by the friction velocity , |} This approximation can be used farther than 5 wall units, but by the error is more than 25%. Buffer layer In the buffer layer, between 5 wall units and 30 wall units, neither law holds, such that: For with the largest variation from either law occurring approximately where the two equations intercept, at . That is, before 11 wall units the linear approximation is more accurate and after 11 wall units the logarithmic approximation should be used, though neither are relatively accurate at 11 wall units. The mean streamwise velocity profile is improved for with an eddy viscosity formulation based on a near-wall turbulent kinetic energy function and the van Driest mixing length equation. Comparisons with DNS data of fully developed turbulent channel flows for showed good agreement. Notes References Further reading External links Definition from ScienceWorld Formula on CFD Online Y+ estimator Fluid dynamics Turbulence