Raytrace simulations and experimental studies of luminescent solar concentrators

The luminescent solar concentrator is a planar, non-tracking device. Originally introduced more than three decades ago, it has yet to establish itself as a means of making photovoltaic solar energy more cost effective. Advances in organic luminescent centres, the emergence of inorganic nanocrystals and the development of new light trapping techniques have created promising opportunities for the LSC. This thesis investigates novel geometries and materials for the practical exploitation of LSCs. The research is based on experimental measurements as well as computational simulations using a Raytrace Model. It is shown both experimentally and computationally that a thin- lm structure produces the same effciency as a homogeneously doped LSC. Two building integrated applications are examined. The rst one is a power generating window employing a Lumogen Violet dye that absorbs short wavelength radiation and is mostly transparent in the visible. Annual yields of over 23 kWh/m2 and a conversion effciency of over 1% are predicted for a 50 cm by 50 cm device. The second BIPV application is the light-bar, which is designed to act as the secondary concentrator in a Venetian blind-like system. With linear Fresnel lenses producing a primary concentration factor of 20, an optimised system could generate nearly 60W/m^2 of power at an effciency of nearly 6% using direct sunlight only. Two novel luminescent materials, nanorods and phycobilisomes have been tested for their potential to reduce re-absorption losses. Despite current practical limitations, these materials are found to be promising due to enhanced Stokes shifts. LSCs with optical concentrations of 10 to 20 could be feasible by addressing the key shortcomings in the form of unabsorbed light and escape cone losses. Their versatility with regards to shape, colour and light absorption makes LSCs particularly relevant for building integrated photovoltaics.Imperial Users Onl

Non Gaussian information of heterogeneity in Soft Matter

Heterogeneity in dynamics in the form of non-Gaussian molecular displacement distributions appears ubiquitously in soft matter. We address the quantification of such heterogeneity using an information-theoretic measure of the distance between the actual displacement distribution and its nearest Gaussian estimation. We explore the usefulness of this measure in two generic scenarios of random walkers in heterogeneous media. We show that our proposed measure leads to a better quantification of non-Gaussianity than the conventional ones based on moment ratios

Functional outcome of patients presenting with isolated medial condyle fractures of femur in a tertiary care hospital

Isolated medial condyle fractures of the femur with an intact lateral condyle are a rare entity. These fractures require prompt diagnosis and anatomical restoration of joint surface to prevent post traumatic arthritis. Till date no suitable implant is available for fixation of these fractures. In this study we have used 4.5 recon plates and CC screws to fix these fractures. We aim to study the functional outcome of these fractures in a group of patients fixed with the above implants. The study was conducted in a group of patients presenting with isolated medial condyle fractures of femur in a tertiary care hospital. An ORIF was done and fractures were fixed with recon plates and lag screws and various parameters were evaluated such as range of motion of knee, average time to weight bearing, Lysholm knee scores, pain perception etc 20 patients were evaluated in the age group of 20-70 in a 5 year period. Majority of the patients had a very good functional range of motion of the knee and nearly 75% achieved union by 12-16 weeks. 70% of the patients had a low VAS score. Overall early diagnosis and prompt anatomical restoration of joint surface in isolated medial condyle fractures of femur led to favourable range of motion of knee, early radiological healing and low VAS score

An extension of Thwaites method for turbulent boundary layers

Thwaites (1949) developed an approximate method for determining the evolution of laminar boundary layers. The approximation follows from an assumption that the growth of a laminar boundary layer in the presence of pressure gradients could be parameterized solely as a function of a flow parameter, $m = \theta^2/\nu \frac{dU_e}{ds}$, thus reducing the von Karman momentum integral to a first-order ordinary differential equation. This method is useful for the analysis of laminar flows, and in computational potential flow solvers to account for the viscous effects. However, for turbulent flows, a similar approximation for turbulent boundary layers subjected to pressure gradients does not yet exist. In this work, an approximate method for determining the momentum thickness of a two-dimensional, turbulent boundary layer is proposed. It is shown that the method provides good estimates of the momentum thickness, when compared to available high-fidelity simulation data, for multiple boundary layers including both favorable and adverse pressure gradient effects, up to the point of separation. In the limit of high Reynolds numbers, it is possible to derive a criterion for the onset of separation from the proposed model which is shown to be in agreement with prior empirical observations (Alber, \textit{$9^{th}$ Aerospace Sciences Meeting, 1971}). The sensitivity of the separation location with respect to upstream perturbations is also analyzed through this model for the NASA/Boeing speed bump and the transonic Bachalo-Johnson bumpComment: 21 pages, 13 figures. Under consideration for publication in J. Fluid Mec

Exact ground state and kink-like excitations of a two dimensional Heisenberg antiferromagnet

A rare example of a two dimensional Heisenberg model with an exact dimerized ground state is presented. This model, which can be regarded as a variation on the kagome lattice, has several features of interest: it has a highly (but not macroscopically) degenerate ground state; it is closely related to spin chains studied by earlier authors; in particular, it is probably the first genuinely two-dimensional quantum system to exhibit domain-wall-like ``kink'' excitations normally found only in one-dimensional systems. In some limits it decouples into non-interacting chains, purely dynamically and not because of weakening of interchain couplings: indeed, paradoxically, this happens in the limit of strong coupling of the chains.Comment: 4 pages, revtex, 5 figures included via epsfi