# y+ vlaues

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 March 13, 2007, 11:58 y+ vlaues #1 Palani Velladurai Guest   Posts: n/a Hello ! I am doing my basic things in CFD, required for my thesis. And If anybody can expalin me , WHAT is the significance of y+ values in boundary layers?...or If you know any nice study material for this one...Please forward to me . Waiting for your replies......

 March 13, 2007, 13:06 Re: y+ vlaues #2 ag Guest   Posts: n/a Google "y+ CFD"

 March 13, 2007, 14:01 Re: y+ vlaues #3 Limonghi Guest   Posts: n/a hello. At the tutorial in CFX in the part of MODELLIGN there is something about Yplus. Yplus referes to a nonedimensional Number that measure the distance from the wall to the fisrt node. It helps to resolve the turbulence near de wall. As a parameter numeric it help you to the convergence. To SST for example its require a Yplus between 20 and 100. Remember that it is a valor near the wall. Be carefull about the flats elements. To control the Yplus control the INFLATION on the mesher.

 March 14, 2007, 07:59 Re: y+ vlaues #4 F.B.Tian Guest   Posts: n/a hi, It was discussed before.the following is some comments: The y+ can be considered similar to the Reynold's number. It is a dimensionless quantity constructed out of the distance from the wall and the friction velocity. As such in turbulence, the y+ is a measure of the resolution of the viscous sub-layer. In general, closer the first layer, lower the y+ and finer the grid to resolve the laminar sublayer. This is the fact most turbulence models concentrate on. The resolution of the laminar sublayer directly influences the wall stress computations and hence the skin friction distribution. The linear profile of the laminar sublayer demands that y+<5 (approx.). However, different turbulence models have their own restricions on the y+ values and this could go down as low as 1. However, I have found that algebraic turbulence models like the Baldwin-Lomax model gives resonably accurate results even for y+ in 7-10 range ( for airfoil computations), though this may not be acceptable to others. For specific details on the y+ in turbulence modelling, you could look into Wilcox. Also, for the physics of y+, I believe a good reference is Tennekes and Lumley. You could also refer to other books on Fluid turbulence for more details.

 March 16, 2007, 04:58 Re: y+ vlaues #5 Jonathan Guest   Posts: n/a Thank You Tian, Yours was the best ever explanation I read on Y+. Few days ago, I posted the same question.

 March 16, 2007, 08:36 Re: y+ vlaues #6 F.B.Tian Guest   Posts: n/a yes,you are right.I just show it to Palani Velladurai,I should said like this"the following is some comments posted few days ago by Jonathan:".

 March 17, 2007, 09:07 Re: y+ vlaues #7 Jonathan Guest   Posts: n/a Can you also explain on Y Star(y*)... I would be lucky if you do it...

 March 20, 2007, 17:03 Re: y+ vlaues #8 Mark Guest   Posts: n/a I agree with John, you made a good discussion. But what kind of y+ values should I expect in regions of incredibly high turbulence? I have a jet of water spraying on a plate and then turning around 180 degrees to go back out along the path it came in (outside the inlet tube). There is an incredible amount of turbulence (the actual device sounds like a freight train even though it's small). I've been poking around trying to fit a good turbulence model to this such that while the exact value isn't dead on from the real thing, it will give a nice trend we could use for future models. Also that plate is incredibly hot. Do boundary layers also effect the temperature profiles to the extent of having problems if the y+ is off?

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