680 research outputs found
Solar rotation rate and its gradients during cycle 23
Available helioseismic data now span almost the entire solar activity cycle
23 making it possible to study solar-cycle related changes of the solar
rotation rate in detail. In this paper we study how the solar rotation rate, in
particular, the zonal flows change with time. In addition to the zonal flows
that show a well known pattern in the solar convection zone, we also study
changes in the radial and latitudinal gradients of the rotation rate,
particularly in the shear layer that is present in the immediate sub-surface
layers of the Sun. In the case of the zonal-flow pattern, we find that the band
indicating fast rotating region close to the equator seems to have bifurcated
around 2005. Our investigation of the rotation-rate gradients show that the
relative variation in the rotation-rate gradients is about 20% or more of their
average values, which is much larger than the relative variation in the
rotation rate itself. These results can be used to test predictions of various
solar dynamo models.Comment: To appear in ApJ. Fig 5 has been corrected in this versio
Solar Rotation Rate During the Cycle 24 Minimum in Activity
The minimum of solar cycle 24 is significantly different from most other
minima in terms of its duration as well as its abnormally low levels of
activity. Using available helioseismic data that cover epochs from the minimum
of cycle 23 to now, we study the differences in the nature of the solar
rotation between the minima of cycles 23 and 24. We find that there are
significant differences between the rotation rates during the two minima. There
are differences in the zonal-flow pattern too. We find that the band of fast
rotating region close to the equator bifurcated around 2005 and recombined by
2008. This behavior is different from that during the cycle 23 minimum. By
auto-correlating the zonal-flow pattern with a time shift, we find that in
terms of solar dynamics, solar cycle 23 lasted for a period of 11.7 years,
consistent with the result of Howe et al. (2009). The autocorrelation
coefficient also confirms that the zonal-flow pattern penetrates through the
convection zone.Comment: Accepted for publication in Ap
Temporal variations of solar rotation rate at high latitudes
Frequency splitting coefficients from Global Oscillation Network Group (GONG)
and Michelson Doppler Imager (MDI) observations covering the period 1995--2001
are used to study temporal variations in the solar rotation rate at high
latitudes. The torsional oscillation pattern in the Sun is known to penetrate
to a depth of about with alternate bands of faster and slower
rotating plasma. At lower latitudes the bands move towards equator with time.
At higher latitudes, however, the bands appear to move towards the poles. This
is similar to the observed pole-ward movement of large scale magnetic fields at
high latitudes. This also supports theoretical results of pole-ward moving
bands at high latitudes in some mean field dynamo models. The polar rotation
rate is found to decrease between 1995 and 1999 after which it has started
increasing.Comment: To appear in ApJ Letter
Multi-criteria Analysis for the Selection of the Best Energy Efficient Option in Urban Water Systems
AbstractThis paper presents the application of multi-criteria decision analysis to select the best energy efficient option for a water supply system. The case study is a part of the Multi-Municipal Water Supply System (MMWSS) for the Algarve region in Portugal. There is a micro-hydropower plant installed in one of the two water treatment plants. The system has two operating schemes due to the seasonality of tourism: one for the high season from June to September; and the other one for the low season from October to May. The aim of the analysis is to compare the energy efficiency of the system for the two operating schemes and for different demands. Energy audits (i.e., hydraulic energy balance along the pipe system) are carried out for each option (pair operating scheme – demand). Different energy efficiency metrics are calculated and two different multi-criteria analysis methods are used and compared to rank the options. Results obtained are discussed and the main conclusions are presented
The Radial Distribution of Magnetic Helicity in the Solar Convective Zone: Observations and Dynamo Theory
We continue our attempt to connect observational data on current helicity in
solar active regions with solar dynamo models. In addition to our previous
results about temporal and latitudinal distributions of current helicity
(Kleeorin et al. 2003), we argue that some information concerning the radial
profile of the current helicity averaged over time and latitude can be
extracted from the available observations. The main feature of this
distribution can be presented as follows. Both shallow and deep active regions
demonstrate a clear dominance of one sign of current helicity in a given
hemisphere during the whole cycle. Broadly speaking, current helicity has
opposite polarities in the Northern and Southern hemispheres, although there
are some active regions that violate this polarity rule. The relative number of
active regions violating the polarity rule is significantly higher for deeper
active regions. A separation of active regions into `shallow', `middle' and
`deep' is made by comparing their rotation rate and the helioseismic rotation
law. We use a version of Parker's dynamo model in two spatial dimensions, that
employs a nonlinearity based on magnetic helicity conservation arguments. The
predictions of this model about the radial distribution of solar current
helicity appear to be in remarkable agreement with the available observational
data; in particular the relative volume occupied by the current helicity of
"wrong" sign grows significantly with the depth.Comment: 12 pages, 8 Postscript figures, uses mn2e.cl
In--out intermittency in PDE and ODE models
We find concrete evidence for a recently discovered form of intermittency,
referred to as in--out intermittency, in both PDE and ODE models of mean field
dynamos. This type of intermittency (introduced in Ashwin et al 1999) occurs in
systems with invariant submanifolds and, as opposed to on--off intermittency
which can also occur in skew product systems, it requires an absence of skew
product structure. By this we mean that the dynamics on the attractor
intermittent to the invariant manifold cannot be expressed simply as the
dynamics on the invariant subspace forcing the transverse dynamics; the
transverse dynamics will alter that tangential to the invariant subspace when
one is far enough away from the invariant manifold.
Since general systems with invariant submanifolds are not likely to have skew
product structure, this type of behaviour may be of physical relevance in a
variety of dynamical settings.
The models employed here to demonstrate in--out intermittency are
axisymmetric mean--field dynamo models which are often used to study the
observed large scale magnetic variability in the Sun and solar-type stars. The
occurrence of this type of intermittency in such models may be of interest in
understanding some aspects of such variabilities.Comment: To be published in Chaos, June 2001, also available at
http://www.eurico.web.co
Modelling pellet flow in single extrusion with DEM
Plasticating single-screw extrusion involves the continuous conversion of loose solid
pellets into a pressurized homogeneous melt that is pumped through a shaping tool. Traditional
analyses of the solids conveying stage assume the movement of an elastic solid plug at a fixed
speed. However, not only the corresponding predictions fail considerably, but it is also well
known that, at least in the initial screw turns, the flow of loose individual pellets takes place.
This study follows previous efforts to predict the characteristics of such a flow using the discrete
element method. The model considers the development of normal and tangential forces resulting
from the inelastic collisions between the pellets and between them and the neighbouring metallic
surfaces. The algorithm proposed here is shown to be capable of capturing detailed features of the
granular flow. The predictions of velocities in the cross- and down-channel directions and of the
coordination number are in good agreement with equivalent reported results. The effect of pellet
size on the flow features is also discussed
Changes in Solar Dynamics from 1995 to 2002
Data obtained by the GONG and MDI instruments over the last seven years are
used to study how solar dynamics -- both rotation and other large scale flows
-- have changed with time. In addition to the well known phenomenon of bands of
faster and slower rotation moving towards the equator and pole, we find that
the zonal flow pattern rises upwards with time. Like the zonal flows, the
meridional flows also show distinct solar activity related changes. In
particular, the anti-symmetric component of the meridional flow shows a
decrease in speed with activity. We do not see any significant temporal
variations in the dynamics of the tachocline region where the solar dynamo is
believed to be operating.Comment: To appear in ApJ, March 1 200
On Predicting the Solar Cycle using Mean-Field Models
We discuss the difficulties of predicting the solar cycle using mean-field
models. Here we argue that these difficulties arise owing to the significant
modulation of the solar activity cycle, and that this modulation arises owing
to either stochastic or deterministic processes. We analyse the implications
for predictability in both of these situations by considering two separate
solar dynamo models. The first model represents a stochastically-perturbed flux
transport dynamo. Here even very weak stochastic perturbations can give rise to
significant modulation in the activity cycle. This modulation leads to a loss
of predictability. In the second model, we neglect stochastic effects and
assume that generation of magnetic field in the Sun can be described by a fully
deterministic nonlinear mean-field model -- this is a best case scenario for
prediction. We designate the output from this deterministic model (with
parameters chosen to produce chaotically modulated cycles) as a target
timeseries that subsequent deterministic mean-field models are required to
predict. Long-term prediction is impossible even if a model that is correct in
all details is utilised in the prediction. Furthermore, we show that even
short-term prediction is impossible if there is a small discrepancy in the
input parameters from the fiducial model. This is the case even if the
predicting model has been tuned to reproduce the output of previous cycles.
Given the inherent uncertainties in determining the transport coefficients and
nonlinear responses for mean-field models, we argue that this makes predicting
the solar cycle using the output from such models impossible.Comment: 22 Pages, 5 Figures, Preprint accepted for publication in Ap
Application of hydraulic transients for leak detection in water supply systems
Abstract The current paper reports the investigation of two transient-based techniques for leak detection in water pipe systems using physical data collected in the laboratory and in quasi-field conditions. The first is the analysis of the leak reflected wave during a transient event and the second is inverse transient analysis (ITA). This was approached through the development of an inverse transient analysis tool and the collection of transient data for the testing and validation of this model. Two experimental programmes were carried out at Imperial College and in cooperation with Thames Water for the validation and testing of these techniques. Evaluation of the presence, location and size of leaks was carried out using the collected data. Transient-based techniques have been shown to be successful in the detection and location of leaks and leak location uncertainties depended on the leak size and location, flow regime and location where the transient event was generated. These leak detection methods are very promising for identifying the general area of the trunk main with leakage, and can be combined with other leak location techniques (e.g. acoustic equipment) to more precisely pinpoint the leak position. Transient-based techniques are particularly important for the diagnosis, monitoring and control of existing water supply systems, not only to detect leaks, but also to better understand the causes of pipe bursts and accidents, particularly when these are due to natural transient events
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