72 research outputs found
Experimental assessment of a new form of scaling law for near-wall turbulence
Scaling laws and intermittency in the wall region of a turbulent flow are
addressed by analyzing moderate Reynolds number data obtained by single
component hot wire anemometry in the boundary layer of a flat plate. The paper
aims in particular at the experimental validation of a new form of refined
similarity recently proposed for the shear dominated range of turbulence, where
the classical Kolmogorov-Oboukhov inertial range theory is inappropriate. An
approach inspired to the extended self-similarity allows for the extraction of
the different power laws for the longitudinal structure functions at several
wall normal distances. A double scaling regime is found in the logarithmic
region, confirming previous experimental results. Approaching the wall, the
scaling range corresponding to the classical cascade-dominated range tends to
disappear and, in the buffer layer, a single power law is found to describe the
available range of scales. The double scaling is shown to be associated with
two different forms of refined similarity. The classical form holds below the
shear scale L s . The other, originally introduced on the basis of DNS data for
a turbulent channel, is experimentally confirmed to set up above L s . Given
the experimental diffulties in the evaluation of the instantaneous dissipation
rate, some care is devoted to check that its one-dimensional surrogate does not
bias the results. The increased intermittency as the wall is approached is
experimentally found entirely consistent with the failure of the refined
Kolmogorov-Oboukhov similarity and the establishment of its new form near the
wall.Comment: 27 pages, 9 figure
Statistical Properties of Turbulence: An Overview
We present an introductory overview of several challenging problems in the
statistical characterisation of turbulence. We provide examples from fluid
turbulence in three and two dimensions, from the turbulent advection of passive
scalars, turbulence in the one-dimensional Burgers equation, and fluid
turbulence in the presence of polymer additives.Comment: 34 pages, 31 figure
Bottleneck effect in three-dimensional turbulence simulations
At numerical resolutions around and above, three-dimensional energy
spectra from turbulence simulations begin to show noticeably shallower spectra
than near the dissipation wavenumber (`bottleneck effect'). This
effect is shown to be significantly weaker in one-dimensional spectra such as
those obtained in wind tunnel turbulence. The difference can be understood in
terms of the transformation between one-dimensional and three-dimensional
energy spectra under the assumption that the turbulent velocity field is
isotropic. Transversal and longitudinal energy spectra are similar and can both
accurately be computed from the full three-dimensional spectra. Second-order
structure functions are less susceptible to the bottleneck effect and may be
better suited for inferring the scaling exponent from numerical simulation
data.Comment: 8 pages, 6 figure
Persistence of small-scale anisotropies and anomalous scaling in a model of magnetohydrodynamics turbulence
The problem of anomalous scaling in magnetohydrodynamics turbulence is
considered within the framework of the kinematic approximation, in the presence
of a large-scale background magnetic field. The velocity field is Gaussian,
-correlated in time, and scales with a positive exponent .
Explicit inertial-range expressions for the magnetic correlation functions are
obtained; they are represented by superpositions of power laws with
non-universal amplitudes and universal (independent of the anisotropy and
forcing) anomalous exponents. The complete set of anomalous exponents for the
pair correlation function is found non-perturbatively, in any space dimension
, using the zero-mode technique. For higher-order correlation functions, the
anomalous exponents are calculated to using the renormalization group.
The exponents exhibit a hierarchy related to the degree of anisotropy; the
leading contributions to the even correlation functions are given by the
exponents from the isotropic shell, in agreement with the idea of restored
small-scale isotropy. Conversely, the small-scale anisotropy reveals itself in
the odd correlation functions : the skewness factor is slowly decreasing going
down to small scales and higher odd dimensionless ratios (hyperskewness etc.)
dramatically increase, thus diverging in the limit.Comment: 25 pages Latex, 1 Figur
A physical-space version of the stretched-vortex subgrid-stress model for large-eddy simulation
Hysteresis phenomenon in turbulent convection
Coherent large-scale circulations of turbulent thermal convection in air have
been studied experimentally in a rectangular box heated from below and cooled
from above using Particle Image Velocimetry. The hysteresis phenomenon in
turbulent convection was found by varying the temperature difference between
the bottom and the top walls of the chamber (the Rayleigh number was changed
within the range of ). The hysteresis loop comprises the one-cell
and two-cells flow patterns while the aspect ratio is kept constant (). We found that the change of the sign of the degree of the anisotropy of
turbulence was accompanied by the change of the flow pattern. The developed
theory of coherent structures in turbulent convection (Elperin et al. 2002;
2005) is in agreement with the experimental observations. The observed coherent
structures are superimposed on a small-scale turbulent convection. The
redistribution of the turbulent heat flux plays a crucial role in the formation
of coherent large-scale circulations in turbulent convection.Comment: 10 pages, 9 figures, REVTEX4, Experiments in Fluids, 2006, in pres
Anomalous scaling of a passive scalar in the presence of strong anisotropy
Field theoretic renormalization group and the operator product expansion are
applied to a model of a passive scalar field, advected by the Gaussian strongly
anisotropic velocity field. Inertial-range anomalous scaling behavior is
established, and explicit asymptotic expressions for the n-th order structure
functions of scalar field are obtained; they are represented by superpositions
of power laws with nonuniversal (dependent on the anisotropy parameters)
anomalous exponents. In the limit of vanishing anisotropy, the exponents are
associated with tensor composite operators built of the scalar gradients, and
exhibit a kind of hierarchy related to the degree of anisotropy: the less is
the rank, the less is the dimension and, consequently, the more important is
the contribution to the inertial-range behavior. The leading terms of the even
(odd) structure functions are given by the scalar (vector) operators. For the
finite anisotropy, the exponents cannot be associated with individual operators
(which are essentially ``mixed'' in renormalization), but the aforementioned
hierarchy survives for all the cases studied. The second-order structure
function is studied in more detail using the renormalization group and
zero-mode techniques.Comment: REVTEX file with EPS figure
Inhibition of human tumour prostate PC-3 cell growth by cannabinoids R(+)-Methanandamide and JWH-015: Involvement of CB2
New System for the Acceleration of the Airflow in Wind Turbines
Background: This patent is based on the wind industry technology called Diffuser Augmented Wind Turbines (DAWTs). This technology consists of a horizontal axis wind turbine, which is housed inside a duct with diverging section in the direction of the free air stream. In this paper, a review of preceding patents related to this technology is carried out.
Objective: This paper presents an innovative patent to improve the performance of horizontal axis wind turbines. In particular, this system is aimed at improving the performance of those turbines that otherwise might not be installed due to the low wind resource existing at certain locations.
Methods: The most innovative elements of this patent are: (1) the semi-spherical grooves, which are mechanized on the surface of the two diffusers in order to guarantee a more energetic boundary layer; (2) the coaxial diffuser, which is located downwind following the first diffuser in order to increase the suction effect on the air mass close to the inlet; (3) the coaxial rings located around the first diffuser outlet, which are used to deflect the external airflow toward the turbine wake; and (4), the selforientating system to orientate the system by the prevailing wind direction.
Results: An application of the patent for increasing the power generated by a horizontal axis wind turbine with three blades is presented. The patent is designed and its performance is evaluated by using a Computational Fluid Dynamics code. The numerical results show that this system rises the airflow going through the rotor of the turbine.
Conclusion: The patented device is an original contribution aimed at enabling a more profitable installation of wind turbines in places where the wind resource is insufficient because of the wind shear caused both by the proximity of the earth and the obstacles on the earth surface.This work was supported by the OASIS Research Project that was cofinanced by CDTI (Spanish Science and Innovation Ministry) and developed with the Spanish companies: Iridium, OHL Concesiones, Abertis, Sice, Indra, Dragados, OHL, Geocisa, GMV, Asfaltos Augusta, Hidrofersa, Eipsa, PyG, CPS, AEC and Torre de Comares Arquitectos S.L and 16 research centres. The authors also acknowledge the partial funding with FEDER funds under the Research Project FC-15-GRUPIN14-004. Finally, we also thank Swanson Analysis Inc. for the use of ANSYS University Research programs as well as the Workbench simulation environment
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