# Quick and easy QUESTION

 Register Blogs Members List Search Today's Posts Mark Forums Read

 December 8, 2005, 15:04 Quick and easy QUESTION #1 Freeman Guest   Posts: n/a While a real wind tunnel test of a car or a general vehicle that is considered as a bluff body, is vortex shedding present in the tests? If so, from my point of view, many simulations and papers that shows many vehicle manufacturers are not correct, as they do (3D) CFD simulations that are steady and the are not... is this statement correct? Thanks to all!

 December 8, 2005, 17:02 Re: Quick and easy QUESTION #2 Mani Guest   Posts: n/a If vortex shedding is present or not depends on the shape of the vehicle as well as on the Reynolds number (i.e. on the speed of the car, and on the air temperature). You are probably right that steady-state solutions will not give you very accurate results, expecially on the drag. Vortex shedding will lead to increased drag as compared o the steady state drag. So, your statement is correct, under the assumption that the Reynolds numbers in actual tests are high enough to result in shedding. Although I don't know any actual results, I would strongly expect that. Especially on very bluff designs like SUV's or trucks! However, I would think that the manufacturers provide the experimentally obtained drag (which naturally includes all flow features) rather than relying solely the CFD result.

 December 9, 2005, 14:20 Re: Quick and easy QUESTIONS #4 Jim_Park Guest   Posts: n/a The manufacturer's results, if measured, were on a 3-d car. Can your question be answered completely without doing the 3-d calculation?

 December 10, 2005, 09:39 Re: Quick and easy QUESTIONS #6 Freeman Guest   Posts: n/a Thank you very much, Mani and Jim_Park. Well, that's true: all manufacturers CFD results I've seen are in 3D and mine are in 2D. They are not comparable, but two questions arise after reading your replies: my 2D Steady, k-e Realizable, 2nd order scheme simulations gave my oscillatory results that showed me the unsteadyness of the case. Q1. I don't know if I've understood well one thing about the 3rd answer you gave me, Mani: althoungh I make plots between 2 iterations made with an "oscillating converged" STEADY RANS solution, they are not showing me nothing REAL, as the oscillation indicates me that I will get real results running an unsteady sim. Is this correct? I wish I could show you those plots, they are really disturbing to me... Q2. Two manufacturer's papers I have show that the 3D simulation finished with an STEADY Realizable and 2nd order schemes (it began with 1st order schemes to get an initial solution), but any paper shows any oscillation in their results (neither CD, Cl nor residuals) so I conlude there is not any vortex shedding present. The question is, it could be possible that because there are "more physics" implicated in a 3D CFD simualtion, it is easier to converge them than 2D ones? I mean, 2D simulations tends to instabilize the flow much more than 3D ones? Thanks a lot. You're really enlighten me!

 December 11, 2005, 16:58 Re: Quick and easy QUESTIONS #7 Jim_Park Guest   Posts: n/a This is not a criticism, just an observation that you've probably made as well. When driving behind a semi-trailer (a 'great bloody lorry' in Great Britain I think) at highway speed, the vortices shedding from the trailer really shake my little Accord. It's definitely unsteady flow behind those vehicles and speeds. Point being that there are flow conditions in the real world where the shedding is real and significant. Not all speeds, but some. If you're observing that some simulations (yours or the transporations folks') show no shedding (unsteady behavior), either the simulation boundary conditions (flow speed, etc) are not in the region where shedding occurs - OR the simulations are not realistic. The lack of realism may be 2D instead of 3D, or the code(s) is/are too diffusive, or the mesh is wrong, or ... .

 December 12, 2005, 12:26 Re: Quick and easy QUESTIONS #8 Freeman Guest   Posts: n/a OK Jim_Park, clear as water! Now I will extend my research to 3D simulations. But imagine 1 minute the following example I propose you to think about: now I go to 3D simulations with the same conditions as I introduced into my 2D simulations (same vel. inlet, turb.int, etc.) and, after converging a 1st order scheme simulation, then I switch to 2nd order and it also converges (as all manufacturer's papers I've seen). If I had this situation, how would it be explained? How a steady RANS in 3D can get a converged result and 2D sim. with the same conditions don't? With your last replies I guess that it could be explained as 2D sim. has (over)predicted a flow instability (vortex shedding) that in 3D -and also with many probabilities (because it is a 3D sim) in the reality- does not exist; and this is because 2D sims. are not as "real" as 3D. Do you agree with the last conclusions? If you want and the last statement was correct, you could give a more formal explanation , it would be realy enlightening to me! Thank you very much Jim_park for your replies. They are helping me a lot with the insights of my sims.

 December 12, 2005, 16:01 Re: Quick and easy QUESTIONS #9 Mani Guest   Posts: n/a >the vortices shedding from the trailer really shake my little Accord. It's definitely unsteady flow behind those vehicles and speeds. How do you know that your Accord does not shake in the steady bound vortices coming off the sides of the truck? The unsteadiness might actually originate as a reaction to the wakes, because of the presence of your car. Like you, I would expect that most likely the wake is unsteady, but if that is from actual shedding of vortices or just from swaying of bound vortices or from other sources is far from "definite"... which brings me to the point: Unsteadiness is not necessarily an indicator of vortex shedding. There could be something else going on that needs to be examined.

 December 12, 2005, 18:38 Re: Quick and easy QUESTIONS #11 Jim_Park Guest   Posts: n/a "How do you know that your Accord does not shake in the steady bound vortices coming off the sides of the truck? The unsteadiness might actually originate as a reaction to the wakes, because of the presence of your car." Caught again. I overstated the conclusion. Although I've felt the shake from a good ways behind the truck and I've observed straps or short pieces of rope flaying around, it's not conclusive. Guilty as charged.

 December 12, 2005, 18:48 Re: Quick and easy QUESTIONS #12 Jim_Park Guest   Posts: n/a The trouble is that I don't know what the manufacturers have published, how good a job they did with errors, resolution, and on and on. I also don't know if the referees were thorough. From what you're seeing, it appears that there are still some puzzles to be resolved. You're in the best position of anyone to work this thing out. There's surely a good paper there someplace. And of course, some valuable knowledge. Good luck.

 December 13, 2005, 16:00 Re: Quick and easy QUESTIONS #13 Freeman Guest   Posts: n/a Ok guys, last posts were really clear! Now I've got a wider knowledge about these fenomena that occurs in my 2D sims. and I'm ready to go and see what will happen in 3D ones. I'm really curious to get 3D results and compare them with 2D to assimilate/verify all discussed in this great thread you've done (at least it was great to me Many thanks Mani and Jim_Park!

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Dob Main CFD Forum 0 October 10, 2006 16:45 Freeman FLUENT 0 December 9, 2005 18:39 Lcw FLUENT 1 January 26, 2005 16:29

All times are GMT -4. The time now is 10:14.