1,689 research outputs found
Shear-Improved Smagorinsky Model for Large-Eddy Simulation of Wall-Bounded Turbulent Flows
A shear-improved Smagorinsky model is introduced based on recent results
concerning shear effects in wall-bounded turbulence by Toschi et al. (2000).
The Smagorinsky eddy-viscosity is modified subtracting the magnitude of the
mean shear from the magnitude of the instantaneous resolved strain-rate tensor.
This subgrid-scale model is tested in large-eddy simulations of plane-channel
flows at two different Reynolds numbers. First comparisons with the dynamic
Smagorinsky model and direct numerical simulations, including mean velocity,
turbulent kinetic energy and Reynolds stress profiles, are shown to be
extremely satisfactory. The proposed model, in addition of being physically
sound, has a low computational cost and possesses a high potentiality of
generalization to more complex non-homogeneous turbulent flows.Comment: 10 pages, 6 figures, added some reference
Aging Illinois White-Tailed Deer by the Tooth Replacement and Wear and the Cementum Annuli Techniques
High Curie temperature Mn 5 Ge 3 thin films produced by non-diffusive reaction
Polycrystalline Mn 5 Ge 3 thin films were produced on SiO 2 using magnetron
sputtering and reactive diffusion (RD) or non-diffusive reaction (NDR). In situ
X-ray diffraction and atomic force microscopy were used to determine the layer
structures, and magnetic force microscopy, superconducting quantum interference
device and ferromagnetic resonance were used to determine their magnetic
properties. RD-mediated layers exhibit similar magnetic properties as MBE-grown
monocrystalline Mn 5 Ge 3 thin films, while NDR-mediated layers show magnetic
properties similar to monocrystalline C-doped Mn 5 Ge 3 C x thin films with
NDR appears as a CMOS-compatible efficient method to
produce good magnetic quality high-curie temperature Mn 5 Ge 3 thin films
EDQNM closure: A homogeneous simulation to support it. A quasi-homogeneous simulation to disprove it
It is known that two-point closures are useful tools for understanding and predicting turbulence. Among the various closures, the Eddy Damped Quasi-Normal Markovian (EDQNM) approach is one of the simplest and, at the same time, most useful. Direct numerical simulations (DNS) can provide information that can be used to test the validity of two-point theories. It is the purpose of the present work to use DNS to validate, or improve upon, EDQNM. A case was selected for which EDQNM is known to give satisfactory results: homogeneous isotropic turbulence. Quantities were then evaluated which may be used to test the assumptions of two-point closure approximations: spectral Lagrangian time scales. The goal was to make a careful and refined study to validate the EDQNM theory. A reference case was built for which EDQNM is likely to give poor results. An attempt to generate a quasi-homogeneous turbulent field containing organized structures, was built by artifically injecting them in the initial conditions. The results of direct simulations using such initial conditions are expected to provide a challenge for EDQNM since this kind of field is simple enough to allow comparisons with two-point theories, but at the same time contains coherent structures which cannot be expected to be accurately accounted for by closures based on expansions about Gaussianity
A Stokes-consistent backflow stabilization for physiological flows
In computational fluid dynamics incoming flow at open boundaries, or \emph{backflow}, often yields to unphysical instabilities for
high Reynolds numbers. It is widely accepted that this is due to the incoming energy
arising from the convection term, which cannot be \emph{a priori} controlled when the velocity field is unknown at the boundary.
In order to improve the robustness of the numerical simulations, we propose a stabilized formulation
based on a penalization of the residual of a weak Stokes problem on the open boundary, whose viscous part controls the
incoming convective energy, while the inertial term contributes to the kinetic energy. We also present different strategies
for the approximation of the boundary
pressure gradient, which is needed for defining the stabilization term.
The method has the advantage that it does not require neither artificial modifications or extensions of the
computational domain. Moreover, it is consistent with the Womersley solution. We illustrate our approach
on numerical examples ~- both academic and real-life -~ relevant to blood and respiratory flows. The results also
show that the stabilization parameter can be reduced with the mesh size
Large-scale bottleneck effect in two-dimensional turbulence
The bottleneck phenomenon in three-dimensional turbulence is generally
associated with the dissipation range of the energy spectrum. In the present
work, it is shown by using a two-point closure theory, that in two-dimensional
turbulence it is possible to observe a bottleneck at the large scales, due to
the effect of friction on the inverse energy cascade. This large-scale
bottleneck is directly related to the process of energy condensation, the
pile-up of energy at wavenumbers corresponding to the domain size. The link
between the use of friction and the creation of space-filling structures is
discussed and it is concluded that the careless use of hypofriction might
reduce the inertial range of the energy spectrum
Environmental dynamics as a structuring factor for microbial carbon utilization in a subtropical coastal lagoon
Laguna de Rocha belongs to a series of shallow coastal lagoons located along South America. It is periodically connected to the sea through a sand bar, exhibiting a hydrological cycle where physicochemical and biological gradients are rapidly established and destroyed. Its most frequent state is the separation of a Northern zone with low salinity, high turbidity and nutrient load, and extensive macrophyte growth, and a Southern zone with higher salinity and light penetration, and low nutrient content and macrophyte biomass. This zonation is reflected in microbial assemblages with contrasting abundance, activity, and community composition. The physicochemical conditions exerted a strong influence on community composition, and transplanted assemblages rapidly transformed to resembling the community of the recipient environment. Moreover, the major bacterial groups responded differently to their passage between the zones, being either stimulated or inhibited by the environmental changes, and exhibiting contrasting sensitivities to gradients. Addition of allochthonous carbon sources induced pronounced shifts in the bacterial communities, which in turn affected the microbial trophic web by stimulating heterotrophic flagellates and virus production. By contrast, addition of organic and inorganic nutrient sources (P or N) did not have significant effects. Altogether, our results suggest that (i) the planktonic microbial assemblage of this lagoon is predominantly carbon-limited, (ii) different bacterial groups cope differently with this constraint, and (iii) the hydrological cycle of the lagoon plays a key role for the alleviation or aggravation of bacterial carbon limitation. Based on these findings we propose a model of how hydrology affects the composition of bacterioplankton and of carbon processing in Laguna de Rocha.This might serve as a starting hypothesis for further studies about the microbial ecology of this lagoon, and of comparable transitional systems.Fil: Alonso, Cecilia. Universidad de la República; UruguayFil: Piccini, Claudia. Instituto de Investigaciones Biológicas "Clemente Estable"; UruguayFil: Unrein, Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); ArgentinaFil: Bertoglio, Florencia. Universidad de la República; UruguayFil: Conde, Daniel. Universidad de la República; UruguayFil: Pernthaler, Jakob. Universitat Zurich; Suiz
Decay of scalar variance in isotropic turbulence in a bounded domain
The decay of scalar variance in isotropic turbulence in a bounded domain is
investigated. Extending the study of Touil, Bertoglio and Shao (2002; Journal
of Turbulence, 03, 49) to the case of a passive scalar, the effect of the
finite size of the domain on the lengthscales of turbulent eddies and scalar
structures is studied by truncating the infrared range of the wavenumber
spectra. Analytical arguments based on a simple model for the spectral
distributions show that the decay exponent for the variance of scalar
fluctuations is proportional to the ratio of the Kolmogorov constant to the
Corrsin-Obukhov constant. This result is verified by closure calculations in
which the Corrsin-Obukhov constant is artificially varied. Large-eddy
simulations provide support to the results and give an estimation of the value
of the decay exponent and of the scalar to velocity time scale ratio
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A tangential regularization method for backflow stabilization in hemodynamics
In computational simulations of fluid flows, instabilities at the Neumann boundaries may appear during backflow regime. It is widely accepted that this is due to the incoming energy at the boundary, coming from the convection term, which cannot be controlled when the velocity field is unknown. We propose a stabilized formulation based on a local regularization of the fluid velocity along the tangential directions on the Neumann boundaries. The stabilization term is proportional to the amount of backflow, and does not require any further assumption on the velocity profile. The perfomance of the method is assessed on a twoand three-dimensional Womersley flows, as well as considering a hemodynamic physiological regime in a patient-specific aortic geometry
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