Wilcox perturbation
 

Hello,

Can you help me with a derivation in Wilcox "Turbulence Modeling for CFD" (1994)?
On pp. 107 Wilcox subsitutes expansions into the momentum equations, but I am not able to do this myself. Is it possible to give some explanation (which terms of the expansions to keep, ...) or maybe some transitions between the expressions?

Thanks.