Constructing cities,deconstructing scaling laws

Cities can be characterized and modelled through different urban measures. Consistency within these observables is crucial in order to advance towards a science of cities. Bettencourt et al. have proposed that many of these urban measures can be predicted through universal scaling laws. We develop a framework to consistently define cities, using commuting to work and population density thresholds, and construct thousands of realizations of systems of cities with different boundaries for England and Wales. These serve as a laboratory for the scaling analysis of a large set of urban indicators. The analysis shows that population size alone does not provide us enough information to describe or predict the state of a city as previously proposed, indicating that the expected scaling laws are not corroborated. We found that most urban indicators scale linearly with city size, regardless of the definition of the urban boundaries. However, when nonlinear correlations are present, the exponent fluctuates considerably.     

Electroabsorption in narrow coupled double quantum wells: Coulombiccoupling effects

The effects of Coulombic coupling between different subband pairs on the electroabsorption spectra of narrow coupled double quantum wells (QWs) have been studied. It is shown via detailed comparison between electroabsorption spectra calculated using a full excitonic Green's function method, a decoupled excitonic Green's function method, a variational method, and experimental data that inclusion of the Coulombic coupling between different subband pairs is required for correct prediction of electroabsorption in the narrow well system. It is also shown that, due to Coulombic coupling, it is necessary to include unbound QW states, above the QW edge, in the simulation of electroabsorption and electrorefraction. These results are of particular significance for the accurate calculation of electrorefraction, by Kramers-Kronig transformation of QW electroabsorption spectra, in coupled QW structures containing narrow QWs     

Competition in the Built Environment: Scaling Laws for Cities,Neighbourhoods and Buildings

Built environments at any spatial scale are represented as sets of like objects—cities, neighbourhoods, buildings but also components such as streets, parks, etc.,—which are distributed spatially according to certain rules that we are only just beginning to detect and measure. Unlike normally distributed attributes of the population, these urban elements often scale according to forces that determine large numbers of small elements and a small number of large ones. While many objects evolve from small to large, not all objects can be large as resources as well as physical limits determine the distribution of their sizes. The theory behind such size distributions is referred to as scaling and the shape of such distributions is quite well-defined by various power and exponential laws. As objects grow and evolve, or are even designed, their form changes qualitatively due to the forces of competition and the constraints on space. Here we explore these ideas for cities, neighbourhoods in towns, and buildings, specifically high buildings, revealing that there are both important differences as well as similarities in their form, function, and structure.     

Spatial aggregation in gravity models: 4. Generalisations and large-scale applications

"This paper is concerned with applying and extending a methodology for analysing spatial aggregation in gravity models developed in three earlier papers to a larger scale and hence more realistic example of spatial interaction than has been treated so far." Seven models are generated and applied to four levels of aggregation of the spatial interaction pattern in Edmonton, Alberta.     

Thermally Self-Consistent Monte Carlo Device Simulations

We present details of a Monte Carlo simulation code which is coupled to a Heat Diffusion Equation (HDE) solver. Through an iterative procedure, which bypasses the differences in electronic and thermal timescales, this coupled code is capable of producing steady-state thermally self-consistent device characteristics. Electronically-generated thermal flux is calculated by monitoring the net rate of phonon emission, which may be resolved both spatially and by phonon type. The thermal solution is extracted through use of a novel analytical thermal resistance matrix technique which avoids calculation of temperatures beyond the electronically important device region while including the large-scale boundary conditions. On application to a GaAs MESFET the expected thermal droop behaviour is obtained in the I-V characteristics and we find a linear relationship between peak lattice temperature and applied source-drain bias. At moderate biases the contribution of intervalley phonons to the thermal power output surpasses that of optical phonons.     

Convection and radiation in layers of low-density fibrous insulants

Attempts to reduce national energy demands have led to increases in insulation thicknesses in roof spaces in northern Europe and North America. Generally it has been assumed that the apparent thermal conductivity of each material used has been a constant and equal to the value obtained in a testing laboratory under different conditions. Examination of the vertical temperature profiles through various horizontal thicknesses of loose-fill mineral wool insulants suggests that convection in the upper surface layers exposed to the free air and radiation contributions result in much larger apparent thermal conductivities than those quoted in the literature and the magnitudes of these effects increase with the thickness of the insulant layer.     

Use of the thermal-probe technique for the measurement of the apparent thermal conductivities of moist materials

The thermal-probe technique has been assessed for use with moist materials. Initial investigations with wet clay specimens showed that the probe diameter had no signoficant effect on the indicated values of the apparent thermal conductivity. Tests to measure the apparent thermal conductivities of aerated concrete blocks, at various moisture contents and distributions, gave results that compared well with other published data.     

Limits to prediction in science and design science

Design, it is argued, is inextricably bound up with the idea of science for, in carrying out design, the present as embodied in scientific knowledge forms the starting point for inventing the future. One bridge between science and design is thus contained in the notion of prediction which reveals the inevitable tensions between these activities. Prediction is concerned with the likely and possible future, design is concerned with the desirable one. Paradoxes contained therein are first explored in methodological terms and then the limits posed by scientific prediction are charted by examples using predictive computer models in urban planning.     

Linear urban models

ABSTRACT A class of linear models is developed in which activities are derived from transformations of each other and exogenous activities. The models are illustrated using spatial distributions of population and employment. Reduced forms are derived and the influence of different transformations on spatial model solutions is explored in terms of the balance of exogenous and endogenous variables, and through analysis of eigenstructures. Ten model types including the traditional Lowry model and Coleman's model of social exchange, are applied to an eight zone representation of Melbourne and the analysis is used to show how model solutions can be spatially independent of their inputs.     

Urban Regeneration as Self-Organisation

Michael Batty redefines regeneration within the context of a wider system of urban reproduction. A spontaneous response to self-organisation, regeneration is a force that can just as easily manifest itself in dereliction and decline. As part of a bottom-up system, what role can there be for intervention by designers and policy-makers? Batty suggests how urban planning needs to identify key points where small change can lead to massive change for the better, so as ‘to plant seeds that do not fall on stony ground’. Copyright © 2012 John Wiley & Sons, Ltd.