John B. Parkinson

Analyses and algorithms for new test reference years and design summer years for the UK

With the increasing use of simulation for building design, test reference years (TRYs) are required for energy analyses and design summer years (DSYs) for assessing natural ventilation in the summer. Previously TRYs and DSYs only existed for three sites in the UK. Also the data was derived from weather data up to 1995. More sites were required and also updated data, as a number of warm years had occurred after 1995. The opportunity was also taken to improve the derivation of the TRY using just the Finkelstein-Schafer statistic and also to improve the algorithm for smoothing between months. New programs had to be written for filling missing values in the lower-quality raw data. This paper describes these programmes, the quality assurance procedures and analyses the years produced. A comparison is made between the 14 sites demonstrating the link between dry bulb temperature, solar radiation and latitude.

Numerical and approximate analytical results for the frustrated spin-1/2 quantum spin chain

We study the T=0 frustrated phase of the 1D quantum spin- 1/2 system with nearest-neighbour and next-nearest-neighbour isotropic exchange known as the Majumdar-Ghosh Hamiltonian. We first apply the coupled-cluster method of quantum many-body theory based on a spiral model state to obtain the ground-state energy and the pitch angle. These results are compared with accurate numerical results using the density matrix renormalization group method, which also gives the correlation functions. We also investigate the periodicity of the phase using the Marshall sign criterion. We discuss particularly the behaviour close to the phase transitions at each end of the frustrated phase.

Simple Physical Model of Collagen Fibrillogenesis Based on Diffusion Limited Aggregation

Type I collagen is a rod-like protein which self-assembles in a regular array to form elongated fibrils. The process of fibril formation, termed fibrillogenesis, is driven by the increase in entropy associated with loss of water from the bound monomers. A model based on diffusion limited aggregation (DLA) was used to investigate some of the mechanisms involved in this process. The aggregates created in the model displayed several features in common with collagen fibrils including an elongated morphology and a preference for tip growth. Analysis of these aggregates revealed a linear relationship between mass and distance from the tip, consistent with experimental observations. Intrafibrillar fluidity was introduced into the model by using a surface diffusion term. This led to the formation of aggregates with more compact morphologies. These results strongly implicate the role of diffusion limited growth in collagen fibril formation.

Phase Transitions in the Spin-Half J1-J2 Model

The coupled cluster method (CCM) is a well-known method of quantum many-body theory, and in this article we present an application of the CCM to the spin-half J1-J2 quantum spin model with nearest- and next-nearest-neighbor interactions on the linear chain and the square lattice. We present results for ground-state expectation values of such quantities as the energy and the sublattice magnetization. The presence of critical points in the solution of the CCM equations, which are associated with phase transitions in the real system, is investigated. Completely distinct from the investigation of the critical points, we also make a link between the expansion coefficients of the ground-state wave function in terms of an Ising basis and the CCM ket-state correlation coefficients. We are thus able to present evidence of the breakdown, at a given value of J2/J1, of the Marshall-Peierls sign rule which is known to be satisfied at the pure Heisenberg point (J2=0) on any bipartite lattice. For the square lattice, our best estimates of the points at which the sign rule breaks down and at which the phase transition from the antiferromagnetic phase to the frustrated phase occurs are, respectively, given by J2/J1 approximate to 0.26 and J2/J1 approximate to 0.61.

Constructing a future weather file for use in building simulation using UKCP09 projections

In the future with climate change, building designers may need to demonstrate to clients that their buildings will continue to provide a comfortable environment under future weather conditions. Building simulation modelling can be used to this end provided that suitable weather files can be constructed. This article describes the use of the latest climate projections from UKCIP (UKCP09 data) and a method of constructing an hourly weather file for the following parameters: dry-bulb temperature, relative humidity, cloud cover, solar irradiation (direct and diffuse), wind speed and wind direction. For a given future scenario, the approach used first selects appropriate months from 3000 years of UKCP09 data to construct a Test Reference Year (TRY). Unfortunately, the UKCP09 hourly data has no wind or cloud data, and the solar irradiance is uncorrected at low sun angles. This article describes algorithms for calculating wind speed and cloud cover from the UKCP09 data. From this data, a TRY can be constructed and formatted to be suitable for use with commonly used simulation packages. Practical applications: Building professionals increasingly seek reassurance on how a proposed building will perform under a future rather than historical climate. This article describes a method of processing the latest future climate projections (UKCP09 data released in June 2009) and generating a Test Reference Year (TRY) with the full complement of weather parameters suitable for use by commonly used building simulation programmes.

Limitations of the CIBSE design summer year approach for delivering representative near-extreme summer weather conditions

Representative, site-specific weather data is a key requirement for building performance simulation. In the UK, such data is available in two formats, Test Reference Years for analysing building services loads under ‘typical’ year conditions and Design Summer Years for estimating summer discomfort of naturally ventilated and free-running buildings. Currently, Design Summer Years are determined as a complete year based on the rank of the average dry bulb temperature from April to September. The simplicity of this approach does not take into account extreme temperature values in individual months or the incident solar radiation, both of which are however of great significance for the summer overheating performance of a building. This paper analyses the implications of this simplified approach for the resulting data. It is shown that there is no consistent relation between the Design Summer Years and the corresponding Test Reference Years and that, for some sites, building performance simulations using Design Summer Year files deliver unreliable results. Practical application : This paper demonstrates that the current approach for deriving Design Summer Years (DSYs) can lead to data series that are not representative for near-extreme summer conditions at a given location. It highlights that a new approach for deriving near-extreme summer years for building performance simulation is needed in order to overcome the inherent shortfalls of the current DSY data.

The design reference year – a new approach to testing a building in more extreme weather using UKCP09 projections

Current practice in building design is to assess a building’s performance using average or typical weather, a test reference year (TRY), and then to see how it performs when ‘stressed’, using a design summer year (DSY). The DSY is an actual year of hourly data which has the third warmest summer in 20 years’ summers. One of the problems with the DSY method is that it does not explicitly take into account solar radiation, or humidity, nor when more extreme weather occurs – it is selected solely on the mean six monthly temperature from April to September. A DSY may actually be cloudier than the average weather of a TRY. This article proposes an alternative approach using a new type of design reference year (DRY) consisting of a year formed from individual more extreme weather months. The DRY is used in simulating the performance of a building and to identify a single critical month for over-heating, or maximum cooling load. This article compares the characteristics of the DSY and proposed DRY using future projected weather data from UKCIP. Practical applications : Building designers are increasingly required by their clients to demonstrate how a proposed building will perform under a future rather than historical climate. This article describes a method of processing the latest future climate projections (UK Climate Impacts Programme’s (UKCIP’s) CP09 data released in June 2009) and generating a design reference year (DRY) for use in building simulation programmes. The DRY is proposed as a replacement for the design summer year (DSY), which has a number of limitations.

Coupled-cluster treatment of the XY-model

We study quantum spin systems in the 1D, 2D square and 3D cubic lattices with nearest-neighbour XY-exchange. We use the coupled-cluster method (CCM) to calculate the ground-state energy, the T = 0 sublattice magnetization and the excited-state energies, all as functions of the anisotropy parameter . We consider the case with S = 1/2 in detail and give some results for higher S. In 1D these results are compared with the exact S = 1/2 results and in 2D with Monte Carlo and series expansions. We obtain critical points close to the expected value and our extrapolated LSUBn results for the ground-state energy are well converged for all except very close to the critical point.

A coupled-cluster treatment of spin- 1/2 systems with nearest- and next-nearest-neighbour interactions

The coupled-cluster method of quantum many-body theory is applied to 1D and 2D spin- 1/2 systems with nearest-neighbour and next-nearest-neighbour isotropic exchange. Several simple approximation schemes based on a Neel model state are investigated and are found to give satisfactory results in the antiferromagnetic regime. Results are given for the ground-state energy, the order parameter and the sublattice magnetization. We obtain evidence of a phase transition.

The S=1 quantum spin chain with equal Heisenberg and biquadratic exchange in a magnetic field

The S=1 quantum spin chain with equal Heisenberg and biquadratic exchange was shown by Sutherland to be integrable. In this paper the equations obtained by Sutherland are solved numerically to obtain the behaviour at zero temperature in a magnetic field. It is found that there is an additional phase change at a lower field than the spin flop field. This new phase change marks the boundary between a regime in which the ground state contains only atoms with Sz=1, 0 and one in which 1, 0 and -1 are present. The form of the elementary excitations in the two regimes is discussed and the possibility of a similar phase change in the pure Heisenberg model is considered.