1,720,962 research outputs found
Anomalous diffusion of inertial particles in random parallel flows: Theory and numerics face to face
No full textThe large-scale transport of inertial particles is investigated by means of Lagrangian simulations. Our main focus is on the possible emergence of anomalous diffusion for the class of random parallel flows. For such flows, a perturbative prediction in the limit of small inertia has recently become available in the literature. Anomalous diffusion was traced back to the possible divergence of the first-order correction to the eddy diffusivity of the tracer limit. Our numerical results show that the conditions for the emergence of anomalous diffusion are affected by resummations of higher-order perturbative contributions which regularize the resulting diffusion process, thus ruling out the occurrence of anomalous diffusion
Minimal model for zero-inertia instabilities in shear-dominated non-Newtonian flows
No full textThe emergence of fluid instabilities in the relevant limit of vanishing fluid inertia (i.e., arbitrarily close to zero
Reynolds number) has been investigated for the well-known Kolmogorov flow. The finite-time shear-induced
order-disorder transition of the non-Newtonian microstructure and the corresponding viscosity change from
lower to higher values are the crucial ingredients for the instabilities to emerge. The finite-time low-to-high
viscosity change for increasing shear characterizes the rheopectic fluids. The instability does not emerge in
shear-thinning or -thickening fluids where viscosity adjustment to local shear occurs instantaneously. The lack of
instabilities arbitrarily close to zero Reynolds number is also observed for thixotropic fluids, in spite of the fact
that the viscosity adjustment time to shear is finite as in rheopectic fluids. Renormalized perturbative expansions
(multiple-scale expansions), energy-based arguments (on the linearized equations of motion), and numerical
results (of suitable eigenvalue problems from the linear stability analysis) are the main tools leading to our
conclusions. Our findings may have important consequences in all situations where purely hydrodynamic fluid
instabilities or mixing are inhibited due to negligible inertia, as in microfluidic applications. To trigger mixing in
these situations, suitable (not necessarily viscoelastic) non-Newtonian fluid solutions appear as a valid answer.
Our results open interesting questions and challenges in the field of smart (fluid) materials
Elastic-like instabilites in non-Newtonian parallel flows
No full textMixing control in fluid environments having very low Reynolds numbers is a crucial need for many practical purposes ranging from biochemistry analysis in microfluidic devices, where mixing has to be rapid and efficient, to lab-on-a-chip applications, where mixing has to be reduced to avoid spurious effects as in microfluidic rheometer applications. The ability to control fluid mixing properties is clearly subjected to a deep understanding of physical mechanisms able to originate such a mixing for very small Reynolds numbers. In this respect, a simple model to capture mesoscopic effects of order-disorder transitions of an underlying non-Newtonian fluid microstructure subjected to shear is proposed and its behaviors investigated (numerically and by means of asymptotic perturbative methods) in relation to the possible emergence of fluid elatic-like instabilities occurring for arbitrarily small flow inertia (i.e. zero Reynolds numbers). A crucial ingredient for instabilities to emerge has been identified in the finite-time response of the network structure to strain where the order-disorder transition corresponds to a change from low-to-high fluid viscosity (and not viceversa). Our results generalizes the concept of ``elastic instabilities'' in viscoelastic fluids to a more general and larger class of non-Newtonian fluids
Explicit expressions for eddy-diffusivity fields and effective large-scale advection in turbulent transport
No full textLarge-scale transport is investigated in terms of new explicit expressions for eddy diffusivities and effective advection obtained from asymptotic perturbative methods. The carrier flow is formed by a large-scale component plus a small-scale contribution mimicking a turbulent flow. The scalar dynamics is observed in its pre-asymptotic regimes (i.e. on scales comparable to those of the large-scale velocity). The resulting eddy diffusivity is thus a tensor field which explicitly depends on the large-scale velocity. Small-scale interactions also cause the emergence of an effective large-scale (compressible) advection field which, as a result of the present study however, turns out to be of negligible importance. Two issues are addressed by means of Lagrangian simulations: quantifying the possible deterioration of the eddy-diffusivity/effective advection description by reducing to zero the spectral gap separating the large-scale velocity component from the small-scale component; comparing the accuracy of our closure against other simple, reasonable, options. Answering these questions is important in view of possible applications of our closure to tracer dispersion in environmental flows
Eddy diffusivities of inertial particles in random Gaussian flows
No full textWe investigate the large-scale transport of inertial particles. We derive explicit analytic expressions for the eddy diffusivities for generic Stokes times. These latter expressions are exact for any shear flow while they correspond to the leading contribution either in the deviation from the shear flow geometry or in the Peclet number of general random Gaussian velocity fields. Our explicit expressions allow us to investigate the role of inertia for such a class of flows and to make exact links with the analogous transport problem for tracer particles.Peer reviewe
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
LAGRANGIAN SIMULATIONS OF DRIFTER TRAJECTORIES ADOPTING A NEW EXPLICIT EXPRESSION FOR EDDY DIFFUSIVITIES
No full textVariations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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