The universal constant for the Lagrangian structure function

The Langevin equation has been used for many years to model the dispersion of passive scalars in turbulent flow. It is a stochastic differential equation for the incremental change of Lagrangian particle velocity as a function of the sum of a deterministic term and a stochastic term. The stochastic term is the product of a coefficient and an incremental Wiener process. The coefficient can be written as (C0ε)1/2, where C0 is a universal constant associated with the Lagrangian structure function and ε is the mean rate of turbulent kinetic energy dissipation. There is considerable uncertainty about the value of C0. The values obtained by different investigators are reviewed. A value of C0=5.7 is calculated for the constant-stress region in the neutral boundary layer.