Some effects of y-axis vibration on visual acuity Final report, Jul. 1966 - Nov. 1967

Side to side head vibration effects on visual acuity measurement

Renormalization group analysis of the Reynolds stress transport equation

The pressure velocity correlation and return to isotropy term in the Reynolds stress transport equation are analyzed using the Yakhot-Orszag renormalization group. The perturbation series for the relevant correlations, evaluated to lowest order in the epsilon-expansion of the Yakhot-Orszag theory, are infinite series in tensor product powers of the mean velocity gradient and its transpose. Formal lowest order Pade approximations to the sums of these series produce a fast pressure strain model of the form proposed by Launder, Reece, and Rodi, and a return to isotropy model of the form proposed by Rotta. In both cases, the model constant are computed theoretically. The predicted Reynolds stress ratios in simple shear flows are evaluated and compared with experimental data. The possibility is discussed of driving higher order nonlinear models by approximating the sums more accurately

50 TeV HEGRA Sources and Infrared Radiation

The recent observations of 50 TeV gamma radiation by HEGRA have the potential of determining the extragalactic flux of infrared radiation. The fact that radiation is observed in the range between 30 and 100 TeV sets an upper limit on the infrared flux, while a cutoff at $E_{\gamma} \approx 50$ TeV fixes this flux with a good accuracy. If the intrinsic radiation is produced due to interaction of high energy protons with gas or low-energy target photons, then an accompaning high-energy neutrino flux is unavoidable. We calculate this flux and underground muon flux produced by it. The muon flux is dominated by muons with energies about 1 TeV and can be marginally detected by a 1 km$^2$ detector like an expanded AMANDA.Comment: 9 pages, latex2e, 3 eps figure

Complexity and hierarchical game of life

Hierarchical structure is an essential part of complexity, important notion relevant for a wide range of applications ranging from biological population dynamics through robotics to social sciences. In this paper we propose a simple cellular-automata tool for study of hierarchical population dynamics

Thermal Fluctuations and Rubber Elasticity

The effects of thermal elastic fluctuations in rubber materials are examined. It is shown that, due to an interplay with the incompressibility constraint, these fluctuations qualitatively modify the large-deformation stress-strain relation, compared to that of classical rubber elasticity. To leading order, this mechanism provides a simple and generic explanation for the peak structure of Mooney-Rivlin stress-strain relation, and shows a good agreement with experiments. It also leads to the prediction of a phonon correlation function that depends on the external deformation.Comment: 4 RevTeX pages, 1 figure, submitted to PR

Microscopic theory for the glass transition in a system without static correlations

We study the orientational dynamics of infinitely thin hard rods of length L, with the centers-of-mass fixed on a simple cubic lattice with lattice constant a.We approximate the influence of the surrounding rods onto dynamics of a pair of rods by introducing an effective rotational diffusion constant D(l),l=L/a. We get D(l) ~ [1-v(l)], where v(l) is given through an integral of a time-dependent torque-torque correlator of an isolated pair of rods. A glass transition occurs at l_c, if v(l_c)=1. We present a variational and a numerically exact evaluation of v(l).Close to l_c the diffusion constant decreases as D(l) ~ (l_c-l)^\gamma, with \gamma=1. Our approach predicts a glass transition in the absence of any static correlations, in contrast to present form of mode coupling theory.Comment: 6 pages, 3 figure

Shell model for rotating turbulence

A modified shell model for rotating turbulence is proposed. The effect of rotation is introduced by a randomized linear term. Randomization is shown to be important in correctly modeling the rotation effect. Numerical simulation shows that the exponent of the energy spectrum in the inertial range changes from –5/3 to –2 as rotation rate increases. The mechanism behind this change is explained by weak turbulence theory and supported by numerical results

Sliding friction between an elastomer network and a grafted polymer layer: the role of cooperative effects

We study the friction between a flat solid surface where polymer chains have been end-grafted and a cross-linked elastomer at low sliding velocity. The contribution of isolated grafted chains' penetration in the sliding elastomer has been early identified as a weakly velocity dependent pull-out force. Recent experiments have shown that the interactions between the grafted chains at high grafting density modify the friction force by grafted chain. We develop here a simple model that takes into account those interactions and gives a limit grafting density beyond which the friction no longer increases with the grafting density, in good agreement with the experimental dataComment: Submitted to Europhys. Letter