7,627 research outputs found
Potential uses for bracken (Pteridium aquilinum (L.) Kuhn) in organic agrculture
Bracken is a weed species due to its toxic nature and adverse effects on agriculture and ecology. This poster reviews research into historical uses for fronds and litter harvested as part of organically approved control methods. The use of bracken as an over winter mulch reduced losses of nitrogen and potassium from bare soil and maintained soil temperatures. Bracken litter was found to be a viable biofuel, with a calorific value comparable to wood and low ash and alkali metal contents. The contents of frond ash were investigated, with high concentrations of potassium found giving them a value as organic fertiliser. The addition of frond ash to soil significantly increased clover growth and number of nodules. The addition of frond ash to soil significantly increased the yield of saleable main crop potatoes. These finding have shown that bracken has a value especially within organic agriculture
Dynamic Power Spectral Analysis of Solar Measurements from Photospheric, Chromospheric, and Coronal Sources
An important aspect in the power spectral analysis of solar variability is the quasistationary and quasiperiodic nature of solar periodicities. In other words, the frequency, phase, and amplitude of solar periodicities vary on time scales ranging from active region lifetimes to solar cycle time scales. Here, researchers employ a dynamic, or running, power spectral density analysis to determine many periodicities and their time-varying nature in the projected area of active sunspot groups (S sub act). The Solar Maximum Mission/Active Cavity Radiometer Irradiance Monitor (SMM/ACRIM) total solar irradiance (S), the Nimbus-7 MgII center-to-wing ratio (R (MgII sub c/w)), the Ottawa 10.7 cm flux (F sub 10.7), and the GOES background x ray flux (X sub b) for the maximum, descending, and minimum portions of solar cycle 21 (i.e., 1980 to 1986) are used. The technique dramatically illustrates several previously unrecognized periodicities. For example, a relatively stable period at about 51 days has been found in those indices which are related to emerging magnetic fields. The majority of solar periodicities, particularly around 27, 150 and 300 days, are quasiperiodic because they vary in amplitude and frequency throughout the solar cycle. Finally, it is shown that there are clear differences between the power spectral densities of solar measurements from photospheric, chromospheric, and coronal sources
Anomalous vortex ring velocities induced by thermally-excited Kelvin waves and counterflow effects in superfluids
Dynamical counterflow effects on vortex evolution under the truncated
Gross-Pitaevskii equation are investigated. Standard longitudinal mutual
friction effects are produced and a dilatation of vortex rings is obtained at
large counterflow. A strong temperature-dependent anomalous slowdown of vortex
rings is observed and attributed to the presence of thermally exited Kelvin
waves. This generic effect of finite-temperature superfluids is estimated using
energy equipartition and orders of magnitude are given for weakly interacting
Bose-Einstein condensates and superfluid
Stationary and non-stationary fluid flow of a Bose-Einstein condensate through a penetrable barrier
We experimentally study the fluid flow induced by a broad, penetrable barrier
moving through an elongated dilute gaseous Bose-Einstein condensate. The
barrier is created by a laser beam swept through the condensate, and the
resulting dipole potential can be either attractive or repulsive. We examine
both cases and find regimes of stable and unstable fluid flow: At slow speeds
of the barrier, the fluid flow is stationary due to the superfluidity of the
condensate. At intermediate speeds, we observe a non-stationary regime in which
the condensate gets filled with dark solitons. At faster speeds, soliton
formation completely ceases and a remarkable absence of excitation in the
condensate is seen again.Comment: 4 pages, 4 figure
An abstract formulation of the concept of entropy
Entropy is presented as a concave function relating two sets of quantities called densities and field. It allows a simple classification of the standard relations of classical thermodynamics and yields a simple derivation of the conditions for concavity of the entropy function. It also allows a formal derivation of the equations of fluid motion. Dissipation, mixtures, and phase changes may also be included in the theory in a natural manner
Direct measurement of quantum phase gradients in superfluid 4He flow
We report a new kind of experiment in which we generate a known superfluid
velocity in a straight tube and directly determine the phase difference across
the tube's ends using a superfluid matter wave interferometer. By so doing, we
quantitatively verify the relation between the superfluid velocity and the
phase gradient of the condensate macroscopic wave function. Within the
systematic error of the measurement (~10%) we find v_s=(hbar/m_4)*(grad phi)
Some exact solutions in moving finite elements
It is shown that when the moving finite elements are used on a number of parabolic problems there are steady-state, stationary, similarity, or travelling-wave solutions that can be found numerically
Energy Loss from Reconnection with a Vortex Mesh
Experiments in superfluid 4He show that at low temperatures, energy
dissipation from moving vortices is many orders of magnitude larger than
expected from mutual friction. Here we investigate other mechanisms for energy
loss by a computational study of a vortex that moves through and reconnects
with a mesh of small vortices pinned to the container wall. We find that such
reconnections enhance energy loss from the moving vortex by a factor of up to
100 beyond that with no mesh. The enhancement occurs through two different
mechanisms, both involving the Kelvin oscillations generated along the vortex
by the reconnections. At relatively high temperatures the Kelvin waves increase
the vortex motion, leading to more energy loss through mutual friction. As the
temperature decreases, the vortex oscillations generate additional reconnection
events between the moving vortex and the wall, which decrease the energy of the
moving vortex by transfering portions of its length to the pinned mesh on the
wall.Comment: 9 pages, 10 figure
Thermodynamic inequalities in superfluid
We investigate general thermodynamic stability conditions for the superfluid.
This analysis is performed in an extended space of thermodynamic variables
containing (along with the usual thermodynamic coordinates such as pressure and
temperature) superfluid velocity and momentum density. The stability conditions
lead to thermodynamic inequalities which replace the Landau superfluidity
criterion at finite temperatures.Comment: 7 pages, 1 figur
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