Mesa thinks you talk too much

(1). How do I get rid of this virus attached to my posting? (2). Stop reading the posting, and you will get your peaceful mind back. You are playing back your old message from your brain. I normally don't keep track of my e-mail, I delete the mail after I have opened it. (3). I must say that, once the message is in your mind, it will last for the rest of your life. Don't read everything people posted here. (4). Anyway, it is not my intention to occupy your brain memory space. I don't know how to erase it for you. Sorry. (5). The only message I have is : " In real world, without a systematic protection approach, blindly running a commercial CFD code is just like reading a posted message, both can be hazardous to your health." (if you go back to read my previous posting, many times, or one hundred times, you are guaranteed to become much smarter. In the first 30 passes, it will become mechanical. In the second 30 passes, it will become automatic. In the last 30 passes, it will start to become innovative. I will have to stop here for my lunch. good luck for the exercise.)

John C. Chien 's MSG on Aug 20

Re: cheers to Jonas (292) - John C. Chien, Fri, 20 Aug 1999, 1:20 p.m.

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(1). It is quite expensive to post it here. It is going to cost a lot world wide. (2). Send him a e-mail next time. (3). Personal issue by e-mail, technical issue by the forum here, not vice versa. ------------------------------------------------------------

Re: cheers to Jonas (253) - John C. Chien, Sun, 22 Aug 1999, 11:24 a.m.

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(1). I have tried to keep this forum going for over a year now. (2). I think it is time for me to step aside so that sponsor of the commercial CFD codes can dedicate their time to answer questions created by the use of their codes (official of non-official, legal or illigal copies). (3). Will meet you sometime in the future. take care. -----------------------------------------------------------

(1). Please Do Not Read my Posting. Thank you.

(1). Please Do Not Read my Posting. Thank you. (2). There is nothing in it.

Thank God you finally got Addy's point for most of your postings.

(1). Who's Addy ?

I thought you know better than everybody. If you don't know, I thought Addy should be your father. But don't trust anybody or anything as Dr.Hrvoje Jasak suggested to you:

Posted By: Dr. Hrvoje Jasak <aeg98@dial.pipex.com>

Tue, 31 Aug 1999, 2:17 a.m.

A few more (cynical) comments to Mr. John C. Chien: 1) Well done, mate, you're thi first person on the planet with a bug-free CFD code (of, I guess, about 200k lines). Maybe you should sell it and make a fortune! 2) Some commercial codes are actually involved with things you see around you every day, so take my advice and: a) stay out of airplanes; b) stay out of cars; c) underground is a big no-no; d) I'd think twice before switching that computer on (ventilation); e) stop breathing, air-conditioning is screwy; f) lawn-mowers have engines and therefore are unsafe; g) electicity? uh-oh; h) even the vegetables (not to mention beer) in your fridge are not safe!! The best thing is to take a dive in the pool and relax... but even your swimming goggles are designed for minimum drag!

A gentleman from NEC (the japanese company) gave a presentation here (penn state) this summer about sliding/interfacing mesh LES code they use for design. one of the cases he presented was of a bullet train entering a tunnel. the results looked pretty good (experimental comparison) and they were able to use it to make practical design changes. Jog\hn has a big beef with commercial codes, i think because he's pissed he didn't jump on the train and get some money. but anyway for engineers who want to do something, instead of worrying about whether their pressure distributions are within 3% of experimental results (especially since most of the time you don't have resources to do experiments anyway), try a reputable commercial code. and John if you don't intend to answer someone's question don't post to that thread. if you have a beef start another thread. Jonas lets us post just about anything we want so if you have something to say, say it but not in a thread started to answer a question

(1). I can only guess that you must be from the vendor side answering questions. (2). If that is the case, it is a good sign for the forum. Don't worry about me, I simply don't exist at all.

(1). Yes, this is exactly what I said before :" It is going to cost a lot world wide." (2). And You have just done that. (3). Emotional approach simply can not solve any CFD problem. (4). The first step to do is to have a open-source CFD code. So, nobody has to keep asking these mechanical questions related to a code. It is not fair for the users to spend a lot of time debugging codes using try-and-error approach. You simply can't sell a black box to general public without any support.

(1). I agree with you. (2). I think it is a good idea. (3). It is a small world, and I'd better spend more time doing my own book reading. Thank you for the idea.

If you were not starting 'emotional' blah blah blah everytime, nobody would.

Guys (and girls!),

Let's not get carried away here: what I really want to make clear is that commercial CFD codes are not perfect, BUT for a large majority of the users they are fit for the purpose, and at least can give ~some~ insight where we can't do experiments (burning real cars in a real tunnel is definitely not a good idea, no matter how much experimental data you get ;-). However, no software I know (with the exception of TeX) is perfect; it may not even be fit for the problem you're trying to solve - all we can do is to try and make it better. So, when you want absolutely the best results and you don't care what it costs (I know such situations!), you cannot replace the "DIY" CFD people: they are highly skilled, very professional and (judging by myself) usually have a huge ego BUT they are able to beat any (general purpose) CFD code for their particular problem. Because of the huge experience of these people, they are usually extremely useful to newsgroups like ours, because "they've been 'round the block a few times" and seen some really weird stuff. Also, in my experience, it is always more fun writing your own (preferably big!) CFD code and beat the commercial opposition into the ground. But the fact remains: a commercial code will have 2 or 3 orders of magnitude more users than "your own" code, no matter how good it is.

I would like to apologise for getting side-tracked, but I think we all need to let John know that we appreciate his experience and would like to hear more about the stuff he's really good at (there's always a lot to learn!). In the future I myself will try to stick to the technical points.

Using STAR-CD code written by Imperial College of England or whatever, I really do'nt see any sign of reaching the STAR. Only thing I can see when I close my eyes is more Penalty! Penalty!! & Penalty!!! for screwing up in design again. Good luck to all STAR-CD fan.

LES, sliding meshes, trains, burning cars..... In an application such as ventilation of a road tunnel it would be a fruitless task to try to define an accurate geometry of the moving/burning cars. In reality the number and speed of the vehicles would vary tremendously. If you accept that you can not define an accurate geometry then you really should:

a) halt the quest for quantative accuracy and go for qualitative CFD design instead.

b) dump LES and sliding meshes and chose a more appropriate 'balanced' selection of numerics. If the geometry itself is 'blurred' or approximated then there really is no point going for a fancy turbulence model for example.

c) have a clear understanding of exactly WHY you are conducting the simulation then use your intuition to define the model.

d) Hey, why not consider representing the (car) occupied space of the tunnel with a porous media, an increase in turbulence and maybe also a collection of point sources of contaminant. If you turn your nose up at such approximations then a place in academia awaits you, if you're nodding your head even slightly then welcome to the wonderful world of (commerical?) CFD application.

Robin.

The question was not if it is usefull to use sliding mesh ... but if it is possible.

In a first stage one can use the sliding mesh approach to see what happens (for some obstacles/cars). Later one should try to find easier ways to achive the same result.

If you start with a simplification you cannot be sure if it works at all.

i agree with you that qualitative results are better than nothing and that the models you describe are appropriate for that purpose but in some cases the need for detailed reesults for detailed design is necessary. you made a comment about LES and i think it was directed at my previous message. i was just trying to relay what the person who made the presentation said not trying to endorse his approach. for the things he was trying to do ( detailed modifications) he did need precise fluid results. he said that his department was having too much problems with the complex turbulence models needed for the RANS so they went to LES because they felt the results were more reliable. i cannot evaluate that statement because i don't know enough about LES to make that judgement. i do know though that RANS results are highly turbulence model dependent. as for the sliding meshes they implemented that capability because the codes are used for design of a wide range of products (from trains to vacuum cleaners were the examples we were shown). i disagree mostly with your point (1) even though in the past i have said that qualitative results are of great value. there are many cases particularly in detail design where precise dense data is required. it would be nice if this wasn't the case (less multi-megabyte log files) but it is. the need to continue improving the accuracy and reliablity of CFD codes is still there and will continue to be for some time yet.

Try this web site to see if it can help you or others

http://icemcfd.com/cfd/CFD_codes.html

(1). I heard that people here are getting rid of the S-code. (2). I guess that there must be reasons. So, they brought in this T-code. The T-code is like an orphan now, because the instructor is trying to push the use of their C-code. But so far it is not operational yet. I can't use any word freely, because of my reputation and my influence.