Computational modeling of wall-bounded particle-laden turbent flows [electronic resource]
Keywords: Mechanical Engineering.

The transport of particles in wall-bounded turbulent flows is of relevance in many industrial, environmental, and biological applications. The physical scenario is particularly rich involving a complex force balance (involving drag, lift, gravity) that induces particle motion, complex interactions between particles and walls, particle collisions and two-way momentum transfer between the fluid and the particles. Many studies in the literature have used both experimental techniques and computational tools to investigate this problem; however, many questions on the relative importance of the various physical effects remain especially in realistic configurations under turbulent conditions, such as in ducts with non-square cross-sections. The objective of this thesis is to develop computational capabilities to study the coupling between flow turbulence and particle dynamics in a range of Reynolds numbers. The detailed analysis carried out in square and squircle cross-section ducts, sheds light on the particle force balance, the impact of collision on particle concentration near walls, and the interplay between particle collision and turbophoresis. Particle preferential accumulation is known to play a major role on the efficiency of particle-based systems. However, the role of turbophoresis, and the resulting increased concentration near solid walls, is not well understood. A unique feature of turbulent flows in non-circular duct, namely the secondary flows of Prandtl's second kind, enhances the transport of momentum, vorticity, and energy from the core of the duct to the corners and creates a distortion in the velocity contours. We performed detailed computational studies to investigate the effect of secondary motion on the flow and particle distribution. Secondary flow transfers particles toward the corners leading to a significant reduction of particle concentration in the core and an even higher preferential concentration of particles in the vicinity of the walls. Different cases are presented where the sharp corners of the duct are gradually smoothed (transition from a square to squircle and, finally, to a circle) to illustrate the importance of the secondary flow on the particle distribution. Particle-particle collisions deeply alter the particle concentration in the near-wall region and provide a mechanism to reduce turbophoresis even in cases when the overall particle loading is very low. Simulations have been carried out in a range of flow conditions and configurations illustrating that, in spite of the relative change of near wall particle concentration, the wall deposition measured experimentally is well reproduced numerically. In addition to using a brute-force collision algorithm we have developed an efficient and scalable stochastic collision model. This strategy introduces fictitious particles that statistically represent the likelihood of finding collision partners in the vicinity of a given particle. An important improvement of our approach, as compared to others available in the literature, is the introduction of a computational procedure to estimate the velocity fluctuations of the fictitious particle rather than requiring user- specified parameters. The simulations using this stochastic collision model compare well with the brute-force approach and lead to considerably more efficient overall computations.