Clustering and collisions of heavy particles in random smooth flows

Finite-size impurities suspended in incompressible flows distribute inhomogeneously, leading to a drastic enhancement of collisions. A description of the dynamics in the full position-velocity phase space is essential to understand the underlying mechanisms, especially for polydisperse suspensions. These issues are here studied for particles much heavier than the fluid by means of a Lagrangian approach. It is shown that inertia enhances collision rates through two effects: correlation among particle positions induced by the carrier flow and uncorrelation between velocities due to their finite size. A phenomenological model yields an estimate of collision rates for particle pairs with different sizes. This approach is supported by numerical simulations in random flows.Comment: 12 pages, 9 Figures (revTeX 4) final published versio

Effect of turbulence on collisions of dust particles with planetesimals in protoplanetary disks

Planetesimals in gaseous protoplanetary disks may grow by collecting dust particles. Hydrodynamical studies show that small particles generally avoid collisions with the planetesimals because they are entrained by the flow around them. This occurs when $St$, the Stokes number, defined as the ratio of the dust stopping time to the planetesimal crossing time, becomes much smaller than unity. However, these studies have been limited to the laminar case, whereas these disks are believed to be turbulent. We want to estimate the influence of gas turbulence on the dust-planetesimal collision rate and on the impact speeds. We used three-dimensional direct numerical simulations of a fixed sphere (planetesimal) facing a laminar and turbulent flow seeded with small inertial particles (dust) subject to a Stokes drag. A no-slip boundary condition on the planetesimal surface is modeled via a penalty method. We find that turbulence can significantly increase the collision rate of dust particles with planetesimals. For a high turbulence case (when the amplitude of turbulent fluctuations is similar to the headwind velocity), we find that the collision probability remains equal to the geometrical rate or even higher for $St\geq 0.1$, i.e., for dust sizes an order of magnitude smaller than in the laminar case. We derive expressions to calculate impact probabilities as a function of dust and planetesimal size and turbulent intensity

Heavy particle concentration in turbulence at dissipative and inertial scales

Spatial distributions of heavy particles suspended in an incompressible isotropic and homogeneous turbulent flow are investigated by means of high resolution direct numerical simulations. In the dissipative range, it is shown that particles form fractal clusters with properties independent of the Reynolds number. Clustering is there optimal when the particle response time is of the order of the Kolmogorov time scale $\tau_\eta$. In the inertial range, the particle distribution is no longer scale-invariant. It is however shown that deviations from uniformity depend on a rescaled contraction rate, which is different from the local Stokes number given by dimensional analysis. Particle distribution is characterized by voids spanning all scales of the turbulent flow; their signature in the coarse-grained mass probability distribution is an algebraic behavior at small densities.Comment: 4 RevTeX pgs + 4 color Figures included, 1 figure eliminated second part of the paper completely revise

Chemical, enantioselective, and sensory analysis of a cholinesterase inhibitor essential oil from coreopsis triloba S.F. Blake (asteraceae)

The fresh leaves of Coreopsis triloba S.F. Blake, collected at Cerro Villonaco in Loja, Ecuador, were investigated with respect to their essential oil (EO). The chemical composition was determined qualitatively through gas chromatography coupled with mass spectrometry (GC-MS) and quantitatively by gas chromatography coupled with flame ionization (GC-FID), using relative response factors (RRF) based on the enthalpy of combustion. The essential oil contained between 92.5% and 93.4% of monoterpene hydrocarbons, with (E)-β-ocimene being the main component (35.2–35.9%), followed by β-phellandrene (24.6–25.0%), α-pinene (15.3–15.9%), myrcene (10.9–11.0%), sabinene (2.2–2.4%), (Z)-β-ocimene (1.5%), and germacrene D (1.2–1.3%). The enantiomeric distribution of α-pinene, β-pinene, limonene, and germacrene D was also determined. The main components responsible for the aroma were identified through aroma extract dilution analysis (AEDA), a gas chromatography-olfactometry (GC-O) based technique, being α-pinene, β-pinene (0.6%), terpinolene (0.1%), α-copaene (0.1–0.3%), β-phellandrene, and (E)-4,8-dimethyl-1,3,7-nonatriene (0.1–0.2%) the main olfactory constituents according to the decreasing factor of dilution (FD) order. The biological tests showed IC50 inhibition values of 42.2 and 6.8 µg/mL for acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), respectively

Anisotropic clustering of inertial particles in homogeneous shear flow

Recently, clustering of inertial particles in turbulence has been thoroughly analyzed for statistically homogeneous isotropic flows. Phenomenologically, spatial homogeneity of particles configurations is broken by the advection of a range of eddies determined by the Stokes relaxation time of the particles which results in a multi-scale distribution of local concentrations and voids. Much less is known concerning anisotropic flows. Here, by addressing direct numerical simulations (DNS) of a statistically steady particle-laden homogeneous shear flow, we provide evidence that the mean shear preferentially orients particle patterns. By imprinting anisotropy on large scales velocity fluctuations, the shear indirectly affects the geometry of the clusters. Quantitative evaluation is provided by a purposely designed tool, the angular distribution function of particle pairs (ADF), which allows to address the anisotropy content of particles aggregates on a scale by scale basis. The data provide evidence that, depending on the Stokes relaxation time of the particles, anisotropic clustering may occur even in the range of scales where the carrier phase velocity field is already recovering isotropy. The strength of the singularity in the anisotropic component of the ADF quantifies the level of fine scale anisotropy, which may even reach values of more than 30% direction-dependent variation in the probability to find two close-by particles at viscous scale separation.Comment: To appear in Journal Fluid Mechanics 200

ASTROD, ASTROD I and their gravitational-wave sensitivities

ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) is a mission concept with three spacecraft -- one near L1/L2 point, one with an inner solar orbit and one with an outer solar orbit, ranging coherently with one another using lasers to test relativistic gravity, to measure the solar system and to detect gravitational waves. ASTROD I with one spacecraft ranging optically with ground stations is the first step toward the ASTROD mission. In this paper, we present the ASTROD I payload and accelerometer requirements, discuss the gravitational-wave sensitivities for ASTROD and ASTROD I, and compare them with LISA and radio-wave PDoppler-tracking of spacecraft.Comment: presented to the 5th Edoardo Amaldi Conference (July 6-11, 2003) and submitted to Classical and Quantum Gravit

The analytic structure of 2D Euler flow at short times

Using a very high precision spectral calculation applied to the incompressible and inviscid flow with initial condition $\psi_0(x_1, x_2) = \cos x_1+\cos 2x_2$, we find that the width $\delta(t)$ of its analyticity strip follows a $\ln(1/t)$ law at short times over eight decades. The asymptotic equation governing the structure of spatial complex-space singularities at short times (Frisch, Matsumoto and Bec 2003, J.Stat.Phys. 113, 761--781) is solved by a high-precision expansion method. Strong numerical evidence is obtained that singularities have infinite vorticity and lie on a complex manifold which is constructed explicitly as an envelope of analyticity disks.Comment: 19 pages, 14 figures, published versio

Vif is a RNA chaperone that could temporally regulate RNA dimerization and the early steps of HIV-1 reverse transcription

HIV-1 Vif (viral infectivity factor) is associated with the assembly complexes and packaged at low level into the viral particles, and is essential for viral replication in non-permissive cells. Viral particles produced in the absence of Vif exhibit structural defects and are defective in the early steps of reverse transcription. Here, we show that Vif is able to anneal primer tRNALys3 to the viral RNA, to decrease pausing of reverse transcriptase during (–) strand strong-stop DNA synthesis, and to promote the first strand transfer. Vif also stimulates formation of loose HIV-1 genomic RNA dimers. These results indicate that Vif is a bona fide RNA chaperone. We next studied the effects of Vif in the presence of HIV-1 NCp, which is a well-established RNA chaperone. Vif inhibits NCp-mediated formation of tight RNA dimers and hybridization of tRNALys3, while it has little effects on NCp-mediated strand transfer and it collaborates with nucleocapsid (NC) to increase RT processivity. Thus, Vif might negatively regulate NC-assisted maturation of the RNA dimer and early steps of reverse transcription in the assembly complexes, but these inhibitory effects would be relieved after viral budding, thanks to the limited packaging of Vif in the virions

Déléguer l'allaitement des veaux laitiers aux vaches ? Résultats d'enquêtes auprès des éleveurs

La séparation précoce mère-veau est une pratique courante dans la plupart des élevages laitiers, mais est sujette à de nombreuses interrogations tant chez les éleveurs (coûts engendrés, travail) que dans la société (bien-être animal). Des pratiques alternatives mises en place par certains éleveurs permettent aux veaux d'être allaités par leur mère ou une autre vache, appelée « nourrice ». Quoiqu’anciennes, ces pratiques demeurent peu connues et leur intégration aux conditions d’élevages modernes questionne. Des enquêtes ont ainsi été réalisées en 2018 pour connaître la motivation des éleveurs, la diversité et les conséquences de ces pratiques sur les résultats techniques. Cent deux élevages, identifiés par des réseaux divers (conseil en élevage, chambres d’agriculture, coopératives…), ont été enquêtés dans le Grand Ouest (n = 50), le Grand Est (n = 41) et le Massif central (n = 11), par téléphone (n = 56) ou sur place (n = 46). La taille médiane des troupeaux est de 60 vaches laitières (8 à 210), principalement constitués de vaches de race Montbéliarde et Holstein. Les élevages sont pour moitié en agriculture biologique (56%), avec une grande diversité de systèmes de traite et de logement (salle de traite en épi, robot, stabulation entravée, libre à logettes…). Trois grandes conduites d'allaitement des veaux, de la naissance à la vente ou au sevrage, ont été identifiées : 1/ les veaux sont allaités exclusivement par leur mère ; 2/ les veaux sont élevés par leur mère puis reçoivent une alimentation lactée artificielle (poudre ou lait entier) ; 3/ les veaux sont élevés par leur mère puis sous une vache nourrice. Elles diffèrent souvent au sein d’un même élevage (n = 62) entre les veaux destinés à la vente (mâles ou femelles) et les velles de renouvellement. La diversité des systèmes d'allaitement se retrouve dans tous les types d'élevages, quelle que soit leur taille ou leur localisation géographique. L'amélioration de la santé des animaux (n = 52), la diminution du temps de travail (n = 47) et des coûts d’élevage (n = 32) sont les principales motivations des éleveurs à mettre en place ces pratiques. Pour 80% d’entre eux, ces pratiques alternatives ont entrainé une diminution de la fréquence des diarrhées des veaux et une amélioration de leur croissance. La séparation tardive vache-veau demeure le point négatif de ces pratiques, avec des périodes de meuglements de 2,4 jours en moyenne. Certains éleveurs, ayant utilisé puis abandonné ces pratiques, pointent aussi d’autres limites (problèmes sanitaires, bâtiments inadaptés). Cette enquête, qui a permis de mieux connaître les pratiques d'allaitement des veaux laitiers par leur mère et sous nourrice, va être complétée par des enquêtes similaires et des expérimentations en fermes, au niveau européen, afin d’avoir une vision plus large de ces pratiques et de leurs impacts