DrAmine
Ansys Employee

You derive the expression from that and you assume that fluctuation velocity is 80% of the u_tau. From that formula you can get u_tau/u_avg from that you can derive u'/u_avg.nFor the length scale, I wrote already The one of the length scale is straightforward knowing the length scale from two-equation models for example. One needs just to relate the length scale to hydraulic diameter. Here one needs again to check a couple of books.n