Turbulence and Passive scalar Advection:

Laminar Flows - Hydrodynamic Theory:

Micro- & Nanofluidics:

Lattice Boltzmann Theory Simulation from nano- to macroscales (Link to my ERC Project):

Lattice Boltzmann on the QPACE Supercomputer (Compressible and Reactive Flows):

Graphics Processing Unit (GPU) Implementation of Lattice Boltzmann models:

Self-Glassiness in Binary Models (Generalized Landau-Ginzburg models):

The
first part of my PhD thesis (advised by R. Benzi and L. Biferale in
Rome University) was devoted to the understanding of one of the most
intriguing and long standing problem in the statistical theory of
turbulence, i.e. intermittency of the dynamical fields and its small
scale universality. Using an exact closure theory based on
time-dependent random multiplicative processes, I performed a
systematic non perturbative calculation to determine the anomalous
scaling exponents in a class of dynamical systems known as 'shell models',
whose intermittency is found numerically very close to the real one. I
also performed non perturbative calculations together with M.M.
Afonso (Institute de Mécanique des fluides de Toulouse),
providing evidence for the recovery of small scale universality in
passive scalar advection against disomogeneous large scale
fluctuations. The issue of universality has also been
addressed in a work (in collaboration with L. Biferale (Rome
University), A. Lanotte (CNR) , M. Cencini (CNR) and F. Toschi
(Eindhoven University)) where, for the first time, we
provided numerical evidence that the zero mode scenario well
known for Kraichnan ensembles of passive scalar advection also applies
to the non linear Navier-Stokes equations stirred by a random forcing
with a power law spectrum. I also gave contribution in the problem of
turbulence modelling from the point of view of kinetic theory: a
relevant long standing question arises concerning the importance of
non-hydrodynamic fields, i.e. high order kinetic moments of the
Boltzmann distribution, sometimes called ghost fields. The influence of
these high order kinetic modes on the dissipative properties of
turbulence dynamics has been studied and controlled in Shell models of
turbulence providing a net evidence that the hydrodynamic manifold is
very robust towards large fluctuations of non hydrodynamic fields.

Laminar Flows - Hydrodynamic Theory:

The
recent blossoming of research in micro-fluidics has prompted renewed
interest in the possibility of slip boundary conditions at the contact
of a liquid with a solid wall. While many experiments have provided
evidence for a violation of the classical no-slip boundary condition at
small spatial scales, the physical mechanisms responsible for this
phenomenon are still unclear. An interesting possibility is the recent
discovery of what appear to be small gas nano-bubbles or pockets
attached to the wall. Along these lines, in collaboration with A.
Prosperetti (Baltimore, USA) and K. Sugiyama (Fluid Eng. Labs, Tokyo
University), I have carried out a rigorous study of slippage properties
arising from boundary heterogeneities and surface roughness. We studied
the nature of the effective velocity boundary condition for liquid flow
over a boundary on which small free-slip islands are randomly
distributed and explored the macroscopic consequences of the existence
of such drag-reducing structures on a solid wall. This was the first
paper in the microfluidics literature of the recent years where the
Stokes equation has been coupled to randomly realized boundary
conditions using ensemble averaging methods. Also, based on
perturbative analysis of the Stokes equations, I was the first one to
propose an analytical estimate of the effect of capillarity on the
boundary condition, an effect that has been reported experimentally but
never controlled from the analytical point of view.

Micro- & Nanofluidics:

In
a series of experiments and numerical simulations in collaboration with
D. Lohse (Twente University, The Netherlands) I studied the motion of
millimetric droplets over super-hydrophobic surfaces. These kind of
materials can exhibit high effective contact angles, therefore
providing low friction mechanism (the so-called Lotus effect) with a
wide range of interdisciplinary applications. When this
super-hydrophobic state breaks down, the drop moves to a collapsed
state with smaller contact angle. This transition was studied with
numerical simulations and led to the first strong evidence of a new
filling infiltration named 'zipping wetting'. These results were
confronted with experiments using ultrahigh-speed imaging up to
50000fps providing state of the art work where numerics, theory and
experiments were joint together. This research was highlighted in the
national Dutch Newspaper (see Journal/media coverage section in my CV).

Lattice Boltzmann Theory Simulation from nano- to macroscales (Link to my ERC Project):

In
collaboration with R. Benzi, L. Biferale (both in Rome University), S.
Succi (CNR) and F. Toschi (Eindhoven University) we have developed a
computational-theoretical approach based on the Lattice Boltzmann
Method (LBM) aimed at the understanding of relevant physical aspects of
small scale hydrodynamcs where confinement may play a relevant
role. I initially studied and quantified boundary condition effects
(slip, mixed boundary conditions for laminar flows) using the LBM
and proved its potentiality for the simulations of single phase
micro-flows at finite Knudsen numbers. I then moved to the description
of the interface physics and isothermal multiphase fluid flows mainly
from the point of view of LBM's diffuse interface hydrodynamics. I
showed how to extend the well know 'Shan-Chen' (SC) model from bulk
descriptions to situations where one needs to include interactions with
solid walls and model the concept of wetting angles. Moreover, I have
been the first to propose the extension of this model to multi-range
potentials where one can achieve better realistic control of the
interface properties emerging at the hydrodynamical level. Also, in
collaboration with X. Shan and H. Chen (EXA Corporation, USA), I proved
analytically that the SC model can be made compliant with an
underlying free energy, thus taking a significant step towards
reconciling / relating the two most used LBM for multiphase flows, i.e.
the SC method and the 'free energy method'. Using the developed
approach, I have provided quantitative evidence that the LBM approach
correctly captures the essential features of a variety of micro,
nano-flows, which cannot be described by atomistic methods for want of
computational power. This led to the -first ever done- quantitative
comparison of The LBM simulations against molecular dynamics
results on wetting transitions for micro-channel flows with
micro-corrugated grooves. Also, studying the old standing Landau-Levich
problem for the onset of entrapment of a liquid film out of a bath, I
made quantitative comparisons between LBM and fully continuous
lubrication hydrodynamics, further extending the prediction of the
latter using the results of LBM in a domain where lubrication
approximation fails. In a recent paper, again in collaboration with X.
Shan and H. Chen, thermal LBM incorporating the effects of
external/internal force fields have been placed within the framework of
discrete kinetic theory, providing striking evidence on how LBM is able
to fully resolve at the same time thermo-hydrodynamical evolutions of
complex flows coupled to a fully resolved diffuse interface dynamics.
Using these ideas, numerical algorithm based on the lattice Boltzmann
method and its
applications for simulations of turbulent convection in presence of
bubbles, showing appreciable changes in the distribution of pressure,
density and temperature
fluctuations inside a convective cell.

In recent works, I provided a clear evidence that LBM with multi-range potentials can capture some non trivial dynamical aspects of soft flowing materials both theoretically and numerically. In my recent papers I also started to explore different theoretical formulations of the lattice Boltzmann equation with finite volumes and/or adaptive meshes.

In recent works, I provided a clear evidence that LBM with multi-range potentials can capture some non trivial dynamical aspects of soft flowing materials both theoretically and numerically. In my recent papers I also started to explore different theoretical formulations of the lattice Boltzmann equation with finite volumes and/or adaptive meshes.

Lattice Boltzmann on the QPACE Supercomputer (Compressible and Reactive Flows):

I
recently started a collaboration with R. Tripiccione, F. Mantovani,
S.F. Schifano, M. Pivanto (Ferrara University), with L. Biferale and A.
Scagliarini (Rome University) and with F. Toschi (Eindhoven
University). We managed to implement a fluid dynamical system on the
Qpace supercomputer. The machine structure is a three-dimensional
torus of identical processing nodes, based on IBM's PowerXCell 8i
processors. These nodes are tightly coupled by an Xilinx Virtex-5-based
FPGA, application-optimized network processor attached to the
PowerXCell 8i processor. The three Identical QPACE supercomputers at
Jülich Research Centre (University of Regensburg and University of
Wuppertal) are as of November 2009 topping the Green500 list of most
energy efficient supercomputers in the world and are at the same time
ranking at 110, 111 and 112 place on the Top500 list of most powerful
supercomputers. In December 2009 we used QPACE to perform the first
large scale physics campaign since the machine was commissioned: the
challenge is twofold:

i) adapting a complex numerical algorithm to the architecture of the Cell

ii) partitioning the computation on a large set of processing elements.

We have studied Rayleigh-Taylor instability with numerical resolutions ever reached before to appreciate the effect of compressibility. We have systematically studied correlation functions and small scale intermittency in both velocity and temperature fields, singling out the effect of the adiabatic gradient in stopping the turbulent fronts while expanding. We are currently performing numerical simulations of reactive flows with the idea to study the emergence of small scale corrections to hydrodynamical fields statistics.

i) adapting a complex numerical algorithm to the architecture of the Cell

ii) partitioning the computation on a large set of processing elements.

We have studied Rayleigh-Taylor instability with numerical resolutions ever reached before to appreciate the effect of compressibility. We have systematically studied correlation functions and small scale intermittency in both velocity and temperature fields, singling out the effect of the adiabatic gradient in stopping the turbulent fronts while expanding. We are currently performing numerical simulations of reactive flows with the idea to study the emergence of small scale corrections to hydrodynamical fields statistics.

Graphics Processing Unit (GPU) Implementation of Lattice Boltzmann models:

In
collaboration with R. Benzi (Rome University), S. Succi and
M.Bernaschi (both in CNR) I have proposed a graphic processing unit GPU
implementation of the multicomponent lattice Boltzmann equation with
multirange interactions for soft-glassy materials. Performance
measurements for flows under shear indicate a GPU/CPU speed up in
excess of 10 for 1024^2 grids. Such significant speed up permits to
carry out multimillion time-steps simulations of 1024^2 grids within
tens of hours of GPU time, thereby considerably expanding the scope of
the glassy Lattice Boltzmann simulations toward the investigation of
long-time relaxation properties of soft-flowing glassy materials.

Self-Glassiness in Binary Models (Generalized Landau-Ginzburg models):

In
collaboration with R. Benzi (Rome University), S. Succi and
M.Bernaschi (both in CNR) I have presented a new phase-field model for
binary fluids exhibiting typical signatures of self-glassiness, such as
long-time relaxation, ageing and long-term dynamical arrest. The
present model allows the cost of building an interface to become
locally zero, while preserving global positivity of the overall surface
tension. An important consequence of this property, which we prove
analytically, is the emergence of compact configurations of fluid
density. Owing to their finite-size support, these ``compactons'' can
be arbitrarily superposed, thereby providing a direct link between the
ruggedness of the free-energy landscape and morphological complexity in
configurational space. The analytical picture is supported by numerical
simulations of the proposed phase-field equation.

Sample LBM code for COST school

Sample LBM code for COST school