Concept

Von Kármán constant

Summary
In fluid dynamics, the von Kármán constant (or Kármán's constant), named for Theodore von Kármán, is a dimensionless constant involved in the logarithmic law describing the distribution of the longitudinal velocity in the wall-normal direction of a turbulent fluid flow near a boundary with a no-slip condition. The equation for such boundary layer flow profiles is: :u=\frac{u_{\star}}{\kappa}\ln\frac{z}{z_0}, where u is the mean flow velocity at height z above the boundary. The roughness height (also known as roughness length) z0 is where u appears to go to zero. Further κ is the von Kármán constant being typically 0.41, and u_\star is the friction velocity which depends on the shear stress τw at the boundary of the flow: :u_\star = \sqrt{\frac{\tau_w}{\rho}}, with ρ the fluid density. The Kármán constant is often used in turbulence modeling, for instance in boundary-layer meteorology to calculate fluxes of momentum, heat and moi
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