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Scholarly Works - Engineering - Energy, Fluid Mechanics and Turbomachinery


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  • ItemOpen AccessPublished version Peer-reviewed
    Turbulent Drag Reduction Using Anisotropic Permeable Substrates.
    (Springer Science and Business Media LLC, 2018) Gómez-de-Segura, G; Sharma, A; García-Mayoral, R; Gómez-de-Segura, G [0000-0002-4572-1077]
    The behaviour of turbulent flow over anisotropic permeable substrates is studied using linear stability analysis and direct numerical simulations (DNS). The flow within the permeable substrate is modelled using the Brinkman equation, which is solved analytically to obtain the boundary conditions at the substrate-channel interface for both the DNS and the stability analysis. The DNS results show that the drag-reducing effect of the permeable substrate, caused by preferential streamwise slip, can be offset by the wall-normal permeability of the substrate. The latter is associated with the presence of large spanwise structures, typically associated to a Kelvin-Helmholtz-like instability. Linear stability analysis is used as a predictive tool to capture the onset of these drag-increasing Kelvin-Helmholtz rollers. It is shown that the appearance of these rollers is essentially driven by the wall-normal permeability Ky+ . When realistic permeable substrates are considered, the transpiration at the substrate-channel interface is wavelength-dependent. For substrates with low Ky+ , the wavelength-dependent transpiration inhibits the formation of large spanwise structures at the characteristic scales of the Kelvin-Helmholtz-like instability, thereby reducing the negative impact of wall-normal permeability.
  • ItemOpen AccessPublished version Peer-reviewed
    Spectral Analysis of the Slip-Length Model for Turbulence over Textured Superhydrophobic Surfaces.
    (Springer Science and Business Media LLC, 2018) Fairhall, CT; García-Mayoral, R; Fairhall, CT [0000-0002-2529-3650]
    We assess the applicability of slip-length models to represent textured superhydrophobic surfaces. From the results of direct numerical simulations, and by considering the slip length from a spectral perspective, we discriminate between the apparent boundary conditions experienced by different lengthscales in the overlying turbulent flow. In particular, we focus on the slip lengths experienced by lengthscales relevant to the near wall turbulent dynamics. Our results indicate that the apparent failure of homogeneous slip-length models is not the direct effect of the texture size becoming comparable to the size of eddies in the flow. The texture-induced signal scatters to the entire wavenumber space, affecting the perceived slip length across all lengthscales, even those much larger than the texture. We propose that the failure is caused by the intensity of the texture-induced flow, rather than its wavelength, becoming comparable to the background turbulence.
  • ItemOpen Access
    Quantifying the role and value of chemical looping combustion in future electricity systems via a retrosynthetic approach
    (Elsevier BV, 2018) Schnellmann, MA; Heuberger, CF; Scott, SA; Dennis, JS; Mac Dowell, N; Schnellmann, Matthias [0000-0002-9025-1481]; Dennis, John [0000-0002-5014-5676]
    Carbon capture and sequestration of CO2 from the combustion of fossil fuels in thermal power plants is expected to be important in the mitigation of climate change. Deployment however falls far short of what is required. A key barrier is the perception by developers and investors that these technologies are too inefficient, expensive and risky. To address these issues, we have developed a novel retrosynthetic approach to evaluate technologies and their design based on the demands of the system in which they would operate. We have applied it to chemical looping combustion (CLC), a promising technology, which enables carbon dioxide emissions to be inherently captured from the combustion of fossil fuels. Our approach has provided unique insight into the potential role and value of different CLC variants in future electricity systems and the likely impact of their integration on the optimal capacity mix, the operational and system cost, and dispatch patterns. The three variants investigated could all provide significant value by reducing the total investment and operational cost of a future electricity system. The minimisation of capital cost appears to be key for the attractiveness of CLC, rather than other factors such as higher efficiency or lower oxygen carrier costs.
  • ItemOpen AccessAccepted version Peer-reviewed
    Scalar flux modeling in turbulent flames using iterative deconvolution
    (American Physical Society (APS), 2018) Nikolaou, ZM; Cant, RS; Vervisch, L; Cant, Stewart [0000-0003-1851-6665]
    In the context of Large Eddy Simulations, deconvolution is an attractive alternative for modelling the un-closed terms appearing in the filtered governing equations. Such methods have been used in a number of studies for non-reacting and incompressible flows, however their application in reacting flows is limited in comparison. Deconvolution methods originate from clearly defined operations, and in theory can be used in order to model any un-closed term in the filtered equations including the scalar flux. In this study, an iterative deconvolution algorithm is used in order to provide a closure for the scalar flux term in a turbulent premixed flame by explicitly filtering the deconvoluted fields. The assessment of the method is conducted a priori using a three-dimensional direct numerical simulation database of a turbulent freely-propagating premixed flame in a canonical configuration. In contrast to most classical a priori studies, the assessment is more stringent as it is performed on a much coarser LES mesh which is constructed using the filtered fields as obtained from the direct simulations. For the conditions tested in this study, deconvolution is found to provide good estimates both of the scalar flux and of its divergence.
  • ItemOpen AccessAccepted version Peer-reviewed
    On the effect of fluid-structure interactions and choice of algorithm in multi-physics topology optimisation
    (Elsevier BV, 2018) Munk, DJ; Kipouros, T; Vio, GA; Parks, GT; Steven, GP; Vio, GA [0000-0001-6540-2180]; Parks, GT [0000-0001-8188-5047]; Steven, GP [0000-0002-0717-4332]
    This article presents an optimisation framework for the compliance minimisation of structures subjected to design-dependent pressure loads. A finite element solver coupled to a Lattice Boltzmann method is employed, such that the effect of the fluid-structure interactions on the optimised design can be considered. It is noted that the main computational expense of the algorithm is the Lattice Boltzmann method. Therefore, to improve the computational efficiency and to assess the effect of the fluid-structure interactions on the fi nal optimised design, the degree of coupling is changed. Several successful topology optimisation algorithms exist with thousands of associated publications in the literature. However, only a small portion of these are applied to real-world problems, with even fewer offering a comparison of methodologies. This is especially important for problems involving fluid-structure interactions, where discrete and continuous methods can provide different advantages. The goal of this research is to couple two key disciplines, fluids and structures, into a topology optimisation framework, which shows fast convergence for multi-physics optimisation problems. This is achieved by offering a comparison of three popular, but competing, optimisation methodologies. The needs for the exploration of larger design spaces and to produce innovative designs make meta-heuristic algorithms less efficient for this task. A coupled analysis, where the fluid and structural mechanics are updated, provides superior results compared with an uncoupled analysis approach, however at some computational expense. The results in this article show that the method is sensitive to whether fluid-structure coupling is included, i.e. if the fluid mechanics are updated with design changes, but not to the degree of the coupling, i.e. how regularly the fluid mechanics are updated, up to a certain limit. Therefore, the computational efficiency of the algorithm can be considerably increased with small penalties in the quality of the objective by relaxing the coupling.
  • ItemOpen AccessAccepted version Peer-reviewed
    A Cold-Startup SSHI Rectifier for Piezoelectric Energy Harvesters with Increased Open-Circuit Voltage
    (Institute of Electrical and Electronics Engineers (IEEE), 2019) Du, S; Amaratunga, GAJ; Seshia, AA; Du, S [0000-0001-6238-4423]; Seshia, AA [0000-0001-9305-6879]
    Piezoelectric vibration energy harvesting has drawn much research interest over the last decade towards the goal of enabling self-sustained wireless sensor nodes. In order to make use of the harvested energy, interface circuits are needed to rectify and manage the energy. Among all active interface circuits, SSHI (synchronized switch harvesting on inductor) and SECE (synchronous electric charge extraction) are widely employed due to their high energy efficiencies. However, the cold-startup issue still remains since an interface circuit needs a stable DC supply and the whole system is completely out of charge at the beginning of implementations or after a certain period of time without input vibration excitation. In this paper, a new cold-startup SSHI interface circuit is presented, which dynamically increases the open-circuit voltage generated from the piezoelectric transducer (PT) in cold-state to start the system under much lower excitation levels. The proposed circuit is designed and fabricated in a 0.18 um CMOS process and experimentally validated together with a custom MEMS (microelectromechanical systems) harvester, which is designed with split electrodes to work with the proposed power extraction circuit. The experiments were performed to start the system from the cold state under variable excitation levels. The results show that the proposed system lowers the required excitation level by at least 50% in order to perform a cold-startup. This aids restarting of the energy harvesting system under low excitation levels each time it enters the cold state.
  • ItemOpen AccessAccepted version Peer-reviewed
    Physics-based Compact Model of Integrated Gate-Commutated Thyristor with Multiple Effects for High Power Application
    (Institution of Engineering and Technology, 2018-06-18) Lyu, Gang; Zhuan, Chijie; Zeng, Rong; Long, T; Palmer, PR; Long, Teng [0000-0003-4401-102X]
    This paper presents a physics-based compact model of integrated gate-commutated thyristor (IGCT) with multiple effects for high power application. The proposed model has both acceptable accuracy and computation time requirement, which is suitable for system level circuit simulation and IGCT’s whole wafer modelling work. First, the development of IGCT model is discussed and the one-dimension phenomenon of IGCT is analyzed in the paper. Second, a physics-based compact model of IGCT is proposed. The proposed model of IGCT includes multiple physical effects that are crucial to IGCTs working in high power applications. These physical effects include the impact ionization effect, moving boundary of depletion region during punch-thourgh (PT) and the local lifetime region. The Fourier series solution is applied for the ambipolar diffusion equation in the base region. Third, the proposed model is implemented in Simulink and compared with the model in Silvaco Atlas, a finite-element (FEM) tool. Finally, the proposed compact model of IGCT is validated by experiments.
  • ItemOpen AccessAccepted version Peer-reviewed
    Monodisperse CNT Microspheres for High Permeability and Efficiency Flow-Through Filtration Applications.
    (Wiley, 2018-03) Copic, Davor; Maggini, Laura; De Volder, Michael; De Volder, Michael [0000-0003-1955-2270]
    Carbon nanotube (CNT)-based filters have the potential to revolutionize water treatment because of their high capacity and fast kinetics in sorption of organic, inorganic, and biological pollutants. To date, CNT filters either rely on CNTs dispersed in liquids, which are difficult to recover and cause safety concerns, or on CNT buckypaper, which offers high efficiency, but suffers from an intrinsic trade-off between filter permeability and capacity. Here, a new approach is presented that bypasses this trade-off and achieves buckypaper-like efficiency combined with filter-column-like permeability and capacity. For this, CNTs are first assembled into porous microspheres and then are packed into microfluidic column filters. These microcolumns exhibit large flow-through filtration efficiencies, while maintaining membrane permeabilities an order of magnitude larger then CNT buckypaper and specific permeabilities double that of activated carbon for similar flowrates (232 000 L m-2 h-1 bar-1 , 1.23 × 10-12 m2 ). Moreover, in a test to remove sodium dodecyl sulfate (SDS) from water, these microstructured CNT columns outperform activated carbon columns. This improved filtration efficiency and permeability is an important step toward a broader implementation of CNT-based filtration devices.
  • ItemOpen AccessAccepted version Peer-reviewed
    Direct and indirect noise generated by entropic and compositional inhomogeneities
    (ASME) Rolland, Erwan; De Dedomenico, Francesca; Hochgreb, Simone; Rolland, Erwan [0000-0002-7021-9038]; Hochgreb, Simone [0000-0001-7192-4786]
    Flow disturbances are generated inside a duct via pulsed injection of helium into a flow of air. This leads to the generation of an acoustic pulse (direct noise), as well as the production of entropic and compositional inhomogeneities which are convected with the mean flow. As these inhomogeneities are convected through a choked nozzle, they generate indirect noise. The resulting acoustic pressure fluctuations are measured experimentally using pressure transducers upstream of the nozzle. Insight obtained from theoretical models and a time-delay analysis can be used to isolate and extract the contributions of direct and indirect noise in the experimental signal. These results are directly compared to existing one-dimensional direct and indirect noise models. The experimental measurement of indirect noise is found to be in good agreement with the theoretical models for entropy noise and compositional noise for a compact one-dimensional isentropic nozzle.
  • ItemOpen AccessAccepted version Peer-reviewed
    Fan-Intake interaction under high incidence
    (ASME International, 2017) Cao, T; Vadlamani, NR; Tucker, PG; Smith, AR; Slaby, M; Sheaf, CTJ; Vadlamani, Rao [0000-0002-8468-5216]; Tucker, Paul [0000-0002-0874-3269]
    In this paper, we present an extensive numerical study on the interaction between the downstream fan and the flow separating over an intake under high incidence. The objectives of this investigation are twofold: (a) to gain qualitative insight into the mechanism of fan–intake interaction and (b) to quantitatively examine the effect of the proximity of the fan on the inlet distortion. The fan proximity is altered using the key design parameter, L/D, where D is the diameter of the intake, and L is the distance of the fan from the intake lip. Both steady and unsteady Reynolds-averaged numerical simulations (RANS) were carried out. For the steady calculations, a low-order fan model has been used, while a full 3D geometry has been used for the unsteady RANS. The numerical methodology is also thoroughly validated against the measurements for the intake-only and fan-only configurations on a high bypass ratio turbofan intake and fan, respectively. To systematically study the effect of fan on the intake separation and explore the design criteria, a simplified intake–fan configuration has been considered. In this fan–intake model, the proximity of the fan to the intake separation (L/D) can be conveniently altered without affecting other parameters. The key results indicate that, depending on L/D, the fan has either suppressed the level of the postseparation distortion or increased the separation-free operating range. At the lowest L/D (∼0.17), around a 5 deg increase in the separation-free angle of incidence was achieved. This delay in the separation-free angle of incidence decreased with increasing L/D. At the largest L/D (∼0.44), the fan was effective in suppressing the postseparation distortion rather than entirely eliminating the separation. Isentropic Mach number distribution over the intake lip for different L/D's revealed that the fan accelerates the flow near the casing upstream of the fan face, thereby decreasing the distortion level in the immediate vicinity. However, this acceleration effect decayed rapidly with increasing upstream distance from the fan-face.
  • ItemOpen AccessAccepted version Peer-reviewed
    Improved multislope MUSCL reconstruction on unstructured grids for shallow water equations
    (Wiley, 2018) Zhao, J; Özgen, I; Liang, D; Hinkelmann, R; Zhao, J [0000-0002-0494-1743]; Özgen, I [0000-0003-4142-9914]
    SummaryIn shallow water flow and transport modeling, the monotonic upstream‐centered scheme for conservation laws (MUSCL) is widely used to extend the original Godunov scheme to second‐order accuracy. The most important step in MUSCL‐type schemes is MUSCL reconstruction, which calculate‐extrapolates the values of independent variables from the cell center to the edge. The monotonicity of the scheme is preserved with the help of slope limiters that prevent the occurrence of new extrema during reconstruction. On structured grids, the calculation of the slope is straightforward and usually based on a 2‐point stencil that uses the cell centers of the neighbor cell and the so‐called far‐neighbor cell of the edge under consideration. On unstructured grids, the correct choice for the upwind slope becomes nontrivial. In this work, 2 novel total variation diminishing schemes are developed based on different techniques for calculating the upwind slope and the downwind slope. An additional treatment that stabilizes the scheme is discussed. The proposed techniques are compared to 2 existing MUSCL reconstruction techniques, and a detailed discussion of the results is given. It is shown that the proposed MUSCL reconstruction schemes obtain more accurate results with less numerical diffusion and higher efficiency.
  • ItemOpen AccessPublished version Peer-reviewed
    Active Quenching Technique for YBCO Tapes: Quench Acceleration and Protection
    (Springer Science and Business Media LLC, 2018) Zhang, X; Geng, J; Shen, B; Li, C; Gawith, JDD; Zhong, Z; Ma, J; Zhang, H; Dong, Q; Coombs, TA; Shen, Boyang [0000-0001-8169-6588]; Gawith, James [0000-0001-7287-3546]; Ma, Jun [0000-0001-6232-1254]; Coombs, Timothy [0000-0003-0308-1347]
    The application of resistive-type superconducting fault current limiters (RSFCLs) in electrical networks is very attractive due to their relative compactness, light weight, and good performance. However, this technology still has drawbacks: asymmetrical quench, uncertain limiting velocity, passive action, and incapability of enlarging capacity. Here, we present an active quenching technique which can potentially solve these problems. The quenching process is triggered using high frequency (HF) AC fields, which are generated by two coupled copper coils attached on both sides of the YBCO sample. Our experiments show that this approach is effective in guaranteeing uniform quench and therefore is expected to be able to significantly extend the service life of the device. Moreover, the quench speed can be considerably increased by the HF field. We find that the performance of acceleration is positively correlated to the transport current, intensity and frequency of the AC field. In addition, a DC magnetic field is added around the sample holder simultaneously with the AC field, to study the field-angular-frequency dependence of the quench time t(B_AC,f_AC,B_DC,θ_DC). Experimental results prove that the DC magnetic field can cooperate with the HF AC field to accelerate quench, which means better performance can be produced with lower costs with the two fields acting together. In all, this technique showed outstanding performance regarding quench acceleration and tape protection. We believe the HF-assisted quenching technology has a promising future in current limiting devices and hope our findings could be helpful for its potential applications.
  • ItemOpen Access
    Flow Field Results of the Cambridge Stratified Swirl Burner Using Laser Doppler Anemometer
    (Department of Engineering, University of Cambridge, 2012-06-08) Zhou, Ruigang; Sweeney, Mark; Hochgreb, Simone
  • ItemOpen Access
    A Tool for the Spectral Analysis of the Laser Doppler Anemometer Data of the Cambridge Stratified Swirl Burner
    (Department of Engineering, University of Cambridge, 2012-06-08) Zhou, Ruigang; Balusamy, Saravanan; Hochgreb, Simone
    A series of flow fields generated by a turbulent methane/air stratified swirl burner are investigated using laser Doppler anemometer (LDA). The LDA provides flow field measurements with comparatively high temporal resolutions. However, processing of the power spectral energy density (PSD) and autocorrelation functions (ACF) of the flow velocity by LDA is complicated by the random, intermittent nature of the LDA signal caused by random arrival of particles at the measuring volume. A tool is developed to overcome this difficulty and the preliminary results are presented in the present report.
  • ItemOpen Access
    Technical Drawings of Cambridge Stratified Swirl Burner (SwB)
    (University of Cambridge & Sandia National Laboratories, 2008) Sweeney, Mark; Barlow, Robert; Hochgreb, Simone