Negative reynolds stresses from Spalart Allmaras model
I have an unbearable question about reynolds stresses. I am using simpleFoam to simulate the flow over an airfoil(2D). Since the Spalart Allamaras (SA) model is most appropiate for this I used it. After that I calculated the Reynolds stresses with the utility R. Now my surprise when Rxx and Ryy have negative and positive values. I run the airfoil2D of the tutorials/incompressible/ras, and again there is change of signs in Rxx and Ryy. This means also that the turbulent kinetic energy, defined as the trace of R has positive and negative values!
Can someone explain me this, what am I doing wrong? I have also tried to find the src where R is calculated but have not managed. Can someone point me where to look for this?
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