A two-fluid closure model, commonly used in engineering simulations, is the drift-velocity model of Simonin. In this work, this model is evaluated for dilute particle-laden pipe flows, using Direct Numerical Simulations (DNS) with particle tracking. The simulations were performed with both heavy and light particles (particle relaxation-times of τ_2~+ = 100 and τ_2~+ = 10, respectively) and with reflecting and absorbing walls as the boundary conditions for the particles, resulting in four different cases. For all the cases, except the combination of heavy particles and absorbing walls, from a pragmatic point of view, the assumptions of local-equilibrium and homogeneous turbulence seem to hold. For the drift-velocity two models were evaluated: (i) a simple Schmidt-number model, and (ii) a more advanced drift-tensor model. From an engineering perspective, for light particles with reflecting walls the Schmidt-number model appears to be the best choice. However, when the particles are heavier or when the walls are absorbing, the more advanced drift-tensor model gives better results. For these cases, provided that there exists good closure models for the time-scales and particle-fluid velocity correlations, a drift-tensor model could be a better option.
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