85 research outputs found
Heat flux scaling in turbulent Rayleigh-B\'enard convection with an imposed longitudinal wind
We present a numerical study of Rayleigh-B\'enard convection disturbed by a
longitudinal wind. Our results show that under the action of the wind, the
vertical heat flux through the cell initially decreases, due to the mechanism
of plumes-sweeping, and then increases again when turbulent forced convection
dominates over the buoyancy. As a result, the Nusselt number is a non-monotonic
function of the shear Reynolds number. We provide a simple model that captures
with good accuracy all the dynamical regimes observed. We expect that our
findings can lead the way to a more fundamental understanding of the of the
complex interplay between mean-wind and plumes ejection in the
Rayleigh-B\'enard phenomenology.Comment: 5 pages, 4 figure
Mesoscopic simulation study of wall roughness effects in micro-channel flows of dense emulsions
We study the Poiseuille flow of a soft-glassy material above the jamming
point, where the material flows like a complex fluid with Herschel- Bulkley
rheology. Microscopic plastic rearrangements and the emergence of their spatial
correlations induce cooperativity flow behavior whose effect is pronounced in
presence of confinement. With the help of lattice Boltzmann numerical
simulations of confined dense emulsions, we explore the role of geometrical
roughness in providing activation of plastic events close to the boundaries. We
probe also the spatial configuration of the fluidity field, a continuum
quantity which can be related to the rate of plastic events, thereby allowing
us to establish a link between the mesoscopic plastic dynamics of the jammed
material and the macroscopic flow behaviour
Statistics of small scale vortex filaments in turbulence
We study the statistical properties of coherent, small-scales,
filamentary-like structures in Turbulence. In order to follow in time such
complex spatial structures, we integrate Lagrangian and Eulerian measurements
by seeding the flow with light particles. We show that light particles
preferentially concentrate in small filamentary regions of high persistent
vorticity (vortex filaments). We measure the fractal dimension of the
attracting set and the probability that two particles do not separate for long
time lapses. We fortify the signal-to-noise ratio by exploiting multi-particles
correlations on the dynamics of bunches of particles. In doing that, we are
able to give a first quantitative estimation of the vortex-filaments
life-times, showing the presence of events as long as the integral correlation
time. The same technique introduced here could be used in experiments as long
as one is capable to track clouds of bubbles in turbulence for a relatively
long period of time, at high Reynolds numbers; shading light on the dynamics of
small-scale vorticity in realistic turbulent flows.Comment: 5 pages, 5 figure
Law of the wall in an unstably stratified turbulent channel flow
We perform direct numerical simulations of an unstably stratified turbulent
channel flow to address the effects of buoyancy on the boundary layer dynamics
and mean field quantities. We systematically span a range of parameters in the
space of friction Reynolds number () and Rayleigh number (). Our
focus is on deviations from the logarithmic law of the wall due to buoyant
motion. The effects of convection in the relevant ranges are discussed
providing measurements of mean profiles of velocity, temperature and Reynolds
stresses as well as of the friction coefficient. A phenomenological model is
proposed and shown to capture the observed deviations of the velocity profile
in the log-law region from the non-convective case
Fluidisation and plastic activity in a model soft-glassy material flowing in micro-channels with rough walls
By means of mesoscopic numerical simulations of a model soft-glassy material,
we investigate the role of boundary roughness on the flow behaviour of the
material, probing the bulk/wall and global/local rheologies. We show that the
roughness reduces the wall slip induced by wettability properties and acts as a
source of fluidisation for the material. A direct inspection of the plastic
events suggests that their rate of occurrence grows with the fluidity field,
reconciling our simulations with kinetic elasto-plastic descriptions of jammed
materials. Notwithstanding, we observe qualitative and quantitative differences
in the scaling, depending on the distance from the rough wall and on the
imposed shear. The impact of roughness on the orientational statistics is also
studied
Mesoscopic model for soft flowing systems with tunable viscosity ratio
We propose a mesoscopic model of binary fluid mixtures with tunable viscosity
ratio based on the two-range pseudo-potential lattice Boltzmann method, for the
simulation of soft flowing systems. In addition to the short range repulsive
interaction between species in the classical single-range model, a competing
mechanism between the short range attractive and mid-range repulsive
interactions is imposed within each species. Besides extending the range of
attainable surface tension as compared with the single-range model, the
proposed scheme is also shown to achieve a positive disjoining pressure,
independently of the viscosity ratio. The latter property is crucial for many
microfluidic applications involving a collection of disperse droplets with a
different viscosity from the continuum phase. As a preliminary application, the
relative effective viscosity of a pressure-driven emulsion in a planar channel
is computed.Comment: 14page
Lattice Boltzmann Methods for thermal flows: continuum limit and applications to compressible Rayleigh-Taylor systems
We compute the continuum thermo-hydrodynamical limit of a new formulation of
lattice kinetic equations for thermal compressible flows, recently proposed in
[Sbragaglia et al., J. Fluid Mech. 628 299 (2009)]. We show that the
hydrodynamical manifold is given by the correct compressible Fourier-
Navier-Stokes equations for a perfect fluid. We validate the numerical
algorithm by means of exact results for transition to convection in
Rayleigh-B\'enard compressible systems and against direct comparison with
finite-difference schemes. The method is stable and reliable up to temperature
jumps between top and bottom walls of the order of 50% the averaged bulk
temperature. We use this method to study Rayleigh-Taylor instability for
compressible stratified flows and we determine the growth of the mixing layer
at changing Atwood numbers up to At ~ 0.4. We highlight the role played by the
adiabatic gradient in stopping the mixing layer growth in presence of high
stratification and we quantify the asymmetric growth rate for spikes and
bubbles for two dimensional Rayleigh- Taylor systems with resolution up to Lx
\times Lz = 1664 \times 4400 and with Rayleigh numbers up to Ra ~ 2 \times
10^10.Comment: 26 pages, 13 figure
Unravelling the role of phoretic and hydrodynamic interactions in active colloidal suspensions
Active fluids comprise a variety of systems composed of elements immersed in a fluid environment which can convert some form of energy into directed motion; as such they are intrinsically out-of-equilibrium in the absence of any external force. A fundamental problem in the physics of active matter concerns the understanding of how the characteristics of autonomous propulsion and agent-agent interactions determine the collective dynamics of the system. We study numerically the suspensions of self-propelled diffusiophoretic colloids, in (quasi)-2d configurations, accounting for both dynamically resolved solute-mediated phoretic interactions and solvent-mediated hydrodynamic interactions. Our results show that the system displays different scenarios at changing the colloid-solute affinity and it develops a cluster phase in the chemoattractive case. We study the statistics of cluster sizes and cluster morphologies for different magnitudes of colloidal activity. Finally, we provide evidences that hydrodynamics plays a relevant role in the aggregation kinetics and cluster morphology, significantly hindering cluster growth
Hydrodynamic and geometric effects in the sedimentation of model run-and-tumble microswimmers
The sedimentation process in an active suspension is the result of the competition between gravity and the autonomous motion of particles. We carry out simulations of run-and-tumble squirmers that move in a fluid medium, focusing on the dependence of the non-equilibrium steady state on the swimming properties. We find that for large enough activity, the density profiles are no longer simple exponentials; we recover the numerical results through the introduction of a local effective temperature, suggesting that the breakdown of the Perrin-like exponential form is a collective effect due to fluid-mediated dynamic correlations among particles. We show that analogous concepts can also fit the case of active non-motile particles, for which we report the first study of this kind. Moreover, we provide evidence of scenarios where the solvent hydrodynamics induces non-local effects which require the full three-dimensional dynamics to be taken into account in order to understand sedimentation in active suspensions. Finally, analyzing the statistics of the orientations of microswimmers, the emergence of a height-dependent polar order in the system is discussed
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