Calculating normalised Reynolds stress

cfd_newb · August 30, 2011, 13:24

Hi - first post, but been lurking on the forums a while! I wonder if I could get some help in simple terms! I'm completely stuck on what I guess should be a simple problem. I've done a search on here, and am still stuck as my results aren't at all what I'm expecting...

I have some Fortran CFD code, and am running incompressible 2D and 3D simulations from it. Ignoring the 3D simulations for now, I have a problem in results generation (post-processing) that I'm trying to figure out.
I want to have plots of normalised Reynolds stress in uu and vv axis for particular points in the simulation. I have output, at every 100 iterations, the values at the relevant points for u,v,density,energy,pressure. I have determined a time-averaged mean for each of these at each point, and can determine the fluctuations quite straight-forwardly from the data. (again, the fluctuations are taken as a time-averaged value for all fluctuations at each point).

The fluid itself in free-space has a constant density (it's air). So looking at this page: http://en.wikipedia.org/wiki/Reynolds_stress , is it simply a case of multiplying the fluctuating components in each specific direction to obtain the Reynolds stresses?

Thanks in advance!

mettler · August 30, 2011, 15:10

I believe so. But, I am not sure you can do that with a compressible fluid (air)..even tho you state that the density is constant. You can check your values for the stresses to see which is dominant in the region of your flow. In the viscous sub-layer the laminar stress should dominate, and as you get closer to the centerline (further from the wall) the turbulent stress should dominate.

August 30, 2011, 13:24	Calculating normalised Reynolds stress	#1
cfd_newb New Member Join Date: Aug 2011 Posts: 1 Rep Power: 0	Hi - first post, but been lurking on the forums a while! I wonder if I could get some help in simple terms! I'm completely stuck on what I guess should be a simple problem. I've done a search on here, and am still stuck as my results aren't at all what I'm expecting... I have some Fortran CFD code, and am running incompressible 2D and 3D simulations from it. Ignoring the 3D simulations for now, I have a problem in results generation (post-processing) that I'm trying to figure out. I want to have plots of normalised Reynolds stress in uu and vv axis for particular points in the simulation. I have output, at every 100 iterations, the values at the relevant points for u,v,density,energy,pressure. I have determined a time-averaged mean for each of these at each point, and can determine the fluctuations quite straight-forwardly from the data. (again, the fluctuations are taken as a time-averaged value for all fluctuations at each point). The fluid itself in free-space has a constant density (it's air). So looking at this page: http://en.wikipedia.org/wiki/Reynolds_stress , is it simply a case of multiplying the fluctuating components in each specific direction to obtain the Reynolds stresses? Thanks in advance!

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August 30, 2011, 15:10		#2
mettler Senior Member Andrew Join Date: Mar 2009 Location: Washington, DC Posts: 211 Rep Power: 18	I believe so. But, I am not sure you can do that with a compressible fluid (air)..even tho you state that the density is constant. You can check your values for the stresses to see which is dominant in the region of your flow. In the viscous sub-layer the laminar stress should dominate, and as you get closer to the centerline (further from the wall) the turbulent stress should dominate.