Yes, Dylan, I have been experiencing some difficulty in learning (again or by the first time) what I have not looked at as much as I would like to in these 20 years past since I took a formal course in Turbulence Modeling - where we could not simulate very practical situations using an open source Fortran77 2D code (similar to a famous one used by Patankar in teaching, restricted to cartesian grids) and an under-stimulated access to Fluent. Not to speak about my poor communication in English skills…Thanks for worrying about my difficulties and helping me!

I think that, In some contexts, the phrase “shear stress calculation” may be not just referring to that one used for obtaining a friction drag coefficient, some kind of head loss, or the hydrodinamical load over a surface (at the post-processing stage of a CFD simulation). It may refers also to that of shear stresses implicitly involved in the diffusive terms of the RANS equations and in the production “source” terms appearing in the k and epsilon/omega transport equations (in an internal product by the deformation rate tensor that amounts to power/volume). Since we are talking about subjects scarcely found in a few textbooks I want to praise a recent and comprehensive one, by Moukalled *et alii*, that I just knew http://www.springer.com/gp/book/9783319168739

About that fourth root of fourth powers of friction velocities, I would like to say that I have nothing against it (is a recommendation of the greats Menter and Esch), but I feel that OpenFOAM developers did not deemed its implementation advantageous.