Edward Dintwa - Academia.edu (original) (raw)

Papers by Edward Dintwa

Research paper thumbnail of Stress intensity factor computation of inclined cracked tension plate using XFEM

One of the major successes in the field of Linear Elastic Fracture Mechanics (LEFM) is the ground... more One of the major successes in the field of Linear Elastic Fracture Mechanics (LEFM) is the groundwork of the stress intensity factor (SIF) computation. The approaches used to carry out SIF values may be analytical, semi-analytical, experimental or numerical. Each one of the above methods has its own benefits however the use of numerical solutions has become the most frequent and popular. Numerous schemes for the numerical computation of SIF have been developed, the J-integral method being the most popular one. In this article we examine the SIFs of an edge cracked two dimensional (2-D) steel plate subjected to tensile loading. Extended finite element (XFEM) computational scheme has been employed to estimate the values of SIF. The SIF values of cracks with different lengths and inclination angles (different configurations) have been examined by utilizing the domain based interaction integral approach. The effect of crack inclination and crack position on SIFs (KI and KII) has also be...

Research paper thumbnail of The discrete element method (DEM) to simulate fruit impact damage during transport and handling: Model building and validation of DEM to predict bruise damage of apples

Postharvest Biology and Technology, 2006

Research paper thumbnail of Development of a Cost-effective Design of a P-V Ventilated Greenhouse Solar Dryer for Commercial Preservation of Tomatoes in a Rural Setting

Advances in Technology Innovation, 2019

Commercial preservation of agro-produce by solar drying entails using large-scale dryer units tha... more Commercial preservation of agro-produce by solar drying entails using large-scale dryer units that are generally not affordable by potential users in a rural setting. The objective of this study was to design a photo-voltaic greenhouse solar drying unit with air convection powered by a photovoltaic system as part of a research study to develop a cost-effective commercial solar dryer for the preservation of tomatoes in a rural setting. The design methodology involved. First, the determination of thermophysical properties of local tomatoes and predicting its drying model. Second, determination of design parameters; design equations, design conditions and assumptions, and psychrometric analysis. Third, application of the design parameters under the climatic conditions of Botswana to produce the embodiment of design with the drawings and specifications for a solar hot air dryer unit of 2000 kg batch-load of wet tomatoes. This drying unit is integrated into a solar collector with a cost-...

Research paper thumbnail of Particle Based Simulations with Demeter++

Particle Based Simulation (PBS) methods, such as the Discrete Element Method (DEM), are the natur... more Particle Based Simulation (PBS) methods, such as the Discrete Element Method (DEM), are the natural choice for simulating phenomena related to granular media. DEM bares close resemblance to Molecular Dynamics (MD) and is ideally suited for solving problems with granular media. Both DEM and MD sum up the forces acting on the particles and integrate Newton's equations of motion over time. In doing so, the technique predicts the path of every particle in the ensemble as time proceeds as well as the forces it experiences. Other PBS methods, like Smooth Particle Hydrodynamics (SPH) or Dissipative Particle Dynamics (DPD) are suited for solving problems in the realm of continuum mechanics with fast moving boundaries. They treat the continuum materials as a collection of material particles. The Laboratory for Agro Machinery and Processing has built a thorough expertise in Particle Based Simulation methods and has developed a C++ software library, called DEMeter++. The reason of existen...

Research paper thumbnail of Development of accurate contact force models for use with Discrete Element Method (DEM) modelling of bulk fruit handling processes

ABSTRACT This thesis is primarily concerned with the development of accurate, simplified and vali... more ABSTRACT This thesis is primarily concerned with the development of accurate, simplified and validated contact force models for the discrete element modelling (DEM) of fruit bulk handling systems. The DEM is essentially a numerical technique to model a system of particles interacting with one another and with the system boundaries through collisions. The specific area of application envisaged is in postharvest agriculture, where DEM could be used in simulation of many unit operations with bulk fruit, such as harvesting, transportation, sorting, storage and processing. Such simulations would provide crucial information to aid equipment and process designs to optimise the systems or to limit physical damage to produce during machine handling, for instance. In a DEM model, the material behaviour at collision is captured by a contact force model relating the local deformation in the contact region to the contact force. Contact force models represent one of the crucial elements of a DEM model. They have a direct bearing on the accuracy with which the material behaviour can be captured by the model. In carrying out the main objective of this project, theoretical, experimental as well as numerical approaches were used in order to, among other things, identify, derive, validate and simplify the relevant contact models proposed. Using the elastic contact force models as bases, mathematical contact force models for viscoelastic spheres are derived, for normal contact, tangential contact as well as torsional contact. The derived model for torsional contact was experimentally validated using a rheometer setup. Still using the rheometer setup, parameters of the derived tangential contact force model were determined. These parameters are: the friction coefficient, the viscous constant and the static shear modulus. The finite element method was used to further investigate the normal and tangential contact force models for elastic spheres. Regarding normal contact, the validity of the Hertz contact model under conditions of frictional contact, non-Hertzian contact surfaces and finite strains was investigated. The effect of friction to the force-deformation behaviour of elastic spheres was found to be negligible. The same is the case for the effects from the non-flat contact surfaces that result when stiffer objects come into contact with the more compliant ones. On the other hand, the study of the effect of finite strains revealed a strong influence of the extent of deformation on the Hertz model. The error arising was found to increase quadratically with increase in the normal deflection. Regarding the tangential contact, because the relevant models currently available in literature are generally either too inaccurate or computationally too costly to be implemented in DEM simulations involving large numbers of particles, an alternative model was derived herein and proposed. The model, it is argued, is simple to implement and can have the same accuracy as the Mindlin-Deresiewicz model. Because the proposed model does not include many different conditions and loading scenarios, it is argued that it will be much more convenient and less costly, computationally. By numerically filtering out all the vibration modes in the transient dynamic analyses of normal collisions of elastic spheres using FEA, an estimation of the relative quantities of the kinetic energy involved in the propagation of elastic waves in the colliding bodies could be made. The proportions varied according to the impact velocity from very low (less 4 %) to substantial proportions of up to 40 %. The relationship with the impact velocity was estimated to be proportional to a power 1/3 of the impact velocity. While this observation does not prescribe the amount of energy dissipation that can occur, it is extremely important because it provides an indication of the quantities of energy that can be available as elastic waves energy during collisions, and which potentially can be lost under some conditions. A significant amount of translational energy loss was found to occur for purely elastic collisions through absorption of energy as elastic waves, depending on the material elastic properties. The restitution coefficients for the purely elastic collisions depend on the impact velocity. Eigenvalue analysis of apple fruit models (based on Jonagold apple properties) yielded frequency modes in the ranges from 87.3 Hz to 849.4 Hz. These results were in good agreement with the ranges of values found in literature. The transient analysis of the viscoelastic collisions of apples, using the time dependent properties obtained from literature revealed no viscous response by the fruits. This is explained by the fact that the time constants reported on, and obtained from stress relaxation experiments are much higher than the typical collision times of the fruits. Because pendulum experiments on apple fruits have revealed the existence of viscous response in apples for these…

Research paper thumbnail of Stress intensity factor computation of inclined cracked tension plate using XFEM

One of the major successes in the field of Linear Elastic Fracture Mechanics (LEFM) is the ground... more One of the major successes in the field of Linear Elastic Fracture Mechanics (LEFM) is the groundwork of the stress intensity factor (SIF) computation. The approaches used to carry out SIF values may be analytical, semi-analytical, experimental or numerical. Each one of the above methods has its own benefits however the use of numerical solutions has become the most frequent and popular. Numerous schemes for the numerical computation of SIF have been developed, the J-integral method being the most popular one. In this article we examine the SIFs of an edge cracked two dimensional (2-D) steel plate subjected to tensile loading. Extended finite element (XFEM) computational scheme has been employed to estimate the values of SIF. The SIF values of cracks with different lengths and inclination angles (different configurations) have been examined by utilizing the domain based interaction integral approach. The effect of crack inclination and crack position on SIFs (KI and KII) has also be...

Research paper thumbnail of Design and Investigation of a Low Cost & Portable Thermal Energy Storage System for Domestic Applications

Environment and Water Resource Management / 837: Health Informatics / 838: Modelling and Simulation / 839: Power and Energy Systems, 2016

In current era, thermal energy storage is a standout amongst the most proficient approaches to st... more In current era, thermal energy storage is a standout amongst the most proficient approaches to store the solar oriented renewable energy for warming and drying the air by energy gathered from sun. The paper exhibits a test examination of a stuffed rock bed solar thermal storage system. A 30 mm crevice is loaded with Polyester insulation in an air sealed dark painted plywood compartment from where heat can be extricated by constrained air convection. This system is able to constantly provide the required heat input into the setup. The heat decay characteristic of the bed is additionally contemplated. The most elevated introductory temperature of the rocks was recorded to 65.5 0 C. Experimental result is likewise dissected by utilizing 3 thermocouples with data logger EasySense TM software. After preparatory thought of specialized, environmental and economic aspects consideration, the effective thermal efficiency of the system is measured to 63.16% with 8.814 MJ integrated energy simply following 5 hour charging period.

Research paper thumbnail of Design and Investigation of a Low Cost & Portable Thermal Energy Storage System for Domestic Applications

Environment and Water Resource Management / 837: Health Informatics / 838: Modelling and Simulation / 839: Power and Energy Systems, 2016

In current era, thermal energy storage is a standout amongst the most proficient approaches to st... more In current era, thermal energy storage is a standout amongst the most proficient approaches to store the solar oriented renewable energy for warming and drying the air by energy gathered from sun. The paper exhibits a test examination of a stuffed rock bed solar thermal storage system. A 30 mm crevice is loaded with Polyester insulation in an air sealed dark painted plywood compartment from where heat can be extricated by constrained air convection. This system is able to constantly provide the required heat input into the setup. The heat decay characteristic of the bed is additionally contemplated. The most elevated introductory temperature of the rocks was recorded to 65.5 0 C. Experimental result is likewise dissected by utilizing 3 thermocouples with data logger EasySense TM software. After preparatory thought of specialized, environmental and economic aspects consideration, the effective thermal efficiency of the system is measured to 63.16% with 8.814 MJ integrated energy simply following 5 hour charging period.

Research paper thumbnail of Development of accurate contact force models for use with Discrete Element Method (DEM) modelling of bulk fruit handling processes

This thesis is primarily concerned with the development of accurate, simplified and validated con... more This thesis is primarily concerned with the development of accurate, simplified and validated contact force models for the discrete element modelling (DEM) of fruit bulk handling systems. The DEM is essentially a numerical technique to model a system of particles interacting with one another and with the system boundaries through collisions. The specific area of application envisaged is in postharvest agriculture, where DEM could be used in simulation of many unit operations with bulk fruit, such as harvesting, transportation, sorting, storage and processing. Such simulations would provide crucial information to aid equipment and process designs to optimise the systems or to limit physical damage to produce during machine handling, for instance. In a DEM model, the material behaviour at collision is captured by a contact force model relating the local deformation in the contact region to the contact force. Contact force models represent one of the crucial elements of a DEM model. Th...

Research paper thumbnail of Particle Based Simulations with Demeter++

III International Symposium on Applications of Modelling as an Innovative Technology in the Agri-Food Chain; MODEL-IT, 2005

Particle Based Simulation (PBS) methods, such as the Discrete Element Method (DEM), are the natur... more Particle Based Simulation (PBS) methods, such as the Discrete Element Method (DEM), are the natural choice for simulating phenomena related to granular media. DEM bares close resemblance to Molecular Dynamics (MD) and is ideally suited for solving problems with granular media. Both DEM and MD sum up the forces acting on the particles and integrate Newton's equations of motion over time. In doing so, the technique predicts the path of every particle in the ensemble as time proceeds as well as the forces it experiences. Other PBS methods, like Smooth Particle Hydrodynamics (SPH) or Dissipative Particle Dynamics (DPD) are suited for solving problems in the realm of continuum mechanics with fast moving boundaries. They treat the continuum materials as a collection of material particles. The Laboratory for Agro Machinery and Processing has built a thorough expertise in Particle Based Simulation methods and has developed a C++ software library, called DEMeter++. The reason of existence of DEMeter++ is to offer an extensible toolkit for the creation of efficient PBS programs in short time. At this moment several research projects involving the simulation of dynamical behavior of granular media and related phenomena are carried out using this DEMeter++ library. The intermediate results of these projects will be presented in this paper.

Research paper thumbnail of Determining tangential contact force model parameters for viscoelastic materials (apples) using a rheometer

Postharvest Biology and Technology, 2004

A computer model, based on discrete element method (DEM), can simulate damage to fruit during han... more A computer model, based on discrete element method (DEM), can simulate damage to fruit during handling. A contact force model is one of the basic ingredients of a DEM model. A rheometer device was used to measure and validate a tangential contact force model for 'Jonagold' apples. The Mindlin and Deresiewicz model for torsion was used to simulate quasi-static torsion and extended to include a viscous term in order to simulate dynamic torsion. The measured parameters were the dynamic friction coefficient, the shear modulus and the viscosity of the apples. For model validation purposes, the angular deformation, the torque, the contact surface and the normal force were measured as well. The model describes, in a satisfactory way, the phenomenon of slip-stick at the contact surface of the apples. By adding the viscous term, the model was improved.

Research paper thumbnail of A discrete element model for simulation of a spinning disc fertilizer spreader I. Single particle simulations

Research paper thumbnail of DEM simulations of the particle flow on a centrifugal fertilizer spreader

Powder Technology, 2009

Usually, the performance of centrifugal spreaders must be evaluated in large halls by capturing t... more Usually, the performance of centrifugal spreaders must be evaluated in large halls by capturing the fertilizer distribution patterns in standardized tests, often carrying a big cost to the manufacturers. In contrast, this paper proposes a first attempt to model a particle flow subjected to a spinning disc using the Discrete Element Method (DEM) starting from the particle outflow of a bin, using flat as well as inclined discs. The model is validated by experiments in two different ways. The first manner is the measurement of the cylindrical mass distribution along the edge of the disc by a device that collects the fertilizer particles in a tray of baskets around the disc. A second method consists of collecting the particles on the ground after their ballistic flight through the air. Both validation methods are relatively cheap and fit into the present statistical or qualitative interpretation of DEM simulations. Additionally, a number of rotational disc speeds is chosen (300-650 rpm) to incorporate velocity dependent effects of the particle flow. It was found that the DEM simulations show a good qualitative and considerable quantitative agreement with the experiments. The deviations between the simulations and experiments are profound at high disc rotational speeds (500-650 rpm) and can be identified as (1) an underestimation of the simulated particle velocities at the edge of the disc and (2) a too low dispersion on the (vertical) simulated particle velocities at the edge of the disc. A parameter study revealed that (1) can be resolved by introducing a velocity dependent friction coefficient, in agreement with literature. The influence of other model parameters such as particle damping and stiffness appears to be small, while the introduction of a rolling friction coefficient to mimic rolling resistance or particle shape does not provide any answer either, and hence reason (2) at this moment must be addressed to unknown external factors such as disc plane vibrations appearing at higher disc speeds.

Research paper thumbnail of Finite element analysis of the dynamic collision of apple fruit

Postharvest Biology and Technology, 2008

... Agricultural materials, and fruit in particular, display the type of behaviour that is accept... more ... Agricultural materials, and fruit in particular, display the type of behaviour that is accepted to be viscoelastic ( [Hamann, 1970] , [Chen and Fridley, 1972] , [Akyurt et al., 1972] , [De Baerdemaeker and Segerlind, 1976] , [Pitt and Chen, 1983] , [Mohsenin, 1986] and [Lu and Puri ...

Research paper thumbnail of On the accuracy of the Hertz model to describe the normal contact of soft elastic spheres

Granular Matter, 2008

The finite element method is used to investigate the validity of the Hertz model for normal conta... more The finite element method is used to investigate the validity of the Hertz model for normal contact under conditions that violate some of the assumptions on which the Hertz model was derived. The conditions investigated are: frictional contact, non-flat contact surface and large strains. The investigations were performed for the case of two elastic spheres in contact as well as that of a sphere in contact with a flat plate. The existence of friction for normal contact does not appear to lead to any erroneous predictions of the force-deflection relationship by the Hertz model. The Hertz model also does not appear to predict significant errors when the contact surface is not flat. However, large strains were found to cause important prediction errors by the Hertz model. Calibration curves for the corrections on the Hertz model are proposed.

Research paper thumbnail of Torsion of viscoelastic spheres in contact

Granular Matter, 2005

The theory of elastic contact between two spherical bodies is used as a basis for an extension to... more The theory of elastic contact between two spherical bodies is used as a basis for an extension to include the contribution of the viscous effects to the total stress for viscoelastic spheres subjected to twisting moments. Expressions relating twisting moment to penetration of slip and penetration of slip to twist angle are derived. Two term power series truncations of the relations are then used to derive approximate expressions for torsional compliance of the bodies. Validation experiments for the extended model were performed by use of a rheometer device. Applications for the model in post-harvest agriculture include extraction of material properties for use in Discrete Element Modelling of mechanical interactions of fruits and other regular shaped produce during machine handling. A specific application is performed involving the use of a rheometer to measure the coefficient of friction for fruit-fruit contact.

Research paper thumbnail of Torsional stiffness of viscoelastic spheres in contact

The European Physical Journal B, 2004

The theory of elastic contact between two spherical bodies according to Hertz, Mindlin and others... more The theory of elastic contact between two spherical bodies according to Hertz, Mindlin and others is used as a basis for an extension to include the contribution of the viscous effects to the total stress for viscoelastic spheres subjected to twisting moments. Expressions relating twisting moment to the radius of a 'stick' region of the contact surface and the radius of the 'stick' region to twist angle are derived. Two term power series truncations of the relations are then used to derive approximate expressions for torsional stiffness of the bodies. Validation of the model was by experiments utilising a rheometer device. Applications for the model in post-harvest agriculture include extraction of material surface properties for use in discrete element modelling of mechanical interactions of fruits and other spheroidal produce during machine handling.

Research paper thumbnail of Validation and Use of an Enzymic Time-Temperature Integrator to Monitor Thermal Impacts Inside a Solid/Liquid Model Food

Biotechnology Progress, 2002

Heat denaturation kinetics of Bacillus licheniformis R-amylase, equilibrated at 81% equilibrium r... more Heat denaturation kinetics of Bacillus licheniformis R-amylase, equilibrated at 81% equilibrium relative humidity at 4°C (BLA81), was studied with help of isothermal and nonisothermal conditions by monitoring the decrease in enthalpy associated with the heat denaturation of the enzyme. Due to its low water content, BLA81 denaturation could be studied in the range of 118-124°C. Two batches of BLA81 were successfully validated under nonisothermal conditions allowing the determinations of process values (reference temperature of 121.1°C) in the range of 1-15 min. In a second step, BLA81 was used as a time-temperature integrator (TTI) to investigate potential differences of process values received by freely moving spherical particles as compared to a centrally fixed particle (single-position impact) inside cans containing water as brine. Results showed that the process value received by freely moving particles can be from 5.6% (4 rpm) to 19.7% (8 rpm) smaller than the process value received by the centrally fixed sphere. This means that evaluating the process value by means of a particle fixed at the critical point in a package can lead to potentially overestimations of the actual process value with possible hazardous quality/safety implications. These results highlight the potentials of the TTI technology to monitor the safety of heat-processed agitated solid/liquid foodstuffs.

Research paper thumbnail of Determination of Parameters of a Tangential Contact Force Model for Viscoelastic Spheroids (fruits) using a Rheometer Device

Biosystems Engineering, 2005

This paper describes a method to experimentally determine the parameters of a tangential contact ... more This paper describes a method to experimentally determine the parameters of a tangential contact force model for spherical viscoelastic objects using a rheometer. The tangential contact force model is a necessary component of a discrete element model to simulate various granular matter systems. In this study, the discrete element model is being applied to simulate postharvest unit operations for fruits or similar agricultural produce. In the first part of the paper, a description of the contact force model is developed by extending the existing models for tangential contact of elastic spheres to include a viscous component. The second part describes the experiments performed and the associated results. The parameters under investigation are the material elastic shear modulus, the material viscosity and the friction coefficient.

Research paper thumbnail of The simulation of the impact damage to fruit during the passage of a truck over a speed bump by means of the discrete element method

Biosystems Engineering, 2008

ABSTRACT The discrete element method (DEM) was used to study fruit damage during transportation. ... more ABSTRACT The discrete element method (DEM) was used to study fruit damage during transportation. The DEM is a particle-based simulation technique which is well suited for the solution of granular material related problems in food and agriculture. In this paper, the application of DEM to food transport problems was demonstrated by simulation of bruising to apples stored in bulk bins during the passage of a truck over a speed bump. The effects of truck load, bulk bin position, suspension type and driving speed on damage were investigated. The simulations showed that higher truck loads lead to less bruising and that apples in bulk bins behind the rear axle suffered more damage than those in bulk bins in front of the rear axle. Furthermore, a considerable reduction in the damage was predicted in simulations where the truck has a soft suspension. Independent of truck load, suspension type and bulk bin position, the commercially significant bruising (i.e. apples with bruise volume of maximum bruise above 500 mm3) was predicted to be insignificant for driving speeds below 20 km h−1. At higher driving speeds, the extent of commercially acceptable bruising depended on various parameters. A reduction in the driving speed, an increase in the truck load and a reduction in the suspension stiffness all helped to reduce the occurrence of fruit damage.

Research paper thumbnail of Stress intensity factor computation of inclined cracked tension plate using XFEM

One of the major successes in the field of Linear Elastic Fracture Mechanics (LEFM) is the ground... more One of the major successes in the field of Linear Elastic Fracture Mechanics (LEFM) is the groundwork of the stress intensity factor (SIF) computation. The approaches used to carry out SIF values may be analytical, semi-analytical, experimental or numerical. Each one of the above methods has its own benefits however the use of numerical solutions has become the most frequent and popular. Numerous schemes for the numerical computation of SIF have been developed, the J-integral method being the most popular one. In this article we examine the SIFs of an edge cracked two dimensional (2-D) steel plate subjected to tensile loading. Extended finite element (XFEM) computational scheme has been employed to estimate the values of SIF. The SIF values of cracks with different lengths and inclination angles (different configurations) have been examined by utilizing the domain based interaction integral approach. The effect of crack inclination and crack position on SIFs (KI and KII) has also be...

Research paper thumbnail of The discrete element method (DEM) to simulate fruit impact damage during transport and handling: Model building and validation of DEM to predict bruise damage of apples

Postharvest Biology and Technology, 2006

Research paper thumbnail of Development of a Cost-effective Design of a P-V Ventilated Greenhouse Solar Dryer for Commercial Preservation of Tomatoes in a Rural Setting

Advances in Technology Innovation, 2019

Commercial preservation of agro-produce by solar drying entails using large-scale dryer units tha... more Commercial preservation of agro-produce by solar drying entails using large-scale dryer units that are generally not affordable by potential users in a rural setting. The objective of this study was to design a photo-voltaic greenhouse solar drying unit with air convection powered by a photovoltaic system as part of a research study to develop a cost-effective commercial solar dryer for the preservation of tomatoes in a rural setting. The design methodology involved. First, the determination of thermophysical properties of local tomatoes and predicting its drying model. Second, determination of design parameters; design equations, design conditions and assumptions, and psychrometric analysis. Third, application of the design parameters under the climatic conditions of Botswana to produce the embodiment of design with the drawings and specifications for a solar hot air dryer unit of 2000 kg batch-load of wet tomatoes. This drying unit is integrated into a solar collector with a cost-...

Research paper thumbnail of Particle Based Simulations with Demeter++

Particle Based Simulation (PBS) methods, such as the Discrete Element Method (DEM), are the natur... more Particle Based Simulation (PBS) methods, such as the Discrete Element Method (DEM), are the natural choice for simulating phenomena related to granular media. DEM bares close resemblance to Molecular Dynamics (MD) and is ideally suited for solving problems with granular media. Both DEM and MD sum up the forces acting on the particles and integrate Newton's equations of motion over time. In doing so, the technique predicts the path of every particle in the ensemble as time proceeds as well as the forces it experiences. Other PBS methods, like Smooth Particle Hydrodynamics (SPH) or Dissipative Particle Dynamics (DPD) are suited for solving problems in the realm of continuum mechanics with fast moving boundaries. They treat the continuum materials as a collection of material particles. The Laboratory for Agro Machinery and Processing has built a thorough expertise in Particle Based Simulation methods and has developed a C++ software library, called DEMeter++. The reason of existen...

Research paper thumbnail of Development of accurate contact force models for use with Discrete Element Method (DEM) modelling of bulk fruit handling processes

ABSTRACT This thesis is primarily concerned with the development of accurate, simplified and vali... more ABSTRACT This thesis is primarily concerned with the development of accurate, simplified and validated contact force models for the discrete element modelling (DEM) of fruit bulk handling systems. The DEM is essentially a numerical technique to model a system of particles interacting with one another and with the system boundaries through collisions. The specific area of application envisaged is in postharvest agriculture, where DEM could be used in simulation of many unit operations with bulk fruit, such as harvesting, transportation, sorting, storage and processing. Such simulations would provide crucial information to aid equipment and process designs to optimise the systems or to limit physical damage to produce during machine handling, for instance. In a DEM model, the material behaviour at collision is captured by a contact force model relating the local deformation in the contact region to the contact force. Contact force models represent one of the crucial elements of a DEM model. They have a direct bearing on the accuracy with which the material behaviour can be captured by the model. In carrying out the main objective of this project, theoretical, experimental as well as numerical approaches were used in order to, among other things, identify, derive, validate and simplify the relevant contact models proposed. Using the elastic contact force models as bases, mathematical contact force models for viscoelastic spheres are derived, for normal contact, tangential contact as well as torsional contact. The derived model for torsional contact was experimentally validated using a rheometer setup. Still using the rheometer setup, parameters of the derived tangential contact force model were determined. These parameters are: the friction coefficient, the viscous constant and the static shear modulus. The finite element method was used to further investigate the normal and tangential contact force models for elastic spheres. Regarding normal contact, the validity of the Hertz contact model under conditions of frictional contact, non-Hertzian contact surfaces and finite strains was investigated. The effect of friction to the force-deformation behaviour of elastic spheres was found to be negligible. The same is the case for the effects from the non-flat contact surfaces that result when stiffer objects come into contact with the more compliant ones. On the other hand, the study of the effect of finite strains revealed a strong influence of the extent of deformation on the Hertz model. The error arising was found to increase quadratically with increase in the normal deflection. Regarding the tangential contact, because the relevant models currently available in literature are generally either too inaccurate or computationally too costly to be implemented in DEM simulations involving large numbers of particles, an alternative model was derived herein and proposed. The model, it is argued, is simple to implement and can have the same accuracy as the Mindlin-Deresiewicz model. Because the proposed model does not include many different conditions and loading scenarios, it is argued that it will be much more convenient and less costly, computationally. By numerically filtering out all the vibration modes in the transient dynamic analyses of normal collisions of elastic spheres using FEA, an estimation of the relative quantities of the kinetic energy involved in the propagation of elastic waves in the colliding bodies could be made. The proportions varied according to the impact velocity from very low (less 4 %) to substantial proportions of up to 40 %. The relationship with the impact velocity was estimated to be proportional to a power 1/3 of the impact velocity. While this observation does not prescribe the amount of energy dissipation that can occur, it is extremely important because it provides an indication of the quantities of energy that can be available as elastic waves energy during collisions, and which potentially can be lost under some conditions. A significant amount of translational energy loss was found to occur for purely elastic collisions through absorption of energy as elastic waves, depending on the material elastic properties. The restitution coefficients for the purely elastic collisions depend on the impact velocity. Eigenvalue analysis of apple fruit models (based on Jonagold apple properties) yielded frequency modes in the ranges from 87.3 Hz to 849.4 Hz. These results were in good agreement with the ranges of values found in literature. The transient analysis of the viscoelastic collisions of apples, using the time dependent properties obtained from literature revealed no viscous response by the fruits. This is explained by the fact that the time constants reported on, and obtained from stress relaxation experiments are much higher than the typical collision times of the fruits. Because pendulum experiments on apple fruits have revealed the existence of viscous response in apples for these…

Research paper thumbnail of Stress intensity factor computation of inclined cracked tension plate using XFEM

One of the major successes in the field of Linear Elastic Fracture Mechanics (LEFM) is the ground... more One of the major successes in the field of Linear Elastic Fracture Mechanics (LEFM) is the groundwork of the stress intensity factor (SIF) computation. The approaches used to carry out SIF values may be analytical, semi-analytical, experimental or numerical. Each one of the above methods has its own benefits however the use of numerical solutions has become the most frequent and popular. Numerous schemes for the numerical computation of SIF have been developed, the J-integral method being the most popular one. In this article we examine the SIFs of an edge cracked two dimensional (2-D) steel plate subjected to tensile loading. Extended finite element (XFEM) computational scheme has been employed to estimate the values of SIF. The SIF values of cracks with different lengths and inclination angles (different configurations) have been examined by utilizing the domain based interaction integral approach. The effect of crack inclination and crack position on SIFs (KI and KII) has also be...

Research paper thumbnail of Design and Investigation of a Low Cost & Portable Thermal Energy Storage System for Domestic Applications

Environment and Water Resource Management / 837: Health Informatics / 838: Modelling and Simulation / 839: Power and Energy Systems, 2016

In current era, thermal energy storage is a standout amongst the most proficient approaches to st... more In current era, thermal energy storage is a standout amongst the most proficient approaches to store the solar oriented renewable energy for warming and drying the air by energy gathered from sun. The paper exhibits a test examination of a stuffed rock bed solar thermal storage system. A 30 mm crevice is loaded with Polyester insulation in an air sealed dark painted plywood compartment from where heat can be extricated by constrained air convection. This system is able to constantly provide the required heat input into the setup. The heat decay characteristic of the bed is additionally contemplated. The most elevated introductory temperature of the rocks was recorded to 65.5 0 C. Experimental result is likewise dissected by utilizing 3 thermocouples with data logger EasySense TM software. After preparatory thought of specialized, environmental and economic aspects consideration, the effective thermal efficiency of the system is measured to 63.16% with 8.814 MJ integrated energy simply following 5 hour charging period.

Research paper thumbnail of Design and Investigation of a Low Cost & Portable Thermal Energy Storage System for Domestic Applications

Environment and Water Resource Management / 837: Health Informatics / 838: Modelling and Simulation / 839: Power and Energy Systems, 2016

In current era, thermal energy storage is a standout amongst the most proficient approaches to st... more In current era, thermal energy storage is a standout amongst the most proficient approaches to store the solar oriented renewable energy for warming and drying the air by energy gathered from sun. The paper exhibits a test examination of a stuffed rock bed solar thermal storage system. A 30 mm crevice is loaded with Polyester insulation in an air sealed dark painted plywood compartment from where heat can be extricated by constrained air convection. This system is able to constantly provide the required heat input into the setup. The heat decay characteristic of the bed is additionally contemplated. The most elevated introductory temperature of the rocks was recorded to 65.5 0 C. Experimental result is likewise dissected by utilizing 3 thermocouples with data logger EasySense TM software. After preparatory thought of specialized, environmental and economic aspects consideration, the effective thermal efficiency of the system is measured to 63.16% with 8.814 MJ integrated energy simply following 5 hour charging period.

Research paper thumbnail of Development of accurate contact force models for use with Discrete Element Method (DEM) modelling of bulk fruit handling processes

This thesis is primarily concerned with the development of accurate, simplified and validated con... more This thesis is primarily concerned with the development of accurate, simplified and validated contact force models for the discrete element modelling (DEM) of fruit bulk handling systems. The DEM is essentially a numerical technique to model a system of particles interacting with one another and with the system boundaries through collisions. The specific area of application envisaged is in postharvest agriculture, where DEM could be used in simulation of many unit operations with bulk fruit, such as harvesting, transportation, sorting, storage and processing. Such simulations would provide crucial information to aid equipment and process designs to optimise the systems or to limit physical damage to produce during machine handling, for instance. In a DEM model, the material behaviour at collision is captured by a contact force model relating the local deformation in the contact region to the contact force. Contact force models represent one of the crucial elements of a DEM model. Th...

Research paper thumbnail of Particle Based Simulations with Demeter++

III International Symposium on Applications of Modelling as an Innovative Technology in the Agri-Food Chain; MODEL-IT, 2005

Particle Based Simulation (PBS) methods, such as the Discrete Element Method (DEM), are the natur... more Particle Based Simulation (PBS) methods, such as the Discrete Element Method (DEM), are the natural choice for simulating phenomena related to granular media. DEM bares close resemblance to Molecular Dynamics (MD) and is ideally suited for solving problems with granular media. Both DEM and MD sum up the forces acting on the particles and integrate Newton's equations of motion over time. In doing so, the technique predicts the path of every particle in the ensemble as time proceeds as well as the forces it experiences. Other PBS methods, like Smooth Particle Hydrodynamics (SPH) or Dissipative Particle Dynamics (DPD) are suited for solving problems in the realm of continuum mechanics with fast moving boundaries. They treat the continuum materials as a collection of material particles. The Laboratory for Agro Machinery and Processing has built a thorough expertise in Particle Based Simulation methods and has developed a C++ software library, called DEMeter++. The reason of existence of DEMeter++ is to offer an extensible toolkit for the creation of efficient PBS programs in short time. At this moment several research projects involving the simulation of dynamical behavior of granular media and related phenomena are carried out using this DEMeter++ library. The intermediate results of these projects will be presented in this paper.

Research paper thumbnail of Determining tangential contact force model parameters for viscoelastic materials (apples) using a rheometer

Postharvest Biology and Technology, 2004

A computer model, based on discrete element method (DEM), can simulate damage to fruit during han... more A computer model, based on discrete element method (DEM), can simulate damage to fruit during handling. A contact force model is one of the basic ingredients of a DEM model. A rheometer device was used to measure and validate a tangential contact force model for 'Jonagold' apples. The Mindlin and Deresiewicz model for torsion was used to simulate quasi-static torsion and extended to include a viscous term in order to simulate dynamic torsion. The measured parameters were the dynamic friction coefficient, the shear modulus and the viscosity of the apples. For model validation purposes, the angular deformation, the torque, the contact surface and the normal force were measured as well. The model describes, in a satisfactory way, the phenomenon of slip-stick at the contact surface of the apples. By adding the viscous term, the model was improved.

Research paper thumbnail of A discrete element model for simulation of a spinning disc fertilizer spreader I. Single particle simulations

Research paper thumbnail of DEM simulations of the particle flow on a centrifugal fertilizer spreader

Powder Technology, 2009

Usually, the performance of centrifugal spreaders must be evaluated in large halls by capturing t... more Usually, the performance of centrifugal spreaders must be evaluated in large halls by capturing the fertilizer distribution patterns in standardized tests, often carrying a big cost to the manufacturers. In contrast, this paper proposes a first attempt to model a particle flow subjected to a spinning disc using the Discrete Element Method (DEM) starting from the particle outflow of a bin, using flat as well as inclined discs. The model is validated by experiments in two different ways. The first manner is the measurement of the cylindrical mass distribution along the edge of the disc by a device that collects the fertilizer particles in a tray of baskets around the disc. A second method consists of collecting the particles on the ground after their ballistic flight through the air. Both validation methods are relatively cheap and fit into the present statistical or qualitative interpretation of DEM simulations. Additionally, a number of rotational disc speeds is chosen (300-650 rpm) to incorporate velocity dependent effects of the particle flow. It was found that the DEM simulations show a good qualitative and considerable quantitative agreement with the experiments. The deviations between the simulations and experiments are profound at high disc rotational speeds (500-650 rpm) and can be identified as (1) an underestimation of the simulated particle velocities at the edge of the disc and (2) a too low dispersion on the (vertical) simulated particle velocities at the edge of the disc. A parameter study revealed that (1) can be resolved by introducing a velocity dependent friction coefficient, in agreement with literature. The influence of other model parameters such as particle damping and stiffness appears to be small, while the introduction of a rolling friction coefficient to mimic rolling resistance or particle shape does not provide any answer either, and hence reason (2) at this moment must be addressed to unknown external factors such as disc plane vibrations appearing at higher disc speeds.

Research paper thumbnail of Finite element analysis of the dynamic collision of apple fruit

Postharvest Biology and Technology, 2008

... Agricultural materials, and fruit in particular, display the type of behaviour that is accept... more ... Agricultural materials, and fruit in particular, display the type of behaviour that is accepted to be viscoelastic ( [Hamann, 1970] , [Chen and Fridley, 1972] , [Akyurt et al., 1972] , [De Baerdemaeker and Segerlind, 1976] , [Pitt and Chen, 1983] , [Mohsenin, 1986] and [Lu and Puri ...

Research paper thumbnail of On the accuracy of the Hertz model to describe the normal contact of soft elastic spheres

Granular Matter, 2008

The finite element method is used to investigate the validity of the Hertz model for normal conta... more The finite element method is used to investigate the validity of the Hertz model for normal contact under conditions that violate some of the assumptions on which the Hertz model was derived. The conditions investigated are: frictional contact, non-flat contact surface and large strains. The investigations were performed for the case of two elastic spheres in contact as well as that of a sphere in contact with a flat plate. The existence of friction for normal contact does not appear to lead to any erroneous predictions of the force-deflection relationship by the Hertz model. The Hertz model also does not appear to predict significant errors when the contact surface is not flat. However, large strains were found to cause important prediction errors by the Hertz model. Calibration curves for the corrections on the Hertz model are proposed.

Research paper thumbnail of Torsion of viscoelastic spheres in contact

Granular Matter, 2005

The theory of elastic contact between two spherical bodies is used as a basis for an extension to... more The theory of elastic contact between two spherical bodies is used as a basis for an extension to include the contribution of the viscous effects to the total stress for viscoelastic spheres subjected to twisting moments. Expressions relating twisting moment to penetration of slip and penetration of slip to twist angle are derived. Two term power series truncations of the relations are then used to derive approximate expressions for torsional compliance of the bodies. Validation experiments for the extended model were performed by use of a rheometer device. Applications for the model in post-harvest agriculture include extraction of material properties for use in Discrete Element Modelling of mechanical interactions of fruits and other regular shaped produce during machine handling. A specific application is performed involving the use of a rheometer to measure the coefficient of friction for fruit-fruit contact.

Research paper thumbnail of Torsional stiffness of viscoelastic spheres in contact

The European Physical Journal B, 2004

The theory of elastic contact between two spherical bodies according to Hertz, Mindlin and others... more The theory of elastic contact between two spherical bodies according to Hertz, Mindlin and others is used as a basis for an extension to include the contribution of the viscous effects to the total stress for viscoelastic spheres subjected to twisting moments. Expressions relating twisting moment to the radius of a 'stick' region of the contact surface and the radius of the 'stick' region to twist angle are derived. Two term power series truncations of the relations are then used to derive approximate expressions for torsional stiffness of the bodies. Validation of the model was by experiments utilising a rheometer device. Applications for the model in post-harvest agriculture include extraction of material surface properties for use in discrete element modelling of mechanical interactions of fruits and other spheroidal produce during machine handling.

Research paper thumbnail of Validation and Use of an Enzymic Time-Temperature Integrator to Monitor Thermal Impacts Inside a Solid/Liquid Model Food

Biotechnology Progress, 2002

Heat denaturation kinetics of Bacillus licheniformis R-amylase, equilibrated at 81% equilibrium r... more Heat denaturation kinetics of Bacillus licheniformis R-amylase, equilibrated at 81% equilibrium relative humidity at 4°C (BLA81), was studied with help of isothermal and nonisothermal conditions by monitoring the decrease in enthalpy associated with the heat denaturation of the enzyme. Due to its low water content, BLA81 denaturation could be studied in the range of 118-124°C. Two batches of BLA81 were successfully validated under nonisothermal conditions allowing the determinations of process values (reference temperature of 121.1°C) in the range of 1-15 min. In a second step, BLA81 was used as a time-temperature integrator (TTI) to investigate potential differences of process values received by freely moving spherical particles as compared to a centrally fixed particle (single-position impact) inside cans containing water as brine. Results showed that the process value received by freely moving particles can be from 5.6% (4 rpm) to 19.7% (8 rpm) smaller than the process value received by the centrally fixed sphere. This means that evaluating the process value by means of a particle fixed at the critical point in a package can lead to potentially overestimations of the actual process value with possible hazardous quality/safety implications. These results highlight the potentials of the TTI technology to monitor the safety of heat-processed agitated solid/liquid foodstuffs.

Research paper thumbnail of Determination of Parameters of a Tangential Contact Force Model for Viscoelastic Spheroids (fruits) using a Rheometer Device

Biosystems Engineering, 2005

This paper describes a method to experimentally determine the parameters of a tangential contact ... more This paper describes a method to experimentally determine the parameters of a tangential contact force model for spherical viscoelastic objects using a rheometer. The tangential contact force model is a necessary component of a discrete element model to simulate various granular matter systems. In this study, the discrete element model is being applied to simulate postharvest unit operations for fruits or similar agricultural produce. In the first part of the paper, a description of the contact force model is developed by extending the existing models for tangential contact of elastic spheres to include a viscous component. The second part describes the experiments performed and the associated results. The parameters under investigation are the material elastic shear modulus, the material viscosity and the friction coefficient.

Research paper thumbnail of The simulation of the impact damage to fruit during the passage of a truck over a speed bump by means of the discrete element method

Biosystems Engineering, 2008

ABSTRACT The discrete element method (DEM) was used to study fruit damage during transportation. ... more ABSTRACT The discrete element method (DEM) was used to study fruit damage during transportation. The DEM is a particle-based simulation technique which is well suited for the solution of granular material related problems in food and agriculture. In this paper, the application of DEM to food transport problems was demonstrated by simulation of bruising to apples stored in bulk bins during the passage of a truck over a speed bump. The effects of truck load, bulk bin position, suspension type and driving speed on damage were investigated. The simulations showed that higher truck loads lead to less bruising and that apples in bulk bins behind the rear axle suffered more damage than those in bulk bins in front of the rear axle. Furthermore, a considerable reduction in the damage was predicted in simulations where the truck has a soft suspension. Independent of truck load, suspension type and bulk bin position, the commercially significant bruising (i.e. apples with bruise volume of maximum bruise above 500 mm3) was predicted to be insignificant for driving speeds below 20 km h−1. At higher driving speeds, the extent of commercially acceptable bruising depended on various parameters. A reduction in the driving speed, an increase in the truck load and a reduction in the suspension stiffness all helped to reduce the occurrence of fruit damage.