From Small-Scale Dynamo to Isotropic MHD Turbulence

We consider the problem of incompressible, forced, nonhelical, homogeneous, isotropic MHD turbulence with no mean magnetic field. This problem is essentially different from the case with externally imposed uniform mean field. There is no scale-by-scale equipartition between magnetic and kinetic energies as would be the case for the Alfven-wave turbulence. The isotropic MHD turbulence is the end state of the turbulent dynamo which generates folded fields with small-scale direction reversals. We propose that the statistics seen in numerical simulations of isotropic MHD turbulence could be explained as a superposition of these folded fields and Alfven-like waves that propagate along the folds.Comment: kluwer latex, 7 pages, 7 figures; Proceedings of the International Workshop "Magnetic Fields and Star Formation: Theory vs. Observations", Madrid, 21-25 April 2003 -- published version (but the e-print is free of numerous typos introduced by the publisher

Protecting the Lawn.

6 p

The Ursinus Weekly, October 13, 1958

Men\u27s student government publishes new rules booklet • Dr. C. N. Parkinson enthralls Forum audience Oct. 7 • Folk singer to appear at museum this Saturday • Dinner held for frosh women in Paisley, Oct. 7 • Joan of Lorraine picked for Fall play; Tryouts on Oct. 13-14 • YM-YW holds annual Fall weekend retreat • APO discusses projects; Cleans college woods • Editorial: Busy • Letters to the editor • Conformists • Little dirty face • Brighter light • U.C. soccer team ties Hill School in pre-season tilt • U.C. cross country team is formed • Sophs tally run in final frame to edge out freshmen • Bear\u27s eleven lose to Johns Hopkins by 12-0 • Business Administration Club holds first meetinghttps://digitalcommons.ursinus.edu/weekly/1368/thumbnail.jp

A systematic correlation between two-dimensional flow topology and the abstract statistics of turbulence

Velocity differences in the direct enstrophy cascade of two-dimensional turbulence are correlated with the underlying flow topology. The statistics of the transverse and longitudinal velocity differences are found to be governed by different structures. The wings of the transverse distribution are dominated by strong vortex centers, whereas, the tails of the longitudinal differences are dominated by saddles. Viewed in the framework of earlier theoretical work this result suggests that the transfer of enstrophy to smaller scales is accomplished in regions of the flow dominated by saddles.Comment: 4 pages, 4 figure

A Cross-Over in the Enstrophy Decay in Two-Dimensional Turbulence in a Finite Box

The numerical simulation of two-dimensional decaying turbulence in a large but finite box presented in this paper uncovered two physically different regimes of enstrophy decay. During the initial stage, the enstrophy, generated by a random Gaussian initial condition, decays as t^{-gamma} with gamma approximately 0.7-0.8. After that, the flow undergoes a transition to a gas or fluid composed of distinct vortices. Simultaneously, the magnitude of the decay exponent crosses over to gamma approximately 0.4. An exact relation for the total number of vortices, N(t), in terms of the mean circulation of an individual vortex is derived. A theory predicting that N(t) is proportional to t^{-xi} and the magnitudes of exponents gamma=2/5 and xi=4/5 is presented and the possibility of an additional very late-time cross-over to gamma=1/3 and xi=2/3 is also discussed.Comment: 11 pages, 7 figure

The Ursinus Weekly, January 12, 1959

Radio station to be set-up with student body consent • Petersen and Drewniak are honored by MAC • Summer study abroad announced by IIE • May Day • Air Force offers commissions to college grads • Frosh present Marooned on Friday, Jan. 16 • Prof. H.C. Symons elected pres. of school board • Dr. I. Progoff speaker at Forum January 14 • YM-YWCA plans for Spring semester • U.C. receives grant from DuPont • Editorial: Radio station • Student opinion • Prof\u27s opinion • Resolutions • Letters to the editor • Soccer team chooses \u2759 captains • Badminton team practices for new season • Courtmen drop close tilt to Fords by 65-64 • Grapplers win over Haverford by 18-16 • Amico speaks • Lester Lanin to be at Sunnybrookhttps://digitalcommons.ursinus.edu/weekly/1375/thumbnail.jp

On Gravitational Waves in Spacetimes with a Nonvanishing Cosmological Constant

We study the effect of a cosmological constant $\Lambda$ on the propagation and detection of gravitational waves. To this purpose we investigate the linearised Einstein's equations with terms up to linear order in $\Lambda$ in a de Sitter and an anti-de Sitter background spacetime. In this framework the cosmological term does not induce changes in the polarization states of the waves, whereas the amplitude gets modified with terms depending on $\Lambda$. Moreover, if a source emits a periodic waveform, its periodicity as measured by a distant observer gets modified. These effects are, however, extremely tiny and thus well below the detectability by some twenty orders of magnitude within present gravitational wave detectors such as LIGO or future planned ones such as LISA.Comment: 8 pages, 4 figures, accepted for publication in Physical Review

The Ursinus Weekly, February 16, 1959

Dr. Goncharoff discusses world youth at Forum Wed. • Seniors to present play by C. George • Curtain Club presents plays • French Club presents Mme. Helene Bordas • YM-YWCA plan for Spring semester • Music room • Freshmen women elected Wed. Feb. 11 to YWCA, WAA, WSGA • Annual Lorelei to be held at Sunnybrook Fri. • May queen to be elected • Editorial: Destruction • Student opinion • Prof\u27s opinion • Girls\u27 b-ball teams win season\u27s start • B-ball squad drops two close tilts • Ursinus whips E-town in wrestling match 30-6 • Letters to the editorhttps://digitalcommons.ursinus.edu/weekly/1377/thumbnail.jp