On the non-uniqueness of Lagrangian stochastic models

Lagrangian stochastic (LS) models are used to predict mean concentrations o f tracer particles within a mechanically ventilated building where the turb ulent flow is three-dimensional and inhomogeneous. The models satisfy the w ell-mixed condition for Gaussian turbulence but produce different statistic s for increments d theta(k)' to the orientation, theta(k)', of the Lagrangi an velocity fluctuation vector. Of the models tested, the best agreement wi th the experimental data is obtained with Thomson's (1987, J. Fluid. Mech. 180, 529-556) model, which is effectively a "zero-spin" model, having [d th eta(k)'; x] approximate to 0. Mean concentrations of tracer-particles are l ess well predicted by the "zero-spin" model of Flesch and Wilson (1992, Bou ndary Layer Meteorol. 61, 349-374), generalized to three dimensions, and by models having [d theta(k)'; x] not equal 0 which produce tracer-particle t rajectories with a preferred handedness. It is suggested that the specifica tion of [d theta(k)'; x] is not sufficient to distinguish the better LS mod els within the well-mixed class. (C) 1999 The Japan Society of Fluid Mechan ics and Elsevier Science B.V. All rights reserved.