4,796 research outputs found
Magnetic reversals in a simple model of MHD
We study a simple magnetohydrodynamical approach in which hydrodynamics and
MHD turbulence are coupled in a shell model, with given dynamo constrains in
the large scales. We consider the case of a low Prandtl number fluid for which
the inertial range of the velocity field is much wider than that of the
magnetic field. Random reversals of the magnetic field are observed and it
shown that the magnetic field has a non trivial evolution linked to the nature
of the hydrodynamics turbulence.Comment: 4 pages, submitted to PR
Influence of Ground Reaction Forces and Joint Velocities on Kicking Velocity
Introduction: Kicking is a vital component in the game of soccer. One major factor that influences the success of a scoring attempt is ball velocity. Ground reaction force (GRF) and joint velocities of the lower extremities are variables of interest for increasing kicking velocity. Previous studies have shown exercise programs used to strengthen the muscles used in kicking have been successful in increasing kicking velocity (Manolopoulos, et al., 2013).[GJR1] To date, no known studies have analyzed the specific relationship between GRF, joint velocity and kicking velocity.
Purpose: The goal of this pilot study was to analyze the influence of ground reaction forces and joint velocities on kicking velocity.
Methods: Four female Division II collegiate soccer players [GJR2] completed 3 instep soccer kicks [GJR3] using their dominant, right leg. Their motion was captured using the Cortex 8.1 Motion Analysis Software. Ball velocity, right and left anterior superior iliac spine (ASIS) and right and left ankle velocities were calculated using the motion analysis software. Ground reaction forces from the plant leg were also measured using force plate data from the Cortex software. Bivariate Pearson correlations with 0.95 confidence intervals were computed using SPSS version 28 for the resultant velocities of the right and left ASIS, right and left ankles, and the velocity of the ball. They were also calculated for the peak GRF in the anteroposterior (X), mediolateral (Y) and vertical (Z) directions and ball velocity. A correlation value of \u3e0.800 or \u3c -0.800 was considered significant.
Results: Significant correlations were found between peak resultant ball velocity and GRF in the X direction (-0.907), GRF in the Y direction (0.867), R ASIS velocity (0.950), R ankle velocity (0.855), and L ankle velocity (0.977). No significant correlations were found between peak resultant ball velocity and GRF in the Z direction (0.788), or peak resultant ball velocity and peak joint velocity of the L ASIS (0.692).
Conclusion: Braking force of the planting leg is shown to correlate significantly with kicking velocity. Although high braking force allows for a faster ball, this can have other implications to injury [GJR4] (Ball, 2012; Jones & Graham-Smith, 2016). Linear velocity of the hip also allows for greater ball velocity. As a pilot study, this study lacks the statistical power to extrapolate the information to larger populations. Therefore, additional studies are needed to further investigate the relationships between kicking mechanics and the resulting ball velocity
Emergence of pointer states in a non-perturbative environment
We show that the pointer basis distinguished by collisional decoherence
consists of exponentially localized, solitonic wave packets. Based on the
orthogonal unraveling of the quantum master equation, we characterize their
formation and dynamics, and we demonstrate that the statistical weights arising
from an initial superposition state are given by the required projection. Since
the spatial width of the pointer states can be obtained by accounting for the
gas environment in a microscopically realistic fashion, one may thus calculate
the coherence length of a strongly interacting gas.Comment: 8 pages, 1 figure; corresponds to published versio
Diffusion and dispersion of passive tracers: Navier-Stokes versus MHD turbulence
A comparison of turbulent diffusion and pair-dispersion in homogeneous,
macroscopically isotropic Navier-Stokes (NS) and nonhelical magnetohydrodynamic
(MHD) turbulence based on high-resolution direct numerical simulations is
presented. Significant differences between MHD and NS systems are observed in
the pair-dispersion properties, in particular a strong reduction of the
separation velocity in MHD turbulence as compared to the NS case. It is shown
that in MHD turbulence the average pair-dispersion is slowed down for
, being
the Kolmogorov time, due to the alignment of the relative Lagrangian tracer
velocity with the local magnetic field. Significant differences in turbulent
single-particle diffusion in NS and MHD turbulence are not detected. The fluid
particle trajectories in the vicinity of the smallest dissipative structures
are found to be characterisically different although these comparably rare
events have a negligible influence on the statistics investigated in this work.Comment: Europhysics Letters, in prin
REACTION MECHANISMS AND RATE LIMITATIONS IN DRY ETCHING OF SILICON DIOXIDE VITH ANHYDROUS HYDROGEN FLUORIDE
A novel dry etching process for silicon dioxide has been developed. This process, carried out at ambient temperature and pressure, uses anhydrous hydrogen fluoride, water vapor in a nitrogen carrier, and a unique processing sequence to achieve etch rates of about 200A/second, with 5 percent or better uniformity.
The overall reaction is a complicated sequence of surface hydration and surface fluorlnation by adsorption, reaction, and product desorptlon. This paper presents two proposed reaction mechanisms and describes how experimental data from a laminar flow reactor were used to evaluate the mechanisms
Cytokine Profiles of Stimulated Blood Lymphocytes in Asthmatic and Healthy Adolescents Cross the School Year
T cell cytokines play an important role in mediating airway inflammation in asthma. The predominance of a Th2 cytokine profile, particularly interleukin (IL)-4 and IL-5, is associated with the pathogenesis and course of asthma. The aim of this study was to test the hypothesis that a stressful life event alters the pattern of cytokine release in asthmatic individuals. Thirteen healthy controls and 21 asthmatic adolescents gave blood samples three times over a semester: midsemester, during the week of final examinations, and 2-3 weeks after examinations. Interferon-γ (IFN-γ), IL-2, IL-4, and IL-5 were measured from supernatants of cells stimulated with PHA/PMA for 24 h. Cells from asthmatic subjects released significantly more IL-5 during the examination and postexamination periods, whereas cells from healthy controls released significantly more IL-2 during the midsemester and examination periods, thereby indicating a bias for a Th2-like pattern in asthmatics and a Th 1-like pattern in healthy controls. IL-4 and IL-5 production showed a marked decrease during and after examinations in healthy controls, whereas this decline was absent in asthmatics. The ratios of IFN-γ:IL-4 and IFN-γ:IL-5 also revealed significant changes in the profile of cytokine release across the semester. These results indicate differential cytokine responses in asthmatics that may become pronounced during periods of cellular activation
Variational bound on energy dissipation in plane Couette flow
We present numerical solutions to the extended Doering-Constantin variational
principle for upper bounds on the energy dissipation rate in turbulent plane
Couette flow. Using the compound matrix technique in order to reformulate this
principle's spectral constraint, we derive a system of equations that is
amenable to numerical treatment in the entire range from low to asymptotically
high Reynolds numbers. Our variational bound exhibits a minimum at intermediate
Reynolds numbers, and reproduces the Busse bound in the asymptotic regime. As a
consequence of a bifurcation of the minimizing wavenumbers, there exist two
length scales that determine the optimal upper bound: the effective width of
the variational profile's boundary segments, and the extension of their flat
interior part.Comment: 22 pages, RevTeX, 11 postscript figures are available as one
uuencoded .tar.gz file from [email protected]
Variational bound on energy dissipation in turbulent shear flow
We present numerical solutions to the extended Doering-Constantin variational
principle for upper bounds on the energy dissipation rate in plane Couette
flow, bridging the entire range from low to asymptotically high Reynolds
numbers. Our variational bound exhibits structure, namely a pronounced minimum
at intermediate Reynolds numbers, and recovers the Busse bound in the
asymptotic regime. The most notable feature is a bifurcation of the minimizing
wavenumbers, giving rise to simple scaling of the optimized variational
parameters, and of the upper bound, with the Reynolds number.Comment: 4 pages, RevTeX, 5 postscript figures are available as one .tar.gz
file from [email protected]
An integrating factor matrix method to find first integrals
In this paper we developed an integrating factor matrix method to derive
conditions for the existence of first integrals. We use this novel method to
obtain first integrals, along with the conditions for their existence, for two
and three dimensional Lotka-Volterra systems with constant terms. The results
are compared to previous results obtained by other methods
Fabrication of 2-2 Connectivity PZT/Thermoplastic Composites for High Frequency Linear Arrays
An alternate approach for fabricating PZT/polymer composites with 2-2 connectivity with fine scales is described. Thin (less than or equal to 20 micrometers) sintered PZT plates and sheets of a thermoplastic polymer film (less than or equal to 10 micrometers) were bonded together via thermal processing. Stack sintering of tape cast PZT generated the necessary PZT plates, while tape cast polymers were used to control the thermoplastic thickness. Composite blocks were cut to required dimensions for linear arrays, electroded, and poled. Electromechanical properties were measured to evaluate the composites. The significance of this fabrication technique is that it is able to generate 2-2 structures at a scale level unachievable by conventional dice-and-fill fabrication methods
- …