Philippe Duffour - Academia.edu (original) (raw)

Papers by Philippe Duffour

The objectives of this study were to investigate the relationship between the ultrasonic vibratio... more The objectives of this study were to investigate the relationship between the ultrasonic vibrations of a specimen, with a vertical surface crack, and the thermal response. The heat required to be liberated by the defect to obtain a detectable surface temperature rise was modelled analytically for energy liberated only at the crack tip and over the sides of a crack. Experiments were carried out on a nickel based superalloy plate with a vertical surface crack. The strain around the defect and thermal image were measured simultaneously. Parameters were varied to change the strain to enable a correlation between the strain and surface temperature rise. From this the energy released from the defect was estimated using an optimisation algorithm.

CRC Press eBooks, Aug 15, 2013

Designing structures with minimal environmental impact is emerging as a serious concern in the co... more Designing structures with minimal environmental impact is emerging as a serious concern in the construction sector. Conventional structural design practice involves designing first for strength, followed by secondary checks on deflections and other serviceability limits states. If these limits are exceeded, the conventional solution has been to add material to increase stiffness. When the design is governed by unpredictable events such as fluctuating loads, strong wind storms or earthquakes, the structure is effectively overdesigned for most of its working life. This paper presents a methodology to design adaptive structures that minimize the whole life energy consumption. The methodology is illustrated on plane pin-jointed trusses, both determinate and indeterminate. Strategically placing actuators allow the internal flow of forces to be homogenized and displacements to be controlled. The actuators only start working when the loads reach a certain threshold. Below this threshold, the structure resists loads mainly passively thereby limiting significantly the operational energy used. It was found that both indeterminate and determinate topologies bring substantial energy savings up to 70% of the total energy. Research and Applications in Structural Engineering, Mechanics and Computation-Zingoni (Ed.

Nucleation and Atmospheric Aerosols, 2007

ABSTRACT In this study a set of cracked steel beams was tested using the thermosonic method of ND... more ABSTRACT In this study a set of cracked steel beams was tested using the thermosonic method of NDT. Each beam was instrumented with a strain gage for the measurement of the vibrations excited. The heat released at the crack was calculated from the vibration records and an experimental estimate of the crack damping. Thus, the local surface temperature rise could be predicted and compared with the infrared camera measurements. The relationship between crack size and level of vibration required for reliable crack detection was discussed.

Renewable Energy, Nov 1, 2021

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A new and innovative master's level course has been developed at University College London (UCL) ... more A new and innovative master's level course has been developed at University College London (UCL) offering a systematic grounding in the behaviour of structural systems. Students are taught how to tackle complicated structural problems using a combination of simpler structural subsystems. In so doing, students are encouraged to examine the underlying physics of the problems and how this might influence the overall structural form and composition of structural systems. To enable confident and intuitive decision making, structural systems are first presented in qualitative terms. Once essential decisions on the composition of the structural system have been made, students are then shown how to provide initial estimates of element sizes and cost comparisons through approximate structural analysis techniques. Furthermore, students are then made aware of how these methods can be used at later project stages to judge the accuracy of solutions to more detailed, computer-aided approaches. The course is organised around a set of structural subsystems. Each is taught in terms of their behaviour, application, methods of approximate analysis, and preliminary design procedures.

Conference proceedings of the Society for Experimental Mechanics, Sep 23, 2020

With the advent of lighter floor systems and the drive to lower the embodied energy in structures... more With the advent of lighter floor systems and the drive to lower the embodied energy in structures, meeting vibration serviceability requirements for floor systems can be a challenge. A number of guidelines have been published in the UK over the past 15 years (e.g. SCI P345, CCIP-016) that have been helpful in providing a consistent methodology. Although these can differ slightly in the detail, they are essentially based around the concept of response factor. Other methodologies have been proposed in continental Europe. The better documented one is the One-Step RMS90 (OS-RMS90) developed as part of the Technical Steel Research of the European Commission (e.g. EUR 24084 EN). Although similar in spirit, OS-RMS90 differs from the UK methodologies in key aspects which can be categorized along three strands (1) Footfall force definition, (2) Floor modelling and estimation of the dynamic response of the floor; (3) Acceptability criteria. This paper proposes a detailed comparison between these methodologies in each of these aspects and concludes by applying the methods on a composite floor case study. Based on this comparison, the merits of various quick assessments are evaluated.

Energy Conversion and Management, Aug 1, 2019

Solar chimney power plants are very large structures with the potential to generate significant a... more Solar chimney power plants are very large structures with the potential to generate significant amounts of electricity. Plant dimensions such as the collector diameter and chimney height and radius are important in determining system performance. The objective of this study is to identify the key parameters that drive performance. Performance was assessed in terms of power output and power output per cost. Using a detailed thermodynamic model, the plant power output was predicted for a wide range of design and operational parameters. It was found that the optimum pressure drop ratio depends on the collector and chimney radius, but not on chimney height, ambient temperature or insolation. The dimensions of the main components must be well matched to achieve best performance. Chimney radii of up to 200 m are essential to achieve maximum power generation up to 900 MW. Optimum power output exists for variation of the collector and chimney radius. However, increasing the chimney height always results in increased power generation. The physical phenomena underpinning high-performing configurations were highlighted. Power and efficiency increase with increased plant dimensions, but technological limits exist for the chimney height. A simple but robust cost model was introduced to identify optimum configurations in terms of power output per cost. Different relative costs between collector and chimney the optimum dimensions were selected. It was concluded that several smaller plants with collector radius about 3000 m are advantageous over one larger plant. Taller chimneys are economically beneficial until the specific chimney costs increase more than quadratically with height. Keywords Solar updraft power plant, Solar thermal chimney, Thermodynamic model, Cost model Nomenclature Chimney Parameters ℎ Chimney radius ℎ Chimney height Δ Pressure drop due to friction in the chimney Power generated Collectors Parameters Collector radius ℎ Collector inlet height ℎ Collector outlet height ℎ Collector height at distance r from inlet Canopy absorptivity Canopy transmissivity Collector annular section plan area Darcy Friction Factor Turbine Parameters x Turbine pressure drop ratio Turbine efficiency Δ Pressure drop available to the turbine Ground Properties Ground absorptivity Ground surface temperature Constant ground temperature at −5 underground Working fluid Properties Air specific heat capacity ̇ Mass flow rate Fluid density Fluid velocity Fluid pressure Friction shear stress or Temperature of the fluid Ambient Properties I Insolation Ambient air temperature at ground level Sky temperature Ambient air pressure at ground level Ambient air density Heat flows

Zenodo (CERN European Organization for Nuclear Research), Dec 1, 2011

The design methodology described in this paper takes a substantial shift from conventional method... more The design methodology described in this paper takes a substantial shift from conventional methods. Traditionally sizing is based on the worst expected load scenario. By contrast to this conventional passive approach the method presented here replaces passive member strategically with active elements (actuators) which are only activated when the loads reach a certain threshold. The structure can withstand low level of loads passively. Above the threshold, actuation comes in to allow the structure to cope with high but rare loading scenarios. Active control introduces operational energy consumption in addition to the energy embodied in a passive design. In this paper we use this dual design to minimize the overall energy required by the structures. This methodology has been used on a simple truss structure and it was showed that it allows significant weight saving compared to conventional passive design. We extend the application of the methodology to a more complex 3D structure. It is confirmed that an optimum activation threshold exists that leads to design that minimises the total energy of the structure. Compared to an optimised passive design we show that the total energy saving is 10-fold.

Journal of Vibration and Control, Dec 5, 2013

This paper investigates the performance of magnetorheological (MR) dampers to mitigate the effect... more This paper investigates the performance of magnetorheological (MR) dampers to mitigate the effect of earthquake loading on civil engineering structures. MR dampers are semi-active fluid dampers containing an MR fluid. They can generate large controllable damping forces by tuning the magnetic field applied to the fluid, which changes its viscosity. Their fast response time and low electric power requirements make them attractive for potential applications in earthquake engineering. In this study the behavior of a prototype MR damper was simulated computationally using the Bouc-Wen model. A new clipped-optimal control strategy, the improved clipped-optimal algorithm (ICA) control model, was developed and aims to reduce the acceleration response of a structure. It was implemented for a scaled three-story building in Simulink. The structural response to seven earthquakes was simulated and compared with two other established control algorithms as well as passive damping. The ICA control algorithm generated the largest reduction in acceleration and displacement response while keeping low the electric power requirements and the force generated by the damper.

Journal of Sound and Vibration, Dec 1, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Journal of Sound and Vibration, Mar 1, 2004

In a companion paper, a theory was presented which allows the study of the linear stability of a ... more In a companion paper, a theory was presented which allows the study of the linear stability of a class of systems consisting of two subsystems coupled through a frictional contact point. A stability criterion in terms of transfer functions was derived and used to simulate the behaviour of generic systems. In the present paper, this approach was pursued and generalized by relaxing in turn certain of the assumptions made earlier. By doing this, it is possible to catalogue systematically all the routes to instability conceivable within the scope of linearity for the class of systems considered. The additional routes to instability identified are as follows. First, the contact point was made compliant by adding a linear contact spring at the interface between the two subsystems. This feature proved to have a significant influence on stability when the contact spring stiffness takes values of the same order of magnitude or lower than that of the average structural stiffness of the system. Second, a route to instability is possible if the system structural damping possesses a slight non-proportional component. The last and most elaborate extension consisted in allowing the coefficient of friction to vary linearly with the sliding speed. Simulation results suggest that a coefficient falling with increasing sliding speed can destabilize an otherwise stable system or can make it even more unstable. In accordance with previous results, a coefficient of friction rising with the sliding speed tends to make a system more stable, although this is not systematic. The theory presented here allows these possible routes to instability to be combined, so that data from vibration measurements or modelling and from frictional measurements can be used directly to predict the region of instability in parameter space.

Journal of Sound and Vibration, Mar 1, 2023

Nucleation and Atmospheric Aerosols, 2005

ABSTRACT Vibro-modulation involves monitoring the amplitude modulation of an ultrasonic vibration... more ABSTRACT Vibro-modulation involves monitoring the amplitude modulation of an ultrasonic vibration field transmitted through a cracked specimen undergoing an additional low frequency structural vibration. If the specimen is undamaged the two vibration fields do not interact. This technique is a potential non-destructive testing method. A model is presented which accounts for this phenomenon quantitatively. Using receptance analysis and an Finite Element model, it is shown that the effect is strongly dependent on the carrier ultrasonic frequency and the system damping.

Journal of Sound and Vibration, Jul 1, 2007

In two earlier papers, a new formalism was derived which led to the prediction of instability for... more In two earlier papers, a new formalism was derived which led to the prediction of instability for two linear systems in sliding contact at a single point. In Duffour and Woodhouse [Instability of systems with a sliding point contact. Part 1: basic modelling 271 (2004) 365-390], predictions were obtained using a friction law featuring a constant coefficient of friction. This formalism was generalised in Duffour and Woodhouse [Instability of systems with a sliding point contact. Part 2: model extensions 271 (2004) 391-410] to include all possible linear routes to instability. This paper presents results from the experimental investigation carried out to test the validity of the theoretical results obtained in the first of the two papers. The rig is of the pin-on-disc type. An instrumented pin was specially designed so that the quantities necessary for the prediction could be measured. It emerged that incipient squeal frequencies observed experimentally could be predicted in 75% of the cases using the simplest formalism presented in Duffour and Woodhouse (2004). The presence of unpredicted squeal frequencies points towards the importance of other effects, such as the disc nominal rotation speed and the value of the normal preload. This study also reveals that the ever-changing nature of friction-induced noises can, to a good extent, be explained by the slight structural variations undergone by any mechanical system.

Solar Energy, May 1, 2016

Solar thermal chimneys (STCs) are renewable energy power plants that require large-scale deployme... more Solar thermal chimneys (STCs) are renewable energy power plants that require large-scale deployment to be economically competitive. This paper presents a steady-state analytical model developed to describe accurately the thermodynamics of the solar collector. The impact of different collector canopy designs on the performance is assessed. Results show that the height of the canopy has a significant effect on plant performance and that the canopy must be sufficiently high at the junction with the chimney to ensure maximum kinetic energy in the flow at the chimney inlet can be reached. A new collector profile with a partially sloped canopy is proposed. It was found to perform at similar levels of maximum power output to the best-performing existing canopy designs, and to be robust under varying environmental conditions. For ease of construction and reduction of associated costs this canopy can be built in stepped annular flat sections with only a minor loss in performance.

Social Science Research Network, 2022

Wind Energy, Jun 28, 2018

Vibration damping in offshore wind turbines is a key parameter to predict reliably the dynamic re... more Vibration damping in offshore wind turbines is a key parameter to predict reliably the dynamic response and fatigue life of these systems. Damping in an OWT originates from different sources, mainly, aerodynamic, structural, hydrodynamic and soil dampings. The difficulties in identifying the individual contribution from each damping source has led to considerable uncertainty and variation in the values recommended. This paper proposes simplified but direct modelling approaches to quantify the different damping contributions from the aerodynamic, hydrodynamic and soil interactions. Results from these models were systemically compared to published values and when appropriate with simulation results from the software package FAST. The range of values obtained for aerodynamic damping confirmed those available in the literature and Blade Element Modelling theory was shown to provide good results relatively efficiently. The influence of couplings between fore-aft and side-side directions on the aerodynamic damping contribution was highlighted. The modelling of hydrodynamic damping showed that this damping is much smaller than usually recommended and could be safely ignored for OWTs. Soil damping strongly depends on the soil specific nonlinear behaviour.

Journal of the Acoustical Society of America, Mar 1, 2006

One implementation of the vibro-modulation technique involves monitoring the amplitude modulation... more One implementation of the vibro-modulation technique involves monitoring the amplitude modulation of an ultrasonic vibration field transmitted through a cracked specimen undergoing an additional low frequency structural vibration. If the specimen is undamaged and appropriately supported, the two vibration fields do not interact. This phenomenon could be used as the basis for a nondestructive testing technique. In this paper, the sensitivity of the technique is investigated systematically on a set of mild steel beams with cracks of different sizes and shapes. A damage index was measured for each crack. The correlation obtained between the crack size and the strength of the modulation is fairly poor. The technique proved extremely sensitive to the initial state of opening and closing of the crack and to the setup due to the modulating effects of contacts between the specimens and the supports. A simple model is proposed which explains the main features observed and approximately predicts the level of sideband obtained experimentally.

Engineering Structures, Jul 1, 2018

Using a previously developed design methodology it was shown that optimal material distribution i... more Using a previously developed design methodology it was shown that optimal material distribution in combination with strategic integration of the actuation system lead to significant whole-life energy savings when the design is governed by rare but strong loading events. The whole-life energy of the structure is made of an embodied part in the material and an operational part for structural adaptation. Instead of using more material to cope with the effect of loads, the actuation system redirects the internal load-path to homogenise the stresses and change the shape of the structure to keep deflections within limits. This paper presents a systematic exploration of the domain in which adaptive two-dimensional pin-jointed structures are beneficial in terms of whole-life energy and monetary costs savings. Two case studies are considered: a vertical cantilever truss representative of a multi-storey building supported by an exoskeleton structure and a simply supported truss beam which is part of a roof system. This exploration takes five directions studying the influence of: (1) the structural topology (2) the characteristics of the load probability distribution (3) the ratio of live load over dead load (4) the aspect ratio of the structure (e.g. height-to-depth) (5) the material energy intensity factor. Results from the main five strands are combined with those from the monetary cost analysis to identify an optimal region where adaptive structures are most effective in terms of both energy and monetary savings. It was found that the optimal region is broadly that of stiffness-governed structures. For the cantilever case, the optimal region covers most of the application domain and it is not very sensitive to either live-to-dead-load or height-to-depth ratios thus showing a wide range of applicability, including ordinary loading scenarios and relatively deep structures.

Journal of Sound and Vibration, Mar 1, 2004

In a companion paper, a theory was presented which allows the study of the linear stability of a ... more In a companion paper, a theory was presented which allows the study of the linear stability of a class of systems consisting of two subsystems coupled through a frictional contact point. A stability criterion in terms of transfer functions was derived and used to simulate the behaviour of generic systems. In the present paper, this approach was pursued and generalized by relaxing in turn certain of the assumptions made earlier. By doing this, it is possible to catalogue systematically all the routes to instability conceivable within the scope of linearity for the class of systems considered. The additional routes to instability identified are as follows. First, the contact point was made compliant by adding a linear contact spring at the interface between the two subsystems. This feature proved to have a significant influence on stability when the contact spring stiffness takes values of the same order of magnitude or lower than that of the average structural stiffness of the system. Second, a route to instability is possible if the system structural damping possesses a slight non-proportional component. The last and most elaborate extension consisted in allowing the coefficient of friction to vary linearly with the sliding speed. Simulation results suggest that a coefficient falling with increasing sliding speed can destabilize an otherwise stable system or can make it even more unstable. In accordance with previous results, a coefficient of friction rising with the sliding speed tends to make a system more stable, although this is not systematic. The theory presented here allows these possible routes to instability to be combined, so that data from vibration measurements or modelling and from frictional measurements can be used directly to predict the region of instability in parameter space.

The objectives of this study were to investigate the relationship between the ultrasonic vibratio... more The objectives of this study were to investigate the relationship between the ultrasonic vibrations of a specimen, with a vertical surface crack, and the thermal response. The heat required to be liberated by the defect to obtain a detectable surface temperature rise was modelled analytically for energy liberated only at the crack tip and over the sides of a crack. Experiments were carried out on a nickel based superalloy plate with a vertical surface crack. The strain around the defect and thermal image were measured simultaneously. Parameters were varied to change the strain to enable a correlation between the strain and surface temperature rise. From this the energy released from the defect was estimated using an optimisation algorithm.

CRC Press eBooks, Aug 15, 2013

Designing structures with minimal environmental impact is emerging as a serious concern in the co... more Designing structures with minimal environmental impact is emerging as a serious concern in the construction sector. Conventional structural design practice involves designing first for strength, followed by secondary checks on deflections and other serviceability limits states. If these limits are exceeded, the conventional solution has been to add material to increase stiffness. When the design is governed by unpredictable events such as fluctuating loads, strong wind storms or earthquakes, the structure is effectively overdesigned for most of its working life. This paper presents a methodology to design adaptive structures that minimize the whole life energy consumption. The methodology is illustrated on plane pin-jointed trusses, both determinate and indeterminate. Strategically placing actuators allow the internal flow of forces to be homogenized and displacements to be controlled. The actuators only start working when the loads reach a certain threshold. Below this threshold, the structure resists loads mainly passively thereby limiting significantly the operational energy used. It was found that both indeterminate and determinate topologies bring substantial energy savings up to 70% of the total energy. Research and Applications in Structural Engineering, Mechanics and Computation-Zingoni (Ed.

Nucleation and Atmospheric Aerosols, 2007

ABSTRACT In this study a set of cracked steel beams was tested using the thermosonic method of ND... more ABSTRACT In this study a set of cracked steel beams was tested using the thermosonic method of NDT. Each beam was instrumented with a strain gage for the measurement of the vibrations excited. The heat released at the crack was calculated from the vibration records and an experimental estimate of the crack damping. Thus, the local surface temperature rise could be predicted and compared with the infrared camera measurements. The relationship between crack size and level of vibration required for reliable crack detection was discussed.

Renewable Energy, Nov 1, 2021

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A new and innovative master's level course has been developed at University College London (UCL) ... more A new and innovative master's level course has been developed at University College London (UCL) offering a systematic grounding in the behaviour of structural systems. Students are taught how to tackle complicated structural problems using a combination of simpler structural subsystems. In so doing, students are encouraged to examine the underlying physics of the problems and how this might influence the overall structural form and composition of structural systems. To enable confident and intuitive decision making, structural systems are first presented in qualitative terms. Once essential decisions on the composition of the structural system have been made, students are then shown how to provide initial estimates of element sizes and cost comparisons through approximate structural analysis techniques. Furthermore, students are then made aware of how these methods can be used at later project stages to judge the accuracy of solutions to more detailed, computer-aided approaches. The course is organised around a set of structural subsystems. Each is taught in terms of their behaviour, application, methods of approximate analysis, and preliminary design procedures.

Conference proceedings of the Society for Experimental Mechanics, Sep 23, 2020

With the advent of lighter floor systems and the drive to lower the embodied energy in structures... more With the advent of lighter floor systems and the drive to lower the embodied energy in structures, meeting vibration serviceability requirements for floor systems can be a challenge. A number of guidelines have been published in the UK over the past 15 years (e.g. SCI P345, CCIP-016) that have been helpful in providing a consistent methodology. Although these can differ slightly in the detail, they are essentially based around the concept of response factor. Other methodologies have been proposed in continental Europe. The better documented one is the One-Step RMS90 (OS-RMS90) developed as part of the Technical Steel Research of the European Commission (e.g. EUR 24084 EN). Although similar in spirit, OS-RMS90 differs from the UK methodologies in key aspects which can be categorized along three strands (1) Footfall force definition, (2) Floor modelling and estimation of the dynamic response of the floor; (3) Acceptability criteria. This paper proposes a detailed comparison between these methodologies in each of these aspects and concludes by applying the methods on a composite floor case study. Based on this comparison, the merits of various quick assessments are evaluated.

Energy Conversion and Management, Aug 1, 2019

Solar chimney power plants are very large structures with the potential to generate significant a... more Solar chimney power plants are very large structures with the potential to generate significant amounts of electricity. Plant dimensions such as the collector diameter and chimney height and radius are important in determining system performance. The objective of this study is to identify the key parameters that drive performance. Performance was assessed in terms of power output and power output per cost. Using a detailed thermodynamic model, the plant power output was predicted for a wide range of design and operational parameters. It was found that the optimum pressure drop ratio depends on the collector and chimney radius, but not on chimney height, ambient temperature or insolation. The dimensions of the main components must be well matched to achieve best performance. Chimney radii of up to 200 m are essential to achieve maximum power generation up to 900 MW. Optimum power output exists for variation of the collector and chimney radius. However, increasing the chimney height always results in increased power generation. The physical phenomena underpinning high-performing configurations were highlighted. Power and efficiency increase with increased plant dimensions, but technological limits exist for the chimney height. A simple but robust cost model was introduced to identify optimum configurations in terms of power output per cost. Different relative costs between collector and chimney the optimum dimensions were selected. It was concluded that several smaller plants with collector radius about 3000 m are advantageous over one larger plant. Taller chimneys are economically beneficial until the specific chimney costs increase more than quadratically with height. Keywords Solar updraft power plant, Solar thermal chimney, Thermodynamic model, Cost model Nomenclature Chimney Parameters ℎ Chimney radius ℎ Chimney height Δ Pressure drop due to friction in the chimney Power generated Collectors Parameters Collector radius ℎ Collector inlet height ℎ Collector outlet height ℎ Collector height at distance r from inlet Canopy absorptivity Canopy transmissivity Collector annular section plan area Darcy Friction Factor Turbine Parameters x Turbine pressure drop ratio Turbine efficiency Δ Pressure drop available to the turbine Ground Properties Ground absorptivity Ground surface temperature Constant ground temperature at −5 underground Working fluid Properties Air specific heat capacity ̇ Mass flow rate Fluid density Fluid velocity Fluid pressure Friction shear stress or Temperature of the fluid Ambient Properties I Insolation Ambient air temperature at ground level Sky temperature Ambient air pressure at ground level Ambient air density Heat flows

Zenodo (CERN European Organization for Nuclear Research), Dec 1, 2011

The design methodology described in this paper takes a substantial shift from conventional method... more The design methodology described in this paper takes a substantial shift from conventional methods. Traditionally sizing is based on the worst expected load scenario. By contrast to this conventional passive approach the method presented here replaces passive member strategically with active elements (actuators) which are only activated when the loads reach a certain threshold. The structure can withstand low level of loads passively. Above the threshold, actuation comes in to allow the structure to cope with high but rare loading scenarios. Active control introduces operational energy consumption in addition to the energy embodied in a passive design. In this paper we use this dual design to minimize the overall energy required by the structures. This methodology has been used on a simple truss structure and it was showed that it allows significant weight saving compared to conventional passive design. We extend the application of the methodology to a more complex 3D structure. It is confirmed that an optimum activation threshold exists that leads to design that minimises the total energy of the structure. Compared to an optimised passive design we show that the total energy saving is 10-fold.

Journal of Vibration and Control, Dec 5, 2013

This paper investigates the performance of magnetorheological (MR) dampers to mitigate the effect... more This paper investigates the performance of magnetorheological (MR) dampers to mitigate the effect of earthquake loading on civil engineering structures. MR dampers are semi-active fluid dampers containing an MR fluid. They can generate large controllable damping forces by tuning the magnetic field applied to the fluid, which changes its viscosity. Their fast response time and low electric power requirements make them attractive for potential applications in earthquake engineering. In this study the behavior of a prototype MR damper was simulated computationally using the Bouc-Wen model. A new clipped-optimal control strategy, the improved clipped-optimal algorithm (ICA) control model, was developed and aims to reduce the acceleration response of a structure. It was implemented for a scaled three-story building in Simulink. The structural response to seven earthquakes was simulated and compared with two other established control algorithms as well as passive damping. The ICA control algorithm generated the largest reduction in acceleration and displacement response while keeping low the electric power requirements and the force generated by the damper.

Journal of Sound and Vibration, Dec 1, 2020

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Journal of Sound and Vibration, Mar 1, 2004

In a companion paper, a theory was presented which allows the study of the linear stability of a ... more In a companion paper, a theory was presented which allows the study of the linear stability of a class of systems consisting of two subsystems coupled through a frictional contact point. A stability criterion in terms of transfer functions was derived and used to simulate the behaviour of generic systems. In the present paper, this approach was pursued and generalized by relaxing in turn certain of the assumptions made earlier. By doing this, it is possible to catalogue systematically all the routes to instability conceivable within the scope of linearity for the class of systems considered. The additional routes to instability identified are as follows. First, the contact point was made compliant by adding a linear contact spring at the interface between the two subsystems. This feature proved to have a significant influence on stability when the contact spring stiffness takes values of the same order of magnitude or lower than that of the average structural stiffness of the system. Second, a route to instability is possible if the system structural damping possesses a slight non-proportional component. The last and most elaborate extension consisted in allowing the coefficient of friction to vary linearly with the sliding speed. Simulation results suggest that a coefficient falling with increasing sliding speed can destabilize an otherwise stable system or can make it even more unstable. In accordance with previous results, a coefficient of friction rising with the sliding speed tends to make a system more stable, although this is not systematic. The theory presented here allows these possible routes to instability to be combined, so that data from vibration measurements or modelling and from frictional measurements can be used directly to predict the region of instability in parameter space.

Journal of Sound and Vibration, Mar 1, 2023

Nucleation and Atmospheric Aerosols, 2005

ABSTRACT Vibro-modulation involves monitoring the amplitude modulation of an ultrasonic vibration... more ABSTRACT Vibro-modulation involves monitoring the amplitude modulation of an ultrasonic vibration field transmitted through a cracked specimen undergoing an additional low frequency structural vibration. If the specimen is undamaged the two vibration fields do not interact. This technique is a potential non-destructive testing method. A model is presented which accounts for this phenomenon quantitatively. Using receptance analysis and an Finite Element model, it is shown that the effect is strongly dependent on the carrier ultrasonic frequency and the system damping.

Journal of Sound and Vibration, Jul 1, 2007

In two earlier papers, a new formalism was derived which led to the prediction of instability for... more In two earlier papers, a new formalism was derived which led to the prediction of instability for two linear systems in sliding contact at a single point. In Duffour and Woodhouse [Instability of systems with a sliding point contact. Part 1: basic modelling 271 (2004) 365-390], predictions were obtained using a friction law featuring a constant coefficient of friction. This formalism was generalised in Duffour and Woodhouse [Instability of systems with a sliding point contact. Part 2: model extensions 271 (2004) 391-410] to include all possible linear routes to instability. This paper presents results from the experimental investigation carried out to test the validity of the theoretical results obtained in the first of the two papers. The rig is of the pin-on-disc type. An instrumented pin was specially designed so that the quantities necessary for the prediction could be measured. It emerged that incipient squeal frequencies observed experimentally could be predicted in 75% of the cases using the simplest formalism presented in Duffour and Woodhouse (2004). The presence of unpredicted squeal frequencies points towards the importance of other effects, such as the disc nominal rotation speed and the value of the normal preload. This study also reveals that the ever-changing nature of friction-induced noises can, to a good extent, be explained by the slight structural variations undergone by any mechanical system.

Solar Energy, May 1, 2016

Solar thermal chimneys (STCs) are renewable energy power plants that require large-scale deployme... more Solar thermal chimneys (STCs) are renewable energy power plants that require large-scale deployment to be economically competitive. This paper presents a steady-state analytical model developed to describe accurately the thermodynamics of the solar collector. The impact of different collector canopy designs on the performance is assessed. Results show that the height of the canopy has a significant effect on plant performance and that the canopy must be sufficiently high at the junction with the chimney to ensure maximum kinetic energy in the flow at the chimney inlet can be reached. A new collector profile with a partially sloped canopy is proposed. It was found to perform at similar levels of maximum power output to the best-performing existing canopy designs, and to be robust under varying environmental conditions. For ease of construction and reduction of associated costs this canopy can be built in stepped annular flat sections with only a minor loss in performance.

Social Science Research Network, 2022

Wind Energy, Jun 28, 2018

Vibration damping in offshore wind turbines is a key parameter to predict reliably the dynamic re... more Vibration damping in offshore wind turbines is a key parameter to predict reliably the dynamic response and fatigue life of these systems. Damping in an OWT originates from different sources, mainly, aerodynamic, structural, hydrodynamic and soil dampings. The difficulties in identifying the individual contribution from each damping source has led to considerable uncertainty and variation in the values recommended. This paper proposes simplified but direct modelling approaches to quantify the different damping contributions from the aerodynamic, hydrodynamic and soil interactions. Results from these models were systemically compared to published values and when appropriate with simulation results from the software package FAST. The range of values obtained for aerodynamic damping confirmed those available in the literature and Blade Element Modelling theory was shown to provide good results relatively efficiently. The influence of couplings between fore-aft and side-side directions on the aerodynamic damping contribution was highlighted. The modelling of hydrodynamic damping showed that this damping is much smaller than usually recommended and could be safely ignored for OWTs. Soil damping strongly depends on the soil specific nonlinear behaviour.

Journal of the Acoustical Society of America, Mar 1, 2006

One implementation of the vibro-modulation technique involves monitoring the amplitude modulation... more One implementation of the vibro-modulation technique involves monitoring the amplitude modulation of an ultrasonic vibration field transmitted through a cracked specimen undergoing an additional low frequency structural vibration. If the specimen is undamaged and appropriately supported, the two vibration fields do not interact. This phenomenon could be used as the basis for a nondestructive testing technique. In this paper, the sensitivity of the technique is investigated systematically on a set of mild steel beams with cracks of different sizes and shapes. A damage index was measured for each crack. The correlation obtained between the crack size and the strength of the modulation is fairly poor. The technique proved extremely sensitive to the initial state of opening and closing of the crack and to the setup due to the modulating effects of contacts between the specimens and the supports. A simple model is proposed which explains the main features observed and approximately predicts the level of sideband obtained experimentally.

Engineering Structures, Jul 1, 2018

Using a previously developed design methodology it was shown that optimal material distribution i... more Using a previously developed design methodology it was shown that optimal material distribution in combination with strategic integration of the actuation system lead to significant whole-life energy savings when the design is governed by rare but strong loading events. The whole-life energy of the structure is made of an embodied part in the material and an operational part for structural adaptation. Instead of using more material to cope with the effect of loads, the actuation system redirects the internal load-path to homogenise the stresses and change the shape of the structure to keep deflections within limits. This paper presents a systematic exploration of the domain in which adaptive two-dimensional pin-jointed structures are beneficial in terms of whole-life energy and monetary costs savings. Two case studies are considered: a vertical cantilever truss representative of a multi-storey building supported by an exoskeleton structure and a simply supported truss beam which is part of a roof system. This exploration takes five directions studying the influence of: (1) the structural topology (2) the characteristics of the load probability distribution (3) the ratio of live load over dead load (4) the aspect ratio of the structure (e.g. height-to-depth) (5) the material energy intensity factor. Results from the main five strands are combined with those from the monetary cost analysis to identify an optimal region where adaptive structures are most effective in terms of both energy and monetary savings. It was found that the optimal region is broadly that of stiffness-governed structures. For the cantilever case, the optimal region covers most of the application domain and it is not very sensitive to either live-to-dead-load or height-to-depth ratios thus showing a wide range of applicability, including ordinary loading scenarios and relatively deep structures.

Journal of Sound and Vibration, Mar 1, 2004

In a companion paper, a theory was presented which allows the study of the linear stability of a ... more In a companion paper, a theory was presented which allows the study of the linear stability of a class of systems consisting of two subsystems coupled through a frictional contact point. A stability criterion in terms of transfer functions was derived and used to simulate the behaviour of generic systems. In the present paper, this approach was pursued and generalized by relaxing in turn certain of the assumptions made earlier. By doing this, it is possible to catalogue systematically all the routes to instability conceivable within the scope of linearity for the class of systems considered. The additional routes to instability identified are as follows. First, the contact point was made compliant by adding a linear contact spring at the interface between the two subsystems. This feature proved to have a significant influence on stability when the contact spring stiffness takes values of the same order of magnitude or lower than that of the average structural stiffness of the system. Second, a route to instability is possible if the system structural damping possesses a slight non-proportional component. The last and most elaborate extension consisted in allowing the coefficient of friction to vary linearly with the sliding speed. Simulation results suggest that a coefficient falling with increasing sliding speed can destabilize an otherwise stable system or can make it even more unstable. In accordance with previous results, a coefficient of friction rising with the sliding speed tends to make a system more stable, although this is not systematic. The theory presented here allows these possible routes to instability to be combined, so that data from vibration measurements or modelling and from frictional measurements can be used directly to predict the region of instability in parameter space.