80 results found
Nightingale M, Santer M, Hewson R, Multiscale Optimisation of Resonant Frequencies for Lattice Based Additive Manufactured Structures, Structural and Multidisciplinary Optimization
This paper introduces a novel methodology for the optimisation of resonant frequencies in three-dimensional lattice structures. The method uses a multiscale approach in which the homogenised material properties of the lattice unit cell are de fined by the spatially varying lattice parameters. Material properties derived from precomputed simulations of the small scale lattice are projected onto response surfaces, thereby describing the large scale metamaterial properties as polynomialfunctions of the small scale parameters. Resonant frequencies and mode shapes are obtained through the eigenvalue analysis of the large scale -finite element model which provides the basis for deriving the frequency sensitivities. Frequency tailoring is achieved by imposing constraints on the resonant frequency for a compliance minimisation optimisation. Asorting method based on the modal assurance criteria allows for specific mode shapes to be optimised whilst simultaneously reducing the impact of localised modes on the optimisation.Three cases of frequency constraints are investigated and compared to an unconstrained optimisation to demonstrate the algorithms applicability. The results show that the optimisation is capable of handling strict frequency constraints and with the use of the modal tracking can even alter the original ordering of the resonant mode shapes. Frequency tailoring allows for improved functionality of compliance minimised aerospace components by avoiding resonant frequencies and hence dynamic stresses.
De Boer G, Raske N, Soltanahmadi S, et al., 2020, A porohyperelastic lubrication model for articular cartilage in the natural synovial joint, Tribology International, Vol: 149, ISSN: 0301-679X
This work focuses on the proposed mechanisms for the lubrication of synovial joints and applies them to an idealised bearing geometry considering a porohyperelastic material (cartilage) rotating against a stationary rigid impermeable surface. The model captures the behaviour of all lubrication regimes including fluid film formation and boundary contact as the load capacity is increased, representing a major advancement in modelling cartilage mechanics. Transient responses in the fluid phase are shown to be faster than those in the solid phase with the former decaying over time as fluid is exuded from the material. The complex behaviour of fluid migrating to and from the lubricating film is captured which leads to a better understanding of the hydration and friction mechanisms observed.
Varas Casado JM, Hewson R, 2020, Multicomplex number class for Matlab, with a focus on the accurate calculation of small imaginary terms for multicomplex step sensitivity calculations, ACM Transactions on Mathematical Software, Vol: 46, Pages: 1-26, ISSN: 0098-3500
A Matlab class for multicomplex numbers was developed with particular attention paid to the robust and accurate handling of smallimaginary components. This is primarily to allow the class to be used to obtainn-order derivative information using the multicomplexstep method for, amongst other applications, gradient-based optimization and optimum control problems. The algebra of multicomplexnumbers is described as is its accurate computational implementation, considering small term approximations and the identification ofprinciple values. The implementation of the method in Matlab is studied, and a class definition is constructed. This new class definitionenables Matlab to handlen-order multicomplex numbers, and perform arithmetic functions. It was found that with this method, thestep size could be arbitrarily decreased toward machine precision. Use of the method to obtain up to the 7th derivative of functions ispresented, as is timing data to demonstrate the efficiency of the class implementation.
de Boer GN, Raske N, Soltanahmadi S, et al., 2020, Compliant-poroelastic lubrication in cartilage-on-cartilage line contacts, TRIBOLOGY-MATERIALS SURFACES & INTERFACES, Vol: 14, Pages: 151-165, ISSN: 1751-5831
Imediegwu C, Murphy R, Hewson R, et al., 2019, Multiscale structural optimization towards three-dimensional printable structures, Structural and Multidisciplinary Optimization, Vol: 60, Pages: 513-525, ISSN: 1615-147X
Murphy RD, Imediegwu C, Hewson R, et al., 2019, Multiscale concurrent optimization towards additively manufactured structures, AIAA Scitech 2019 Forum, Publisher: American Institute of Aeronautics and Astronautics
This work establishes a robust concurrent multiscale optimization framework which facilitates the precise functional-grading of mechanical properties within structures, over two scales.The novelty lies in the concurrent nature of the response surface which connects the small-scalegeometry to the large-scale domain. A concurrent implementation enables an efficient application of computational resources, such that a large number of design variables can be usedwithout a significant computational penalty. This framework also takes advantage of the process flexibility and precision of additive manufacturing techniques to ensure that all optimizedstructures are manufacturable and suitable for an aerospace based application. A complianceminimization case is compared against a standard topology optimization algorithm, resultingin superior functional values and demonstrates the efficacy of the presented approach. A further application of this framework is highlighted through a target deformation case, where acomplex deformation field is obtained through simple loading conditions. Results from both ofthe example problems indicate that this framework has potential within the field of adaptivestructures, to inspire a new generation of multifunctional designs.
Amigo RCR, Prado DS, Paiva JL, et al., 2018, Topology optimisation of biphasic adsorbent beds for gas storage, Structural and Multidisciplinary Optimization, Vol: 58, Pages: 2431-2454, ISSN: 1615-147X
Adsorption is a retention mechanism of fluid molecules on solid surfaces and presents a wide range of applications, such as fuel storage, refrigeration and separation processes. This work describes the modelling of gas adsorption on porous media and presents an optimisation approach for the design of adsorption systems based on biphasic adsorbent beds by topology optimisation. A comprehensive formulation for the adsorption phenomenon is presented, detailing the derivation of governing equations and respective weak forms and discretisation for the implementation of the finite element method (FEM). A new topology optimisation material model based on offset hyperbolic tangents is introduced. The derivation of sensitivities is presented in detail, based on a transient adjoint problem. A diverse set of optimised adsorbed natural gas (ANG) tanks, considering real material properties of activated carbon and steel, is presented. Results indicate the suitability of the method in optimising the distribution of phases across adsorbent beds and show that biphasic ANG tanks can perform significantly better than traditional tanks.
Imediegwu C, Murphy R, Hewson RW, et al., 2018, Multiscale structural and thermal optimization towards 3D printable structures, The 9th International Conference on Computational Methods
Gao L, Hua Z, Hewson R, et al., 2018, Elastohydrodynamic lubrication and wear modelling of the knee joint replacements with surface topography, Biosurface and Biotribology, Vol: 4, Pages: 18-23, ISSN: 2405-4518
This numerical study predicted wear of lubricated total knee replacements with the existing of textured surface and the possibility of surface designs to reduce wear. In the first part, a wear model of metal-on-polyethylene total knee replacement was developed. The medial and lateral knee compartments was accounted for separately, with the contact force and motion during walking cycles applied. An adapted Archard wear formula was employed where the wear factor was an exponential function of the `Lambda ratio' (film thickness to the average roughness). Wear of the soft bearing surface (polyethylene insert) was simulated with regularly geometry update until a steady-state wear observed. In the second part, the effect of surface topography of the knee replacements was investigated. The surface texturing techniques have shown promising benefit to machine components in many areas of engineering practice. The texture parameters were designed using the Taguchi method for the geometry, size, and distribution of the micro dimples. It was observed that the lateral compartment may benefit from surface texturing if dimples were properly designed, while the texturing showed hardly advantageous effect on the medial surface in terms of lubrication enhancement and wear reduction. Some results were presented in the 6th World Tribology Conference.
Imediegwu C, Murphy R, Hewson RW, et al., 2018, The design of a lattice-based periodic microstructure model towards 3D printable optimized structures, 10th European Solid Mechanics Conference
Navadeh N, Hewson RW, Fallah AS, 2018, Dynamics of transversally vibrating non-prismatic Timoshenko cantilever beams, Engineering Structures, ISSN: 0141-0296
Gao L, Hua Z, Hewson RW, 2018, Can a “pre-worn” bearing surface geometry reduce the wear of metal-on-metal hip replacements? – A numerical wear simulation study, Wear, Vol: 406-407, Pages: 13-21, ISSN: 0043-1648
Total Hip Replacement (THR) is generally a highly successful treatment for late stage hip joint diseases and wear, however, wear continues to be one of the major causes of metal-on-metal THR's failure. Hip replacements typically experience a two-stage wear; a higher initial wear rate in the beginning followed by a lower steady-state one with the surface profile changed. This alludes to the potential use of a cup with a non-spherical interior cavity with an initial geometry similar to a worn surface which may benefit from lower wear rate. In this paper wear is numerically simulated with a cup having a non-spherical geometry inspired by the initial stage of wear.A wear model was recently developed by the authors for the THR, which considered the lubricated contact in both elastohydrodynamic lubrication (EHL) and mixed lubrication regime, rather than a dry contact used in most of other studies of wear modelling in the academic literature. In this study the wear model has been updated by introducing the ‘λ ratio’ (the ratio of film thickness to surface roughness) and addressing the non-Newtonian shear-thinning properties of the synovial fluid. This wear model was able to describe the non-linear wear evolution process due to the change of worn profiles. Firstly the wear of a spherical hip joint was simulated until a steady-state wear rate is achieved. Then a non-spherical joint was proposed in which the cup bearing geometry was generated by the previously predicted worn profile from the spherical joint. At last the wear of this “pre-worn” hip bearing was simulated and compared to the spherical one. Approximately 40% reduction in the steady-state wear rate and 50% in the total accumulated wear has been observed in the non-spherical hip joint. This study presented a full numerical analysis of the relationship between lubrication, wear reduction and the geometry change, and quantitatively suggested the optimal geometry to reduce running-in wea
Wang H, Kow J, Raske N, et al., 2017, Robust and high-performance soft inductive tactile sensors based on the Eddy-current effect, Sensors and Actuators A: Physical, Vol: 271, Pages: 44-52, ISSN: 0924-4247
Tactile sensors are essential for robotic systems to interact safely and effectively with the external world, they also play a vital role in some smart healthcare systems. Despite advances in areas including materials/composites, electronics and fabrication techniques, it remains challenging to develop low cost, high performance, durable, robust, soft tactile sensors for real-world applications. This paper presents the first Soft Inductive Tactile Sensor (SITS) which exploits an inductance-transducer mechanism based on the eddy-current effect. SITSs measure the inductance variation caused by changes in AC magnetic field coupling between coils and conductive films. Design methodologies for SITSs are discussed by drawing on the underlying physics and computational models, which are used to develop a range of SITS prototypes. An exemplar prototype achieves a state-of-the-art resolution of 0.82 mN with a measurement range over 15 N. Further tests demonstrate that SITSs have low hysteresis, good repeatability, wide bandwidth, and an ability to operate in harsh environments. Moreover, they can be readily fabricated in a durable form and their design is inherently extensible as highlighted by a 4 × 4 SITS array prototype. These outcomes show the potential of SITS systems to further advance tactile sensing solutions for integration into demanding real-world applications.
Taherkhani AR, Gilkeson CA, Gaskell PH, et al., 2017, Aerodynamic CFD based optimization of police car using Bezier curves, SAE International Journal of Materials and Manufacturing, Vol: 10, Pages: 85-93, ISSN: 1946-3987
This paper investigates the optimization of the aerodynamic design of a police car, BMW 5-series which is popular police force across the UK. A Bezier curve fitting approach is proposed as a tool to improve the existing design of the warning light cluster in order to reduce drag. A formal optimization technique based on Computational Fluid Dynamics (CFD) and moving least squares (MLS) is used to determine the control points for the approximated curve to cover the light-bar and streamline the shape of the roof. The results clearly show that improving the aerodynamic design of the roofs will offer an important opportunity for reducing the fuel consumption and emissions for police vehicles. The optimized police car has 30% less drag than the non-optimized counter-part.
Boer GD, Raske N, Wang H, et al., 2017, Design Optimisation of a Magnetic Field Based Soft Tactile Sensor., Sensors, Vol: 17, ISSN: 1424-2818
This paper investigates the design optimisation of a magnetic field based soft tactile sensor, comprised of a magnet and Hall effect module separated by an elastomer. The aim was to minimise sensitivity of the output force with respect to the input magnetic field; this was achieved by varying the geometry and material properties. Finite element simulations determined the magnetic field and structural behaviour under load. Genetic programming produced phenomenological expressions describing these responses. Optimisation studies constrained by a measurable force and stable loading conditions were conducted; these produced Pareto sets of designs from which the optimal sensor characteristics were selected. The optimisation demonstrated a compromise between sensitivity and the measurable force, a fabricated version of the optimised sensor validated the improvements made using this methodology. The approach presented can be applied in general for optimising soft tactile sensor designs over a range of applications and sensing modes.
de Boer G, Hewson R, Bryant M, et al., 2017, An investigation into the contact between soft elastic and poroelastic bodies rotating under load, Tribology - Materials, Surfaces & Interfaces, Pages: 1-9, ISSN: 1751-5831
Raske N, Hewson RW, Kapur N, et al., 2017, A predictive model for discrete cell gravure roll Coating, Physics of Fluids, Vol: 29, Pages: 062101-062101, ISSN: 1089-7666
A heterogeneous multiscale model for discrete cell gravure roll coating is presented along with experimental results for the purpose of model validation. The cell volume, generalized cell shape, and the gravure patterning are considered in the model which is based on a multiscale description of the flow in the coating bead. The inclusion of a web-to-roll contact term accounts for the special gravure case when the web-roll separation tends to zero. The results show how the coating bead responds to changes in operating conditions. These are presented as profile plots of the fluid properties and coating bead shape.
Gao L, Hua Z, Hewson RW, et al., 2017, EHL and Wear Modelling of the Knee Joint Replacements with Surface Topography, The 6th World Tribology Conference
Wang H, de Boer G, Kow J, et al., 2017, A low-cost soft tactile sensing array using 3D hall sensors, Procedia Engineering, Vol: 168, Pages: 650-653, ISSN: 1877-7058
Tactile sensors are essential for robotic systems to safely interact with the external world and to precisely manipulate objects. Existing tactile sensors are typically either expensive or limited by poor performance, and most are not mechanically compliant. This work presents MagTrix, a soft tactile sensor array based on four 3D Hall sensors with corresponding permanent magnets. MagTrix has the capability to precisely measure triaxis force (1 mN resolution) and to determine contact area. In summary, the presented tactile sensor is robust, low-cost, high-performance and easily customizable to be integrated into a range of robotic and healthcare applications.
de Boer GN, Gao L, Hewson RW, et al., 2016, Heterogeneous multiscale methods for modelling surface topography in EHL line contacts, Tribology International, Vol: 113, Pages: 262-278, ISSN: 1879-2464
A multiscale method for the Elastohydrodynamic Lubrication (EHL) of line contacts is derived based on the Heterogeneous Multiscale Methods.Periodicity applies to the topographical features and lubricant flow, data is homogenised over a range of variables at amicro-scale and coupled into amacro-scalemodel. This is achieved using flow factorsas calculated from metamodels, which themselves evolve with the solution procedure. Results are givenfor an idealised topography and illustrate significant deviationsfrom smooth surface assumptions as quantified by the flow factors. Improvements in the accuracy andefficiency with previous work and large fluctuations due to micro-EHL are also presented.Validation of the multiscale method with a deterministic topography is provided demonstrating good accuracy and efficiency.
Wang H, de Boer G, Kow J, et al., 2016, Design Methodology for Magnetic Field-Based Soft Tri-Axis Tactile Sensors, Sensors, Vol: 16, ISSN: 1424-8239
Tactile sensors are essential if robots are to safely interact with the external world and to dexterously manipulate objects. Current tactile sensors have limitations restricting their use, notably being too fragile or having limited performance. Magnetic field-based soft tactile sensors offer a potential improvement, being durable, low cost, accurate and high bandwidth, but they are relatively undeveloped because of the complexities involved in design and calibration. This paper presents a general design methodology for magnetic field-based three-axis soft tactile sensors, enabling researchers to easily develop specific tactile sensors for a variety of applications. All aspects (design, fabrication, calibration and evaluation) of the development of tri-axis soft tactile sensors are presented and discussed. A moving least square approach is used to decouple and convert the magnetic field signal to force output to eliminate non-linearity and cross-talk effects. A case study of a tactile sensor prototype, MagOne, was developed. This achieved a resolution of 1.42 mN in normal force measurement (0.71 mN in shear force), good output repeatability and has a maximum hysteresis error of 3.4%. These results outperform comparable sensors reported previously, highlighting the efficacy of our methodology for sensor design.
Gao L, Hewson RW, 2016, Development of A Wear Model for LubricatedMetal-On-Metal Hip Joints, 5th International Conference on Biomedical Engineering and Biotechnology 2016
de Boer GN, Wang H, Ghajari M, et al., 2016, Force and Topography Reconstruction Using GP and MOR for the TACTIP Soft Sensor System, 17th Annual Conference, TAROS 2016, Publisher: Springer International Publishing, Pages: 65-74, ISSN: 0302-9743
Bach C, Jebari R, Viti A, et al., 2016, Composite stacking sequence optimization for aeroelastically tailored forward-swept wings, Structural and Multidisciplinary Optimization, Vol: 55, Pages: 105-119, ISSN: 1615-1488
A method for stacking sequence optimization and aeroelastic tailoring of forward-swept composite wings is presented. It exploits bend-twist coupling to mitigate aeroelastic divergence. The method is intended for estimating possible weight savings in preliminary aircraft design stages. A structural beam model of the composite wingbox is derived from anisotropic shell theory and the governing aeroelastic equations are presented for a spanwise discretized forward swept wing. Optimization of the system to reduce wing mass is undertaken for sweep angles of -35 degrees to 0 degrees and Mach numbers from 0.7 to 0.9. A subset of lamination parameters (LPs) and the number of laminate plies in each pre-defined direction (restricted to0, +/-45 and 90 degrees}) serve as design variables. A bi-level hybrid optimization approach is employed, making use of a genetic algorithm (GA) and a subsequent gradient-based optimizer. Constraints are implemented to match lift requirements and prevent aeroelastic divergence, excessive deformations, airfoil stalling and structural failure. A permutation GA is then used to match specific composite ply stacking sequences to the optimum design variables with a limited number of manufacturing constraints considered for demonstration purposes. The optimization results in positive bend-twist coupling and a reduced structural mass. Results are compared to an uncoupled reference wing with quasi-isotropic layups and with panel thickness alone the design variables. For a typical geometry and a forward sweep of -25 degrees at Mach 0.7, a wingbox mass reduction of 13% was achieved.
de boer GN, Gao L, Hewson RW, et al., 2016, A multiscale method for optimising surface topography in elastohydrodynamic lubrication (EHL) using metamodels, Structural and Multidisciplinary Optimization, Vol: 54, Pages: 483-497, ISSN: 1615-1488
The frictional performance of a bearing is ofsignificant interest in any mechanical system wherethere are lubricated surfaces under load and in relativemotion. Surface topography plays a major role in determiningthe coefficient of friction for the bearing becausethe size of the fluid film and topography are of a comparableorder. The problem of optimising topography forsuch a system is complicated by the separation in scalesbetween the size of the lubricated domain and that ofthe topography, which is of at least one order of magnitudeor more smaller. This paper introduces amultiscale method for optimising the small scale topographyfor improved frictional performance of the largescale bearing. The approach fully couples theelastohydrodynamic lubrication at both scales betweenpressure generated in the lubricant and deformation ofthe bounding surfaces. Homogenised small scale data isused to inform the large scale model and is representedusing Moving Least Squares metamodels calibrated bycross validation. An optimal topography for a minimumcoefficient of friction for the bearing is identified andcomparisons made of local minima in the response,where very different topographies with similar frictionalperformance are observed. Comparisons of the optimaltopography with the smooth surface model demonstratedthe complexity of capturing the non-linear effect of topographyand the necessity of the multiscale method incapturing this. Deviations from the smooth surface modelwere quantified by the metamodel coefficients andshowed how topographies with a similar frictional performancehave very different characteristics.
Chakladar ND, Gao L, Hewson R, et al., 2016, Evolution of wear and roughness in mixed lubrication regime, The 6th World Tribology Conference
Gao L, Dowson D, Hewson RW, 2016, A numerical study of non-Newtonian transient elastohydrodynamic lubrication of metal-on-metal hip prostheses, Tribology International, Vol: 93, Pages: 486-494, ISSN: 1879-2464
This paper presents a comprehensive numerical study of transient non-Newtonian elastohydrodynamic lubrication of metal-on-metal hip prosthesis subjected to two different gait cycles. The shear-thinning property of the synovial fluid was found to have a significant effect on the lubricating film, in terms of both the magnitude and location of the minimum film thickness, and more generally the film thickness distribution. A range of clearances between the acetabular cup and femoral head was investigated and the shear-thinning effect was more pronounced in the hip replacements with smaller clearances.
Gao L, Dowson D, Hewson RW, 2015, Predictive wear modelling of the articulating metal-on-metal hip replacements, Journal of Biomedical Materials Research - Part B Applied Biomaterials, Vol: 105, Pages: 497-506, ISSN: 1552-4981
The lubrication regime in which artificial hip joints operate adds complexity to the prediction of wear, as the joint operates in both the full fluid film regime, specifically the elastohydrodynamic lubrication (EHL) regime, and the mixed or boundary lubrication regimes, where contact between the bearing surfaces results in wear. In this work a wear model is developed which considers lubrication for the first time via a transient EHL model of metal‐on‐metal hip replacements. This is a framework to investigate how the change in film thickness influences the wear, which is important to further investigation of the complex wear procedure, including tribo‐corrosion, in the lubricated hip implants. The wear model applied here is based on the work of Sharif et al. who adapted the Archard wear law by making the wear rate a function of a relative film thickness nominalized by surface roughness for examining wear of industrial gears. In this work the gait cycle employed in hip simulator tests is computationally investigated and wear is predicted for two sizes of metal-on‐metal total hip replacements. The wear results qualitatively predict the typical wear curve obtained from experimental hip simulator tests, with an initial ‘running‐in period’ before a lower wear rate is reached. The shape of the wear scar has been simulated on both the acetabular cup and femoral head bearing surfaces.
Taherkhani AR, de Boer GN, Gaskell PH, et al., 2015, Aerodynamic drag reduction of emergency response vehicles, Advances in Automobile Engineering, Vol: 4, ISSN: 2167-7670
This paper presents the first experimental and computational investigation into the aerodynamics of emergency response vehicles and focuses on reducing the additional drag that results from the customary practice of adding light-bars onto the vehicles’ roofs. A series of wind tunnel experiments demonstrate the significant increase in drag that results from the light bars and show these can be minimized by reducing the flow separation caused by them. Simple potential improvements in the aerodynamic design of the light bars are investigated by combining Computational Fluid Dynamics (CFD) with Design of Experiments and metamodelling methods. An aerofoil-based roof design concept is shown to reduce the overall aerodynamic drag by up to 20% and an analysis of its effect on overall fuel consumption indicates that it offers a significant opportunity for improving the fuel economy and reducing emissions from emergency response vehicles. These benefits are now being realised by the UK’s ambulance services.
Raske N, Hewson RW, Kapur N, et al., 2015, A predictive model for discrete cell gravure roll coating, 11th European Coating Symposium 2015
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