Is yPlusRAS correct?

miro2000 · August 11, 2013, 11:39

Hello everyone,

i know there are some threads about whether this utility calculates y+ or y*, but those threads are pretty old afaik. So i apologise in advance if this was answered recently.

So, I'm using OF 2.2.0 to calculate flow around a aerofoil in 3D. I believe that my mesh is OK. I'm running simpleFoam and KomegaSST.
My y+ ranges cca [29,300], at least that's what yPlusRAS says.

Omega diverges when I use "linearUpwind grad(omega)" scheme. (upwind works fine)

I'm suspecting that is happening because calculated value of yPlusRAS is not correct. I tried compiling yPlus and yPlusRANS(i think) utility that can be found on the forums but they don't seem to work with 2.2.0.

Thank you for reading and have a cool day.
- miro

edit: my Re is 4e6

immortality · August 11, 2013, 13:27

Hi
for your mesh that y+ is overall bigger than 30 (high-Re) yPlusRAS utility that works with wall functions and calculates y* in fact is correct and works fine.
if you want to model with low-Re mesh (if you want to model separation very well for example) tell me so that I send you the proper utility.

miro2000 · August 11, 2013, 13:59

Hello and thank you for your interest.

Forgive my ignorance (I am still a student), but i don't think I'm using low-Re mesh, since i was trying to achieve y+ > 30, and in other words am not calculating what's happening inside the boundary layer directly in an area where viscous forces are greater than inertial.

So if I understand correctly yPlusRAS is not good for my model and it would be great if you would sent me your utility.

immortality · August 11, 2013, 14:15

no yPlusRAS works correctly for high-Re mesh.
you better set turbulent terms to first order upwind instead of second order,they are only models and aren't a lot precise in fact that you want to use second order.just set a lower value for their tolerances in fvSolution for more accuracy.

miro2000 · August 11, 2013, 14:31

OK, i will try that.
I am curios why is it diverging for second order upwind though...
My mesh should be OK, it passes meshCheck and looks nice, and calculation converges nicely for first order, and when i switch it to second o.u. omega gets bounded after 3 iterations..

Currently I'm looking into the boundary conditions as a possible cause.

Thanks for help,
- miro

August 11, 2013, 11:39	Is yPlusRAS correct?	#1
miro2000 Member Miro Join Date: Jan 2013 Location: Europe Posts: 51 Rep Power: 13	Hello everyone, i know there are some threads about whether this utility calculates y+ or y, but those threads are pretty old afaik. So i apologise in advance if this was answered recently. So, I'm using OF 2.2.0 to calculate flow around a aerofoil in 3D. I believe that my mesh is OK. I'm running simpleFoam and KomegaSST. My y+ ranges cca [29,300], at least that's what yPlusRAS says. Omega diverges when I use "linearUpwind grad(omega)" scheme. (upwind works fine) I'm suspecting that is happening because calculated value of yPlusRAS is not correct. I tried compiling yPlus and yPlusRANS(i think) utility that can be found on the forums but they don't seem to work with 2.2.0. Thank you for reading and have a cool day. - miro edit: my Re is 4e6 Last edited by miro2000; August 11, 2013 at 12:59. Reason: Re*

August 11, 2013, 13:27		#2
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 26	Hi for your mesh that y+ is overall bigger than 30 (high-Re) yPlusRAS utility that works with wall functions and calculates y* in fact is correct and works fine. if you want to model with low-Re mesh (if you want to model separation very well for example) tell me so that I send you the proper utility. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

August 11, 2013, 14:15		#4
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 26	no yPlusRAS works correctly for high-Re mesh. you better set turbulent terms to first order upwind instead of second order,they are only models and aren't a lot precise in fact that you want to use second order.just set a lower value for their tolerances in fvSolution for more accuracy. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

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August 11, 2013, 13:59		#3
miro2000 Member Miro Join Date: Jan 2013 Location: Europe Posts: 51 Rep Power: 13	Hello and thank you for your interest. Forgive my ignorance (I am still a student), but i don't think I'm using low-Re mesh, since i was trying to achieve y+ > 30, and in other words am not calculating what's happening inside the boundary layer directly in an area where viscous forces are greater than inertial. So if I understand correctly yPlusRAS is not good for my model and it would be great if you would sent me your utility.

August 11, 2013, 14:31		#5
miro2000 Member Miro Join Date: Jan 2013 Location: Europe Posts: 51 Rep Power: 13	OK, i will try that. I am curios why is it diverging for second order upwind though... My mesh should be OK, it passes meshCheck and looks nice, and calculation converges nicely for first order, and when i switch it to second o.u. omega gets bounded after 3 iterations.. Currently I'm looking into the boundary conditions as a possible cause. Thanks for help, - miro