Rheology of suspensions of rigid-rod like particles in turbulent channel flow

Abstract

The rheological behaviour of Brownian rigid-rod particles (fibres) in turbulent channel flow is investigated by numerical means. From a direct numerical simulation (DNS) of turbulent channel flow, Lagrangian time traces of the velocity derivative tensor, as experienced by small inertia-free particles, are generated. The response of the conformation distribution function of ensembles of passive Brownian fibres along these Lagrangian pathes is computed by a stochastic simulation using Jeffery’s equation (1922) and the rheological theory of dilute suspensions of fibres in Newtonian solvents of [Int. J. Multitphase Flow 1 (2) (1974) 195]. Results are presented for the influence and importance of turbulent fluctuations and Brownian motion on the moments of the distribution function and stresses generated by the presence of the particles. The stresses peak in the buffer layer and it is found that stress levels as well as rms of stresses rise quickly with the square of the fibre aspect ratio. At aspect ratios of more than about r=100, the stress distributions remain qualitative similar. Smaller fibres generate higher average stress levels but lower stress fluctuation levels than larger fibres for which the effect of Brownian motion is weak. Large normal stresses always come in hand with large shear stresses showing that, in such suspensions possible drag reduction will always be connected to a significant stress deficit.