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-   -   How to calculate the first grid spacing (ds) using the NASA formula online? (http://www.cfd-online.com/Forums/main/64715-how-calculate-first-grid-spacing-ds-using-nasa-formula-online.html)

Byron JI May 20, 2009 23:03

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

ashish_singh May 21, 2009 06:14

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.

Byron JI May 21, 2009 06:54

Thanks a lot, ashish_singh

I think I got it.

Byron

radiatn May 22, 2009 00:40

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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