# y+ in a train surface

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 January 26, 2009, 05:10 y+ in a train surface #1 Lottar Guest   Posts: n/a Hi!! I'm working with a train model and i wanted to prepare the grid in order to have a y+ close to the surface equal to 100 (because of other studies performed and capacity...) anyway, i know that the computation of the size cell should be like that: d/2=y=(y+·viscosity)/u_tao where u_tao is calculated as sqrt(Cf·u_infinity/2) my question is what is Cf and how to get this?? i know there are expressions depending of the flow (laminar or turbulent) but for a U_infinity=40 m/s which one should i use??? thanks in advance!!! Lottar

 January 26, 2009, 05:57 Re: y+ in a train surface #2 Magellan Guest   Posts: n/a Hi Lottar u+ and y+ non-dimensional forms are usually used for turbulent flows, with the definitions: u+ = u_mean/u_star y+ = y*u_star*rho/mue with wall friction velocity u_star = sqrt(tau/rho) and tau = mue*du/dy @ the wall (y=0) The link to the skin friction is: Cf = 2*tau/(rho*U^2) You should calculate also the Reynolds number (Re=rho*L*U/mue) to see what flow regime you have (laminar or turbulent). But as I mentioned at the beginning u+ and y+ values are mostly used for turbulent flows, so far as I know. Hope this helps! M.

 January 26, 2009, 06:06 Re: y+ in a train surface #3 Lottar Guest   Posts: n/a Thanks Magellan!! But in this case is difficult to know which is du/dy at y=0 and so on, so the best way to get y+ i found was with Cf=0.0588/Re^(1/5) since a turbulent boundary layer as u say will appear and the surface on the train could be considered as a flat plate... what do u think guys? cheers!

 January 26, 2009, 08:25 Re: y+ in a train surface #4 Timon Guest   Posts: n/a Hi Lottar, Just make a coarse approximation, create a grid accordingly, perform a simulation and adjust your grid based on the computed and desired y+ values.

 January 26, 2009, 18:14 Re: y+ in a train surface #5 Lottar Guest   Posts: n/a Ok, i'll try in this way...thanks Timon!

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