1,669 research outputs found
Coherent structures in fully-developed pipe turbulence
A turbulent mean profile for pipe flow is prescribed which closely matches
experimental observations. The nature of perturbations superimposed upon this
profile is then considered. Optimal growth calculations predict two distinct
classes of structures, clearly associated with near-wall and large-scale
structures. Quantitative correspondence of the spanwise wavelength of
wall-structures with experimental observations is very good. The response to
harmonic forcing is also considered, and the linear growth tested with direct
numerical simulation of forced turbulence. Despite the very simple eddy
viscosity assumption, this linear approach predicts well the surprisingly large
growth of outer-scale modes in the bulk flow.
Un profil moyen turbulent est prescrit dans une conduite cylindrique, en
adequation avec les observations experimentales. Nous considerons ensuite la
nature des perturbations a cet ecoulement synthetique. Le calcul des
croissances optimales predit deux types de structures, associees respectivement
aux structures de proche-paroi et de grande echelle. Un excellent accord
quantitatif est trouve avec les resultats experimentaux quant a la longueur
d'onde transversale. La reponse harmonique est egalement etudiee, et la
croissance lineaire observee comparee a des simulations numeriques directes de
turbulence forcee. Malgre de l'hypothese simple de type `Eddy viscosity', cette
approche lineaire predit efficacement la croissance spectaculaire des modes de
grande echelle au coeur de l'ecoulement.Comment: 5 pages; Congres Francais de Mecanique, Marseille (2009
A note on the stability of slip channel flows
We consider the influence of slip boundary conditions on the modal and
non-modal stability of pressure-driven channel flows. In accordance with
previous results by Gersting (1974) (Phys. Fluids, 17) but in contradiction
with the recent investigation of Chu (2004) (C.R. Mecanique, 332), we show that
slip increases significantly the value of the critical Reynolds number for
linear instability. The non-modal stability analysis however reveals that the
slip has a very weak influence on the maximum transient energy growth of
perturbations at subcritical Reynolds numbers. Slip boundary conditions are
therefore not likely to have a significant effect on the transition to
turbulence in channel flows
Increasing the agricultural sustainability of closed agrivoltaic systems with the integration of vertical farming: A case study on baby-leaf lettuce
The photovoltaic (PV) greenhouses are closed agrivoltaic (CA) systems that allow the production of energy and food on the same land, but may result in a yield reduction when the shading of the PV panels is excessive. Adopting innovative cropping systems can increase the yield of the CA area, generating a more productive and sustainable agrosystem. In this case study we quantified the increase of land productivity derived from the integration of an experimental vertical farm (VF) for baby leaf lettuce inside a pre-existing commercial CA. The mixed system increased the yield by 13 times compared to the CA and the average LER was 1.31, but only 12 % of the energy consumption was covered by the CA energy. To achieve the energy self-sufficiency and avoid the related CO2 emissions, the VF area should not exceed 7–18 % of the CA area, depending on the PV energy yield and the daily light integral (DLI) of the LED lighting, meaning a land consumption from 5 to 14 times higher than the VF area. The support of the PV energy was essential for the profitability of the VFCA. Design features and solutions were proposed to increase the agronomic and economic sustainability of the VFCA. The VFs can be considered a possible answer for the reconversion of the actual underutilized CAs with high PV cover ratios into productive and efficient cropping systems, but a trade-off between energy production and land consumption should be identified to ensure an acceptable environmental sustainability of the mixed system
Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow
The magnetorotational (MRI) dynamo has long been considered one of the
possible drivers of turbulent angular momentum transport in astrophysical
accretion disks. However, various numerical results suggest that this dynamo
may be difficult to excite in the astrophysically relevant regime of magnetic
Prandtl number (Pm) significantly smaller than unity, for reasons currently not
well understood. The aim of this article is to present the first results of an
ongoing numerical investigation of the role of both linear and nonlinear
dissipative effects in this problem. Combining a parametric exploration and an
energy analysis of incompressible nonlinear MRI dynamo cycles representative of
the transitional dynamics in large aspect ratio shearing boxes, we find that
turbulent magnetic diffusion makes the excitation and sustainment of this
dynamo at moderate magnetic Reynolds number (Rm) increasingly difficult for
decreasing Pm. This results in an increase in the critical Rm of the dynamo for
increasing kinematic Reynolds number (Re), in agreement with earlier numerical
results. Given its very generic nature, we argue that turbulent magnetic
diffusion could be an important determinant of MRI dynamo excitation in disks,
and may also limit the efficiency of angular momentum transport by MRI
turbulence in low Pm regimes.Comment: 7 pages, 6 figure
Agricultural sustainability estimation of the European photovoltaic greenhouses
The integration of the photovoltaic (PV) energy in the greenhouse farm has raised concerns on the agricultural sustainability of this specific agrosystem in terms of crop planning and management, due to the shading cast by the PV panels on the canopy. The PV greenhouse (PVG) can be classified on the basis of the PV cover ratio (PVR), that is the ratio of the projected area of PV panels to the ground and the total greenhouse area. In this paper, we estimated the yield of 14 greenhouse horticultural and floricultural crops inside four commercial PVG types spread in southern Europe, with PVR ranging from 25 to 100%. The aim of the work is to identify the PVG types suitable for the cultivation of the considered species, based on the best trade-off between PV shading and crop production. The daily light integral (DLI) was used to compare the light scenarios inside the PVGs to the crop light requirements, and estimate the potential yield. The structures with a PVR of 25% were compatible with the cultivation of all considered species, including the high light demanding ones (tomato, cucumber, sweet pepper), with an estimated negligible or limited yield reduction (below 25%). The medium light species (such as asparagus) with an optimal DLI lower than 17 mol m−2 d−1 and low light crops can be cultivated inside PVGs with a PVR up to 60%. Only low light demanding floricultural species with an optimal DLI lower than 10 mol m−2 d−1, such as poinsettia, kalanchoe and dracaena, were compatible inside PVGs with a PVR up to 100%. Innovative cropping systems should be considered to overcome the penalizing light scenarios of the PVGs with high PVR, also implementing LED supplementary lighting. This paper contributes to identify the sustainable PVG types for the chosen species and the alternative crop managements in terms of transplantation period and precision agriculture techniques, aimed at increasing the crop productivity and adaptability inside the PVG agrosystems
Growth and instability of a laminar plume in a strongly stratified environment
Experimental studies of laminar plumes descending under gravity into stably stratified environments have shown the existence of a critical injection velocity beyond which the plume exhibits a bifurcation to a coiling instability in three dimensions or a sinuous instability in a Hele-Shaw flow. In addition, flow visualization has shown that, prior to the onset of the instability, a stable base flow is established in which the plume penetrates to a depth significantly smaller than the neutral buoyancy depth. Moreover, the fresh water that is viscously entrained by the plume recirculates within a ‘conduit’ whose boundary with the background stratification appears sharp. Beyond the bifurcation, the buckling plume takes the form of a travelling wave of varying amplitude, confined within the conduit, which disappears at the penetration depth. To determine the mechanisms underlying these complex phenomena, which take place at a strikingly low Reynolds number but a high Schmidt number, we study here a two-dimensional arrangement, as it is perhaps the simplest system which possesses all the key experimental features. Through a combination of numerical and analytical approaches, a scaling law is found for the plume’s penetration depth within the base flow (i.e. the flow where the instability is either absent or artificially suppressed), and the horizontal cross-stream velocity and concentration profile outside the plume are determined from an asymptotic analysis of a simplified model. Direct numerical simulations show that, with increasing flow rate, a sinuous global mode is destabilized giving rise to the self-sustained oscillations as in the experiment. The sinuous instability is shown to be a consequence of the baroclinic generation of vorticity, due to the strong horizontal gradients at the edge of the conduit, a mechanism that is relevant even at very low Reynolds numbers. Despite the strength of this instability, the penetration depth is not significantly affected by it, instead being determined by the properties of the plume in the vicinity of the source. This scenario is confirmed by a local stability analysis. A finite region of local absolute instability is found near the source for sinuous modes prior to the onset of the global instability. Sufficiently far from the source the flow is locally stable. Near the onset of the global instability, varicose modes are also found to be locally, but only convectively, unstable
Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?
In Keplerian accretion disks, turbulence and magnetic fields may be jointly
excited through a subcritical dynamo process involving the magnetorotational
instability (MRI). High-resolution simulations exhibit a tendency towards
statistical self-organization of MRI dynamo turbulence into large-scale cyclic
dynamics. Understanding the physical origin of these structures, and whether
they can be sustained and transport angular momentum efficiently in
astrophysical conditions, represents a significant theoretical challenge. The
discovery of simple periodic nonlinear MRI dynamo solutions has recently proven
useful in this respect, and has notably served to highlight the role of
turbulent magnetic diffusion in the seeming decay of the dynamics at low
magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a
common regime in accretion disks. The connection between these simple
structures and the statistical organization reported in turbulent simulations
remained elusive, though. Here, we report the numerical discovery in moderate
aspect ratio Keplerian shearing boxes of new periodic, incompressible,
three-dimensional nonlinear MRI dynamo solutions with a larger dynamical
complexity reminiscent of such simulations. These "chimera" cycles are
characterized by multiple MRI-unstable dynamical stages, but their basic
physical principles of self-sustainment are nevertheless identical to those of
simpler cycles found in azimuthally elongated boxes. In particular, we find
that they are not sustained at low Pm either due to subcritical turbulent
magnetic diffusion. These solutions offer a new perspective into the transition
from laminar to turbulent instability-driven dynamos, and may prove useful to
devise improved statistical models of turbulent accretion disk dynamos.Comment: 12 pages, 8 figures, submitted to A&
Diverse Linguistic Development in Prelingually Deaf Children with Cochlear Implants
The advent of cochlear implants has enormously improved the quality of sensory perception in deaf children. Notwithstanding these advantages, the current literature shows a substantial variability in language proficiency among implanted children. This case series explores the variability of language acquisition in congenitally deaf children with cochlear implants. We report 4 prelingually deaf children (mean age=10.5; SD=1.08), affected by a genetically determined bilateral deafness, due to GJB2 gene mutation Cx26. Each implanted child underwent a systematic assessment of speech perception and production, as well as of lexical, morphologic, and syntactic skills in both comprehension and production. Notwithstanding similar clinical histories and similarly good postimplant pure-tone audiometry, two of the four children fared very poorly in speech audiometry, whereas the other two children gained very good results. We suggest that the language impairment detected in (some) implanted children may not be fully accounted for by pure auditory thresholds and that may be the outcome of concomitant damage to core components of the child's linguistic brain
A study of the influence of the gauge group on the Dyson-Schwinger equations for scalar-Yang-Mills systems
The particular choice of the gauge group for Yang-Mills theory plays an
important role when it comes to the influence of matter fields. In particular,
both the chosen gauge group and the representation of the matter fields yield
structural differences in the quenched case. Especially, the qualitative
behavior of the Wilson potential is strongly dependent on this selection.
Though the algebraic reasons for this observation is clear, it is far from
obvious how this behavior can be described besides using numerical simulations.
Herein, it is investigated how the group structure appears in the
Dyson-Schwinger equations, which as a hierarchy of equations for the
correlation functions have to be satisfied. It is found that there are
differences depending on both the gauge group and the representation of the
matter fields. This provides insight into possible truncation schemes for
practical calculations using these equations.Comment: 47 page
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