23,269 research outputs found
The late-time development of the Richtmyer–Meshkov instability
Measurements have been made of the growth by the Richtmyer–Meshkov instability of nominally single-scale perturbations on an air/sulfur hexafluoride (SF6) interface in a large shock tube. An approximately sinusoidal shape is given to the interface by a wire mesh which supports a polymeric membrane separating the air from the SF6. A single shock wave incident on the interface induces motion by the baroclinic mechanism of vorticity generation. The visual thickness delta of the interface is measured from schlieren photographs obtained singly in each run and in high-speed motion pictures. Data are presented for delta at times considerably larger than previously reported, and they are tested for self-similarity including independence of initial conditions. Four different initial amplitude/wavelength combinations at one incident shock strength are used to determine the scaling of the data. It is found that the growth rate decreases rapidly with time, ddelta/dt[proportional]t–p (i.e., delta[proportional]t1–p), where 0.67<~p<~0.74 and that a small dependence on the initial wavelength lambda0 persists to large time. The larger value of the power law exponent agrees with the result of the late-time-decay similarity law of Huang and Leonard [Phys. Fluids 6, 3765–3775 (1994)]. The influence of the wire mesh and membrane on the mixing process is assessed
Omnipresent long-period intensity oscillations in open coronal structures
Quasi-periodic propagating disturbances in coronal structures have been
interpreted as slow magneto-acoustic waves and/or periodic upflows. Here we aim
to understand their nature from the observed properties using a three-hour
imaging sequence from AIA/SDO in two different temperature channels. We also
compare the characteristics with a simple wave model. We searched for
propagating disturbances in open-loop structures at three different locations;
a fan loop-structure off-limb, an on-disk plume-like structure and the
plume/interplume regions in the north pole of the sun. In each of the subfield
regions chosen to cover these structures, the time series at each pixel
location was subjected to wavelet analysis to find the different periodicities.
We then constructed powermaps in three different period ranges. We also
constructed space-time maps for the on-disk plume structure to estimate the
propagation speeds in different channels. We find propagating disturbances in
all three structures. Powermaps indicate that the power in the long-period
range is significant up to comparatively longer distances along the loop than
that in the shorter periods. This nature is observed in all three structures. A
detailed analysis on the on-disk plume structure gives consistently higher
propagation speeds in the 193 \AA channel and also reveals spatial damping
along the loop. The amplitude and the damping length values are lower in hotter
channels, indicating their acoustic dependence. These properties can be
explained very well with a propagating slow-wave model. We suggest that these
disturbances are more likely to be caused by propagating slow magneto-acoustic
waves than by high-speed quasi-periodic upflows. We find that intensity
oscillations in longer periods are omnipresent at larger heights even in active
regions.Comment: accepted for publication in A &
Magnetotransport in the low carrier density ferromagnet EuB_6
We present a magnetotransport study of the low--carrier density ferromagnet
EuB_6. This semimetallic compound, which undergoes two ferromagnetic
transitions at T_l = 15.3 K and T_c = 12.5 K, exhibits close to T_l a colossal
magnetoresistivity (CMR). We quantitatively compare our data to recent
theoretical work, which however fails to explain our observations. We attribute
this disagreement with theory to the unique type of magnetic polaron formation
in EuB_6.Comment: Conference contribution MMM'99, San Jos
Soft Computing Based Cavity Temperature Control of Plastic Injection Molding system
The plastic parts having complex three dimensional structures are produced by Plastic injection molding system. Thequality of the product is determined by controlling the temperature of the mold cavity. The mold cavity temperature controlwith the conventional ON/OFF, PI, and PID controllers have several disadvantages. This paper proposes the method toreduce settling time and undershoot in cavity temperature control with selected evolutionary algorithms. The controllerparameters are optimized with Genetic Algorithm (GA) and Particle Swarm Optimization (PSO) Algorithm for PID and I-PDcontrollers by considering Mean Square Error (MSE) as fitness function. Compared to conventional methods theparameter optimization using soft computing methods such as GA and PSO improves the performance indices of PID andI-PD controllers
Geomagnetic storm effects on GPS based navigation
The energetic events on the sun, solar wind and subsequent effects on the Earth's geomagnetic field and upper atmosphere (ionosphere) comprise space weather. Modern navigation systems that use radio-wave signals, reflecting from or propagating through the ionosphere as a means of determining range or distance, are vulnerable to a variety of effects that can degrade the performance of the navigational systems. In particular, the Global Positioning System (GPS) that uses a constellation of earth orbiting satellites are affected due to the space weather phenomena. <br><br> Studies made during two successive geomagnetic storms that occurred during the period from 8 to 12 November 2004, have clearly revealed the adverse affects on the GPS range delay as inferred from the Total Electron Content (TEC) measurements made from a chain of seven dual frequency GPS receivers installed in the Indian sector. Significant increases in TEC at the Equatorial Ionization anomaly crest region are observed, resulting in increased range delay during the periods of the storm activity. Further, the storm time rapid changes occurring in TEC resulted in a number of phase slips in the GPS signal compared to those on quiet days. These phase slips often result in the loss of lock of the GPS receivers, similar to those that occur during strong(&gt;10 dB) L-band scintillation events, adversely affecting the GPS based navigation
Remote surface inspection system
This paper reports on an on-going research and development effort in remote surface inspection of space platforms such as the Space Station Freedom (SSF). It describes the space environment and identifies the types of damage for which to search. This paper provides an overview of the Remote Surface Inspection System that was developed to conduct proof-of-concept demonstrations and to perform experiments in a laboratory environment. Specifically, the paper describes three technology areas: (1) manipulator control for sensor placement; (2) automated non-contact inspection to detect and classify flaws; and (3) an operator interface to command the system interactively and receive raw or processed sensor data. Initial findings for the automated and human visual inspection tests are reported
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Thaan Vuzha Nilam Tharisu: the land without a farmer becomes barren
This report forms a part of the international research project on policy and sustainable agriculture, Policies that Work for Sustainable Agriculture and Regenerated Rural Economies. The report details the findings of one of the constituent studies, undertaken by an Indian NGO, the Society for People's Education and Economic Change (SPEECH). The Importance of this project is that it concentrated on rainfed rather than irrigated agriculture - i.e. the sharp end of rural development in India, and that the focus was very much on the micro-level, looking at policy as seen from the ground. One of the recurring themes throughout the research was the importance of appreciating people as individuals, and in this spirit some of the personal qualities of the research team are shared. The research covers events in sites in the Virudhunagar district of Tamilnadu: Tiruchuli Panchayat Union and the Villur chain of tanks. This is an essentially rural area, where the need for sustainable forms of agriculture and rural livelihoods is clear. The political landscape is fractured and complex (§2.3), and the officials with the responsibility of implementing policy face significant obstacles and disincentives in doing so in response to the needs of local communities
Visualisation of an entangled channel spin-1 system
Co-variance matrix formalism gives powerful entanglement criteria for
continuous as well as finite dimensional systems. We use this formalism to
study a mixed channel spin-1 system which is well known in nuclear reactions. A
spin-j state can be visualized as being made up of 2j spinors which are
represented by a constellation of 2j points on a Bloch sphere using Majorana
construction. We extend this formalism to visualize an entangled mixed spin-1
system.Comment: 4 pages,4 figure
Effects of the size of cosmological N-Body simulations on physical quantities -- I: Mass Function
N-Body simulations are a very important tool in the study of formation of
large scale structures. Much of the progress in understanding the physics of
galaxy formation and comparison with observations would not have been possible
without N-Body simulations. Given the importance of this tool, it is essential
to understand its limitations as ignoring these can easily lead to interesting
but unreliable results. In this paper we study the limitations due to the
finite size of the simulation volume. We explicitly construct the correction
term arising due to a finite box size and study its generic features for
clustering of matter and also on mass functions. We show that the correction to
mass function is maximum near the scale of non-linearity, as a corollary we
show that the correction to the number density of haloes of a given mass
changes sign at this scale; the number of haloes at small masses is over
estimated in simulations. This over estimate results from a delay in mergers
that lead to formation of more massive haloes. The same technique can be used
to study corrections to other physical quantities. The corrections are
typically small if the scale of non-linearity is much smaller than the
box-size. However, there are some cases of physical interest in which the
relative correction term is of order unity even though a simulation box much
larger than the scale of non-linearity is used. Within the context of the
concordance model, our analysis suggests that it is very difficult for present
day simulations to resolve mass scales smaller than 100 solar masses accurately
and the level of difficulty increases as we go to even smaller masses, though
this constraint does not apply to multi-scale simulations.Comment: 10 pages, 6 figure, MNRAS format. Accepted for publication in the
MNRA
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