MRC Laboratory of Molecular Biology
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On the stratification and induced flow in an emptying-filling box driven by a plane vertically distributed source of buoyancy
No full textA theoretical model is presented for the steady multi-layered flow induced by a plane vertically distributed buoyancy source producing a turbulent wall plume in a ventilated box. While aspects of the stratification and rate of fluid exchange between box and exterior have been studied previously, the streamline pattern and velocity field have not been considered until now, despite having potentially important practical implications for achieving comfort in naturally ventilated buildings and for the indoor spread of airborne contagions. The boundary condition at the wall for each layer is established by deducing the turbulent entrainment rate. Using conformal mapping techniques and Poisson's integral theorem, closed-form solutions for the streamfunction of the induced flow in each layer are established. While the flow near the ceiling was overlooked in the classic model for the multi-layered stratification, after considering the possible flow scenarios, the stratification is re-evaluated herein by incorporating an entraining ceiling current. With a markedly thinner top layer, the refined stratification matches well with the available experimental observations, the restrictions we place on the applicability of the model overcoming the previous over-prediction in the number of interfaces. The magnitude of the dimensionless flow velocity, independent of the wall buoyancy flux and physical scale of the box, decreases significantly with the number of layers. Three types of layer, each with a distinct induced flow pattern, are distinguished and their implications for room airflow considered. Notably, the flow in the base layer represents a continual and smooth flushing of air between the inlet opening and the wall plume, whereas an intermediate layer is almost entirely comprised of near-stagnant air
Spatio-temporal mixed membership models for criminal activity
No full textWe suggest a probabilistic approach to study crime data in London and highlight the benefits of defining a statistical joint crime distribution model which provides insights into urban criminal activity. This is achieved by developing a hierarchical mixture model for observations, crime occurrences over a geographical study area, that are grouped according to multiple time stamps and crime categories. The mixture components correspond to spatial crime distributions over the study area and the goal is to infer, based on the observations, how and to what degree the latent distributions are shared across the groups
Defects and Passivation Mechanism of the Suboxide Layers at SiO/4H-SiC (0001) Interface: A First-Principles Calculation
No full textThe origination of poor quality remains debating at the as-grown SiO2/4H-SiC (0001) interface during the thermal oxidation process. A low electron density layer (SiOx(0.3<x<2) is observed at the Si-terminated SiO2/4H-SiC (0001) interface in experiment, different from the previous reports on carbon-related defects. In this article, the SiO2/4H-SiC (0001) interface modeled with an interfacial SiOx(0.3<x<2) suboxide layer (1 nm) is systematically studied by first-principles calculations. According to the calculated electronic structures, the Si-Si antibonding structures in the SiOx layer are the dominating defects that cause obvious gap states, while the oxygen and silicon vacancy are not. The energy positions of the defect states are located at 0.72 eV above the valence band maximum (VBM), which indicates the Si-Si defect is the potential influence factor for the p-channel Silicon carbide (SiC)-MOSFET. We further study several passivation schemes by introducing nitrogen, phosphorus, boron, and aluminum at the interface to analyze the passivation mechanism. According to the calculated results of the passivation models, nitrogen passivation is an effective method, fully removing the gap states from Si-Si defects by forming Si3N, while boron passivation works for vacancy defect at the suboxide layers. The theoretical results prove the dominated defect structures at the interfacial SiOx layer and suggest that a combination passivation strategy (nitrogen and boron) may be an effective method to further improve the SiO2/4H-SiC interface
Proof of concept study for fuselage boundary layer ingesting propulsion
No full textKey results from the EU H2020 project CENTRELINE are presented. The research activities undertaken to demonstrate the proof of concept (technology readiness level-TRL 3) for the so-called propulsive fuselage concept (PFC) for fuselage wake-filling propulsion integration are discussed. The technology application case in the wide-body market segment is motivated. The developed performance bookkeeping scheme for fuselage boundary layer ingestion (BLI) propulsion integration is reviewed. The results of the 2D aerodynamic shape optimization for the bare PFC configuration are presented. Key findings from the high-fidelity aero-numerical simulation and aerodynamic validation testing, i.e., the overall aircraft wind tunnel and the BLI fan rig test campaigns, are discussed. The design results for the architectural concept, systems integration and electric machinery pre-design for the fuselage fan turbo-electric power train are summarized. The design and performance implications on the main power plants are analyzed. Conceptual design solutions for the mechanical and aerostructural integration of the BLI propulsive device are introduced. Key heuristics deduced for PFC conceptual aircraft design are presented. Assessments of fuel burn, NOx emissions, and noise are presented for the PFC aircraft and benchmarked against advanced conventional technology for an entry-into-service in 2035. The PFC design mission fuel benefit based on 2D optimized PFC aero-shaping is 4.7%
Analysis of the rate of combustion of biomass char in a fluidised bed of CLOU particles
No full textThis research concerns the combustion of biomass char in a fluidised bed using Chemical Looping Combustion with Oxygen Uncoupling (CLOU). To evaluate the influence of the CLOU material on the rate of combustion, an analytical model has been developed, based on a simplified scenario, i.e. gaseous mass transfer external to the fuel particle was taken as a stagnant system, ignoring advective flow. The combustion of a char particle was modelled as a shrinking particle. Results from the model were compared with experiments performed by combusting char from birch-wood in a fluidised bed (i.d. 30 mm) of an active oxygen carrier (CuO supported on mayenite) or inert SiO2 sand. The experiments were undertaken with a partial pressure of oxygen, pO2, close to the equilibrium pressure of O2 of the Cu-based oxygen carrier. Despite the same pO2 for both experiments, the presence of the reaction of oxygen uncoupling from the oxygen carrier resulted in a significant increase in the combustion rate of char. As a result, at 1173 K, the burn-out time of 0.1 g of char particles with the oxygen carrier was around five times faster than with SiO2. The results from the analytical model of CLOU agreed with the experimental observations despite the simplified assumption of a stagnant system, viz. a system in which the mass transfer boundary layer, δ, is infinite. This is because the char combustion in CLOU depends, in fact, on gaseous mass transfer across an effective boundary thickness, δe, rather than a conventional boundary thickness for the system, δ. At 1023 K when the oxygen uncoupling did not occur, the enhancement in the rate of reaction due to the presence of the CLOU particle was less significant. The model was used to investigate the apparent enhancement of the combustion rate in CLOU, and possible explanations, including improved mass transfer, are discussed
Power-efficient adaptive behavior through a shape-changing elastic robot
No full textThe adaptive morphology of a robot, such as shape adaptation, plays a significant role in adapting its behaviors. Shape adaptation should ideally be achieved without considerable cost, like the power required to deform the robot’s body, and therefore, it is reasonably considered as the last resort in classical rigid robots. However, the last decade has seen an increasing interest in soft robots: robots that can achieve deformability through their inherent material properties or structural compliance. Nevertheless, the dynamics of these types of robots is often complex and therefore it is difficult to substantiate whether the cost like the required power for changing its shape will be worthwhile to achieve the desired behavior. This article presents an approach in the development and analysis of a shape-changing locomoting robot, which relies on the ability of elastic beams to deform and vibrate. Through a proper use of elastic materials and the robot’s vibration-based dynamics, it will be shown both analytically and experimentally how shape adaptation can be designed such that it leads to desirable behaviors, with better power efficiency compared to when the robot solely relies on changing its control input. The results encourage emerging direction in robotics that investigates approaches to change robots’ behaviors through their adaptive morphology
The efficiency of eco-friendly corrosion inhibitors in protecting steel reinforcement
No full textReinforcing steel is used extensively in buildings to provide strength and integrity to the concrete structure. This material is, however, highly susceptible to corrosion in chloride-contaminated environments, which increases the risk of structural instability and failure. This work characterises the mechanisms and efficiency of corrosion protection offered by sodium nitrate, casein, and two amino acids (11–aminoundecanoic acid, and p–aminobenzoic acid) in simulated concrete pore solutions with different contents of chloride ions. The performance of each inhibitor in the critical chloride concentration (Ccirt) was investigated using electrochemical techniques. Open circuit potential and linear polarisation were used to identify the Ccrit in synthetic pore solutions. Potentiodynamic polarisation and electrochemical impedance spectroscopy were performed to evaluate the corrosion activities and the passivation mechanism of inhibitors in Ccrit. Results indicate that reinforcing steel can be protected through an appropriate selection of corrosion inhibitors. Among of the inhibitors studied here, casein demonstrated the highest corrosion inhibition efficiency with minimum current density of 9.19 × 10−8 μA/cm2 and inhibitor efficiency of more than 80%. Casein provides passivity to the reinforcing steel in the presence of the Ccirt in the pore solution
A Review on the Developments of Peridynamics for Reinforced Concrete Structures
No full textConcrete is the most widely used man made material in the world. Reinforced with steel, it forms a key enabler behind our rapidly urbanising built environment. Yet despite its ubiquity, the failure behaviour of the material in shear is still not well understood. Many different shear models have been proposed over the years, often validated against sets of physical tests, but none of these has yet been shown to be sufficiently general to account for the behaviour of all possible types and geometries of reinforced concrete structures. A key barrier to a general model is that concrete must crack in tension, and in shear such cracks form rapidly to create brittle failure. Peridynamics (PD) is a non-local theory where the continuum mechanics equilibrium equation is reformulated in an integral form, thereby permitting discontinuities to arise naturally from the formulation. On the one hand, this offers the potential to provide a general concrete model. On the other hand, PD models for concrete structures have not focussed on applications with reinforcement. Moreover, a robust model validation that assesses the strengths and weakness of a given model is missing. The objectives of this paper are twofold: (1) to evaluate the benchmark tests involving shear failure for RC structures; and (2) to review the most recent PD theory and its application for reinforced concrete (RC) structures. We investigate these models in detail and propose benchmark tests that a PD model should be able to simulate accurately
Limits to making L-shape ring profiles without ring growth
No full textA novel ring rolling process is proposed to flexibly produce shaped rings without circumferential ring growth, potentially saving material and energy as well as reducing upstream and downstream processing requirements. In this paper, six circumferential constraint rolls are used constrain circumferential growth and enable L-shape profiles to be developed through axial material flow, via a compressive hoop stress. Process limits were studied in 22 experiments on lead rings and a set of axisymmetric thermally coupled simulations on a high value engineering material, Inconel 718. Profile depths of 75 % of the original wall thickness were achieved in a range of rings and operating conditions, and material savings of up to 60 % demonstrated over rectilinear rolling. There was no evidence of cracking or void formation, unlike processes where under-deformed regions are stretched circumferentially and are vulnerable to cracking. In several cases a non-circular ring shape developed, limiting the achievable profile depth especially for small wall thicknesses, large reductions in thickness per pass, or large profile heights. The constraint roll forces when this ‘collapse’ occurs was studied and an upper bound predicted by a plastic hinge model. The thermal simulations showed that in all except 4 cases reheats would be required to keep within safe temperature bounds, thus suggesting an optimum reduction in thickness per pass to avoid both excessive cooling and collapse
Harmonic linearisation of aerodynamic loads in a frequency-domain model of a floating wind turbine
No full textWhile detailed aero-servo-hydro-elastic simulation codes for modelling floating wind turbines (FWTs) are available, they achieve high accuracy at the expense of calculation speed. For conceptual design and optimisation, fast solutions are needed, and equivalent linearisation techniques combined with frequency-domain analysis offers to capture the complex behaviour of FWTs in fast, approximate models. The main aim of this paper is to apply a harmonic linearisation approach to model the aerodynamic loading within a complete coupled model of a FWT, quantifying its performance, and where accuracy is unsatisfactory, to give insight into the causes. Two linearised models are derived from a coupled nonlinear aero-hydro-servo-elastic model, using the OC3-Hywind FWT as a test case: the typical tangent linearisation derived by numerical perturbation of the nonlinear model and the harmonic linearisation yielding improved representation of the aerodynamic loads. Comparisons against nonlinear time-domain simulations from Bladed show that it is possible to create a frequency-domain model of a FWT, including a flexible structure, aeroelastic rotor loads and the effect of the control system, with reasonable accuracy. The biggest source of errors is the presence of additional harmonics caused by nonlinear interactions between loads at different frequencies, rather than inaccurate linearisation of the magnitudes of forces. The computational cost of the harmonic linearisation implemented varies, but in most cases is ∼10× slower than the tangent linearisation and ∼100× faster than the time domain solution