The Potential of Freeform Construction Processes

The level of automation technology and processes control found in modern day construction lags significantly behind other industries such as automotive and aerospace. The construction industry has health and safety issues and still uses traditional methods of procurement. These problems are compounded by diminishing skills in the labour force. Methods of production must change if these issues are to be resolved and Freeform Construction is a collection of processes that could have potential impact. This paper outlines some of the major issues facing construction and sets a context with examples of digital fabrication in construction. Freeform Construction is defined and potential applications are presented and related to application scale. The viability of two potential applications are investigated in terms of cost.Mechanical Engineerin

Extremely Anisotropic Scintillations

A small number of quasars exhibit interstellar scintillation on time-scales less than an hour; their scintillation patterns are all known to be anisotropic. Here we consider a totally anisotropic model in which the scintillation pattern is effectively one-dimensional. For the persistent rapid scintillators J1819+3845 and PKS1257-326 we show that this model offers a good description of the two-station time-delay measurements and the annual cycle in the scintillation time-scale. Generalising the model to finite anisotropy yields a better match to the data but the improvement is not significant and the two additional parameters which are required to describe this model are not justified by the existing data. The extreme anisotropy we infer for the scintillation patterns must be attributed to the scattering medium rather than a highly elongated source. For J1819+3845 the totally anisotropic model predicts that the particular radio flux variations seen between mid July and late August should repeat between late August and mid November, and then again between mid November and late December as the Earth twice changes its direction of motion across the scintillation pattern. If this effect can be observed then the minor-axis velocity component of the screen and the orientation of that axis can both be precisely determined. In reality the axis ratio is finite, albeit large, and spatial decorrelation of the flux pattern along the major axis may be observable via differences in the pairwise fluxes within this overlap region; in this case we can also constrain both the major-axis velocity component of the screen and the magnitude of the anisotropy.Comment: 5 pages, 4 figures, MNRAS submitte

Pentagonal puckering in a sheet of amorphous graphene

Ordered graphene has been extensively studied. In this paper we undertake a first density functional study of it topologically disordered analogues of graphene, in the form of a random network, consisting predominantly of hexagonal rings, but also including pentagons and heptagons. After some preliminaries with crystalline material, we relax various random network models and find that the presence of carbon pentagons induce local curvature, thus breaking the initial planar symmetry, in some analogy with the case of fullerenes. Using density functional theory to calculate the total energy, we find that while the planar state is locally stable, there is a puckered state that has lower energy. The scale of the puckering is consistent with that expected with local maxima and minima associated with pentagons surrounded by larger rings; forming local "buckyball domes"

Breaking internal waves and turbulent dissipation

We explore what might be discovered about the breaking of progressive internal waves and the consequent mixing by following some of the methodologies and techniques used to study surface wave breaking. It is suggested that breaking is most likely to occur in wave groups, where the wave field is locally amplified. In a stratified fluid of uniform buoyancy frequency, N, the breaking regions of internal wave groups extend in approximately horizontal directions. Two classes of breaking, “convective overturn” and “shear instability,” are possible in progressive internal waves propagating in uniform stratification with no mean shear. Convective overturning and associated static instability occur at all wave frequencies, but only if the wave slope, s = am, exceeds unity, where a is the wave amplitude and m is the vertical wavenumber. Self-induced shear instability may take place in waves with slopes s \u3c 1, and therefore less than the slopes required for convective overturn, but only when a wave-related Richardson number is sufficiently small; to achieve this, the wave frequency must be close to the inertial frequency. Equations are derived to express the energy dissipated in breaking or the strength of breaking in terms of the characteristics of a breaking wave. A particular measure of breaking analogous to that used to quantify surface wave breaking is ΛI(cb)dcb, the mean area of the fronts of breaking regions, projected onto the vertical and per unit volume, that are produced by internal breakers traveling at speeds between cb and cb + dcb. Estimates are made of the values of ΛI required to sustain a vertical eddy diffusion coefficient of Kρ = 10–5 m2 s–1 through the breaking of internal waves of typical amplitude by convective overturn (with s \u3e 1) and by the self-induced shear instability of near-inertial waves when s \u3c 1. Values of ΛI are of order 1.0 × 10–2 m–1 (i.e., a vertical surface area of about 10 cm × 10 cm in each cubic meter). The predictions are tested by using them to find the fraction of the water column in which turbulence occurs and by comparing the predicted values with existing observations. Additional theoretical studies and laboratory experiments are required to test the proposed analytical relations. Existing sea-going measurement techniques are reviewed and further observations are suggested to advance the understanding of breaking internal waves

Foam triangles

Foam patches left by waves breaking as they approach a smooth and gently sloping beach from a near-normal direction sometimes have the distinctly triangular shape that has been studied by Turner and Turner (2011). Explanations of the size of the angle at the apex of the triangles observed by Turner and Turner are suggested in terms of physical processes that determine the speed at which the point of breaking travels along a wave crest. These explanations differ from the entrainment model proposed by Turner and Turner (2011). The range of sizes of the apex angles can most likely be explained in terms of the directional spreading of waves approaching the surf zone

The relation between the duration and shape of internal wave groups

This paper discusses the effect of the shape of internal wave groups on their “duration” or “lifetime”—how long they retain their form before their component waves disperse. The methodology devised by Smith and Brulefert (2010) to study the dispersion of surface wave groups is extended to examine the dispersion of internal wave groups. To provide tangible examples, it is supposed that wave groups of ellipsoidal shape, symmetrical about a vertical plane, are generated in a uniformly stratified thermocline by moving periodic disturbances perturbing the base of an overlying mixed layer. The dispersion relation for internal waves is used to determine the group duration, taken as the time required for the volume of the group to approximately double through the fastest separation of its component waves. As well as allowing the orientation (inclination of their larger, major axis to the horizontal) and aspect ratio (that of the minor to major axis in a vertical plane) of wave groups to vary, their lifetimes are compared in two particular cases: Case A in which the length of the initial minor axes in the vertical plane of the groups is the same, and Case B in which groups are initially composed of the same number of waves. Two-dimensional groups and “three-dimensional groups” (the latter predominantly two-dimensional but of limited extent in one horizontal direction) are considered. As has been found for surface waves, the duration of internal wave groups does indeed depend upon their shape. In both Cases, groups with relatively small aspect ratio and, in Case B, groups with many waves usually have greater lifetimes than relatively large aspect ratio groups with few waves. Two-dimensional groups have greater lifetimes than three-dimensional groups. In many cases, the groups with the longest lifetime have their longer (major) axis inclined at an angle to the horizontal that is close to the inclination of the group velocity vector; in these cases the lines of constant phase of waves composing the groups are not (as is found for some surface wave groups) slanted with respect to the major axis of the groups, but parallel. Some long-lifetime groups are found to have their major axes inclined to the horizontal at an angle that is very close to that of the front of a packet of waves generated by the moving periodic disturbance at the foot of the mixed layer. In Appendix B it is shown that the ratio, np/n or nm/n, of the number of waves that would be recorded as the group passes a fixed point or a vertical mooring, to the number of waves contained, instantaneously, within a wave group, depends on the shape of the group and on the ratio of the dominant wave phase speed to the group velocity of the group. A simple model described in Appendix C suggests how such slanted wave groups can be generated in the thermocline by moving, but transient, disturbances. The orientation of “scars,” regions left by waves breaking in the wave groups, is examined in Appendix D. Except for near inertial waves with small aspect ratio, scars are generally close to being horizontal