183 research outputs found
CFD Analysis of Helicopter Wakes in Ground Effect
The paper presents CFD results for the wake of a helicopter flying a low altitude at different advance ratios.
The wakes are assessed in terms of topology and velocity magnitudes. The structure of the wake near ground
changes rapidly with the advance ratio and its decay appears to be faster than what is suggested by theoretical
analyses. The results show clear the potential of modern CFD for use in helicopter safety and highlights the
need for detailed surveys of helicopter wakes using full-scale physical experiments
Experimental Investigation of Helicopter Noise While Approaching an Elevated Helipad
The present paper describes a test campaign performed to investigate the noise footprint emitted by a helicopter in an idealised urban context, reproducing the approach to an elevated helipad. The test campaign was performed in Politecnico di Milano’s anechoic chamber and was finalised to investigate the effects produced only by helicopter noise. The set up consisted of a two-blade main rotor helicopter model and an aluminium rectangular prism model reproducing the landing building. Ground observer perceptions were recorded by means of a surface microphone and a realistic landing trajectory was approximated as a succession of fixed point measurements. Collected data were analysed through acoustic spectra and sound maps. Spectra were used to comprehend physical phenomena, such as reflection, diffraction and shielding, and to analyse the different contributions of helicopter noise. A sound map analysis enabled us to obtain a global perspective of the involved phenomena and to understand th extent to which people close to a building are stressed by a helicopter approaching an elevated urban helipad. Moreover, the experimental database, obtained over a free geometry, can be considered a useful tool for the validation of aeroacoustic solvers with different levels of fidelity
Shear and Breathing Modes of Layered Materials.
Layered materials (LMs), such as graphite, hexagonal boron nitride, and transition-metal dichalcogenides, are at the center of an ever-increasing research effort, due to their scientific and technological relevance. Raman and infrared spectroscopies are accurate, non-destructive approaches to determine a wide range of properties, including the number of layers, N, and the strength of the interlayer interactions. We present a general approach to predict the complete spectroscopic fan diagrams, i.e., the relations between frequencies and N for the optically active shear and layer-breathing modes of any multilayer comprising N ≥ 2 identical layers. In order to achieve this, we combine a description of the normal modes in terms of a one-dimensional mechanical model, with symmetry arguments that describe the evolution of the point group as a function of N. Group theory is then used to identify which modes are Raman- and/or infrared-active, and to provide diagrams of the optically active modes for any stack composed of identical layers. We implement the method and algorithms in an open-source tool to assist researchers in the prediction and interpretation of such diagrams. Our work will underpin future efforts on Raman and infrared characterization of known, and yet not investigated, LMs
CFD Analysis of Helicopter Wakes in Ground Effect
The paper presents CFD results for the wake of a helicopter flying a low altitude at different advance ratios. The wakes are assessed in terms of topology and velocity magnitudes. The structure of the wake near ground changes rapidly with the advance ratio and its decay appears to be faster than what is suggested by theoretical analyses. The results show clear the potential of modern CFD for use in helicopter safety and highlights the need for detailed surveys of helicopter wakes using full-scale physical experiments
Giga-Hertz quantized charge pumping in bottom gate defined InAs nanowire quantum dots
Semiconducting nanowires (NWs) are a versatile, highly tunable material
platform at the heart of many new developments in nanoscale and quantum
physics. Here, we demonstrate charge pumping, i.e., the controlled transport of
individual electrons through an InAs NW quantum dot (QD) device at frequencies
up to GHz. The QD is induced electrostatically in the NW by a series of
local bottom gates in a state of the art device geometry. A periodic modulation
of a single gate is enough to obtain a dc current proportional to the frequency
of the modulation. The dc bias, the modulation amplitude and the gate voltages
on the local gates can be used to control the number of charges conveyed per
cycle. Charge pumping in InAs NWs is relevant not only in metrology as a
current standard, but also opens up the opportunity to investigate a variety of
exotic states of matter, e.g. Majorana modes, by single electron spectroscopy
and correlation experiments.Comment: 21 page
Gain-of-Function STIM1 L96V Mutation Causes Myogenesis Alteration in Muscle Cells From a Patient Affected by Tubular Aggregate Myopathy
Tubular Aggregate Myopathy (TAM) is a hereditary ultra-rare muscle disorder characterized by muscle weakness and cramps or myasthenic features. Biopsies from TAM patients show the presence of tubular aggregates originated from sarcoplasmic reticulum due to altered Ca2+ homeostasis. TAM is caused by gain-of-function mutations in STIM1 or ORAI1, proteins responsible for Store-Operated-Calcium-Entry (SOCE), a pivotal mechanism in Ca2+ signaling. So far there is no cure for TAM and the mechanisms through which STIM1 or ORAI1 gene mutation lead to muscle dysfunction remain to be clarified. It has been established that post-natal myogenesis critically relies on Ca2+ influx through SOCE. To explore how Ca2+ homeostasis dysregulation associated with TAM impacts on muscle differentiation cascade, we here performed a functional characterization of myoblasts and myotubes deriving from patients carrying STIM1 L96V mutation by using fura-2 cytofluorimetry, high content imaging and real-time PCR. We demonstrated a higher resting Ca2+ concentration and an increased SOCE in STIM1 mutant compared with control, together with a compensatory down-regulation of genes involved in Ca2+ handling (RyR1, Atp2a1, Trpc1). Differentiating STIM1 L96V myoblasts persisted in a mononuclear state and the fewer multinucleated myotubes had distinct morphology and geometry of mitochondrial network compared to controls, indicating a defect in the late differentiation phase. The alteration in myogenic pathway was confirmed by gene expression analysis regarding early (Myf5, Mef2D) and late (DMD, Tnnt3) differentiation markers together with mitochondrial markers (IDH3A, OGDH). We provided evidences of mechanisms responsible for a defective myogenesis associated to TAM mutant and validated a reliable cellular model usefull for TAM preclinical studies
The Physics of Kondo Impurities in Graphene
This article summarizes our understanding of the Kondo effect in graphene,
primarily from a theoretical perspective. We shall describe different ways to
create magnetic moments in graphene, either by adatom deposition or via
defects. For dilute moments, the theoretical description is in terms of
effective Anderson or Kondo impurity models coupled to graphene's Dirac
electrons. We shall discuss in detail the physics of these models, including
their quantum phase transitions and the effect of carrier doping, and confront
this with existing experimental data. Finally, we point out connections to
other quantum impurity problems, e.g., in unconventional superconductors,
topological insulators, and quantum spin liquids.Comment: 27 pages, 8 figs. Review article prepared for Rep. Prog. Phys. ("key
issues" section). (v2) Final version as publishe
How close can one approach the Dirac point in graphene experimentally?
The above question is frequently asked by theorists who are interested in
graphene as a model system, especially in context of relativistic quantum
physics. We offer an experimental answer by describing electron transport in
suspended devices with carrier mobilities of several 10^6 cm^2V^-1s^-1 and with
the onset of Landau quantization occurring in fields below 5 mT. The observed
charge inhomogeneity is as low as \approx10^8 cm^-2, allowing a neutral state
with a few charge carriers per entire micron-scale device. Above liquid helium
temperatures, the electronic properties of such devices are intrinsic, being
governed by thermal excitations only. This yields that the Dirac point can be
approached within 1 meV, a limit currently set by the remaining charge
inhomogeneity. No sign of an insulating state is observed down to 1 K, which
establishes the upper limit on a possible bandgap
Large-area epitaxial monolayer MoS2
Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics, and energy harvesting. Large-area growth methods are needed to open the way to applications. Control over lattice orientation during growth remains a challenge. This is needed to minimize or even avoid the formation of grain boundaries, detrimental to electrical, optical, and mechanical properties of MoS2 and other 2D semiconductors. Here, we report on the growth of high-quality monolayer MoS2 with control over lattice orientation. We show that the monolayer film is composed of coalescing single islands with limited numbers of lattice orientation due to an epitaxial growth mechanism. Optical absorbance spectra acquired over large areas show significant absorbance in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though the interaction between the growth substrate and MoS2 is strong enough to induce lattice alignment via van der Waals interaction, we can easily transfer the grown material and fabricate devices. Local potential mapping along channels in field-effect transistors shows that the single-crystal MoS2 grains in our film are well connected, with interfaces that do not degrade the electrical conductivity. This is also confirmed by the relatively large and length-independent mobility in devices with a channel length reaching 80um
Aharonov-Bohm interferences from local deformations in graphene
One of the most interesting aspects of graphene is the tied relation between
structural and electronic properties. The observation of ripples in the
graphene samples both free standing and on a substrate has given rise to a very
active investigation around the membrane-like properties of graphene and the
origin of the ripples remains as one of the most interesting open problems in
the system. The interplay of structural and electronic properties is
successfully described by the modelling of curvature and elastic deformations
by fictitious gauge fields that have become an ex- perimental reality after the
suggestion that Landau levels can form associated to strain in graphene and the
subsequent experimental confirmation. Here we propose a device to detect
microstresses in graphene based on a scanning-tunneling-microscopy setup able
to measure Aharonov-Bohm inter- ferences at the nanometer scale. The
interferences to be observed in the local density of states are created by the
fictitious magnetic field associated to elastic deformations of the sample.Comment: Some bugs fixe
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