# General questions on grid-based computing

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 May 26, 2000, 10:38 Re: General questions on grid-based computing #4 John C. Chien Guest   Posts: n/a (1). You are talking about a very big issue, that is the grid generation in general. (2). In 2-D, you have only a few lines or curves to act as the boundary of the geometry. So, you can easily create a mesh or subdivide the domain with lines or curves and generate a multi-block mesh. (3). In 2-D, if you decide to take the numerical mesh generation approach instead of the algebraic method, then you have to solve a couple of partial differential equations. Then, the computing time of the mesh generation would be the time to solve the set of partial differential equations. So, it is faster to use algebraic mesh generation methods. (4). Since the mesh generation requires the distributions of nodal points along the boundary curves, the construction of the geometry, the visualization of the mesh and interactive operation with the mesh code, you will have to include these in the time estimation. Even though some mesh generation codes can automatically generate the final mesh, it is not push button operation. (5). The movement of a mesh, either due to the boundary conditions, the solutions, or the smoothing operations, require the similar operations as in the mesh generation. In other words, one can use either algebraic methods or numerical methods to readjust the mesh points or cells. (6). In 3-D, the problem is still related to the complexity of the geometry itself. Unless you are dealing with straight lines and flat plane surfaces, the geometry modeling alone will take one most of his time in generating the mesh. This is because a quality mesh does not like a highly skewed cell, and in order to reach this goal, both the geometry topology and the mesh topology (multi-block) must be studied in great detail first. (7). Then even after the creation of the first mesh, the subsequent adjustment to the mesh is even more difficult. In some cases, the nodes distribution along the boundary curve must be re-adjusted from patch to patch. And this can only be done looking at the 2-D screen and trying to change the setting with mouse. (8). So, if you are using the numerical grid generation apporach, the mesh generation will be slow. Otherwise, you would be spending a lot of time trying to identify which curve is which on the flat 2-D screen among many interconnecting curves, vertices, surfaces. (9). So, the 3-D geometry (you need this part ) and mesh generation is a very difficult problem to solve. So far, the approach is interactive, and it requires a lot of experience. (10). The unstructured mesh approach will reduce the overall time, but still, if you get some bad cells in a complex mesh problem, the solution to improve it is sometimes hard to find.

 May 26, 2000, 11:11 Re: General questions on grid-based computing #5 Steve Amphlett Guest   Posts: n/a I guess this is a troll. Are we supposed to all go marching to www.aplied-scientific.com and discover grid-less CFD?

 May 26, 2000, 11:46 Re: General questions on grid-based computing #6 CONSULTANT Guest   Posts: n/a Hey, Wait a minute ! WHAT IS GOING ON HERE.... Adrin Gharakhani, Sc.D. the person who started this message chain - ISN'T HE FROM APPLIED-SCIENTIFIC ITSELF ????? Check http://www.applied-scientific.com/Employment.html CONSULTANT

 May 26, 2000, 12:49 Re: General questions on grid-based computing, only the question is important. #7 John C. Chien Guest   Posts: n/a (1). In the cfd-forum, only the question and answer is important. The name is not. (2). Sometimes, it is possible to find a closed form solution. (3). Sometimes, it is possible to use singularity distribution to obtain a solution. (4). And in most cases, we have to subdivide the complex computational domain into tiny cells or grids to search for approximate solutions. (5). So, a name is nameless, unless the Q & A promotes the use of CFD. And even after that, a name is nearly nameless. (6). By the way, geometry and mesh is the essential part of modern CFD. The grid without a solution is only geometry, and the mesh independent solution will stay unchanged regardless of how one refine its mesh. So, the mesh becomes invisible. And the mesh is important because most of the time, the solution is mesh dependent. It's not only visible, but also affect the solution.

 May 26, 2000, 14:09 Re: General questions on grid-based computing - No Offence Please... #8 CONSULTANT Guest   Posts: n/a NO Offence, Please. I just pointed out the coincidence. I agree that just the contents are important and not the names. Even though people who have visited Applied-Scientific homepage might have noticed the coincidence, I just thought of pointing it out. Well. Let me stop here on this subject. CONSULTANT

 May 26, 2000, 15:37 Re: General questions on grid-based computing #9 Adrin Gharakhani Guest   Posts: n/a > I guess this is a troll. Are we supposed to all go marching to www.aplied-scientific.com and discover grid-less CFD? This is the most unprofessional insult on this forum, yet,and absolutely irrelevant to the questions I asked. Adrin Gharakhani

 May 26, 2000, 15:44 Re: General questions on grid-based computing #10 Adrin Gharakhani Guest   Posts: n/a > the person who started this message chain - ISN'T HE FROM APPLIED-SCIENTIFIC ITSELF ????? Yes. Was this information ever hidden from anyone? (it's clear from the email address in these messages) As John Chien astutely pointed out, does it matter who posted the questions? I asked these questions because I needed to know what the state-of-the-art is, since I've been away from it for 10 years now! And yes ... Adrin Gharakhani, Sc.D.

 May 26, 2000, 16:09 Re: General questions on grid-based computing #11 Adrin Gharakhani Guest   Posts: n/a Thanks for the valuable input Adrin Gharakhani

 May 26, 2000, 16:25 Re: General questions on grid-based computing #12 Adrin Gharakhani Guest   Posts: n/a > O(N^2/3) (you spend most of the time on the surface mesh generation). Very interesting! Is the concern the correct specification of the geometry and making sure there are no incompatibilities, or does it somehow have to do with volumetric meshing? That is, do you need a good surface mesh in order to avoid degeneracy within the fluid domain or is there something more? > More important for the time spent on grid generation is the problem you want to solve ... I am looking into LES ... Obviously, if one is to claim accuracy one needs, at the very least, to reduce numerical diffusion - even with LES. So, I'd like to have adaptivity with automatic mesh generation before implementing any LES. The problems of interest are unsteady flows in complex geometries. Then, in this case, what percentage of the flow simulation would be spent on automatic mesh generation and what percentage on adaptivity? (as a rough estimate) Perhaps, directly related to the latter is the question of the poisson solver. Are there complications in this case for parallelization? > When you do surface mesh adaptation, how do you keep your original definition of the surface (which may be a CAD drawing or analytical functions in space) Is this the complexity in surface meshing you alluded to earlier? Thanks for the input Adrin Gharakhani

 May 26, 2000, 16:27 Re: General questions on grid-based computing #13 Adrin Gharakhani Guest   Posts: n/a Thanks for the input Adrin Gharakhani

 May 26, 2000, 17:29 Re: General questions on grid-based computing #14 Jonas Larsson Guest   Posts: n/a Eh, the original message didn't mention any web site and included a few very specific questions that certainly are of general interest to most CFDers - one of the best questions I've seen here lately. Adrin has also contributed a lot to this forum before by answering other peoples questions. Hmm, I just realised I fell for your "troll" by answering your post .

 May 27, 2000, 00:19 Re: General questions on grid-based computing #16 CONSULTANT Guest   Posts: n/a I am SORRY that I didn't notice your e-mail I-D. I should have checked before sending out that message. The moment I saw the web-address of Applied-Scientifice, I visited the site and I checked out the employment page. I was surprised and sent-out my message, thinking that it might have been a coincidence, in an act of impulse. It was not my intention to divert the issue from genuine cfd discussions. You can see from the very few messages that I had posted. Sorry if I have hurt your feelings. CONSULTANT

 May 27, 2000, 02:21 Re: General questions on grid-based computing #17 Adrin Gharakhani Guest   Posts: n/a Apology, of course, accepted. Now back to what really matters - the original questions Adrin Gharakhani

 June 2, 2000, 15:16 Re: General questions on grid-based computing, a very big issue. #20 John C. Chien Guest   Posts: n/a (1). I am not trying to answer your questions. I am just making some comments. (2). As I said before, this is a very big issue. (3). The grid quality issue was studied for more than 20 years with the "numerical mesh generation". In this approach, the mesh quality will be much better than that produced by the algebraic methods. The cost of the computing time is relatively high. you will have to solve several partial differential equations. The mesh quality is only part of the issue. And much of the effort and progress has been done by Joe Thompson and associates. We can say that this is the top of the line approach. (4). As I said before, a mesh without solution is just geometry. Even in that case, generating hundred of thousands of x,y and z in space itself is not trivial. (5). In order to make some sense out of a mesh, a good solution has to be generated. The adaptive mesh becomes the other part of the issue. This also has been studied for more than 20 years. (6). 3-D geometry is not a simple issue. A good 3-D mesh takes time to generate. A meaningful 3-D mesh which can provide accurate 3-D solution requires knowledge about the solution itself. (7). If one is still looking for a solution, then he has very little knowledge about it. Then he will have very little information to guide him to generate a good mesh. (8). Thus, the fine quality of "numerical mesh generation" remains largely a geometry issue. And the adaptive mesh without a good solution for guidance, simply does not know the right place to go. (9). If you adapt the mesh to the wrong solution, are you going to get back the right solution? (10). The problem is "we don't know the solution, because we are looking for it". And if you have the solution, then we would not be looking for it. And when a joke cease to be a joke, the problem is solved.

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