Y+ and accuracy of Pressure Coefficient of a wing
 

Hi All,

Forgot to ask a question in my previous thread.

The 2D analysis I am doing here has a Y+ value of 2 (min) and 11 (max), however the S-A Turbulence model says that a Y+ of 30 should be used.

Does it mean that the pressure coeff. are not right! or should I stick with the current value. The adapt option in Fluent doesnt seem to coarse the grid for Y+ of 30.

Thanks in advance,

Mick

Re: Y+ and accuracy of Pressure Coefficient of a w
 

Spallart-Almaras model is much better suited for y+=1. So adapt your mesh to this value. A value of y+=5-6 is very wrong (and I bet that's the medium value for your model)!

Observation: if you got a structured mesh, then coarsening is impossible (this is available only for unstructured meshes)!!

Razvan

Re: Y+ and accuracy of Pressure Coefficient of a w
 

Hello Razwan,

Many Thanks for the reply. Yes you are right, I am using a structured grid close to the wing.

According to your advice I adapted the mesh to Y+=1 by using min value=1 and max value=1. Correct me if I did it the wrong way. Now, when I plot the turbulence for wall Y+, I dont see any lines that are close to 1. It is in a region of about 5 and 12!

What have I done wrong here?

Many Thanks in advance.

Mick

Re: Y+ and accuracy of Pressure Coefficient of a w
 

Use y+= (min(0.8),max(1.2)). It will not work otherway. Also, activate dinamic adaption, and set 10-20 iterations for adaption. This tehnique (adaption) is not so great after all, because it introduces a high gradient of cell volume right where it isn't needed: in the boundary layer! So, for better precision I use this type of adaption only in desperate cases. What you should try to do is to obtain a good grid right from the start. To do this, follow my instructions:

- make first a 2d grid for a flat plate, and calculate flow over this plate with BC for your final case (same velocity, temperature, pressure, etc)

- plot y+ values on the plate: this way you will find out if you have chosen the right size for first cell at the wall; if you did not succeed at the first shot, try again until you figure out what cell size would be needed to obtain a desired y+ value

- take this value and make a good grid for your final geometry.

Best wishes, Razvan