Doubt about application of RST Turbulent model.

Dolly

I've read through some papers from Launder et al (1975), Speziale et al (1991), talking about development of a second order closure model for turbulent flow. However, the model constants are always derived from homogenous turbulent flow. I wonder if this is really useful for real application as most of the real flow seems to be non-homogenous.

Anyone can explain to me what is "isotropic decay" experiment & "return to isotropy" experiment?

Thank you for your help.