4,129 research outputs found
Transport properties of heavy particles in high Reynolds number turbulence
The statistical properties of heavy particle trajectories in high Reynolds
numbers turbulent flows are analyzed. Dimensional analysis assuming Kolmogorov
scaling is compared with the result of numerical simulation using a synthetic
turbulence advecting field. The non-Markovian nature of the fluid velocity
statistics along the solid particle trajectories is put into evidence, and its
relevance in the derivation of Lagrangian transport models is discussed.Comment: 30 pages, 11 eps figures included. To appear in Physics of Fluid
Efficient prediction of broadband trailing edge noise and application to porous edge treatment
Trailing edge noise generated by turbulent flow traveling past an edge of an
airfoil is one of the most essential aeroacoustic sound generation mechanisms.
It is of great interest for noise problems in various areas of industrial
application. First principle based CAA with short response time are needed in
the industrial design process for reliable prediction of spectral differences
in turbulent-boundary-layer trailing-edge noise due to design modifications. In
this paper, an aeroacoustic method is studied, resting on a hybrid CFD/CAA
procedure. In a first step RANS simulation provides a time-averaged solution,
including the mean-flow and turbulence statistics such as length-scale,
time-scale and turbulence kinetic energy. Based on these, fluctuating sound
sources are then stochastically generated by the Fast Random Particle-Mesh
Method to simulate in a second CAA step broadband aeroacoustic sound. From
experimental findings it is well known that porous trailing edges significantly
lower trailing edge noise level over a large range of frequencies reaching up
to 8dB reduction. Furthermore, sound reduction depends on the porous material
parameters, e.g. geometry, porosity, permeability and pore size. The paper
presents first results for an extended hybrid CFD/CAA method including porous
materials with prescribed parameters. To incorporate the effect of porosity, an
extended formulation of the Acoustic Perturbation Equations with source terms
is derived based on a reformulation of the volume averaged Navier-Stokes
equations into perturbation form. Proper implementation of the Darcy and
Forchheimer terms is verified for sound propagation in homogeneous and
anisotropic porous medium. Sound generation is studied for a generic symmetric
NACA0012 airfoil without lift to separate secondary effects of lift and camber
on sound from those of the basic edge noise treatments.Comment: 37 page
Non Asymptotic Properties of Transport and Mixing
We study relative dispersion of passive scalar in non-ideal cases, i.e. in
situations in which asymptotic techniques cannot be applied; typically when the
characteristic length scale of the Eulerian velocity field is not much smaller
than the domain size. Of course, in such a situation usual asymptotic
quantities (the diffusion coefficients) do not give any relevant information
about the transport mechanisms. On the other hand, we shall show that the
Finite Size Lyapunov Exponent, originally introduced for the predictability
problem, appears to be rather powerful in approaching the non-asymptotic
transport properties. This technique is applied in a series of numerical
experiments in simple flows with chaotic behaviors, in experimental data
analysis of drifter and to study relative dispersion in fully developed
turbulence.Comment: 19 RevTeX pages + 8 figures included, submitted on Chaos special
issue on Transport and Mixin
Turbulence-resolving simulations of wind turbine wakes
Turbulence-resolving simulations of wind turbine wakes are presented using a
high--order flow solver combined with both a standard and a novel dynamic
implicit spectral vanishing viscosity (iSVV and dynamic iSVV) model to account
for subgrid-scale (SGS) stresses. The numerical solutions are compared against
wind tunnel measurements, which include mean velocity and turbulent intensity
profiles, as well as integral rotor quantities such as power and thrust
coefficients. For the standard (also termed static) case the magnitude of the
spectral vanishing viscosity is selected via a heuristic analysis of the wake
statistics, while in the case of the dynamic model the magnitude is adjusted
both in space and time at each time step. The study focuses on examining the
ability of the two approaches, standard (static) and dynamic, to accurately
capture the wake features, both qualitatively and quantitatively. The results
suggest that the static method can become over-dissipative when the magnitude
of the spectral viscosity is increased, while the dynamic approach which
adjusts the magnitude of dissipation locally is shown to be more appropriate
for a non-homogeneous flow such that of a wind turbine wake
Enviromental patterns and intermittent cascades
Real environmental flows are non-homogeneous, of fundamental interest is to determine and quantify turbulent diffusion from the available conditions of the flow, because the role of buoyancy and rotation modify the flow topology with often the dominant scale occurring when these two forces are in equilibrium. In geophysical flows both in the Atmosphere and the Ocean, the main forcing occurs at the Rossby deformation Radius with both direct and inverse energy cascades [1,2]. The role of the spectra of steady and decaying turbulence is important as well as its scale to scale conditions, so that a large range of scales has to be taken into account. When mixing and dispersion processes are studied, the behaviour of reactants or pollutants is seen to depend of both the intermittency of the vorticity and energy spectra. If irreversible molecular mixing has to be accounted, the range of scales spans from hundreds of Kilometres to the Bachelor or Kolmogorov sub millimeter scales. It is important to evaluate mixing and compare with oscillating grid experiments, Redondo [3], across a density interface measuring entrainment and grid decaying non steady mixing. These experiments are evaluated and compared with results of a Kinematic simulation model, Castilla [4]. The local vorticity is evaluated confirming the trapping of tracers in the strong vertical regions in 2D flows, but showing also that hyperdiffusion may also occur. Intermittency was evaluated using numerical evaluation of higher order moments in different types of 2D and 3D turbulence.Peer ReviewedPostprint (published version
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