478 research outputs found
Minimum DC link Voltages for the Generator Bridge Converter of a SCIG Based Variable Speed Wind Turbine with Fully Rated Converters
Squirrel Cage Induction Generator (SCIG) based variable speed wind turbine with Fully Rated Converters (FRC) is a popular choice in the industry for the modern multi mega-watt wind turbines. Typical FRC system uses a fixed DC link voltage that allows operation in all steady state and dynamic operating conditions while allowing the modulation index of the PWM scheme to vary. However, the analysis made in this paper shows that at steady state, in the maximum power point tracking region where the turbine is operated at variable speeds with generator controlled using Rotor Flux Oriented Control (R-FOC), it is possible to operate the Generator Bridge (GB) converter with significantly lower DC link voltages than the fixed value used, by maintaining maximum modulation index in the PWM scheme. This paper presents a methodology of determining the minimum DC link voltages for such a system supported by simulation results showing the successful operation of a GB converter with minimum DC link voltages in the maximum power point tracking region
Surface Instability of Icicles
Quantitatively-unexplained stationary waves or ridges often encircle icicles.
Such waves form when roughly 0.1 mm-thick layers of water flow down the icicle.
These waves typically have a wavelength of 1cm approximately independent of
external temperature, icicle thickness, and the volumetric rate of water flow.
In this paper we show that these waves can not be obtained by naive
Mullins-Sekerka instability, but are caused by a quite new surface instability
related to the thermal diffusion and hydrodynamic effect of thin water flow.Comment: 11 pages, 5 figures, Late
NMR Experiments on a Three-Dimensional Vibrofluidized Granular Medium
A three-dimensional granular system fluidized by vertical container
vibrations was studied using pulsed field gradient (PFG) NMR coupled with
one-dimensional magnetic resonance imaging (MRI). The system consisted of
mustard seeds vibrated vertically at 50 Hz, and the number of layers N_ell <= 4
was sufficiently low to achieve a nearly time-independent granular fluid. Using
NMR, the vertical profiles of density and granular temperature were directly
measured, along with the distributions of vertical and horizontal grain
velocities. The velocity distributions showed modest deviations from
Maxwell-Boltzmann statistics, except for the vertical velocity distribution
near the sample bottom which was highly skewed and non-Gaussian. Data taken for
three values of N_ell and two dimensionless accelerations Gamma=15,18 were fit
to a hydrodynamic theory, which successfully models the density and temperature
profiles including a temperature inversion near the free upper surface.Comment: 14 pages, 15 figure
Bifurcations of a driven granular system under gravity
Molecular dynamics study on the granular bifurcation in a simple model is
presented. The model consists of hard disks, which undergo inelastic
collisions; the system is under the uniform external gravity and is driven by
the heat bath. The competition between the two effects, namely, the
gravitational force and the heat bath, is carefully studied. We found that the
system shows three phases, namely, the condensed phase, locally fluidized
phase, and granular turbulent phase, upon increasing the external control
parameter. We conclude that the transition from the condensed phase to the
locally fluidized phase is distinguished by the existence of fluidized holes,
and the transition from the locally fluidized phase to the granular turbulent
phase is understood by the destabilization transition of the fluidized holes
due to mutual interference.Comment: 35 pages, 17 figures, to be published in PR
A nonlinear hydrodynamical approach to granular materials
We propose a nonlinear hydrodynamical model of granular materials. We show
how this model describes the formation of a sand pile from a homogeneous
distribution of material under gravity, and then discuss a simulation of a
rotating sandpile which shows, in qualitative agreement with experiment, a
static and dynamic angle of repose.Comment: 17 pages, 14 figures, RevTeX4; minor changes to wording and some
additional discussion. Accepted by Phys. Rev.
Targeting Potential Drivers of COVID-19: Neutrophil Extracellular Traps
Coronavirus disease 2019 (COVID-19) is a novel, viral-induced respiratory disease that in âŒ10-15% of patients progresses to acute respiratory distress syndrome (ARDS) triggered by a cytokine storm. In this Perspective, autopsy results and literature are presented supporting the hypothesis that a little known yet powerful function of neutrophils-the ability to form neutrophil extracellular traps (NETs)-may contribute to organ damage and mortality in COVID-19. We show lung infiltration of neutrophils in an autopsy specimen from a patient who succumbed to COVID-19. We discuss prior reports linking aberrant NET formation to pulmonary diseases, thrombosis, mucous secretions in the airways, and cytokine production. If our hypothesis is correct, targeting NETs directly and/or indirectly with existing drugs may reduce the clinical severity of COVID-19
Drug Use Mediates the Relationship Between Depressive Symptoms and Adherence to ART Among Recently Incarcerated People Living with HIV
Depression is a known risk factor for antiretroviral therapy (ART) non-adherence, but little is known about the mechanisms explaining this relationship. Identifying these mechanisms among people living with HIV (PLHIV) after release from prison is particularly important, as individuals during this critical period are at high risk for both depression and poor ART adherence. 347 PLHIV recently released from prison in North Carolina and Texas were included in analyses to assess mediation of the relationship between depressive symptoms at 2 weeks post-release and ART adherence (assessed by unannounced telephone pill counts) at weeks 9â21 post-release by the hypothesized explanatory mechanisms of alcohol use, drug use, adherence self-efficacy, and adherence motivation (measured at weeks 6 and 14 post-release). Indirect effects were estimated using structural equation models with maximum likelihood estimation and bootstrapped confidence intervals. On average, participants achieved 79% ART adherence. The indirect effect of depression on adherence through drug use was statistically significant; greater symptoms of depression were associated with greater drug use, which was in turn associated with lower adherence. Lower adherence self-efficacy was associated with depressive symptoms, but not with adherence. Depression screening and targeted mental health and substance use services for depressed individuals at risk of substance use constitute important steps to promote adherence to ART after prison release
Photonic band gaps in materials with triply periodic surfaces and related tubular structures
We calculate the photonic band gap of triply periodic bicontinuous cubic
structures and of tubular structures constructed from the skeletal graphs of
triply periodic minimal surfaces. The effect of the symmetry and topology of
the periodic dielectric structures on the existence and the characteristics of
the gaps is discussed. We find that the C(I2-Y**) structure with Ia3d symmetry,
a symmetry which is often seen in experimentally realized bicontinuous
structures, has a photonic band gap with interesting characteristics. For a
dielectric contrast of 11.9 the largest gap is approximately 20% for a volume
fraction of the high dielectric material of 25%. The midgap frequency is a
factor of 1.5 higher than the one for the (tubular) D and G structures
Identify : the investigation and detection of urological neoplasia in patients referred with suspected urinary tract cancer : a multicentre cohort study
Funding Action Bladder Cancer UK, The Rosetree Trust, The Urology Foundation. None had any involvement in the protocol stage of the study.Peer reviewedPublisher PD
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