces and possibly define a smaller mesh size in order to handle all the surface blocking features.
Look pa, no 0-Hz modes!Full disclosure…the multi-zone mesher did an adequate job but didn’t exactly capture all of the details of my contact patches. It did well enough with a body sizing and manual source definition in order to ‘mostly’ bond each component to the board.
- Use the hex-dominant mesher. Wow, that was hard for me to say. I’m a bit of a meshing snob, and the hex dominant mesher was immature when it was released way back when. There were a few instances when it was good, but for the most part, it typically created a good surface mesh and a nightmare volume mesh. People have been telling me to give it another shot, and for the most part…they’re right. It’s much, much better. However, for this model, it has a hard time because of the aspect ratio. I get the following message when I apply a hex dominant control:
- The warning is right…the mesh looks decent on the surface but upon further investigation I get some skewed tets/pyramids. If I reduce the element size I can significantly reduce the amount of poorly formed elements:
- That’s going on the refrigerator door tonight!
And…no 0-Hz modes!
- Lastly…go back to DesignModeler or SpaceClaim and slice/dice the model and use a multi-body part.
3 operations, ~2 minutes of work (I was eating at the same time)
That’s a purdy mesh! (Note: most of the lower-quality elements, .5 and under, are because there are 2-elements through thickness, reducing the element size or using a single element thru-thickness would fix that right up)
Phew…this was a long one. Sorry about that. Get me talking about meshing and look what happens. Again, the take-away from all of this should be that the thin sweeper is a great tool. Just be aware of its limitations and you’ll be able to avoid some of these ‘odd’ behaviors (it’s not all that odd when you understand what happens behind the scenes).
This video shows you a new capability in ANSYS v15.0 that allows multiple parts to be simultaneously meshed on multiple CPU cores…with no additional licenses required!