Comparison of Three k-e Turbulence Models for Predicting Ventilation Air Jets

Abstract

Published data on unconfined plane-wall jets and plane-free jets were reviewed and used to assess the accuracy of numerical simulations, A plane-free jet was numerically simulated using the standard k-e model and four nonuniform grid patterns (70 x32,100 x52,120 x 60, and 120 x 74), The solution for a plane-free jet with adequate grid resolution was in good agreement with the published data, A plane-wall jet was numerically simulated using five different grids (70 x 32, 100 x 52, 120 x 60, 120 x 74, and 120 x 92) and three k-e turbulence models (the standard k-e model. Lam and Bremhorst low Reynolds number model, and Lam and Bremhorst low Reynolds number model with wall functions). The simulations predicted velocity decay and velocity profile well, but overpredicted the jet spread and entrainment ratio by 20 to 40%, indicating the need for a better turbulence model for wall jet predictions. Keywords. Ventilation, Numerical, Grid pattern. J et-ventilated structures are widely used in both agricultural and nonagricultural systems. Numerous research has been done on ventilation air flows involving jets as well as on jets themselves. Because jets have been studied extensively, they provide good test cases to estimate the effects of using different turbulence models and grids. Turbulence modeling is an important part of numerical simulation. Currently, most of the ventilation simulations use one of three k-e models (k is turbulent kinetic energy, e is the dissipation rate of turbulent kinetic energy): the standard k-e model (Launder and Spalding, 1974), Lam and Bremhorst (1981) low Reynolds number model, or the Lam and Bremhorst low Reynolds number with wall functions (Chen et al., 1990). Aldiough most of the studies reported agreement with the corresponding experimental results using the three models, detailed comparison of the performances of the three models is not available in a ventilation setting. Figure 1 is a sketch of a plane-free jet and a plane-wall jet. A plane-free jet is a two-dimensional jet fliat is not attached to any solid surface. If a two-dimensional jet is attached to a solid surface, the jet is a plane-wall jet. For ventilation applications, the jet characteristics of interest include the velocity profile, velocity decay (or peak velocity), jet spread, and entrainment ratio. The velocity