Since the pioneering work of Reynolds (1883), much effort has been allocated on the topic of laminar-turbulent transition regime in a single-phase flow, with special focusing on the unstable and intermittent natures of this regime (Mullin, 2011). The transition regime of dispersed flows carried less attention even though dispersed flows are used in many industrial processes. As for suspensions of neutrally buoyant particles, Matas et al. (2003) observed changes in the values of the critical Reynolds numbers depending on both the solid volume fraction and the particle-to-pipe sizeratio. Typically, the transition occurs at lower Reynolds numbers when the flow carries macro-sized particles at dilute to moderate concentrations (up to 25%). On the contrary, the critical Reynolds numbers of the onset of transition is shifted towards greater values when particles are micro-sized and their concentration is higher. In this work, we aim at understanding the mechanisms lying behind the shift of the laminar-turbulent transition regime down to lower critical Reynolds numbers in suspension flows of macro-sized particles. Fully-coupled numerical simulations are used to investigate the interactions between neutrally-buoyant finite-size particles and a transitional channel flow. To our knowledge, other than the simulations of Shao et al. (2012) and Garcia-Villalba et al. (2012) performed in turbulent channel flows, there are no direct numerical simulations performed on fluctuating suspension flows in channels or pipes with finite-size particles. The numerical method chosen for this work is the Force-Coupling Method (FCM) (Maxey and Patel, 2001, Lomholt and Maxey 2003). It is fully-resolved in the sense that the fluid equations are solved at a length-scale smaller than the particle radius. In a first step, the laminarization process of a single-phase flow initially turbulent at Re=6000 is statistically characterized (Re is based on the average flow velocity, the channel height and the kinematic viscosity). In a second step, particles are randomly added to the fluctuating channel flow at a solid volume fraction of 5%, the size ratio of particle diameter to channel height being 1/16. The starting point of the calculation of the suspension flow is a snapshot taken from the single-phase flow case at Re=1625 (the smallest Reynolds number at which the flow does not relaminarize).
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