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-   -   What's delta x plus? (http://www.cfd-online.com/Forums/fluent/117900-whats-delta-x-plus.html)

ringtail May 17, 2013 01:14

What's delta x plus?
 
Hi all,

Now I am reading the user's guide of ANSYS FLUENT 14.5, for some criteria about mesh generation which is for LES simulation.
It mentions, "it is typically recommended to use a mesh with a grid spacing scaling with delta x+ =40, delta y+=1, delta z+=20, where x is the streamwise, y the wall normal and z the spanwise direction (for example, channel flow)." in page 694.

I have no idea about " dleta x +", and what's the difference between "delta y+" and "y+"?

Any suggestion will be greatly appreciated. Thanks in
advance.

RodriguezFatz May 17, 2013 01:52

What they mean is:
delta x+ = x+
delta y+ = y+
delta z+ = z+
But this is a frowzy formulation...

Edit: And x+ is the wall spacing in x direction and is normalized on the same physical value as y+. The same for z+.

ringtail May 17, 2013 03:45

Quote:

Originally Posted by RodriguezFatz (Post 428161)
What they mean is:
delta x+ = x+
delta y+ = y+
delta z+ = z+
But this is a frowzy formulation...

Edit: And x+ is the wall spacing in x direction and is normalized on the same physical value as y+. The same for z+.

After read the paper "grid-point requirements for large eddy simulation: Chapman’s estimates revisited", now I guess, maybe the difference between
is :
all the variable with a plus is derived from similar formula;
delta means space between any two adjoin mesh
layer;
and without delta, it means space between the first mesh layer and
the wall.

However, I'm not sure if it is correct.


RodriguezFatz May 17, 2013 05:23

Yes, you are right, this is a better explanation.
But, saying y+ is located at the first grid point (what nearly everyone does), is pretty inaccurately anyway, since y+ is a normalized variable calculated by the physical "y". So basically you can use any "y" value to calculate a related "y+" value. The common usage seems to be pretty much slang.


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