First this paper describes in detail an apparatus for heat-transport measurements in shallow horizontal layers of fluid at low temperatures Then high-precision results of convective heat transport as a function of the Rayleigh number R are presented for cylindrical cells of aspect ratio L = 208,472 and 57 The present paper concentrates on the long-time behaviour of Boussinesq systems Non-Boussinesq effects, transient effects near the convective onset, and time-dependent states are described elsewhere (Walden & Ahlers 1981 Ahlers et al 1981 Ahlers 1980b and references therein) The measurements show that the convective onset near the critical Rayleigh number Rc is sharp within the experimental resolution of about 01 % of the Nusselt number N even in laterally finite containers Values of R and of the initial slopes of N(R), are obtained and compared with predictions for different flow patterns Over a wider range of R and for L = 57 and 472, N was found within experimental resolution to be a unique, continuous function of R For L = 208, hysteretic transitions are revealed by N(R) near R ≈ 3 and R ≈ 10 For L = 472, the effect of impulsive heating was studied and revealed complicated, long-lived, but surprisingly repro- ducible transients

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112082003322

Heat transport and temporal evolution of fluid flow near the Rayleigh-Bénard instability in cylindrical containers

After a brief review in the introduction of the major breakthroughs in the study of Rayleigh-Benard convection (RBC) since the experiments of Henri Benard, a few selected topics are presented in more detail. The effect of on the bifurcation to convection is discussed because experimental work on this is quite recent and as yet incomplete. Examples of spatio-temporal chaos are examined because this interesting nonlinear state is as yet incompletely understood. The effect of rotation on RBC is presented because some of the experimental results disagree with modern theories.

Experiments with Rayleigh-Bénard Convection

We report experimental results for the power spectra, variance, skewness and kurtosis of temperature fluctuations in turbulent Rayleigh–Benard convection (RBC) of a fluid with Prandtl number in cylindrical samples with aspect ratios (diameter over height ) of 0.50 and 1.00. The measurements were primarily for the radial positions and . In both cases, data were obtained at several vertical locations . For all locations, there is a frequency range of about a decade over which the spectra can be described well by the power law . For all and , the value is less than one near the top and bottom plates and increases as or increase from 0.01 to 0.5. This differs from the finding for (He et al., Phys. Rev. Lett., vol. 112, 2014, 174501) and the expectation for the downstream velocity of turbulent wall-bounded shear flow (Rosenberg, J. Fluid Mech., vol. 731, 2013, pp. 46–63), where is found or expected in an inner layer ( ) near the wall but in the bulk. The variance is described better by a power law than by the logarithmic dependence found or expected for and for turbulent shear flow. For both , we found that, independent of Rayleigh number, near the sidewall ( ), where plumes primarily rise or fall and the large-scale circulation (LSC) dynamics is most influential. This result agrees with a model due to Priestley (Turbulent Transfer in the Lower Atmosphere, 1959, University of Chicago Press) for convection over a horizontal heated surface. However, we found along the sample centreline ( ), where there are relatively few plumes moving vertically and the LSC dynamics is expected to be less important; that result is consistent with one of two possible interpretations by Adrian (Intl J. Heat Mass Transfer, vol. 39, 1996, pp. 2303–2310) of a model due to Libchaber et al. (J. Fluid Mech., vol. 204, 1989, pp. 1–30). We discuss the composite nature of fluctuations in turbulent RBC, with contributions from intrinsic background fluctuations, plumes, the stochastic dynamics of the LSC, and the sloshing and torsional mode of the LSC. None of the models advanced so far explicitly consider all of these contributions.

On the nature of fluctuations in turbulent Rayleigh–Bénard convection at large Prandtl numbers

The UNIX operating system, a general-purpose time-sharng system, has, without marketing, advertising, or technical support, become widely used by universities and scientific research establishments. It is the de facto standard of comparison for such systems and has spawned a small industry of suppliers of UNIX variants and look-alikes. This article attempts to uncover the reasons for its success and to draw some lessons for the future of operating systems.

https://science.sciencemag.org/content/sci/215/4534/779.full.pdf

The UNIX Operating System: A Model for Software Design.

Patterns are ubiquitous in the world that surrounds us. They can form via bifurcations, for instance from the spatially uniform state, as a control parameter is varied. Their nature generally is determined by nonlinear terms in the relevant equations of motion, and thus their elucidation is a non-trivial goal in nonlinear physics. In the early 1970's, there was a revival of interest in the condensed-matter physics community in chaos and pattern formation in nonlinear dissipative systems. Experimentalists and theorists brought the tools of their field to bear on these challenging problems. Mostly in terms of his own experiences, the author of this paper reviews some of the issues that have been addressed, some of the techniques that have been applied, and some of the progress that has been made by experimentalists during the two-and-a-half decades since then, and the relationship which these results have to our present-day understanding of nonlinear systems.

/pdf/over-two-decades-of-pattern-formation-a-personal-perspective-r5332abb41.pdf

Over two decades of pattern formation, a personal perspective

Microcomputers, operating as satellite processors in a UNIX∗ system, are at work in our laboratory collecting data, controlling apparatus and machinery, and analyzing results. The system combines the benefits of low-cost hardware and sophisticated UNIX software. Software tools have been developed that accomplish timing and synchronization; data acquisition, storage, and archiving; command signal generation; and on-line interaction with the operator. Mechanical testing, magnetic measurements, and collecting and analyzing data from low-temperature convective studies are now routine. The system configurations used and the benefits derived are discussed.

http://ieeexplore.ieee.org/iel7/6731005/6770388/06770418.pdf

BC Wonsiewicz

Papers

UNIX time-sharing system: Microcomputer control of apparatus, machinery, and experiments

Microcomputer control of apparatus, machinery, and experiments