96 research outputs found
The Statistics of Supersonic Isothermal Turbulence
We present results of large-scale three-dimensional simulations of supersonic
Euler turbulence with the piecewise parabolic method and multiple grid
resolutions up to 2048^3 points. Our numerical experiments describe
non-magnetized driven turbulent flows with an isothermal equation of state and
an rms Mach number of 6. We discuss numerical resolution issues and demonstrate
convergence, in a statistical sense, of the inertial range dynamics in
simulations on grids larger than 512^3 points. The simulations allowed us to
measure the absolute velocity scaling exponents for the first time. The
inertial range velocity scaling in this strongly compressible regime deviates
substantially from the incompressible Kolmogorov laws. The slope of the
velocity power spectrum, for instance, is -1.95 compared to -5/3 in the
incompressible case. The exponent of the third-order velocity structure
function is 1.28, while in incompressible turbulence it is known to be unity.
We propose a natural extension of Kolmogorov's phenomenology that takes into
account compressibility by mixing the velocity and density statistics and
preserves the Kolmogorov scaling of the power spectrum and structure functions
of the density-weighted velocity v=\rho^{1/3}u. The low-order statistics of v
appear to be invariant with respect to changes in the Mach number. For
instance, at Mach 6 the slope of the power spectrum of v is -1.69, and the
exponent of the third-order structure function of v is unity. We also directly
measure the mass dimension of the "fractal" density distribution in the
inertial subrange, D_m = 2.4, which is similar to the observed fractal
dimension of molecular clouds and agrees well with the cascade phenomenology.Comment: 15 pages, 19 figures, ApJ v665, n2, 200
Imaging the onset of the resonance regime in low-energy NO-He collisions
At low energies, the quantum wave-like nature of molecular interactions
result in unique scattering behavior, ranging from the universal Wigner laws
near zero Kelvin to the occurrence of scattering resonances at higher energies.
It has proven challenging to experimentally probe the individual waves
underlying these phenomena. We report measurements of state-to-state integral
and differential cross sections for inelastic NO-He collisions in the 0.2 - 8.5
cm range with 0.02 cm resolution. We study the onset of the
resonance regime by probing the lowest-lying resonance dominated by s and p
waves only. The highly structured differential cross sections directly reflect
the increasing number of contributing waves as the energy is increased. A new
NO-He potential calculated at the CCSDT(Q) level was required to reproduce our
measurements.Comment: 14 pages, 4 figure
Quantum state resolved molecular dipolar collisions over four decades of energy
Collisions between cold polar molecules represent a fascinating research
frontier, but have proven hard to probe experimentally. We report measurements
of inelastic cross sections for collisions between NO and ND 3 molecules at
energies between 0.1 and 580 cm-1 , with full quantum state resolution. At
energies below the 100 cm-1 well depth of the interaction potential, we
observed backward glories originating from peculiar U-turn trajectories. At
energies below 0.2 cm-1, we observed a breakdown of the Langevin capture model,
which we interpreted in terms of a suppressed mutual polarization during the
collision, effectively switching off the molecular dipole moments. Scattering
calculations based on an ab initio NO-ND3 potential energy surface revealed the
crucial role of near-degenerate rotational levels with opposite parity in
low-energy dipolar collisions
Observation of correlated excitations in bimolecular collisions
Whereas collisions between atoms and molecules are largely understood,
collisions between two molecules have proven much harder to study. In both
experiment and theory, our ability to determine quantum state-resolved
bimolecular cross sections lags behind their atom-molecule counterparts by
decades. For many bimolecular systems, even rules of thumb -- much less
intuitive understanding -- of scattering cross sections are lacking. Here, we
report the measurement of state-to-state differential cross sections on the
collision of state-selected and velocity-controlled nitric oxide (NO) radicals
and oxygen (O2) molecules. Using velocity map imaging of the scattered NO
radicals, the full product-pair correlations of rotational excitation that
occurs in both collision partners from individual encounters are revealed. The
correlated cross sections show surprisingly good agreement with quantum
scattering calculations using ab initio NO-O2 potential energy surfaces. The
observations show that the well-known energy-gap law that governs atom-molecule
collisions does not generally apply to bimolecular excitation processes, and
reveal a propensity rule for the vector correlation of product angular momenta.Comment: Received: 06 September 2017 Accepted: 20 December 2017 Published
online: 19 February 2018, Nature Chemistry 201
Evolution Semigroups in Supersonic Flow-Plate Interactions
We consider the well-posedness of a model for a flow-structure interaction.
This model describes the dynamics of an elastic flexible plate with clamped
boundary conditions immersed in a supersonic flow. A perturbed wave equation
describes the flow potential. The plate's out-of-plane displacement can be
modeled by various nonlinear plate equations (including von Karman and Berger).
We show that the linearized model is well-posed on the state space (as given by
finite energy considerations) and generates a strongly continuous semigroup. We
make use of these results to conclude global-in-time well-posedness for the
fully nonlinear model.
The proof of generation has two novel features, namely: (1) we introduce a
new flow potential velocity-type variable which makes it possible to cover both
subsonic and supersonic cases, and to split the dynamics generating operator
into a skew-adjoint component and a perturbation acting outside of the state
space. Performing semigroup analysis also requires a nontrivial approximation
of the domain of the generator. And (2) we make critical use of hidden
regularity for the flow component of the model (in the abstract setup for the
semigroup problem) which allows us run a fixed point argument and eventually
conclude well-posedness. This well-posedness result for supersonic flows (in
the absence of rotational inertia) has been hereto open. The use of semigroup
methods to obtain well-posedness opens this model to long-time behavior
considerations.Comment: 31 page
Perfect Fluid Theory and its Extensions
We review the canonical theory for perfect fluids, in Eulerian and Lagrangian
formulations. The theory is related to a description of extended structures in
higher dimensions. Internal symmetry and supersymmetry degrees of freedom are
incorporated. Additional miscellaneous subjects that are covered include
physical topics concerning quantization, as well as mathematical issues of
volume preserving diffeomorphisms and representations of Chern-Simons terms (=
vortex or magnetic helicity).Comment: 3 figure
Brain antigens in functionally distinct antigen-presenting cell populations in cervical lymph nodes in MS and EAE
Drainage of central nervous system (CNS) antigens to the brain-draining cervical lymph nodes (CLN) is likely crucial in the initiation and control of autoimmune responses during multiple sclerosis (MS). We demonstrate neuronal antigens within CLN of MS patients. In monkeys and mice with experimental autoimmune encephalomyelitis (EAE) and in mouse models with non-inflammatory CNS damage, the type and extent of CNS damage was associated with the frequencies of CNS antigens within the cervical lymph nodes. In addition, CNS antigens drained to the spinal-cord-draining lumbar lymph nodes. In human MS CLN, neuronal antigens were present in pro-inflammatory antigen-presenting cells (APC), whereas the majority of myelin-containing cells were anti-inflammatory. This may reflect a different origin of the cells or different drainage mechanisms. Indeed, neuronal antigen-containing cells in human CLN did not express the lymph node homing receptor CCR7, whereas myelin antigen-containing cells in situ and in vitro did. Nevertheless, CLN from EAE-affected CCR7-deficient mice contained equal amounts of myelin and neuronal antigens as wild-type mice. We conclude that the type and frequencies of CNS antigens within the CLN are determined by the type and extent of CNS damage. Furthermore, the presence of myelin and neuronal antigens in functionally distinct APC populations within MS CLN suggests that differential immune responses can be evoked
Comparative Membranome Expression Analysis in Primary Tumors and Derived Cell Lines
Despite the wide use of cell lines in cancer research, the extent to which their surface properties correspond to those of primary tumors is poorly characterized. The present study addresses this problem from a transcriptional standpoint, analyzing the expression of membrane protein genes - the Membranome – in primary tumors and immortalized in-vitro cultured tumor cells. 409 human samples, deriving from ten independent studies, were analyzed. These comprise normal tissues, primary tumors and tumor derived cell lines deriving from eight different tissues: brain, breast, colon, kidney, leukemia, lung, melanoma, and ovary. We demonstrated that the Membranome has greater power than the remainder of the transcriptome when used as input for the automatic classification of tumor samples. This feature is maintained in tumor derived cell lines. In most cases primary tumors show maximal similarity in Membranome expression with cell lines of same tissue origin. Differences in Membranome expression between tumors and cell lines were analyzed also at the pathway level and biological themes were identified that were differentially regulated in the two settings. Moreover, by including normal samples in the analysis, we quantified the degree to which cell lines retain the Membranome up- and down- regulations observed in primary tumors with respect to their normal counterparts. We showed that most of the Membranome up-regulations observed in primary tumors are lost in the in-vitro cultured cells. Conversely, the majority of Membranome genes down-regulated upon tumor transformation maintain lower expression levels also in the cell lines. This study points towards a central role of Membranome genes in the definition of the tumor phenotype. The comparative analysis of primary tumors and cell lines identifies the limits of cell lines as a model for the study of cancer-related processes mediated by the cell surface. Results presented allow for a more rational use of the cell lines as a model of cancer
The VANDELS ESO public spectroscopic survey
VANDELS is a uniquely deep spectroscopic survey of high-redshift galaxies with the VIMOS spectrograph on ESO’s Very Large Telescope (VLT). The survey has obtained ultradeep optical (0.48 < λ < 1.0 μ m) spectroscopy of ≃2100 galaxies within the redshift interval 1.0 ≤ z ≤ 7.0, over a total area of ≃0.2 deg2 centred on the CANDELS Ultra Deep Survey and Chandra Deep Field South fields. Based on accurate photometric redshift pre-selection, 85 per cent of the galaxies targeted by VANDELS were selected to be at z ≥ 3. Exploiting the red sensitivity of the refurbished VIMOS spectrograph, the fundamental aim of the survey is to provide the high-signal-to-noise ratio spectra necessary to measure key physical properties such as stellar population ages, masses, metallicities, and outflow velocities from detailed absorption-line studies. Using integration times calculated to produce an approximately constant signal-to-noise ratio (20 < tint< 80 h), the VANDELS survey targeted: (a) bright star-forming galaxies at 2.4 ≤ z ≤ 5.5, (b) massive quiescent galaxies at 1.0 ≤ z ≤ 2.5, (c) fainter star-forming galaxies at 3.0 ≤ z ≤ 7.0, and (d) X-ray/Spitzer-selected active galactic nuclei and Herschel-detected galaxies. By targeting two extragalactic survey fields with superb multiwavelength imaging data, VANDELS will produce a unique legacy data set for exploring the physics underpinning high-redshift galaxy evolution. In this paper, we provide an overview of the VANDELS survey designed to support the science exploitation of the first ESO public data release, focusing on the scientific motivation, survey design, and target selection
Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics
We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions
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