# NACA4415 Airfoil / Y+ Value Lift'n'Drag

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 June 5, 2017, 15:34 NACA4415 Airfoil / Y+ Value Lift'n'Drag #1 New Member   Join Date: Jun 2017 Posts: 6 Rep Power: 6 Hello everyone, I do a student research project at the moment. The topic is to calculate the lift and drag-coefficients of a NACA4415 Airfoil with/without winglet and to create the polar-diagramms. I use the k-w model, the freestream velocity is 95 km/h the length of the airfoil 1.5m. I asked a prof of mine in which way the Y+ Value is important to calculate the coefficients. He meant its ok to have a Y+ between 30-70 or 30-100. We have had a training course where i was said to use a Y+ less than 1. This would be very important. On the internet I found out that a Y+ less than 1 would be important at heat transfer and the drag-coefficient. Is it really necessary to have such a low Y+ for right results? Thank You

 June 12, 2017, 02:22 #2 Senior Member   Colinda Join Date: May 2012 Location: Brussels Posts: 139 Rep Power: 12 Indeed for a correct drag prediction it is important to have low y+ values IF you use a turbulence model without wall functions. But be careful; it is not the only thing that is important. Very often the focus is so much on Y+ that the rest is forgotten. E.g. the expansion ratio should not be too high either. Also it would be important to have a sufficiently fine mesh in the wake. It would be wise to perform a mesh dependency study.

 June 12, 2017, 04:23 #3 New Member   Join Date: Jun 2017 Posts: 6 Rep Power: 6 Thank you for the fast answer. I'm using a turbulence model with wall condition. I will try to use a Wall function 1 or 2 mm (or should it be less for example 0,5?) thick with 5 Layer. I would choose a lowest thickness round about 0.05mm. That gives me a Y+ < 1 on the Wing Surface. I have big Expansion ratios from the Wall. I made 3 areas. The inner with 50mm, middle 250 and outher 1250mm, for reasons of better Performance. I could also use more areas to get a lower ratio. The Flow room is 10 times the lenght of the Wing. Thats important for the Lift Report. Sent from my D5803 using CFD Online Forum mobile app

 June 12, 2017, 04:51 #4 Senior Member   Colinda Join Date: May 2012 Location: Brussels Posts: 139 Rep Power: 12 When you write "wall condition" do you mean wall boundary condition? A wall function is different: in the viscous sublayer it allows to approximate the boundary layer by a function instead of solving the Navier-Stokes equations inside it. It allows to work with higher Y+ values at the wall outside of the viscous sub-layer. Expansion ratios should be really small in the boundary layer (approx. 1.2 to 1.5 would be good)

 June 12, 2017, 05:11 #5 New Member   Join Date: Jun 2017 Posts: 6 Rep Power: 6 Yes thats what i meant. I understood it wrong because I was so focused on the mesh. I will look it up if a wall function is used i'm not sure yet. So to get a lower ratio i should create a continous change in the mesh size, getting fine at the Wing Surface? Sent from my D5803 using CFD Online Forum mobile app

 June 18, 2017, 13:31 #6 New Member   Join Date: Jun 2017 Posts: 6 Rep Power: 6 Ok i looked it up, i'm using a wall function. Also the graphs of Lift and drag follow the right pattern, so i think its correct, thank you now i understand it better! Sent from my D5803 using CFD Online Forum mobile app colinda1 likes this.

 Tags y+;lift;drag;airfoil;naca