ALBERT

Beschreibung: Energy and linear limits are calculated for the Poiseuille–Couette spiral motion between concentric cylinders which rotate rigidly and rotate and slide relative to one another. The addition of solid rotation can bring the linear limit down to the energy limit with coincidence achieved in the limit of infinitely fast rotation. If the differential rotation is also added, the solid rotation rate need be only finite to achieve near coincidence. Sufficient conditions for non-existence of sub-linear instability are derived. The basic spiral character of the instability is analysed and the results compared with the experiments of Ludwieg (1964). © 1970, Cambridge University Press. All rights reserved.

Beschreibung: A generalized eddy-viscosity function νT, is introduced in order to express the Reynolds stress in an incompressible dusty gas as a linear combination of the Kronecker and rate-of-strain tensors. On the basis of Saffman's dusty-gas model a transport equation for the eddy viscosity is derived from the general turbulence energy equations, thereby introducing two additional functions, the specific turbulence kinetic energy E1, and a scale variable s. In order to determine the three variables modified Prandtl–Wieghardt relation among them is accepted and a transport equation for s is postulated in the same manner as in the clean-gas turbulence transport model (firstly proposed by Harlow & Nakayama 1967) but with the inclusion of an additional term accounting for the dust particles stabilizing action. We are considering values of loading (mass ratio of particles) of order of unity, with particle/gas density ratios of order of 103 and volume concentrations of the order of 10−3, so that particle–particle interactions are neglected. Supposing that the particles nearly follow the gas motion, following well at large scales and poorly at small, an application of the theory to problem of numerical calculations of the dusty-gas parameters such as mean velocity profile of turbulent pipe flow is given. © 1980, Cambridge University Press. All rights reserved.

Beschreibung: The fluid mechanics of self-propelling, slender uniflagellar micro-organisms is examined theoretically. The mathematical analysis of these motions is based upon the Stokes equations, and the body is represented by a continuous distribution of stokeslets and doublets of undetermined strength. Since the body is self-propelling, additional constraints on the total force and moment upon it are applied. A system of singular integral and auxiliary equations, in which the propulsive velocity and viscous force per unit length are the unknowns, is derived. The vector integral equation is decomposed into near- and far-field contributions, and the solution is determined by a straightforward iterative procedure. The flagella considered are of constant radius and are restricted to planar undulations. The analysis is applied to a small amplitude wave form of infinite length, and a third-order analytic solution is obtained. By means of numerical computation, the method is extended to large amplitude wave forms of both infinite and finite length. The validity and accuracy of the solution method, the effect of local curvature, and an approximate model for an attached cell body-proper are evaluated in light of alternative theories. The solution method is systematically applied to a variety of wave-form shapes representative of actual flagella. For a sinusoidal wave form, the variations in propulsive velocity, power output and propulsive efficiency are examined as functions of the number of wavelengths on the flagellum, the amplitude and the flagellar radius. Wave forms of variable amplitude and variable wavelength are also considered. Among the significant results are the effect of the cell body on pitching, the significant differences between constant frequency and constant phase-speed undulations for variable wavelength wave forms, and comparisons with other pertinent theories. © 1980, Cambridge University Press. All rights reserved.

Beschreibung: Measurements of turbulent flow through a constricted tube with a contoured 75 % constriction have been performed with a laser-Doppler anemometer. One motivation for these studies was the problem of flow through arterial stenoses and consequently a range of relatively low Reynolds numbers (5000-15000) was employed. Velocity profiles, r.m.s. turbulence velocities, and energy spectra were recorded along with determinations of the wall pressure variation and length of recirculation region. Results show extremely high levels of turbulence and considerable turmoil within the so-called recirculation region. A method for determining the reattachment point in an intensely turbulent flow is given which should offer improvement over flow visualization techniques. Similarity in turbulence energy spectra measured at various radial positions is found at several axial locations, but does not occur in the region immediately downstream of separation. The downstream recovery of the turbulence velocity to upstream values takes place slowly. A secondary motivation of providing experimental data useful in evaluating turbulence models for computational fluid dynamics led to extensive measurements of radial and axial r.m.s. turbulence velocity components at a Reynolds number of 15000. © 1980, Cambridge University Press. All rights reserved.

Beschreibung: The nonlinear evolution and breakdown of laminar flow in the boundary layer on a flat plate is examined with the aim of making a closer comparison of theory and experiment than has been attempted previously. The importance of three-dimensionality is emphasized. It is concluded that many features of the nonlinear instability are consistent with existing linear and weakly nonlinear theories even as breakdown is approached. The development of the secondary instability, or ‘spike’, is also considered and suggestions for an improved theory of its growth are made. © 1980, Cambridge University Press. All rights reserved.

Beschreibung: A magnetic field is shown to be asymptotically (t→∞) decaying in a flow of finite conductivity with v = Cr, where C = Cξ(t) is a random matrix. The decay is exponential, and its rate does not depend on the conductivity. However, the magnetic energy increases exponentially owing to growth of the domain occupied by the field. The spatial distribution of the magnetic field is a set of thin ropes and (or) layers. © 1984, Cambridge University Press. All rights reserved.

Beschreibung: During December 1978 the Scott Polar Research Institute (SPRI) undertook its sixth and final season of long-range airborne radio echo sounding (RES) and magnetometry in Antarctica in cooperation with the US National Science Foundation Division of Polar Programmes (NSF-DPP) and the Electromagnetics Institute of the Technical University of Denmark (TUD). The 1978–79 season (DF–79) was part of a long-term programme of glaciological and geophysical investigation of the Antarctic ice sheet (Drewry and Meldrum 1978a; 1978b). The broad aims have been to extend a 100-km grid network over Antarctica for a study of surface form, thickness, internal structure, rheology, thermodynamics, electrical properties, basal conditions and processes, and sub-ice geology.

Beschreibung: Three units, correlatable with recent Lake Champlain, late-glacial marine Champlain Sea, and proglacial Lake Vermont sediments, have been identified from about 200 km of high-resolution seismic reflection profiles and eight piston cores collected in southern Lake Champlain. Lake Vermont deposits are nonfossiliferous and range from thin to absent nearshore and on bedrock highs to more than 126 m thick near Split Rock Point. Champlain Sea sediments contain marine foraminifers and ostracodes and are fairly uniform in thickness (20–30 m). Recent Lake Champlain sediments range in thickness from 0 to 25 m. Average sedimentation rates for Lake Vermont are considerably higher (4–8 cm/yr) than those for the Champlain Sea (0.8–1.2 cm/yr) and Lake Champlain (0.14–0.15 cm/yr). Bedrock, till, and deltaic and alluvial deposits were also identified on the acoustic records but were not sampled. An unconformity separating Champlain Sea deposits from Lake Champlain deposits is associated with numerous benches at water depths of 20–30 m. These benches, the alluvial deposits, and the onset of deltaic deposition are probably associated with a low water level stillstand at the close of the Champlain Sea episode.

Beschreibung: The final effort of the CLIMAP project was a study of the last interglaciation, a time of minimum ice volume some 122,000 yr ago coincident with the Substage 5e oxygen isotopic minimum. Based on detailed oxygen isotope analyses and biotic census counts in 52 cores across the world ocean, last interglacial sea-surface temperatures (SST) were compared with those today. There are small SST departures in the mid-latitude North Atlantic (warmer) and the Gulf of Mexico (cooler). The eastern boundary currents of the South Atlantic and Pacific oceans are marked by large SST anomalies in individual cores, but their interpretations are precluded by no-analog problems and by discordancies among estimates from different biotic groups. In general, the last interglacial ocean was not significantly different from the modern ocean. The relative sequencing of ice decay versus oceanic warming on the Stage 6/5 oxygen isotopic transition and of ice growth versus oceanic cooling on the Stage 5e/5d transition was also studied. In most of the Southern Hemisphere, the oceanic response marked by the biotic census counts preceded (led) the global ice-volume response marked by the oxygen-isotope signal by several thousand years. The reverse pattern is evident in the North Atlantic Ocean and the Gulf of Mexico, where the oceanic response lagged that of global ice volume by several thousand years. As a result, the very warm temperatures associated with the last interglaciation were regionally diachronous by several thousand years. These regional lead-lag relationships agree with those observed on other transitions and in long-term phase relationships; they cannot be explained simply as artifacts of bioturbational translations of the original signals.