MRC Laboratory of Molecular Biology
CUED - Cambridge University Engineering DepartmentNot a member yet
45551 research outputs found
Sort by
Accounting for the influence of the free surface on the vibration response of underground railway tunnels: A new iterative method
No full textThis paper presents a new method for calculating the ground-borne vibration from shallow underground railways. The method is based on an iterative wave-scattering approach, which decouples the problem into two sub-systems: (1) a tunnel embedded in a full-space and (2) a half-space domain for the soil alone. The Pipe-in-Pipe (PiP) model is first used to find the soil response remote from the tunnel, in the absence of a free surface. The reflected wave-field approaching the soil-tunnel interface from the free surface is then computed using a Boundary-Element Method (BEM) model, before applying this as an external load on the tunnel wall to calculate the revised response. The process is repeated for multiple iterations until convergence is achieved. To the authors' knowledge, this is the first time that an iterative approach has been applied to an elastodynamic problem. The results from this iterative PiP-BEM model are compared with those of a coupled Finite-Element-Boundary-Element Method (FEM-BEM) model and found to agree well over the frequency range typically associated with ground-borne vibration
Asymmetric Carbon Nanohorn Enabled Soft Capacitors with High Power Density and Ultra-Low Cutoff Frequency
No full textFlexible capacitors are a promising power source for foldable and biological electronic devices. Although various materials and device structures have been explored, they are still limited by low energy densities and slow rate capabilities compared to their rigid counterparts. Here, asymmetric carbon nanohorns are proposed as an active material to fabricate flexible solid-state carbon wire (CW)-based electrochemical supercapacitors (ss-CWECs) which exhibit high power density and ultra-low cutoff frequency. By controlling the electric arc reaction at low temperature (77 K), asymmetric single-wall carbon nanohorns (SWCNHs) are synthesized with high yield. Based on microscopy and electrochemical characterization, the fundamental reaction mechanism in polyvinyl-based electrolyte system is elucidated, as being associated with deprotonation reaction at acid, base, and elevated temperature conditions. Additionally, by using activated carbon, multi-walled carbon nanotubes, and SWCNHs as hybrid electrode materials (5:1:1), remarkable specific length capacitance of 48.76 mF cm−1 and charge–discharge stability (over 2000 times cycles) of ss-CWECs are demonstrated, which are the highest reported to date. Furthermore, a high-pass filter for eliminating ultra-low electronic noise is demonstrated, which enables an optical Morse Code communication system to be operated. Current results confirm the SWCNHs as promising materials for high-performance soft electronics and energy storage applications
Decision-making in driver-automation shared control: A review and perspectives
No full textShared control schemes allow a human driver to work with an automated driving agent in driver-vehicle systems while retaining the driver s abilities to control. The human driver, as an essential agent in the driver-vehicle shared control systems, should be precisely modeled regarding their cognitive processes, control strategies, and decision-making processes. The interactive strategy design between drivers and automated driving agents brings an excellent challenge for human-centric driver assistance systems due to the inherent characteristics of humans. Many open-ended questions arise, such as what proper role of human drivers should act in a shared control scheme? How to make an intelligent decision capable of balancing the benefits of agents in shared control systems? Due to the advent of these attentions and questions, it is desirable to present a survey on the decision making between human drivers and highly automated vehicles, to understand their architectures, human driver modeling, and interaction strategies under the driver-vehicle shared schemes. Finally, we give a further discussion on the key future challenges and opportunities. They are likely to shape new potential research directions
Turbulence Modelling for Complex Flows in Turbomachinery
No full textThis research aims to contribute the efforts of reducing the predictive uncertainties of Computational Fluid Dynamics (CFD) so that it can be a more reliable tool for modelling complex flows in turbomachinery. To achieve this goal, a three-dimensional (3D) flow phenomenon that embodies many key features of turbomachinery flows, that is, the corner separation flow often found in axial compressor stators, is selected as the research object. Based on the understanding of the underlying physics of this complex flow, the RANS turbulence closure models and a hybrid RANS-LES method are developed and extended. The improved RANS and hybrid RANS-LES models are shown to be able to predict the corner separation flows with significantly improved accuracy, and are expected to be applicable to other types of flows driven by the similar physical mechanisms as those of the corner separation flows. The commonly used RANS turbulence models (namely Spalart-Allmaras (SA) and Menter’s Shear Stress Transport (SST) models) have been found to overpredict the size of corner separation. The physical reason is partly attributed to the underestimation of turbulence mixing between the main flow and the endwall boundary layer. This makes the endwall boundary layer unable to withstand the bulk adverse pressure gradient, and in turn leads to its premature separation near the endwall. The flow characteristics within the compressor stator cascade are then studied to facilitate understanding the physical mechanisms that drive the formation of 3D flow structures, and the physical reasons that lead to the RANS modelling uncertainties. The source terms in the SA and SST models that control mixing are identified and modified, in order to increasing the mixing process and strengthening the endwall flow. The modified turbulence models are validated to give better predictions of the extent of corner separation under various operating conditions. Further, the results of an investigation into the effects of explicit non-linear turbulence modelling on anisotropic turbulence flows are presented. In compressors, the flow is subject to 3D shear and the effects due to turbulence anisotropy shall not be ignored. It is found that increasing the Reynolds stress anisotropy within the corner region contributes to counter-rotating streamwise vortices being generated in the corner region. This leads to the higher-momentum fluid in the main flow passage being entrained into the corner region and thus enhances mixing between the corner flow and the main flow. The flow within the corner region is energized and thus is more resistant to separation. Although the proposed modifications can predict corner separations with significantly better accuracy, the results are still subject to the limitations of RANS in capturing separated flows. To further reduce the modelling uncertainties, the hybrid RANS-LES method is resorted to allow for at least some larger scales of turbulence being resolved in the regions where RANS does not work well. In this thesis, an effort is presented towards a high-fidelity hybrid RANS-LES simulation for 3D complex separated flows. The hybrid RANS-LES method is developed to resolve turbulence where necessary, while avoiding the higher cost of performing large eddy simulation (LES) everywhere. That is, reasonably modelling the near-wall turbulence behaviour in the hybrid RANS-LES context by proposing a physical-based RANS-LES blending function. The suggested future work is presented in the final chapter, which outlines the necessity of investigation into the turbulence anisotropic effects on the tip-leakage flow modelling, and the potential applicability of the proposed model variants in the prediction of the blockage due to corner separation in the multi-stage environment
Soot emission simulations of a single sector model combustor using incompletely stirred reactor network modeling
No full textThe simulation of soot evolution is a problem of relevance for the development of low-emission aero-engine combustors. Apart from detailed CFD approaches, it is important to also develop models with modest computational cost so a large number of geometries can be explored, especially in view of the need to predict engine-out soot particle size distributions (PSDs) to meet future regulations. This paper presents an approach based on Incompletely Stirred Reactor Network (ISRN) modeling that simplifies calculations, allowing for the use of very complex chemistry and soot models. The method relies on a network of Incompletely Stirred Reactors (ISRs), which are inhomogeneous in terms of mixture fraction but characterized by homogeneous conditional averages, with the conditioning performed on the mixture fraction. The ISRN approach is demonstrated here for a single sector lean-burn model combustor operating on Jet-A1 fuel in pilot-only mode, for which detailed CFD and experimental data are available. Results show that reasonable accuracy is obtained at a significantly reduced computational cost. Real fuel chemistry and a detailed physicochemical sectional soot model are consequently employed to investigate the sensitivity of ISRN predictions to the chemical mechanism chosen and to provide an estimate of the soot particle size distribution at the combustor exit
Chemical looping electricity storage
No full textDeveloping grid-scale energy storage technologies is the key element for broader deployment of renewable sources of energy. This paper examines a simple cycle which makes use of a thermo-chemical store, with a view to achieving high storage capacity by using the chemical looping concept. Results show that a Chemical Looping Electricity Storage (CLES) system can achieve a very high capacity, in the range of 250–350 kWh/m3, second only to hydrogen electricity storage systems. Its round-trip efficiency (40–55%) is potentially higher than that of the hydrogen electricity storage systems. By achieving a higher capacity than pumped thermal energy storage and higher round-trip efficiency than that of hydrogen systems, CLES has the potential to fill out the gap between these two grid-scale storage technologies. Thus, this system may play an important role in our future energy mix because, unlike hydrogen storage, it can achieve a high storage capacity without a huge penalty on its round-trip efficiency
Numerical modelling of structures adjacent to retaining walls subjected to earthquake loading
No full textIn an urban environment, it is often necessary to locate structures close to existing retaining walls due to congestion in space. When such structures are in seismically active zones, the dynamic loading attracted by the retaining wall can increase. In a novel approach taken in this paper, finite element-based numerical analyses are presented for the case of a flexible, cantilever sheet pile wall with and without a structure on the backfill side. This enables a direct comparison of the influence exerted by the structure on the dynamic behaviour of the retaining wall. In this paper, the initial static bending moments and horizontal stresses prior to application of any earthquake loading are compared to Coulomb’s theory. The dynamic behaviour of the retaining wall is compared in terms of wall-top accelerations and bending moments for different earthquake loadings. The dynamic structural rotation induced by the differential settlements of the foundations is presented. The accelerations generated in the soil body are considered in three zones, i.e., the free field, the active and the passive zones. The differences caused by the presence of the structure are highlighted. Finally, the distribution of horizontal soil pressures generated by the earthquake loading behind the wall, and in front of the wall is compared to the traditional Mononobe-Okabe type analytical solutions
Model Reduction Methods for Complex Network Systems
No full textNetwork systems consist of subsystems and their interconnections and provide a powerful framework for the analysis, modeling, and control of complex systems. However, subsystems may have high-dimensional dynamics and a large number of complex interconnections, and it is therefore relevant to study reduction methods for network systems. Here, we provide an overview of reduction methods for both the topological (interconnection) structure of a network and the dynamics of the nodes while preserving structural properties of the network. We first review topological complexity reduction methods based on graph clustering and aggregation, producing a reduced-order network model. Next, we consider reduction of the nodal dynamics using extensions of classical methods while preserving the stability and synchronization properties. Finally, we present a structure-preserving generalized balancing method for simultaneously simplifying the topological structure and the order of the nodal dynamics
Theoretical estimation of size effects on the electronic transport in tailored graphene nanoribbons.
No full textFocusing on the potential applications of tailored graphene nanoribbons (t-GNRs), in this work, we systematically study size effects on the electronic transport in t-GNR-based molecular junctions. As a result of the manufacturing error generated during the processing or synthesis of t-GNRs using techniques such as ion beam lithography, the final dimensions of the as-fabricated devices often deviate from the design values, giving rise to a size distribution around the mean value which could considerably affect the device performance. To simulate the effects of the manufacturing error, a series of t-GNR-based junctions with various dimensions have been modelled and systematically investigated using density functional theory (DFT) coupled with the non-equilibrium Green's function (NEGF). For junctions that consist of an acene chain connected with two graphene nanosheets, it is found that the chain length has little influence on the electronic transport and that, on the other hand, the junction conductivity is significantly altered by its width due to the different number and nature of the electron transfer pathways. Furthermore, increasing the width of the junction leads to a clear odd-even variation of decreasing amplitude in its transport behavior. These findings underpin further fundamental and device-based studies of t-GNRs
Thermally stimulated relaxation and behaviors of oxygen vacancies in SrTiO<inf>3</inf> single crystals with (100), (110) and (111) orientations
No full textThe strontium titanate (SrTiO3) single crystals with different orientations of (100), (110) and (111) were investigated using thermally stimulated depolarization current (TSDC) measurements, which has been proved to be an effective strategy to fundamentally study the relationship between relaxation phenomena and defect chemistry in dielectrics. The origins of different relaxations in SrTiO3 crystals were identified and the activation energy of oxygen vacancies was estimated from TSDC measurements. It was further found that oxygen-treated SrTiO3 crystals exhibit different relaxation behaviors. Noticeable changes of thermal relaxation associated with oxygen vacancies have taken place in relation to the crystalline anisotropy. The SrTiO3 (110) samples display higher concentration and activation energy of oxygen vacancies. First-principles calculations were carried out on SrTiO3 (110) crystals to study the effect of oxygen vacancy on different surface microstructure. From the resulting minimum formation energy of 0.63 eV, it demonstrates that the oxygen vacancies tend to form on the TiO-terminated surfaces. Considering the band structure, oxygen vacancies near the surface contribute to the transition of crystal from insulator to metallic characteristic