70 research outputs found
Direct numerical simulation of turbulent channel flow over porous walls
We perform direct numerical simulations (DNS) of a turbulent channel flow
over porous walls. In the fluid region the flow is governed by the
incompressible Navier--Stokes (NS) equations, while in the porous layers the
Volume-Averaged Navier--Stokes (VANS) equations are used, which are obtained by
volume-averaging the microscopic flow field over a small volume that is larger
than the typical dimensions of the pores. In this way the porous medium has a
continuum description, and can be specified without the need of a detailed
knowledge of the pore microstructure by indipendently assigning permeability
and porosity. At the interface between the porous material and the fluid
region, momentum-transfer conditions are applied, in which an available
coefficient related to the unknown structure of the interface can be used as an
error estimate. To set up the numerical problem, the velocity-vorticity
formulation of the coupled NS and VANS equations is derived and implemented in
a pseudo-spectral DNS solver. Most of the simulations are carried out at
and consider low-permeability materials; a parameter study is
used to describe the role played by permeability, porosity, thickness of the
porous material, and the coefficient of the momentum-transfer interface
conditions. Among them permeability, even when very small, is shown to play a
major role in determining the response of the channel flow to the permeable
wall. Turbulence statistics and instantaneous flow fields, in comparative form
to the flow over a smooth impermeable wall, are used to understand the main
changes introduced by the porous material. A simulations at higher Reynolds
number is used to illustrate the main scaling quantities.Comment: Revised version, with additional data and more in-depth analysi
Bridging Polymeric Turbulence at different Reynolds numbers: From Multiscaling to Multifractality
The addition of polymers modifies a flow in a non-trivial way that depends on
fluid inertia (given by the Reynolds number Re) and polymer elasticity
(quantified by the Deborah number De). Using direct numerical simulations, we
show that polymeric flows exhibit a Re and De dependent multiscaling energy
spectrum. The different scaling regimes are tied to various dominant
contributions -- fluid, polymer, and dissipation -- to the total energy flux
across the scales. At small scales, energy is dissipated away by both polymers
and the fluid. Fluid energy dissipation, in particular, is shown to be more
intermittent in the presence of polymers, especially at small Re. The more
intermittent, singular nature of energy dissipation is revealed clearly by the
multifractal spectrum
Morphology of clean and surfactant-laden droplets in homogeneous isotropic turbulence
We perform direct numerical simulations of surfactant-laden droplets in
homogeneous-isotropic turbulence with Taylor Reynolds number
. Effects of surfactant on the droplet and local flow
statistics are well approximated using a lower, averaged value of surface
tension, allowing us to extend the framework developed by Kolmogorov (1949) and
Hinze (1955) for surfactant-free bubbles to surfactant-laden droplets. We find
the Kolmogorov-Hinze scale () is indeed a pivotal length scale in the
droplets' dynamics, separating the coalescence-dominated and the
breakage-dominated regimes in the droplet size distribution. We see that
droplets smaller than have spheroid-like shapes, whereas larger droplets
have long convoluted filamentous shapes with diameters equal to . As a
result, droplets smaller than have areas that scale as , while
larger droplets have areas that scale as , where is the droplet
equivalent diameter. We further characterise the filamentous droplets by
computing the number of handles (loops of the dispersed phase extending into
the carrier phase) and voids (regions of the carrier phase enclosed by the
dispersed phase) on each droplet. The number of handles per unit length of
filament () scales inversely with surface tension, while the
number of voids is independent of surface tension. Handles are indeed an
unstable feature of the interface and are destroyed by the restoring effect of
surface tension, whereas voids can move freely inside the droplets.Comment: 31 pages, 13 figure
Collective dynamics of dense hairy surfaces in turbulent flow
Flexible filamentous beds interacting with a turbulent flow represent a
fundamental setting for many environmental phenomena, e.g., aquatic canopies in
marine current. Exploiting direct numerical simulations at high Reynolds number
where the canopy stems are modelled individually, we provide evidence on the
essential features of the honami/monami collective motion experienced by hairy
surfaces over a range of different flexibilities, i.e., Cauchy number. Our
findings clearly confirm that the collective motion is essentially driven by
fluid flow turbulence, with the canopy having in this respect a fully-passive
behavior. Instead, some features pertaining to the structural response turn out
to manifest in the motion of the individual canopy elements when focusing, in
particular, on the spanwise oscillation and/or on sufficiently small Cauchy
numbers
The effect of particle anisotropy on the modulation of turbulent flows
We investigate the modulation of turbulence caused by the presence of
finite-size dispersed particles. Bluff (isotropic) spheres vs slender
(anisotropic) fibers are considered to understand the influence of the object
shape on altering the carrier flow. While at a fixed mass fraction - but
different Stokes number - both objects provide a similar bulk effect
characterized by a large-scale energy depletion, a scale-by-scale analysis of
the energy transfer reveals that the alteration of the whole spectrum is
intrinsically different. For bluff objects, the classical energy cascade is
shrinked in its extension but unaltered in the energy content and its typical
features, while for slender ones we find an alternative energy flux which is
essentially mediated by the fluid-solid coupling.Comment: 11 pages, 6 figure
The impact of porous walls on the rheology of suspensions
We study the effect of isotropic porous walls on a plane Couette flow laden
with spherical and rigid particles. We perform a parametric study varying the
volume fraction between and , the porosity between and
and the non-dimensional permeability between and We
find that the porous walls induce a progressive decrease in the suspension
effective viscosity as the wall permeability increases. This behavior is
explained by the weakening of the wall-blocking effect and by the appearance of
a slip velocity at the interface of the porous medium, which reduces the shear
rate in the channel. Therefore, particle rotation and the consequent velocity
fluctuations in the two phases are dampened, leading to reduced particle
interactions and particle stresses. Based on our numerical evidence, we provide
a closed set of equations for the suspension viscosity, which can be used to
estimate the suspension rheology in the presence of porous walls
Large is different: Nonmonotonic behavior of elastic range scaling in polymeric turbulence at large Reynolds and Deborah numbers
We use direct numerical simulations to study homogeneous and isotropic turbulent flows of dilute polymer solutions at high Reynolds and Deborah numbers. We find that for small wave numbers k, the kinetic energy spectrum shows Kolmogorov-like behavior that crosses over at a larger k to a novel, elastic scaling regime, E(k) ∼ k−ξ, with ξ ≈ 2.3. We study the contribution of the polymers to the flux of kinetic energy through scales and find that it can be decomposed into two parts: one increase in effective viscous dissipation and a purely elastic contribution that dominates over the nonlinear flux in the range of k over which the elastic scaling is observed. The multiscale balance between the two fluxes determines the crossover wave number that depends nonmonotically on the Deborah number. Consistently, structure functions also show two scaling ranges, with intermittency present in both of them in equal measure.journal articl
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