# How to choose a BL dimensions..?

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 September 18, 2007, 17:40 How to choose a BL dimensions..? #1 Cyril Guest   Posts: n/a Hi, I'm working on a plane fuselage.. trying to simulate the fluid dynamics around it and find the size of the layer (to dimension the air inlet). I'd like to have your advice for 2 things: - Is the size of the volume enough for my calculations? (and do you know a method or some principles to determine a good volume (not too big, not too small) ?) - How to determine a good size for the BL? Do you think the one on my drawing is big enough ? Thanks for your help! Cyril

 September 18, 2007, 18:06 Re: How to choose a BL dimensions..? #2 Cyril Guest   Posts: n/a ...here is a pict of my geometry:

 September 19, 2007, 03:30 Re: How to choose a BL dimensions..? #3 mAx Guest   Posts: n/a Hi Cyril, I would say, your dimensions are to small (walls are to close to your fuselage. I would expand your box with a factor of 3. But I am not an extern-aero specialist. It should be confirm (or not), by another user Regarding your BL, you have to estimate your y+ and adjust the first layer size. http://www.cfd-online.com/Links/tools.html#yplus

 September 19, 2007, 08:05 Re: How to choose a BL dimensions..? #4 Cyril Guest   Posts: n/a Y+....... ok i'll try to find some informations on it. Could it be the reason why i have a "turbulent viscosity limited to viscosity ratio of 1.00000e+5 in 3921 cells" ??

 September 19, 2007, 08:10 Re: How to choose a BL dimensions..? #5 Cyril Guest   Posts: n/a what is exactly the y+...? Because the 2 tools on the link you gave me need a y+ input.. and i don't know what to put... Thanks

 September 19, 2007, 08:34 Re: How to choose a BL dimensions..? #6 mAx Guest   Posts: n/a it's a dimensionless variable which describes the boundary layer thickness. cf: log-law of y+ in turbulence theory. If you are interested, I can send you theory about it (in french ;o) ) Regarding the Turbulent Viscosity Warning, check that you well described the turbulent parameters in the BC.

 September 19, 2007, 08:36 Re: How to choose a BL dimensions..? #7 Cyril Guest   Posts: n/a Je veux bien cette doc ! Merci

 September 19, 2007, 08:42 Re: How to choose a BL dimensions..? #8 Kasper Guest   Posts: n/a y+ is a non dimensional distance from the wall (your fuselage surface) See a description here: http://www.cfd-online.com/Wiki/Dimen..._wall_distance If you want to use wall functions, let's say a "log law" you need to make sure that the first cell is in the logaritmic layer. This layer is normally starting at y+=30. So you should put y+=30 in the grid-space-calculator and further fill in the Re and length that you have based your Re on. Then you get a distance wherein your first cell should be. This is if you want to use wall functions. If you want to compute the boundary layer solution you should go as close as y+=5 and have 10 cells within this distance. y+=5 is probably a very small distance, it could be 0.0001m for instance. Then you should put y+=5 and make your grid have 10 cells with in the distance you compute using the grid-space-calculator. Hope this is enough information. BR Kasper Skriver - Denmark

 September 19, 2007, 08:52 Re: How to choose a BL dimensions..? #9 jack Guest   Posts: n/a moi aussi je serai bien interesse. Merci

 September 19, 2007, 08:58 Re: How to choose a BL dimensions..? #10 Kasper Guest   Posts: n/a I have one in danish explaining how to calculate the correlation between y+ and y, even with a small example..

 September 19, 2007, 21:03 Re: How to choose a BL dimensions..? #11 Cyril Guest   Posts: n/a hum... i'd need to learn danish then

 September 21, 2007, 08:42 Re: How to choose a BL dimensions..? #12 Cyril Guest   Posts: n/a Hi, I've just used the first link (3rd tool) with the total conditions (101325Pa/15°C (ground) Mach0.8, y+=5, Ref Length=20m (length of my fuselage) and i've found ds=1.3e-5m, Q=101325 and Re=2.3e8... I find these results a bit strange.. First if I understood well, ds should be the length of the first BL cell... 0.113mm is very small for a plane that is 20meters long! My computer's never gonna be able to compute that. Then the total pressure Q=the static pressure... (the Re is quite the same as the one I found using the formula Re=SpeedxLength/nu) Could you help me to "analyse" these results? Thanks mAx !

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