Difference between ICEM and Gridgen
 

Could some one please tell me the differences between the way ICEM handles the bottom up approach of meshing and the way Gridgen does it.

I mean I see that even ICEM can support a bottom up approach like you create some initial blocking , then delete it but after such initiation of the block, ICEM makes it possible to build blocks from the vertices by using the option - " From Vertices/Faces " in the Create Block options also it is possible to extrude faces of a block in ICEM so how does this differ from the bottom up approach used by gridgen.

Yes, you can do bottom up in ICEM CFD, but it is still quite different from Gridgen. Gridgen (it has been a few years for me and I have not tried point-wise) has you build the edges and faces and then smooth them. From what I recal, you can actually build all the separate faces and then turn it into a block, There is also the extrude option, etc.

Maybe John C. or some other Gambit user can chime in with the basic process and comment more on the differences...

ICEM CFD Hexa can create "blocks" from corners or from faces, but those are meant to fill in gaps between other blocks. This is usually done as part of some elaborate blocking strategy that involves deleting blocks or to join two previously separate blockings... You can also start with 2D blocking and extrude or rotate it into 3D blocking, or you can take a 3D blocking and extrude a face to create an extra block... One of the support guys in our office has done some impressive bottom up blocking with the ICEM CFD "create elements" option and then converted those elements into blocks.

But 9 times out of 10, the top down process is faster and more stable (at least with ICEM CFD Hexa) than the bottom up process...

Never having used ICEM Hexa, I'm not in a position to compare and contrast the two methods.

However, I will respectfully disagree with Simon's assertion that the top down process is faster and more stable than bottom up in the majority of cases.

Well, both approaches have advantages and disadvantages. In top-down approach you can make the good topology and mesh at very fast pace. However for very complex geometries it is tedious to find the suitable topology in top-down approach for newbie.
In bottom up approach, you have the visual feeling while building the topology from connectors, domain and then blocks. So some times it is very much easy to work on the bottom up approach for new comers.
Another advantage of bottom-up approach is that, you can make the hybrid mesh very easily and with higher degree of control. So whenever you find it difficult to mesh the particular portion of geometry with hexa, you can use the tetra meshing with no special treatment.
ICEM bottom-top approach is not meant for the complete solution, it just complement the top-down approach. When used in standalone mode it may be proves to be difficult and poor approach when compared to gridgen's bottom-up (or gambit or ANSYS meshing) approach.
I agree with the Simon's comments that the top-down approach is very fast 9 out 10 times than bottom-up approach. Keep in mind this is only possible if you are expert enough in the top-down approach other wise it may be disastrous. On the other hand in bottom-top approach you will always reach the destination, no matter how much time it takes (maintaining the quality is difficult in bottom-up approach).
Suppose if there are two experts in bottom up approach (say Gridgen) and top-down approach (say ICEM CFD) and if we compare the speed (and quality obviously) then we term this racing as "racing of rabbit and turtle". So if rabbit keep on working then it will win the race otherwise he may not even be able to finish the race.

Comparison is possible when the things are same. Very difficult to compare to different approaches and software. If you are new, never use any software before, you must go to ICEM (hard to learn and time expensive) but high quality of meshing within few hours or even in minutes if you train yourself as an expert.

Gridgen and ICEM have different approaches. One may convenient with bottom-up approach and the other with top-down.

User has full control of meshing in Gridgen, can decide topology, know what is going-on, where is the problem, easily find the issues in the mesh and can easily solve that. Pointwise is more powerful than Gridgen except anisotropic meshing as it is not available in the current release, will be in Pointwise 17.

User is fully dependent on what is available in ICEM. Its automatic process, if anything goes wrong during blocking you have to delete everything and start from the scratch.

I am new to ICEM, may be I'm wrong, It's my experience, so don't mind.
Personally, I like ICEM. I have been working in Gridgen/Pointwise for last 7 years.

Best Regards

Ali

Just to be clear... When I said...

I was comparing ICEM CFD top down and ICEM CFD Bottom up...

I personally think the same point compares more generally, but I do not have sufficient expertise in actually using the other codes to state that outright ;)

When I did more customer demos years ago, we converted many users from "bottom up" methods based on our speed. Once you know some key tricks, like how to use Ogrids, top down is very powerful. The ability to move the blocking verts around and to apply a blocking topology to a variety of models is also a powerful speed advantage... But as FAR said, our bottom up has mostly been to "compliment" our top down...

The exception is the newer "MultiZone" technology. It is somewhat like bottom up, but since it is so automated, it doesn't feel that way. I heard that Pointwise has (or is developing) something similar based on their own technology...

In the end, having a few strong competitors in the market keeps us all working and is better for the end users.

Thank you for your answer, Far. This answer is very helpful information for me. I learnt ICEM and will need to usually generate complex geometry grids for aircraft design. Now I have both ICEM and Gridgen available to use. I recently heard about that Gridgen is better for complex geometry design. But according to your answer, shall I conclude that, even for very complex geometry structured grids generation, if I become an expert in ICEM, there won't be any need for me to learn Gridgen? Since you provide a lot very helpful comments on this forum, I trust the information from you very much.

And btw, what do see in the future of ICEM and Gridgen?

And I see that Gridgen has a lot of shortcuts which are really helpful on accelerating the grids generation operation speed. But i didn't find out shortcuts available in ICEM. So I regard this as a (small?) disadvantage of ICEM compared with Gridgen. Correct me if I'm wrong at this point of view.