Im having trouble with meshing in general
 

Hello guys, I'm fairly new and fresh to the CFD world. I just graduated as an Aerospace Engineer and I'm trying to get better at Ansys. I took one course in CFD simulation however it does not cover the many cases you might face in the Aerospace Industry. I am having general trouble generating meshes as I don't know what should be placed in areas such as the local sizing mesh size and the other options such as surface and volume mesh in (Fluent Meshing). Currently, I have a case that I'm running to get better and also participating in a competition. The case is our self-designed UCAV. During the meshing, I'm having trouble such as having parts of the wing and tail chipped off. I managed to fix the tail problem by decreasing the local sizing on it however the wing problem just arises out of nowhere for some reason. Also, Ansys Fluent meshing seems a bit moody as the mesh for the wing was perfect previously however when I fixed the problem for the tail it decided to ruin the mesh for the wing. I don't know if this is normal when starting in CFD or not but its driving me crazy not being able to "learn" this from anywhere as every video I watch or course I come across don't explain the things I want to understand such as how to calculate the mesh size for my specified body and a good tutorial on Spaceclaim and Fluent Meshing.

So any good advice from the experienced people here would be appreciated.

Thanks.

Welcome to the most difficult part of CFD. Meshing is unfortunately as much of an art as it is a science. I can pass along a few tips I've learned over the years which may or may not help.

1) Geometry quality is king. The majority of trouble in meshing I see from students, and in my own work, come from poor geometries. Most meshing software can deal with limited geometric errors through mesh defeaturing, but these can only go so far. The majority of my time when setting up a problem is in Spaceclaim checking every face and edge. You can use some of the built in tools like "Bad Faces", "Inexact Edges", or "Sharp Faces". These can help you identify geometry errors before getting into meshing. Another big aspect of geometry prep is using your engineering intuition to know where you can approximate geometry and when it needs to be explicit. A good example of this is adding a small radius to the trailing edge of a wing. In the real world a trailing edge is never perfectly sharp. Adding a small radius can help solve some discontinuities in boundary layers and other mesh refinement. Use your intuition to learn where to add radii, where to remove small features, or other geometric changes to make meshing easier.

2) There are no rules. One of the most challenging problems with meshing is knowing how much to refine and when the mesh is "Good enough". Unfortunately there are no quick shortcuts to determining a proper mesh. Beyond estimating Y+ values, the rest of the mesh quality is mostly determined through a "guess and check" process often referred to as a Mesh Refinement or Mesh Independence Study. Learning how to perform one can save you a lot of time fussing around with large, needlessly refined meshes.

3) Try different meshing softwares. Fluent meshing is relatively new to the game and, in my experience, pretty limited in usability. Try using the mechanical meshing in workbench or even a third party software which can export a .msh file for Fluent. Each software has its strengths and weaknesses. You may find one works better for one problem and one works better for another.

4) Learn the math. With all CFD knowing the practice without understanding the theory can lead to a lot of frustration and poor results. With meshing there is no rules, but you can learn what the mesh generation is trying to calculate. Learn how the mesh metrics (Orthogonal quality, skewness, etc.) are calculated. Know the differences between quad, tri, or poly elements and which are best for certain applications. Learn the rules that do exist like how to estimate the first layer thickness for Y+ and how the Y+ value is being used for the actual calculation. Finally, not for the faint of heart, you can dive down the rabbit hole of mesh generation algorithms to see exactly how a software descritizes geometry and generates a mesh.

I hope these tips are at all helpful. As I said earlier, mesh generation is as much an art as a science. There's really no better way to learn then just trying things yourself and learning from your experience. For practical examples YouTube or the Ansys Tutorials are great resources. For the more fundamentals of CFD and meshing algorithms there are a few good books out there. One book I often refer back to is Computational Fluid Dynamics: A Practical Approach by Jiyuan Tu. It provides a practical dive into the math of both the physics and the mesh generation.