A reversible infinite HMM using normalised random measures

Copyright 2014 by the author(s). We present a nonparametric prior over reversible Markov chains. We use completely random measures, specifically gamma processes, to construct a countably infinite graph with weighted edges. By enforcing symmetry to make the edges undirected we define a prior over random walks on graphs that results in a reversible Markov chain. The resulting prior over infinite transition matrices is closely related to the hierarchical Dirichlet process but enforces reversibility. A reinforcement scheme has recently been proposed with similar properties, but the de Finetti measure is not well characterised. We take the alternative approach of explicitly constructing the mixing measure, which allows more straightforward and efficient inference at the cost of no longer having a closed form predictive distribution. We use our process to construct a reversible infinite HMM which we apply to two real datasets, one from epigenomics and one ion channel recording.

Multiview stereo via volumetric Graph-Cuts and occlusion robust photo-consistency.

This paper presents a volumetric formulation for the multi-view stereo problem which is amenable to a computationally tractable global optimisation using Graph-cuts. Our approach is to seek the optimal partitioning of 3D space into two regions labelled as "object" and "empty" under a cost functional consisting of the following two terms: (1) A term that forces the boundary between the two regions to pass through photo-consistent locations and (2) a ballooning term that inflates the "object" region. To take account of the effect of occlusion on the first term we use an occlusion robust photo-consistency metric based on Normalised Cross Correlation, which does not assume any geometric knowledge about the reconstructed object. The globally optimal 3D partitioning can be obtained as the minimum cut solution of a weighted graph.

Comparison of different scheduling techniques for H-infinity controllers

Several approaches to designing schedule H-infinity control systems are compared. These include a controller switching approach and also parameter scheduling of an observer representation of the controller. They are illustrated by application to a Generic VSTOI. Aircraft Model (GVAM) supplied by The Royal Aerospace Establishment (RAE) at Bedford. The switched design has been tested on the simulator at RAE Bedford. The linear H-infinity designs make use of a loop-shaping followed by robust stabilisation to additive perturbations of a normalised coprime factorisation of the shaped plans. The different scheduling approaches are compared with respect to achieved robust stability levels. performance and complexity of implementation.

Alternative uses of partitioning interwell tracer tests for DNAPL source zone characterisation

The heterogeneous nature of the subsurface and associated DNAPL morphologies often poses the greatest limitation to source zone clean-up strategies. Hence, detailed site characterisation techniques are required. The data presented in this paper has been collected from a series of laboratory 2-D tank experiments and numerical simulations of Partitioning Interwell Tracer Tests (PITT) in a wide range of aquifer conditions and DNAPL morphologies. Alternative uses of tracer breakthrough data have been developed In order to characterise the mass flux generated from the DNAPL source. By combining the laboratory and numerical data, a relationship between normalised mass flux and tracer-based average source zone DNAPL saturation has been established. Knowledge of such a relationship allows remediation targets to be identified, clean-up efficiencies to be evaluated, and increases the accuracy of any risk assessment.

Effects of turbulent reynolds number on the performance of algebraic flame surface density models for large eddy simulation in the thin reaction zones regime: A direct numerical simulation analysis

A direct numerical simulation (DNS) database of freely propagating statistically planar turbulent premixed flames with a range of different turbulent Reynolds numbers has been used to assess the performance of algebraic flame surface density (FSD) models based on a fractal representation of the flame wrinkling factor. The turbulent Reynolds number Ret has been varied by modifying the Karlovitz number Ka and the Damköhler number Da independently of each other in such a way that the flames remain within the thin reaction zones regime. It has been found that the turbulent Reynolds number and the Karlovitz number both have a significant influence on the fractal dimension, which is found to increase with increasing Ret and Ka before reaching an asymptotic value for large values of Ret and Ka. A parameterisation of the fractal dimension is presented in which the effects of the Reynolds and the Karlovitz numbers are explicitly taken into account. By contrast, the inner cut-off scale normalised by the Zel'dovich flame thickness ηi/δz does not exhibit any significant dependence on Ret for the cases considered here. The performance of several algebraic FSD models has been assessed based on various criteria. Most of the algebraic models show a deterioration in performance with increasing the LES filter width. © 2012 Mohit Katragadda et al.

Dynamic compression of foam supported plates impacted by high velocity soil

The response of back-supported buffer plates comprising a solid face sheet and foam core backing impacted by a column of high velocity particles (sand slug) is investigated via a lumped parameter model and coupled discrete/continuum simulations. The buffer plate is either resting on (unattached) or attached to a rigid stationary foundation. The lumped parameter model is used to construct maps of the regimes of behaviour with axes of the ratio of the height of the sand slug to core thickness and the normalised core strength. Four regimes of behaviour are identified based on whether the core compression ends prior to the densification of the sand slug or vice versa. Coupled discrete/continuum simulations are also reported and compared with the lumped parameter model. While the model predicted regimes of behaviour are in excellent agreement with numerical simulations, the lumped parameter model is unable to predict the momentum transmitted to the supports as it neglects the role of elasticity in both the buffer plate and the sand slug. The numerical calculations show that the momentum transfer is minimised for intermediate values of the core strength when the so-called "soft-catch" mechanism is in play. In this regime the bounce-back of the sand slug is minimised which reduces the momentum transfer. However, in this regime, the impulse reduction is small (less than 10% of that transferred to a rigid structure). For high values of the core strength, the response of the buffer plate resembles a rigid plate with nearly no impulse mitigation while at low values of core strength, a slap event occurs when the face sheet impinges against the foundation due to full densification of the foam core. This slap event results in a significant enhancement of the momentum transfer to the foundation. The results demonstrate that appropriately designed buffer plates have potential as impulse mitigators in landmine loading situations. © 2013 Elsevier Ltd. All rights reserved.

Modelling of jacked piles in centrifuge

Pile jacking is a common installation method for displacement piles due to the low noise and vibration involved in the installation procedure. Problems may occur when modelling jacked piles in the centrifuge, especially for friction piles, due to scaling effects which can be minimised, provided that the pile diameter is greater than fifty times the mean grain size. A series of centrifuge tests of jacked piles was performed in both dry and saturated fine sand. Piles were installed using three different methods. The effect of the installation method on the head load required is assessed. The influence of the normalised roughness was also tested by testing one rough and one smooth pile. Finally, cyclic shearing at constant depth was performed to establish the shaft load reduction due to friction fatigue. © 2014 Taylor & Francis Group.

A parametrically excited vibration energy harvester

In the arena of vibration energy harvesting, the key technical challenges continue to be low power density and narrow operational frequency bandwidth. While the convention has relied upon the activation of the fundamental mode of resonance through direct excitation, this article explores a new paradigm through the employment of parametric resonance. Unlike the former, oscillatory amplitude growth is not limited due to linear damping. Therefore, the power output can potentially build up to higher levels. Additionally, it is the onset of non-linearity that eventually limits parametric resonance; hence, this approach can also potentially broaden the operating frequency range. Theoretical prediction and numerical modelling have suggested an order higher in oscillatory amplitude growth. An experimental macro-sized electromagnetic prototype (practical volume of ∼1800 cm3) when driven into parametric resonance, has demonstrated around 50% increase in half power band and an order of magnitude higher peak power density normalised against input acceleration squared (293 μW cm-3 m-2 s4 with 171.5 mW at 0.57 m s-2) in contrast to the same prototype directly driven at fundamental resonance (36.5 μW cm-3 m-2 s4 with 27.75 mW at 0.65 m s-2). This figure suggests promising potentials while comparing with current state-of-the-art macro-sized counterparts, such as Perpetuum's PMG-17 (119 μW cm-3 m-2 s4). © The Author(s) 2013.

Masonry response to tunnelling: a sensitivity study on the effect of cracking and building weight

Predicting damage to masonry structures due to tunnelling-induced ground movements remains a challenge for practising design engineers. Useful simplified procedures exist, but more detailed analysis has the potential to improve these procedures. This paper considers the use of finite element modelling, including non-linear constitutive laws for the soil and the structure, to simulate damage to a simple masonry structure subjected to tunnelling in sand. The numerical model is validated through comparison with the results of a series of centrifuge tests and used to perform a sensitivity study on the effect of building weight and masonry damage on the structural response. Results show a direct correlation between the weight of the structure, normalised to the relative stiffness between the structure and the soil, and the modification of the settlement profile. By including a cracking model for the masonry, the reduction in structural stiffness caused by progressive masonry damage is also proven to affect the building deflection.