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-   -   Could you comare StarCD with CFX 5?Help, please... (https://www.cfd-online.com/Forums/cfx/18673-could-you-comare-starcd-cfx-5-help-please.html)

Suteh October 9, 2001 15:24

Could you comare StarCD with CFX 5?Help, please...
 
Dear collegues, Could you give view on StarCD and CFX 5? What's "better" and "more strongly"? Our university want to buy licence of one of those but we want to do the best choice. Our specialists and I too didn't use those CFD codes. We deal with heat exchanger tasks (especially complicated external streamline, free surface and porous solids with thermal flux). I'm impatiencing yours answers. Thanks.

Ribeiro October 10, 2001 09:18

Re: Could you comare StarCD with CFX 5?Help, pleas
 
It's hard to say which CFD code is "better" or "more strongly". I think we could say that it is suited to model a process or not.

I don't know well the StarCD but I always hear good words about it.

I use CFX-5 and I know Fluent. I could say that for your needs the CFX-5 is the better choice. Besides the models and multiphase flow the CFX solver is completely coupled and parallel. Thus you have results faster than others softwares. I have seen benchmarks that prove this.

This is a personal opinion but I guess CFX more friendly...

I hope it helps.

Suteh October 10, 2001 16:46

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Thanks Ribeiro. I was glad to get your answer. Those were usful. And I've got some quastions. (If you don't mind) Has CFX 5.4.1 advanced post-processor? I wonder shall I use other codes as one? What duration do you spend to get result of task of average complication? What part does preparing take of the duration?

Ribeiro October 10, 2001 17:33

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Ok, Suteh (It's really don't bore me!)

Well, a new version of CFX-5 will be released on october 2001, CFX-5.5. This version has much more features than CFX-5.4.1. The post-processor is the CFX-Post. Very very good. It is a real advance.

Of course, if you have money to spend, two or more codes are recommended. With this you can compare and take the potentialities of each code.

I have done a watergate simulation (2 phases: air and water) with more or less 300,000 cells. The setup part was really fast (without CFX knowledge would spend 1 afternoon) and the real transient simulation for 10 s take 1 night (8 to 10 hours). The same problem with CFX-4 (a seggregate solver as Fluent and Star-CD). I took 4 to 5 days.

I hope it helps.

Bart Prast October 11, 2001 04:36

Re: Could you comare StarCD with CFX 5?Help, pleas
 
I work(ed)with both starcd and cfx 5. To put it in a nutshell: CFX solver is superior and really fast. It is easy to get a solution quickly (automated unstructured meshing) STAR-CD is more of an expert tool. Takes longer to generate a grid but is very flexible and complete (physical modelling wise). CFX is developing rapidly to include more models (turbulence, multiphase). For our application (transonic interior flows) I like CFX 5 best.

Suteh October 11, 2001 15:38

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Hi, Ribeiro Thanks you. Have you ever used nonstationary boundary condition doing any task by CFX? I've heard there's no that possibility. Is it correct?

Suteh October 11, 2001 15:45

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Hi, Bart Prast, thanks.

Robin Steed October 11, 2001 22:48

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Hi Suteh,

I am a CFX employee, so you can take my comments with the necessary grain of salt :). Before coming to CFX a year ago however, I was a Star-CD user and thus have a good base for comparison.

A thorough discussion of the two codes could go on forever, and as Ribeiro has pointed out, the question is really "What code can do your job well?". I will point out some of the major differences which come to my mind.

Others may want to comment on these, and given there are many aspects to a CFD code, I will break my discussion into 3 main categories: i) Pre-processing, ii) Solver, iii) Post-processing.

Here we go...

Robin Steed October 11, 2001 23:14

Pre-processing
 
Geometry: Pro-Star has some geometry import and creation utilities, but tools are limited.

The CFX-5 pre-processor, CFX-Build, is based on MSC-Patran. As such, it includes a complete CAD package and all the geometry import capabilities of MSC-Patran.

Units: StarCD users must take care to enter values in a consistent set of units. This is a major source for error.

Model geometry and boundary conditions may be specified in <bi>any</bi> units. All boundary conditions, fluid properties, etc. have units attached. A list of default units are provided at each dialog, but the user may specify the units by assembling them from other base units, or an expression whose results are in the correct units. This unit conversion/conservation capability is found throughout CFX-5.

Expressions: There is no facility to enter expressions for boundary conditions in StarCD, other than perhaps using a user FORTRAN routine.

In CFX-5, expressions may be specified using the CFX Expression Language (CEL) and used for boundary conditions. CEL has access to many solver variables and advanced functions. Functions include standard trig functions, exponentials, step functions and others. In CFX-5.5 it is also possible to write an expression for one boundary which is dependant on an averaged quantity from another boundary!

Meshing: StarCD includes some rudimentary meshing tools, but users typically depend on a third-party meshing tool to do the job.

CFX-5 includes an advanced advancing front mesh generator which allows generous control of meshing parameters. The CFX-5 mesher will also extrude the surface mesh for a number of layers (called inflation) to provide better discretization in boundary layers.

These would be the major differences which come to my mind. As a general comment, I find the CFX-Build pre-processor to be much more intuitive than Pro-Star. If I had to choose the most important difference, it would be the unit conversion/conservation capability.

Robin

Robin Steed October 12, 2001 00:04

Solver
 
The most significant technology of any CFD solution is the solver. Provided that a user can set up the necessary grid and boundary conditions, the solver can make or break any simulation. Here are a few differences between the CFX-5 solver and StarCD.

Fundamentals: StarCD solves finite volume based equations on a staggered grid. That is to say that the Momentum equations are solved at cell vertices and the pressure equation is solved at the cell center. There is no coupling between the equations, so although iterations do not take long, convergence will not be achieved quickly.

CFX-5's is a <u>coupled</u> element based finite volume method. The Pressure and Momentum equations are coupled and solved at the same location.

Solver: StarCD employs an iterative solver. By iterative I mean that the solver sweeps through the entire grid at each iteration. As a consequence, the solution time will increase exponentially with grid size.

CFX-5 employs an unstructured multigrid solver. Between each timestep, the multigrid solver agglomerates cells into successively coarser grids, solving an error equation at each level until at the coarsest level a direct solver may be used. This results in rapid convergence and a constant solution time per node (ie double the grid size and the solution only takes twice as long, not 2^4).

Execution: The StarCD solver must be compiled for every new grid. Not only does this require that you have the necessary compiler to run StarCD.

CFX-5 does not. Enough said!

Extension: If a StarCD user wishes to introduce a source term, or solve any additional equations in the course of a run, they must modify a User Fortran routine and recompile.

CFX-5 provides the CFX Expression Language (CEL) and CFX Command Language (CCL) to extend the capability of the code. Within the pre-processor, or from the definition file, a user may write an expression (with units of course:) to be evaluated in the course of a run. This expression may interact with momentum, energy, pressure equations as desired. Prior to CFX-5.5 (which introduces combustion among other features), combustion equations could be entered and solved by CCL. If a user must perform complicated logic, the CCL provides a junction-box capability to link to pre-compiled shared libraries prepared by the user (CFX-5.5).

Parillelization: StarCD runs in parallel, but the set up is painful and is next to impossible to run on multiple platforms.

CFX-5's parillelization has been implemented at a highly abstracted level (developer speak:). The parallel performance is superb, in some cases providing super-linear performance! Parallel setup is very simple; users simply click on a button and select the systems they wish to run on (mix and match Irix, Sun, NT, Linux...). Memory overhead is also very small in parallel, thus x processors results in 1/x the serial memory requirement for each processor.

Discretization: StarCD is very difficult to converge using a second order scheme and next to impossible to start 2nd order without a good initial guess. UDS is generally the desired scheme, particularly when a tetrahedral mesh is used.

CFX-5 features a 2nd order high resolution discretization (basically a bounded second order scheme). The solver can be started from scratch using this scheme and performs beautifully. We do not recommend converging solutions using UDS as first order results will be unreliable

I could go on, but I'll be lucky if you read this far. Suffice it to say that the solver is very accurate, robust and fast.

Robin

Robin Steed October 12, 2001 00:19

Post-processing
 
So you set up your problem and ran it to convergence, what about the results? A post-processor is vital for extracting the results of your solution.

I won't go far here. Having used Pro-Star, I can confidently say that Ensight and Fieldview's bread and butter is StarCD users. Pro-Star is poor at best; graphic objects are difficult to create and manipulate, quantitative post processing is ridiculous. Only single operations may be performed at a time, therefore to evaluate a simple expression, the user must break that expression into a sequence of +'s /'s *'s and so on. I've seen machine language which is more readable (well not really, but close :)

CFX-5's old post-processor, Visualize, had excellent graphic capabilities but was poor when it came to quantitative results (though still much better than Pro-Star). CFX-Post, the next-generation post-processor for CFX-5 is unbelievable! As with the rest of CFX-5, units are obeyed. Users can create whatever expressions they wish, and units are calculated automatically. Expressions are entered in the same CEL used elsewhere in CFX-5 (pre, solver, post).

Macros for CFX-Post (known as session files) can be created automatically by recording the users actions. The standard language is CCL and CEL, but programming structures in macros may be added by using PERL launguage constructs (why re-invent the wheel, Eh?).

(Oh yeah, I forgot to mention that these PERL constructs (known as Power Syntax) can also be used in the solver definition file to execute PERL commands from the solver!)

Well I think that is plenty for now. If any of you have more questions or comments, I encourage you to respond.

Best regards, Robin

Bart Prast October 12, 2001 04:16

Re: Post-processing
 
Being an CFX 5 user (and former STAR-CD user) I agree with 90% of your story. However, the fact that STAR-CD relied on user FORTRAN was a BIG plus for me. To be able to write your own subroutines outside the CFD package is very good (and will be in CFX 5). I'm not quite convinced that CEL is very convinient if you have large and complex source terms f.i.

But the solver is superb!

Robin Steed October 12, 2001 08:56

Re: Post-processing
 
Hi Bart,

You're absolutely right. The lack of user fortran up until 5.5 has been a hinderance. However, other than doing very complicated logic (beyond step functions), there is little one needs user fortran for.

Robin

Suteh October 14, 2001 07:08

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Hi, Robin Steed I'm sorry my answer is belated. Thank you for detailed and very interesting answer. It has helped me seriously to recommend to my chief to choose CFX (undoubtedly). Besides we've had talks with CFX distributor. They gave me and my colleagues useful information. Best regards.

Robin Steed October 14, 2001 20:58

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Hi Suteh,

Glad I could help.

Best regards, Robin

Ribeiro October 15, 2001 09:31

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Hi, Suteh

What you mean nonstationary boundary? The boundary properties vary with time or it has a velocity?

regards

Ribeiro

pop October 15, 2001 10:59

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Robin, very through comparison. Thanks.

Can you comment in the same manner about CFX and Fluent. My university in "thinking" about Fluent however I thing it is very weak solver. What do you think?

Pop

cfd guy October 15, 2001 11:10

Re: Solver
 
Mr. Steed,
As a new user in CFX-5, I would like to discuss some aspects about your comments. . .
In Pre-Processing, at least the way I've been building my solids, I still could not obtain an inflated layer near the walls (prisms elements) and I still don't know its reasons. So I have been running my cases only with tetraedral elements, which is generated inside the geometry given a Maximum Edge Length (BUILD 5). Yes, I can run hexaedral meshes too, I also have been greatly solving CFX-4 meshes in CFX-5, but sometimes your geometry cannot be broken in several blocks to have a structured mesh using BUILD-4. Now, we're acquiring CFX-HEXA hoping that those issues will be gone and also hoping that this pre-processor will be as useful as BUILD (4 or 5) are to us.

In the discretization aspect, unfortunately the 2nd order scheme cannot always be started from the scratch. We work here with an specific problem which we must start the 2nd order discretization from a 1st order results file.
To finalize, I would like to discuss one more thing. When I work with CFX-4, segregated solver, imagine that I spend 'n' days to achieve a converged solution, a transient turbulent flow in a mesh containing 50k cells (a good number of cells to my problem). We had migrated to CFX-5, due its coupled solver which is much more faster thant the segregated one and due a lot of features, etc... Also imagine that I built a tetraedral mesh with 70k cells, which I classify a coarse grid to this case, and I run in paralel (2 jobs in the same computer).
So, if I submit these two jobs in two computers (PC with about 800MHz and 512Mb RAM each). In one computer, running CFX-4, the same case and mesh commented above, I spend 'n' days to solve it, and in another computer, a dual pc, running CFX-5 paralel (2 jobs), solving the same case but in a tetraedral mesh. I can say that, in my short experience time as CFX-5 user, I spend '2n' days to solve the same case in this code. Why two times more than CFX-4 and running in paralel mode?
I agree with you when the coupled solver is much more faster than the segregated, but is the coupled solver more stable? This case mentioned, in CFX-4 I use 10^-2 as timestep, whereas I must use timesteps in order of 10^-4 to converge the solution in CFX-5, that's why it costs much more days to me. If you ask me if I performed a benchmark, given the same mesh (hexaedral), comparing CFX 4 against 5, in terms of computational efforts, I still not had time to do it.
Well, I guess I'll be going now... Please, let's expand on this... Regards,
cfd guy


Dan Williams October 15, 2001 15:35

Re: Solver
 
> In the discretization aspect, unfortunately the 2nd
: order scheme cannot always be started from the
: scratch. We work here with an specific problem which
: we must start the 2nd order discretization from a
: 1st order results file.

Odd, I'm not sure what you might be doing to have this problem. Maybe your timestep is too large? With the second order scheme it's not a good idea to use more than 1/5th of the characteristic L/V scale for your problem. Additionally, if you are running incompressible flows, try a fixed blend factor of 0.75 instead. The monotone scheme really only matters for compressible flows, and occasionally the "switchiness" of the non-linear limiter can create convergence problems, allthough it's rare.

As far as your comparison with CFX-4 goes it sounds to me like your comparing apples and oranges. You really need to take the exact same grid in both codes and solve both problems using the same convergence criteria. CFX-4 will be faster per iteration, but how you define convergence and select your timestep will affect time to convergence.

CFX-5 should be able to handle timesteps which are on the order of 1/5th to 1/3rd of the characterisPOST http://www.cfd-online.com:80/Forum/cfx.cgi?post HTTP/1.0 Content-length: 1645 Content-type: application/x-www-form-urlencoded Cookie: CFX User Forum

Dan Williams October 15, 2001 15:37

Re: Solver
 
> In the discretization aspect, unfortunately the 2nd
: order scheme cannot always be started from the
: scratch. We work here with an specific problem which
: we must start the 2nd order discretization from a
: 1st order results file.

Odd, I'm not sure what you might be doing to have this problem. Maybe your timestep is too large? With the second order scheme it's not a good idea to use more than 1/5th of the characteristic L/V scale for your problem. Additionally, if you are running incompressible flows, try a fixed blend factor of 0.75 instead. The monotone scheme really only matters for compressible flows, and occasionally the "switchiness" of the non-linear limiter can create convergence problems, allthough it's rare.

As far as your comparison with CFX-4 goes it sounds to me like your comparing apples and oranges. You really need to take the exact same grid in both codes and solve both problems using the same convergence criteria. CFX-4 will be faster per iteration, but how you define convergence and select your timestep will affect time to convergence. Keep in mind that the two codes use completly different approaches to numerics (CFX-4 being cell-centred and segPOST http://www.cfd-online.com:80/Forum/cfx.cgi?post HTTP/1.0 Content-length: 1772 Content-type: application/x-www-form-urlencoded Cookie: CFX User Forum

Dan Williams October 15, 2001 16:05

Re: Solver
 
> In the discretization aspect, unfortunately the 2nd
: order scheme cannot always be started from the
: scratch. We work here with an specific problem which
: we must start the 2nd order discretization from a
: 1st order results file.

Odd, I'm not sure what you might be doing to have this problem. Maybe your timestep is too large? With the second order scheme it's not a good idea to use more than 1/5th of the characteristic L/V scale for your problem. Additionally, if you are running incompressible flows, try a fixed blend factor of 0.75 instead. The monotone scheme really only matters for compressible flows, and occasionally the "switchiness" of the non-linear limiter can create convergence problems, allthough it's rare.

As far as your comparison with CFX-4 goes it sounds to me like your comparing apples and oranges. You really need to take the exact same grid in both codes and solve both problems using the same convergence criteria. CFX-4 will be faster per iteration, but how you define convergence and select your timestep will affect time to convergence. Keep in mind that the two codes use completly different approaches to numerics (CFX-4 being cell-centred and segPOST http://www.cfd-online.com:80/Forum/cfx.cgi?post HTTP/1.0 Content-length: 1766 Content-type: application/x-www-form-urlencoded Cookie: CFX User Forum

John C. Chien October 15, 2001 19:52

Re: Solver
 
(1). My experience with CFX-TASCflow was: It is very difficult to get the calculation started, especially when two-layer turbulence model was used for general flow problems. (wall function might be faster but in most cases it is not appropriate, thus useless) (2). The code would fail right at the begining of iterations, and very small time steps are required (sometimes 10^-6) with many steps of constant restart using different time steps. (3). If you are lucky, once you have converging solution, most of the time it will converge faster.( but this does not include the initial trial-and-error to get it started) (4). I have used the code for almost two years day and night, so it is not my theory. (4). I would say that for simple problem, with no flow separation, you will get faster solution with a fine mesh. Coupled or implicit solver is always slower for coarse mesh solution .(fewer mesh points) (5). My feeling is, coupled, accurate, implicit methods are good for converging solutions. These are not good methods for bringing the solutions into converging states. (6). The only meaningful approach is to do head-on contest, using various codes or methods on the identical problem, which is the so-called benchmark test. (7) in other words, it is meaningless to talk about the advantages of a code over another one without a well-defined benchmark test, where the total time and accuracy of the solution must be addressed.

Robin Steed October 15, 2001 23:48

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Well Pop,

I can't comment about Fluent as extensively as StarCD, as I have had only limited exposure to it. Fluent is certainly better than StarCD in terms of pre and post processing. I would still put CFX above it in both, but not as strongly as I did for Star.

Discretization

The solver is a different story, however. Fluent, like StarCD, solves segregated equations iteratively on a staggered grid.

CFX-5's coupled multigrid solver will converge much more quickly and provide a more accurate solution. CFX has the most advance discretization schemes and solvers available.

Judging Convergence Another point is convergence. Judging convergence is very important. StarCD provides requires an absolute residual to judge convergence by. This makes it very difficult to be sure you have sufficiently converged, as the necessary absolute residual will change from one simulation to another.

Fluent does slightly better; by default Fluent will normalize by the largest residual in the first 5 iterations. There is also an option to choose an iteration residual to normalize by. The problem with this approach is also it's dependance on simulation; also, if you restart, or start with a better initial guess, you cannot be sure just how good your convergence is.

If you think about it, what you really want to know is how far off you are from machine round-off error. If your residuals are approaching machine round-off, you will not converge further (except perhaps to run with double precision). If you have no perspective of how far off your are, you may perceive that your run has stalled, but really it just a limitation of the computer.

In CFX-TASCflow and CFX-5, the residual is normalized by the largest (magnitude +ve or -ve) of coefficients in each coefficient matrix, or by the range of coefficients. Actually, it's still more complicated than that, but suffice it to say that a normalized residual of 1e-7 is always approaching round-off. For most simulations, a maximum normalized residual of 1e-4 is sufficient, 1e-5 is being anal, and 1e-6 ... well you get the idea. RMS residuals tend to be an order of magnitude smaller. Basically, this makes it easier for you to know when your solution is numerically accurate (physical accuracy will depend on grid density and discritization ...). Convergence can be further judged by conservation.

There are other differences too, such as the availability of total pressure boundary conditions, mass flow boundary conditions which can be used for compressible flow, among others.

That said, Fluent does have it's pro's. It has many more user oriented models than CFX-5. CFX-5 is catching up quickly in this area. CFX-5.5 (to be released later this month) introduces many new models. It's been late coming, CFX-5 development was focused on the core solver up until 5.4.1, but all the pieces are now in place and they are really rolling.

The entire architecture is focused on expansion; the parallelization of the solver has been implemented at a highly abstracted level, resulting extremely good parallel performance. The influence of ASC on CFX is noticeable by the numerical methods behind the solver. Our turbulence modelling is done by none other than Dr. Florian Menter (inventor of the k-omega-SST model).

The pace of development behind CFX-5 is tremendous. CFX-5.5 is awesome, and there's a lot of great stuff already planned for 5.6!

If anyone else has other comparisons I welcome their comments.

Best regards, Robin

cfd guy October 16, 2001 10:54

Re: Solver
 
Thanks Dan,
Someday I'll try a fixed blend factor.
I'll try decreasing, even more, my timestep when I start from the scratch using 2nd order scheme. But I'm afraid that I will spend even more time than I restart using a previous 1st order solution.
Regards,
cfd guy

Joern Beilke October 16, 2001 13:18

Re: Could you comare StarCD with CFX 5?Help, pleas
 
When I read your comments then I get the impression, that you never found out, what you can do with Prostar for Pre- and Postprocessing.

It is also interesting that you forgot to mention Proam (Samm) - the "auomatic" meshing tools around StarCD. Is this some sort of intention or do you just have a very limited insight in StarCD.


John C. Chien October 16, 2001 14:05

Re: Solver
 
(1). Well, I think he is very happy, because the job of a salesman is to get you keep using his products, the longer the better. Make sure that you don't run out of problems.(2). As long as they keep inventing new versions of the code, and you keep using their codes, then someone should be very happy, right? (3). It is very much like an "ideal perpetual machine".

Robin Steed October 16, 2001 15:08

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Joern,

No need to be confrontational here :) I declared from the start that I am a CFX employee and to take it as you wish. It's been just over a year since I last used Star, so they may have introduced new features since then. Proam was not available when I last used Star.

I am fully aware of the pre and post processing capabilities of Prostar and stand behind my statements. If you have anything constructive you are certainly welcome. I am not going to tell you that CFX is a panacea, every code is limited in some way and we certainly have our limitations. I am simply offerening my perspective.

Kind regards, Robin

Joern Beilke October 16, 2001 18:02

Re: Could you comare StarCD with CFX 5?Help, pleas
 
SAMM (now ProAM) is available since about 1997.


convergence man October 16, 2001 18:22

Re: Could you comare StarCD with CFX 5?Help, pleas
 
In fairness to StarCD, I would think it more professional to leave the abuse/applause to the user's of StarCD and CFX as opposed to the 'biased' opinions of a competitor... It would be as a Boeing engineer pointing out the failings of the (General Dynamics) F-16 to the pilot of the F-16, provided the pilot was an experienced flyer.

Besides, Robin, you appear much more as an authority on CFX and perhaps should stick with that. Thanks for your inputs there and keep up the good work...


John C. Chien October 16, 2001 18:38

Re: Could you comare StarCD with CFX 5?Help, pleas
 
(1). Are you sure that your university knows how to use the commercial CFD codes properly? (2). Making a copy of a commercial CFD code available on a computer really does not solve any of your problems at all. (3). You must have qualified engineers in the first place. Since you and your specialists don't use commercial codes, do you use any CFD codes at all? (4). It is really very cheap to have two codes at the same time, if you really want to solve your problem. And this will prevent unnessary promotion of codes in the forum. Most companies using commercial CFD codes have more than one commercial CFD codes. This is necessary, because each code has its own limitations. And based on the training of the users, not everyone like to use the same code. This can be very ineffective. (5). Since I am writing already, why not give your specialists a chance to use a various codes for a while on their own problems first, then ask their opinion about their own preference of the code. Does your policy allow this approach?

Bart Prast October 17, 2001 07:59

Re: Could you comare StarCD with CFX 5?Help, pleas
 
I used pro-am and was very dissapointed with the quality of the grids it produced. It always needed a lot of repairing to get rid of a lot of bad cells. I'm now using CFX5 (saved us a LOT of grid generation time: days versus hours)

Suteh October 17, 2001 17:38

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Our university has limited funds. We can't use two or tree codes (at least legal). Commercial code license costs too expensive for us. And even our sponsor can "overpower" only one code (although that's dolefully). Of course, we've got own "narrow" programs for some necessary task. But that's not that I want. You try to claim me on promotion of codes in the forum. It's too simply to write (but promotion's usually done otherwise completely). I'd like to know users opinions (and I cost money of our sponsor). You have had the chance to claim other users in false (if you want to do it). Besides my actions will force one creator to run down competitors (maybe) for user benefit.

Dan Williams October 17, 2001 17:58

Re: Solver
 
CFD Guy (Do you have a real name?):

You shouldn't have to endlessly reduce your timestep. With a reasonable initial guess and proper boundary conditions setup, 1/5th to 1/3rd of the characteristic L/V scale should work just fine.

Dan.

Dan Williams October 17, 2001 17:59

Re: Solver
 
John,

Most of the time, startup problems are almost entirely related to the quality of the initial guess. What were you typically using? For example, if it was a zero velocity field, then you might have serious problems starting up a compressible flow.

Dan.

John C. Chien October 17, 2001 21:58

Re: Solver
 
(1). I think, you are right. I am always dealing with very complex problems.

John C. Chien October 17, 2001 22:02

Re: Could you comare StarCD with CFX 5?Help, pleas
 
(1). Yes, the cost is one thing. And the proper matching between the code and the user's experience is another important factor to get the right solution.

star-cd user October 18, 2001 07:27

Re: Solver
 
There are a few inaccuracies in your comments about STAR-CD.

1. Fundamentals: STAR-CD is the first commercial code which uses collocated (non-staggered) variable arrangement for finite volume solution method. 2. Solver: It has a capability of using AMG solver. 3. Discretisation: The 2nd order discretisation scheme MARS is very robust and bounded scheme which guarantees the 2nd order accuracy for all types of meshes. 4. Extension:STAR-CD user coding facility is the most popular and used feature amongst users.

.......

star-cd user October 18, 2001 07:54

Re: Could you comare StarCD with CFX 5?Help, pleas
 
1. Discretisation: Again you are wrong here. Fluent does not use staggered grid. 2. Judging Convergence: Steady State convergence check in STAR-CD is not based on absolute residuals but on normalised residuals using appropriate normalisation factors for every variable. Convergence to 1e-3 level is normally sufficient for most enginnering problems. Machine accuracy can easily be achieved by lowering the residual tolerance.

Roued October 18, 2001 12:05

Re: Could you comare StarCD with CFX 5?Help, pleas
 
Hi,

Since this discussion has been going so far I would like to give my comments as well.

I have been working with CFD in different areas both as a scientist and now as a commercial user. And first with CFX , then Star and now CFX again, but not for long. I was working on a national research program using cfd to understand different aspects of ventilation inside buildings. In this research program two universities and a national research institut work together. Back in 1994, we all started to use cfx, but was general unsatisfied with the pre-/post processor and user support so after three years we split. I went to use star and the two universities got fluent. At a must better price then when using cfx. since we are talking about the overall cost of using cfd.

At first I think that it would be the best if you would make a tryout of the two software, and also known what your area of research will be and get a good discussion with the salesman for demonstration etc. Do not choose the software just on what people say ! Get to know the software !

The overall cost of the software can be very very different. I am speaking both as a scientist and commercial user of CFD software. During my talks with the salesman from both star and cfx, - cfx has by far been the most expensive software.

Furthermore to use a commercial cfd software as john pointed out, experiance are needed !!! To get to known the kind of cfd software can take from 3 - 6 mouth, going from a beginnger to a more experiance user. This will in the end cost you some time, before you will feel at home in the software.

The pre-processing step be one of the mejor time consumning parts of using CFD. Both star and cfx has pro's and con's. But star will let you do very complicated things with the mesh, but an external meshing software can be of an advantage when working with CAD files. But building complex mesh in cfx5 is not at all easy !!! One major advantage of star has been that you are able to generate a meshed block bit by bit to finally have a hole geoemtry and then adding them together. This will bcome a obtion in cfx 5.5, but I do not know how if will work.

In cfx5 you have no detailed control over the tet meshing. Which can make a greate deal of different from working as a scientist and to a commercial user.

This was my few cent's

Regards

Roued

not a cfx user October 18, 2001 13:40

Re: Solver
 
This is interesting: one claims to be the ex-star user and even does not get the basics right. Can I trust with your CFX info?


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