1,721,020 research outputs found
Fluid Mechanics for Mechanical Engineers
No full textThis textbook describes the fundamentals of the phenomena of fluid dynamics in the context of engineering instances. It is designed to replace introductory books and notes on the subject for first-level engineering courses as well as higher-level courses or for professional use. The use of this book requires the basic knowledge of mathematics and physics normally delivered in the early years of undergraduate study. However, the extensive use of examples and solved exercises proposes a parallel intuitive route to understanding the necessary mathematical formalisms. It proves that a new fluid dynamics text should not contain new ideas or formalisms, but should present the material in a modern and intuitive way. The approach chosen is primarily practical, so that that readers can practice by solving the proposed problems and examples in order to be prepared to solve the new problems they will encounter in their academic and professional activities. It serves as a teaching tool for courses in basic fluid dynamics, advanced fluid dynamics, turbulence, and aerodynamics
Turbulence Modulation by Slender Fibers
Full text linkIn this paper, we numerically investigate the turbulence modulation produced by long flexible fibres in channel flow. The simulations are based on an Euler–Lagrangian approach, where fibres are modelled as chains of constrained, sub-Kolmogorov rods. A novel algorithm is deployed to make the resolution of dispersed systems of constraint equations, which represent the fibres, compatible with a state-of-the-art, Graphics Processing Units-accelerated flow-solver for direct numerical simulations in the two-way coupling regime on High Performance Computing architectures. Two-way coupling is accounted for using the Exact Regularized Point Particle method, which allows to calculate the disturbance generated by the fibers on the flow considering progressively refined grids, down to a quasi-viscous length-scale. The bending stiffness of the fibers is also modelled, while collisions are neglected. Results of fluid velocity statistics for friction Reynolds number of the flow (Formula presented.) and fibers with Stokes number (Formula presented.) = 0.01 (nearly tracers) and 10 (inertial) are presented, with special regard to turbulence modulation and its dependence on fiber inertia and volume fraction (equal to (Formula presented.) · (Formula presented.) and (Formula presented.) · (Formula presented.)). The non-Newtonian stresses determined by the carried phase are also displayed, determined by long and slender fibers with fixed aspect ratio (Formula presented.), which extend up to the inertial range of the turbulent flow
Orientational Dynamics of Long Flexible Fibers in Wall-Bounded Turbulence
No full textIn this paper, we study numerically the role of fiber length and flexibility on the orientational dynamics of slender fibers in turbulent channel flow. We consider fibers of different flexibility at varying aspect ratio, up to lengths being comparable to the channel height. These fibers are constructed by constraining a large number of sub-Kolmogorov rods in a single chain, alongside a bending stiffness torque that allows to prescribe a finite value of the fiber rigidity. To perform our analysis, we carried out a series of one-way coupled direct numerical simulations of a fiber-laden channel flow at fixed shear Reynolds number: Reτ=300, based on the half height of the channel. By calculating the orientational statistics of the suspended fibers, we find that shorter fibers, with length O(10-1) when normalized by the channel half height, tend to exhibit a nearly-isotropic orientation distribution near the channel center, as would fibers suspended in homogeneous isotropic turbulence. As the fiber length is increased (up to values comparable to the channel half height), however, deviations from the isotropic orientation distribution become more and more significant. When the fibers are more rigid, these deviations are dampened and it is also observed that the tumbling rate of the fiber is lowered on average
Influence of Particle Anisotropy and Motility on Preferential Concentration in Turbulence
No full textThe simplest numerical framework to study turbulent particle dispersion assumes that particles can be modeled as point-like spheres brought about by the flow. In spite of its simplicity, this framework has led to significant advancements in the study of particle-turbulence interactions. In this paper we examine how particle dispersion in dilute turbulent suspensions changes when particles are non-spherical (elongated) and may actively move within the fluid (motile). In particular, we show how elongation and motility add to particle inertia to modulate preferential concentration. Results for particles suspended in wall-bounded turbulence are presented to highlight effects on wall accumulation and segregation, which represent the macroscopic manifestation of preferential concentration
Particle capture by drops in turbulent flow
No full textWe examine the process of particle capture by large deformable drops in turbulent channel flow. We simulate the solid-liquid-liquid three-phase flow with an Eulerian-Lagrangian method based on direct numerical simulation of turbulence coupled with a phase-field model, to capture the interface dynamics, and Lagrangian tracking of small (sub-Kolmogorov) particles. Drops have the same density and viscosity of the carrier liquid, and neutrally buoyant, quasi-inertialess, solid particles are one-way coupled with the other phases. Our results show that particles are transported towards the interface by jetlike turbulent motions and, once close enough, are captured by interfacial forces in regions of positive surface velocity divergence. These regions appear to be well correlated with high-enstrophy flow topologies that contribute to enstrophy production via vortex compression or stretching. Examining the turbulent mechanisms that bring particles to the interface, we have been able to derive a simple transport model for particle capture. The model is based on a single turbulent transport equation in which the only parameter scales with the turbulent kinetic energy of the fluid measured in the vicinity of the drop interface, and its predictions of the overall capture efficiency agree remarkably well with numerical results
Morphodynamics of melting ice over turbulent warm water streams
Full text linkWe investigate the morphodynamics of an ice layer over a turbulent stream of warm water using numerical simulations. At low water speeds, characteristic streamwise undulations appear, which can be explained by the Reynolds analogy between heat and momentum transfer. As the water speed increases, these undulations combine with spanwise ripples of a much greater length scale. These ripples are generated by a melting mechanism controlled by the instability originating from the ice–water interactions, and, through a melting/freezing process, they evolve downstream with a migration velocity much slower than the turbulence characteristic velocity
Deformation of flexible fibers in turbulent channel flow
No full textIn this paper, we examine from a statistical point of view the deformation of flexible fibers in turbulent channel flow. Fibers are longer than the Kolmogorov length scale of the carrier flow and have finite inertia. Our aim is to examine the effect of local shear and turbulence anisotropy on fiber twisting and bending, when shape effects add to the inertial bias. To these aims, we use an Eulerian–Lagrangian approach based on direct numerical simulation of turbulence in dilute flow conditions. Fibers are modelled as chains of sub-Kolmogorov rods (referred to as elements hereinafter) interconnected by holonomic constraints that enable relative rotation of neighbouring elements. Statistics are computed from simulations at shear Reynolds number Re=150, based on the channel half height, for fibers with different aspect ratio, lambda_r (defined as the ratio between the length l_r of each element r composing the fiber and its cross-sectional radius, a), and different inertia, parameterized by the Stokes number of the element, St_r . We show that bending of flexible fibers is in general stronger in the bulk of the flow, where they are subject to turbulent velocity fluctuations only. Near the wall, fibers are more easily stretched by the mean shear, especially for large-enough inertia (St_r=5 in our simulations). In spite of this different dynamics, which is connected to the anisotropy of the flow, we find that the fiber end-to-end distance reaches a steady state regardless of fiber
location with respect to the wall
Effect of roughness on elongated particles in turbulent channel flow
No full textIn this paper, we examine the deposition and resuspension of rigid elongated particles transported by turbulence in a channel bounded by two-dimensional roughness. To analyze these processes, we use an Euler–Lagrange approach based on Direct Numerical Simulation of the carrier phase and Lagrangian Particle Tracking of the dispersed phase. Four different channel configurations are considered: One is the classical channel flow bounded by smooth flat walls, whereas the other three correspond to a channel with walls of increasing roughness. The roughness shape is obtained by superimposition of sinusoidal functions with different amplitude k and is characterized by the mean absolute value of the amplitude, k̄= 0.012, 0.024 and 0.050 (k̄= 0 for a smooth wall). The friction Reynolds number is Reτ=150 for all cases. Particles are modeled as prolate ellipsoids and classified according to their aspect ratio λ. Three different particles sets are considered: λ=1, corresponding to the reference case of spheres, λ=3, corresponding to slightly elongated particles, and λ=10, corresponding to long fiber-like particles. The particle response time is St+=5 for all sets. In turbulent flow bounded by smooth walls, particles are known to accumulate preferentially in the near-wall region, leaving the central region of the channel scarcely populated. Wall roughness produces a completely different scenario: Particles exhibit a more homogeneous distribution along the wall-normal direction. We show that the aspect ratio does not affect the preferential distribution and the velocity statistics of the particles. The effect of elongation, however, becomes important for their preferential orientation, which is much weaker than in the smooth-walls case, in the near-wall region, while recovering the smooth-walls case in the outer region of the channel. This finding supports the validity of Townsend's similarity hypothesis, namely that the bulk flow dynamics are unaffected by the roughening of the bounding walls
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