# How to calculate the first grid spacing (ds) using the NASA formula online?

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 May 20, 2009, 23:03 How to calculate the first grid spacing (ds) using the NASA formula online? #1 New Member   JI Bai-feng Join Date: May 2009 Posts: 2 Rep Power: 0 hi ,everyone. As all known, the Enhanced wall treatment needs that the y+ must be equal to 1 or below 5. Some useful formula tools are aviable online, one is provided by NASA,as the following URL: http://geolab.larc.nasa.gov/APPS/YPlus/ For a given Reynolds number, Mach number and desired y+ value, this page will estimate the spacing normal to a solid surface required to yield 1 grid point in the laminar sublayer. Now , my problem is what is the Ref. Length meaning actually ? R.Mah likes this.

 May 21, 2009, 06:14 #2 New Member   ashish Join Date: May 2009 Posts: 3 Rep Power: 10 Hi Byron, Reference length is used to normalize length dimensions of geometry and also use to post process the data It depends on the kind of problem you are dealing with. If it aircraft aerodynamics related problem, you must choose chord length of wing as a reference length. In automobile case people either use length of body or height of the body for reference length. Any way usually we need Y+ < 1 for aerospace kind of applications where viscous calculations are important. It can be explained by a simple example too. If you have a cylinder of diameter D, then D will be your reference length to calculate ds for corresponding y+ value. R.Mah likes this.

 May 21, 2009, 06:54 #3 New Member   JI Bai-feng Join Date: May 2009 Posts: 2 Rep Power: 0 Thanks a lot, ashish_singh I think I got it. Byron

 May 22, 2009, 00:40 #4 Member   Radiatn Join Date: Mar 2009 Posts: 34 Rep Power: 10 I tried that formula a few years ago and my result showed that the mesh layer size suggested by that formula is >5 times bigger than what should have been meshed. But that depends on the turbulence model you choose as well.

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