6,570 research outputs found
Comparison of the computed flow field around a bubble growing at an orifice using PIV techniques
For bubbles growing rapidly at orifices, the inertia of the liquid displacement and the resultant liquid flow field contribute to the production of an inertia force which tends to retard bubble movement. It is therefore the purpose of this paper to report on a study to examine the validity of liquid velocity fields predicted by potential flow methods and measurements made using Particle Image Velocimetry (PIV) techniques. Air bubbles are generated in water at atmospheric conditions from a 1 mm diameter orifice. The process is transient and occurs over a period of approximately 80 msecs. Therefore a combination of high speed video techniques and PIV image processing has been used to determine the liquid velocity vector fields during the bubble growth, detachment and translation periods. This paper will present a summary of the experimental techniques and the theoretical model and discuss the results of the study
A PIV and CFD analysis of natural convection ice melting
The melting of a vertical ice cylinder in water is investigated in this paper. The experiments were carried out in a water-filled cylindrical Perspex barrel with adiabatic walls for Rayleigh numbers of 0.22x108 and 0.475x108. The ice crystal is suspended in the water and experimental images of the natural convection melting process were obtained using both shadowgraphy and particle image velocimetry (PIV) techniques. This data is compared with a numerical model which attempts to capture the melt-front on a fixed computational grid. The numerical model takes into account the density inversion effects in the water. The results show the applicability of PIV to this type of flow and demonstrate a simple numerical model to effectively resolve the melting phenomenon
Introducing instrumentation and data acquisition to mechanical engineering students using LabVIEW
For several years, LabVIEW has been used within the Department of Mechanical Engineering at the University of Strathclyde as the basis for introducing the basic concepts and practice of data acquisition, and more generally, instrumentation, to postgraduate engineering students and undergraduate project students. The objectives of introducing LabVIEW within the curriculum were to expose students to instrumentation and experimental analysis, and to create courseware that could be used flexibly for a range of students. It was also important that staff time for laboratory work be kept to manageable levels. A course module was developed which allows engineering students with very little or no previous knowledge of instrumentation or programming to become acquainted with the basics of programming, experimentation and data acquisition. The basic course structure has been used to teach both undergraduates and postgraduates as well as laboratory technical staff. The paper describes the objectives of the use of LabVIEW for teaching, the structure of the module developed, and the response of students who have been subjected to the course, and how it is intended to expand the delivery to greater student numbers
The Struve-Sahade effect in the optical spectra of O-type binaries I. Main-sequence systems
We present a spectroscopic analysis of four massive binary systems that are
known or are good candidates to display the Struve-Sahade effect (defined as
the apparent strengthening of the secondary spectrum of the binary when the
star is approaching, and the corresponding weakening of the lines when it is
receding).
We use high resolution optical spectra to determine new orbital solutions and
spectral types of HD 165052, HD 100213, HD 159176 and DH Cep. As good knowledge
of the fundamental parameters of the considered systems is necessary to examine
the Struve-Sahade effect. We then study equivalent width variations in the
lines of both components of these binaries during their orbital cycle.
In the case of these four systems, variations appear in the equivalent widths
of some lines during the orbital cycle, but the definition given above can any
longer be valid, since it is now clear that the effect modifies the primary
spectrum as much as the secondary spectrum. Furthermore, the lines affected,
and the way in which they are affected, depend on the considered system. For at
least two of them (HD 100213 and HD 159176) these variations probably reflect
the ellipsoidal variable nature of the system.Comment: 12 pages, 20 figures, in press A&
External lightning protection system for wind turbine blades - aerodynamics.
Many wind turbine blade manufacturers have installed lightning protection systems (particularly the down conductor) internally. Having the down conductor internally within the blades would indeed preserve their aerodynamic performance. However, the blades are, as a consequence, vulnerable to damage and burn resulting from lightning strikes. Owing to this, the authors believe that by having the down conductor on the external surface of the blade, the incidence of blade damage would be reduced. The authors have not found any literature in the public domain that quantifies the effect of having an external down conductor on the aerodynamic performance. Hence, in this paper, a study of the effects of an externally mounted lightning conductor has been undertaken. Simulation studies were carried out using the computational fluid dynamics numerical method available in the COMSOL Multiphysics software package. The results of studies on single conductor arrangement have shown that the degradation on aerodynamic performance is least at the trailing edges of the blade. However, it may not be adequate for lightning protection. Therefore, using a similar numerical modelling methodology, simulations were extended and investigated on multiple conductor arrangements where conductors? locations were varied on an aerofoil surfaces. The results of the aerodynamic modelling suggested that a four conductor arrangement may be the best option as it gives more coverage for lightning protection of the wind turbine blades while still having the least reduction (of around 25%) on lift to drag ratio
The radial velocity curve of HD153919 (4U1700-37) revisited
We have re-analysed all available high-resolution ultraviolet IUE spectra of
the high-mass X-ray binary HD153919/4U1700-37. The radial velocity
semi-amplitude of 20.6 +/- 1.0 km/s and orbital eccentricity of 0.22 +/- 0.04
agree very well with the values obtained earlier from optical spectra. They
disagree with earlier conclusions for the same data reduced by Heap & Corcoran
(1992) and by Stickland & Lloyd (1993).Comment: 6 pages, latex, figure included, Astronomy & Astrophysics, in pres
External lightning protection system for wind turbine blades - power performance
Previous studies conducted by the authors on the aerodynamic performance of turbine blades have shown unfavourable effects of externally mounted lightning down conductors. Owing to this, one could ask whether or not externally mounted lightning down conductors could linearly affect the power performance of the whole wind turbine as well. Therefore, this paper presents a study of the power performance of one wind turbine with an externally mounted lightning down conductor. An untwisted blade profile of NACA 4418 with and without external conductor was used on the full wind turbine model. Numerical simulations were carried out on turbine power output derived from Blade Element Momentum (BEM) theory. The results are compared and discussed. The preliminary results indicate that degradation in power performance may not be significant
Modelling CO emission from Mira's wind
We have modelled the circumstellar envelope of {\it o} Ceti (Mira) using new
observational constraints. These are obtained from photospheric light scattered
in near-IR vibrational-rotational lines of circumstellar CO molecules at 4.6
micron: absolute fluxes, the radial dependence of the scattered intensity, and
two line ratios. Further observational constraints are provided by ISO
observations of far-IR emission lines from highly excited rotational states of
the ground vibrational state of CO, and radio observations of lines from
rotational levels of low excitation of CO. A code based on the Monte-Carlo
technique is used to model the circumstellar line emission.
We find that it is possible to model the radio and ISO fluxes, as well as the
highly asymmetric radio-line profiles, reasonably well with a spherically
symmetric and smooth stellar wind model. However, it is not possible to
reproduce the observed NIR line fluxes consistently with a `standard model' of
the stellar wind. This is probably due to incorrectly specified conditions of
the inner regions of the wind model, since the stellar flux needs to be larger
than what is obtained from the standard model at the point of scattering, i.e.,
the intermediate regions at approximately 100-400 stellar radii (2"-7") away
from the star. Thus, the optical depth in the vibrational-rotational lines from
the star to the point of scattering has to be decreased. This can be
accomplished in several ways. For instance, the gas close to the star (within
approximately 2") could be in such a form that light is able to pass through,
either due to the medium being clumpy or by the matter being in radial
structures (which, further out, developes into more smooth or shell-like
structures).Comment: 18 pages, 3 figures, accepted for publication in Ap
Light Curve Analysis of Hipparcos Data for the Massive O-type Eclipsing Binary UW CMa
Hipparcos photometric data for the massive O-type binary UW CMa were analysed
within the framework of the Roche model. Photometric solutions were obtained
for five mass ratios in the range. The system is found to
be in a contact configuration. Independently of , the best-fitting model
solutions correspond to the orbital inclination and the
temperature of the secondary component , at the fixed
temperature of the primary . Considering that the spectrum of the
secondary is very weak, photometric solutions corresponding to the contact
configuration favor the mass ratio smaller than unity (in which case the
luminosity of the secondary is smaller than that of the primary). The absolute
parameters of the system are estimated for different values of the mass ratio.Comment: 18 pages, 6 figure
Lag-Optimized Blood Oxygenation Level Dependent Cerebrovascular Reactivity Estimates Derived From Breathing Task Data Have a Stronger Relationship With Baseline Cerebral Blood Flow
Published: 15 June 2022Cerebrovascular reactivity (CVR), an important indicator of cerebrovascular health,
is commonly studied with the Blood Oxygenation Level Dependent functional MRI
(BOLD-fMRI) response to a vasoactive stimulus. Theoretical and empirical evidence
suggests that baseline cerebral blood flow (CBF) modulates BOLD signal amplitude
and may influence BOLD-CVR estimates. We address how acquisition and modeling
choices affect the relationship between baseline cerebral blood flow (bCBF) and
BOLD-CVR: whether BOLD-CVR is modeled with the inclusion of a breathing task,
and whether BOLD-CVR amplitudes are optimized for hemodynamic lag effects. We
assessed between-subject correlations of average GM values and within-subject spatial
correlations across cortical regions. Our results suggest that a breathing task addition to
a resting-state acquisition, alongside lag-optimization within BOLD-CVR modeling, can
improve BOLD-CVR correlations with bCBF, both between- and within-subjects, likely
because these CVR estimates are more physiologically accurate. We report positive
correlations between bCBF and BOLD-CVR, both between- and within-subjects. The
physiological explanation of this positive correlation is unclear; research with larger
samples and tightly controlled vasoactive stimuli is needed. Insights into what drives
variability in BOLD-CVR measurements and related measurements of cerebrovascular
function are particularly relevant when interpreting results in populations with altered
vascular and/or metabolic baselines or impaired cerebrovascular reserve.This work was supported by the Center for Translational Imaging
at Northwestern University. The authors disclosed receipt of
the following financial support for the research, authorship,
and/or publication of this article: This research was supported by
the Eunice Kennedy Shriver National Institute of Child Health
and Human Development of the National Institutes of Health
[K12HD073945]. KZ was supported by an NIH-funded training
program [T32EB025766]. SM was supported by the European
Union’s Horizon 2020 research and innovation program [Marie
Skłodowska-Curie grant agreement No. 713673] and a fellowship
from La Caixa Foundation [ID 100010434, fellowship code
LCF/BQ/IN17/11620063]. CC-G was supported by the Spanish
Ministry of Economy and Competitiveness [Ramon y Cajal
Fellowship, RYC2017-21845], the Basque Government [BERC
2018-2021 and PIBA_2019_104], and the Spanish Ministry
of Science, Innovation and Universities [MICINN; PID2019-
105520GB-100]
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