438 research outputs found
Drag reduction in a turbulent boundary layer with sinusoidal riblets
Get PDFWe report on an experimental investigation on the effect of sinusoidal riblets on the near-wall characteristics of a turbulent boundary layer. The investigated riblets are characterized by a fixed wavelength and two different values of the amplitude. We comment on the flow field organization via hot wire anemometry, planar and stereoscopic particle image velocimetry experiments; furthermore, we infer on the friction drag, directly measured with a load cell, comparing the sinusoidal riblets to the reference case of riblets aligned with the mean flow (longitudinal riblets) and the Smooth case.We show that the sinusoidal riblets generally yield higher drag reduction, attaining values as large as 10%, compared with the longitudinal riblets that are limited to 8% under the same conditions. We demonstrate that the drag reduction is associated with an overall attenuation of the turbulence intensity in the buffer layer. Furthermore, we provide statistical evidence of the fact that the sinusoidal riblets are responsible for an attenuation of the Reynolds shear stresses that contribute the most to turbulence production. From the detection of the accelerated events in the buffer layer, we show that the sinusoidal riblets lead to a weakening of the intensity of the events in the streamwise plane and an enhancement of the spanwise induced motion. We relate this mechanism to that responsible for drag reduction when using spanwise wall oscillations, suggesting a possible effect of a secondary alternating vorticity in the grooves of the sinusoidal riblets
Wall bounded flows manipulation using sinusoidal riblets
Get PDFWe experimentally investigate the effects of microgrooves on the development of a zero pressure gradient turbulent boundary layer. Starting from the well-known streamwise aligned riblets, we look at the effect of wavy riblets, characterized by a sinusoidal pattern in the mean flow direction. We perform hot wire experiments as well as particle image velocimetry to get some insights on the effect of the sinusoidal shape on the near wall organisation of the boundary layer. The statistical analysis clearly shows that the wavy pattern has a strong influence on the near wall structure of the boundary layer. The statistical analysis performed using the VITA technique reveals that the coherent structures that characterize the turbulent boundary layer are attenuated by the geometry manipulation. Furthermore, the POD reconstructed velocity fields, measured with PIV, reveal that the manipulation tampers with the momentum exchange occurring between the near wall and the outer region of the boundary layer, hence suggesting a modified turbulence production cycle
A perturbative approach to the Bak-Sneppen Model
Get PDFWe study the Bak-Sneppen model in the probabilistic framework of the Run Time
Statistics (RTS). This model has attracted a large interest for its simplicity
being a prototype for the whole class of models showing Self-Organized
Criticality. The dynamics is characterized by a self-organization of almost all
the species fitnesses above a non-trivial threshold value, and by a lack of
spatial and temporal characteristic scales. This results in {\em avalanches} of
activity power law distributed. In this letter we use the RTS approach to
compute the value of , the value of the avalanche exponent and the
asymptotic distribution of minimal fitnesses.Comment: 4 pages, 3 figures, to be published on Physical Review Letter
Dynamics of Fractures in Quenched Disordered Media
Full text linkWe introduce a model for fractures in quenched disordered media. This model
has a deterministic extremal dynamics, driven by the energy function of a
network of springs (Born Hamiltonian). The breakdown is the result of the
cooperation between the external field and the quenched disorder. This model
can be considered as describing the low temperature limit for crack propagation
in solids. To describe the memory effects in this dynamics, and then to study
the resistance properties of the system we realized some numerical simulations
of the model. The model exhibits interesting geometric and dynamical
properties, with a strong reduction of the fractal dimension of the clusters
and of their backbone, with respect to the case in which thermal fluctuations
dominate. This result can be explained by a recently introduced theoretical
tool as a screening enhancement due to memory effects induced by the quenched
disorder.Comment: 7 pages, 9 Postscript figures, uses revtex psfig.sty, to be published
on Phys. Rev.
Generalized Dielectric Breakdown Model
Full text linkWe propose a generalized version of the Dielectric Breakdown Model (DBM) for
generic breakdown processes. It interpolates between the standard DBM and its
analog with quenched disorder, as a temperature like parameter is varied. The
physics of other well known fractal growth phenomena as Invasion Percolation
and the Eden model are also recovered for some particular parameter values. The
competition between different growing mechanisms leads to new non-trivial
effects and allows us to better describe real growth phenomena.
Detailed numerical and theoretical analysis are performed to study the
interplay between the elementary mechanisms. In particular, we observe a
continuously changing fractal dimension as temperature is varied, and report an
evidence of a novel phase transition at zero temperature in absence of an
external driving field; the temperature acts as a relevant parameter for the
``self-organized'' invasion percolation fixed point. This permits us to obtain
new insight into the connections between self-organization and standard phase
transitions.Comment: Submitted to PR
Invasion Percolation with Temperature and the Nature of SOC in Real Systems
Full text linkWe show that the introduction of thermal noise in Invasion Percolation (IP)
brings the system outside the critical point. This result suggests a possible
definition of SOC systems as ordinary critical systems where the critical point
correspond to set to 0 one of the parameters. We recover both IP and EDEN
model, for , and respectively. For small we find a
dynamical second order transition with correlation length diverging when .Comment: 4 pages, 2 figure
Aeroelastic-structural coupling in antenna prototype for windy open-space
Get PDFThe interaction between wind and an antenna prototype for the low-frequency radio telescope of the Square Kilometer Array (SKA) is experimentally tested in the wind tunnel of the Politecnico di Torino. The tests aim to predict the antenna behaviour during working conditions, i.e. mounted by means of five contact points to a metal grid on sandy ground in the Australian desert. The wind tunnel is characterised by a circular test section having a diameter equal to 3 m and a length equal to 5 m. The height and the distance between the lateral legs of the antenna are equal respectively to 2.2 m and 1.5 m. The tests were performed at increasing wind speed up to 110 km/h. The system under analysis is an aluminium antenna composed by four parts arranged in axial symmetry and each one made of fifteen rods and small plates/wire elements. A numerical parametric model of the system is developed to numerically study the dynamic behaviour of the antenna in the frequency range of interest. The model is able to handle very high modal density and closed spaced modes in multiplicity of four because of the symmetric structure as well as the different shapes of the elements forming the antenna. The wind tunnel results emphasise the fluid-structure coupling of aerodynamics modes and the critical aspects of the boundary conditions for a good prediction of the oscillations amplitudes
Phase separation in systems with absorbing states
Full text linkWe study the problem of phase separation in systems with a positive definite
order parameter, and in particular, in systems with absorbing states. Owing to
the presence of a single minimum in the free energy driving the relaxation
kinetics, there are some basic properties differing from standard phase
separation. We study analytically and numerically this class of systems; in
particular we determine the phase diagram, the growth laws in one and two
dimensions and the presence of scale invariance. Some applications are also
discussed.Comment: Submitted to Europhysics Let
Growing Cayley trees described by Fermi distribution
Full text linkWe introduce a model for growing Cayley trees with thermal noise. The
evolution of these hierarchical networks reduces to the Eden model and the
Invasion Percolation model in the limit , respectively.
We show that the distribution of the bond strengths (energies) is described by
the Fermi statistics. We discuss the relation of the present results with the
scale-free networks described by Bose statistics
Statistical properties of fractures in damaged materials
Full text linkWe introduce a model for the dynamics of mud cracking in the limit of of
extremely thin layers. In this model the growth of fracture proceeds by
selecting the part of the material with the smallest (quenched) breaking
threshold. In addition, weakening affects the area of the sample neighbour to
the crack. Due to the simplicity of the model, it is possible to derive some
analytical results. In particular, we find that the total time to break down
the sample grows with the dimension L of the lattice as L^2 even though the
percolating cluster has a non trivial fractal dimension. Furthermore, we obtain
a formula for the mean weakening with time of the whole sample.Comment: 5 pages, 4 figures, to be published in Europhysics Letter
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