Law of the Wall describes the mean velocity profile in a turbulent boundary layer.
Three layers exist:
viscous sublayer (y⁺ < 5): U⁺ = y⁺;
buffer layer (5–30);
log-law overlap region (y⁺ > 30):
U⁺ = (1/κ) ln(y⁺) + B
where y⁺ = y u_τ/ν is the wall unit and U⁺ = U/u_τ.
The von Kármán constant κ ≈ 0.41 and intercept B ≈ 5.0 are universal across flows.
The friction Reynolds number Re
τ = δ u_τ/ν sets the extent of the log region.