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June 2, 2006, 14:59 
y+ for NACA0012 transonic simulations

#1 
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Hi,
I'm doing bidimensional transonic simulations over the NACA 0012 using a structured Ogrid. My Euler results are good, so I start my full NavierStokes simulation from the Euler result using the Baldwin & Lomax model. The final result is not bad, but the y+ for j=2 is too high(from 1 to 25), and theoretically there are no grid points on the viscous sublayer(that is, y+ less than 5). I start by decreasing the j=2 distance to 0.000003 (based on airfoil chord of 1), but the y+ values are still very high. What are the normal grid distances from the wall to j=2 for these type of simulations? Is there any way to reduced the y+ values without changing the flow properties? (Standard sea level temp and pressure) Thanks for your suggestions! 

June 4, 2006, 05:38 
Re: y+ for NACA0012 transonic simulations

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Dear Oscar Arias,
The typical y+ for algebraic models is expected to be around 3, so as to have a good resolution of the viscous sublayer. The y+ does depend on flow( through the friction velocity), but often in most simulations can be fixed reasonably apriori, through the grid, by adjusting the sdistance to the first layer, j=2 from the wall. Typically any value of the order of 1e4 to 1e5, with a reasonably good strectching factor of say 1.051.1 should do the job. Your choice of 3e6 is good, but also do look at the stretching ratio you have provided. A bad y+ reflects severely in the skin friction distribution, though y+ of 710 for BL seems to give reasonably accurate results from my experience. Regards, Ganesh 

June 5, 2006, 08:41 
Re: y+ for NACA0012 transonic simulations

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Thanks a lot for your answer Ganesh.
My Strechting values are on that range(from 1.04 to 1.1). I was testing my program with that laminar case of the boundary layer over a flat plate, trying to obtain the blasius velocity distribution. The result is similar to that found by Blasius(ploting u/Uinf VS eta ), but the friction coefficient is not good. I think that maybe there's an error on the derivates calculation. For the derivates I'm using the divergence theorem with an auxiliary cell around the node. The problem is at the wall. How should be the properties for the ghost volumes(say j=0)? Is there a better way to calculated the derivates?...at least for the wall? Thanks 

June 6, 2006, 02:35 
Re: y+ for NACA0012 transonic simulations

#4 
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Dear Oscar,
The stretching values seem good enough for the simulation. The computation of the derivatives via Gauss divergence theorem is also right. On the wall, such a procedure would involve a covolume consiting of the nodes forming an edge and the centroid of the cell. You could try this out if possible. How bad is the skin friction coefficient? You could also check to see if the expression you have used in computing the sfc is reight. For the ghost volumes, all properties are the same as the "true" neighbouring cell on the body, except for the normal velocity which is reversed to satisfy "nopenetration" bc for solid walls. A ghost cell procedure may be necessary for invsicid flux computation, but no ghost cells need be used for the viscous fluxes. In fact, a "half" covolume consisting of the nodes of the edge on the wall and the centroid of the cell it belongs to can be used to compute the derivatives and hence the viscous fluxes on the wall. Hope this helps Regards, Ganesh 

June 8, 2006, 08:09 
y+ for NACA0012 transonic simulations

#5 
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Thanks Ganesh for your comments.
To make good use of your help, I would like to send you some pictures of the way that I compute the derivates, Blasius solution and skin friction coefficient for the flat plate!...is that posible?. best regards 

June 8, 2006, 09:38 
Re: y+ for NACA0012 transonic simulations

#6 
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Dear Oscar,
Sure. I would definitely like to have a look at the computations. Regards, Ganesh 

June 8, 2006, 09:44 
Re: y+ for NACA0012 transonic simulations

#7 
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Using the Forum response message is not possible to send pictures......can you send me an email with an alternative email address.....?
Thanks 

June 9, 2006, 01:27 
Re: y+ for NACA0012 transonic simulations

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