Faculty of Engineering
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Publication Open Access Robustness of digital filters with respect to limit-cycle behavior under coefficient perturbations(International Society for Optics and Photonics, 1996-06-07) Kulasekere, E. C; Premaratne, K; Bauer, P. HA digital filter which has been designed to be limit cycle free may exhibit limit cycles at the implementation stage. This is due to the inability to implement filter coefficients exactly in hardware when they are quantized to satisfy available wordlength requirements. Given a digital filter which is limit cycle free under zero input conditions, the work below presents an algorithm which finds a region in the coefficient space, about the nominal filter coefficient values, where in the filter remains limit cycle free. Furthermore the results of the algorithm will also indicate the availability of other machine representable numbers for the coefficients that fall within this robustness region. Hence one may even choose shorter wordlength registers for coefficient storage if the corresponding grid falls within the constructed robustness region.Publication Embargo An exhaustive search algorithm for checking limit cycle behavior of digital filters(IEEE, 1996-10) Premaratne, K; Kulasekere, E. C; Bauer, P. H; Leclerc, L. JThe presence of limit cycles that may arise in fixed-point arithmetic implementation of a digital filter can significantly impair its performance. This paper presents an algorithm to determine the presence/absence of such limit cycles. For generality, the filter is taken to be in its state-space formulation. The algorithm is applicable independent of filter order, type of quantization nonlinearity, and whether the accumulator is single or double length. It may be utilized to construct limit cycle free regions in filter coefficient space. Once a filter is determined to be limit cycle free, a technique that provides a robustness region in coefficient space where all filters remain limit cycle free is also presented.Publication Embargo Component part standardization: A way to reduce the life-cycle costs of products(Elsevier, 1999-04-20) Perera, H. S. C; Nagarur, N; Tabucanon, M. TThis paper studies the effects of component part standardization on life-cycle costs. Three possible situations for component standardization are discussed. A summary is presented of various costs in different phases of the product life cycle and how they are shared among the manufacturer, user, and society. Then, we explain how the component standardization reduces the costs of different phases of the product life cycle. Finally, we point out some of the possible disadvantages of component standardization.Publication Open Access Comfort conditions for built environments in Sri Lanka(Institution of Engineers, 1999) Jayasinghe, M.T.RThe neutral temperatures for different locations in Sri Lanka have been established using actual climatic data. Based on these neutral temperatures, 'standard comfort zone for each of these locations can be identified on the psychrometric chart. Field measurements have been carried out for the validation of comfort zones for Sri Lankan conditions. The effects of physiological cooling at relatively high internal air velocities have been highlighted. In order to check the applicability of standard modification techniques for the comfort zones to take account of elevated internal air velocities, surveys have been carried out at two different velocities. The need for additional boundaries to standard modifications to suit Sri Lankan conditions have been highlighted. This provides a method of extending the boundaries of these comfort zones thereby accommodating higher levels of dry bulb temperatures and humidites in the built environments. This fact can be utilised as the basis of minimising the energy demand in buildings either air conditioned or not, by making use of combined modes at different internal air velocities.Publication Open Access Passive techniques for residential buildings in low altitudes of Sri Lanka(Institution of engineers: Colombo, 1999) Jayasinghe, M.T.R; Attalage, R. AThe role of passive techniques, which utilises a certain number of passive elements to maintain the thermal comfort within the built environments of residential buildings at low atlitudes of Sri Lanka, has been highlighted. The possibility of defining a unique neutral temperature of 26°C for low altitutes of Sri Lanka has been utilised in a rational way in presenting simplified set of rules of thumb, that can be used by the designers at early stages. The effects of orientation, shading devices, window sizes, roofing materials, insulation, colour of the exterior and interior walls, use of courtyards to maximise natural ventilation, and arrangement of roof to minimise structural cooling are shown with suitable examples.Publication Embargo Elastodynamic Green's functions of orthotropic half plane(American Society of Civil Engineers, 1991-03-01) Rajapakse, R.K.N.D; Wang, Y.The dynamic response of an orthotropic elastic half plane subjected to a set of time‐harmonic buried loadings is investigated. The governing differential equations are established in terms of displacements and a general solution is derived using Fourier integral transforms with respect to the x‐coordinate. The boundary‐value problems corresponding to time‐harmonic vertical and horizontal loads acting in the interior of the half plane are solved. Explicit analytical solutions are presented for displacements and stresses due to buried uniformly distributed and concentrated loadings. Some characteristics of the analytical solution are investigated, and its numerical evaluation is also discussed. Selected numerical results for displacements and stresses of isotropic, ice, layered soil, and cadmium half‐plane regions are presented. A discussion of these numerical solutions is presented to investigate the influence of the degree of material anisotropy, frequency of excitation, and the type of loading on the response of the elastic half plane.Publication Open Access Potential of biomass fuel conservation in selected Asian countries(Pergamon, 1999-07-01) Bhattacharya, S. C; Attalage, R. A; Leon, M Augustus; Amur, G. Q; Salam, P Abdul; Thanawat, CThe potential of savings in the biomass consumed for energy in seven Asian countries—China, India, Pakistan, Nepal, Philippines, Sri Lanka and Vietnam—is estimated, if the centuries-old traditional methods of biomass use are reconsidered and an efficient, rational use is implemented. The present pattern and share of biomass consumption of different traditional biomass energy devices are recorded. The efficiency of traditional technologies and that of improved ones—technologies which are practically applicable or already in use somewhere else—are compared and the potential of biomass savings is calculated. The total biomass saving potential in all seven countries together has been estimated at 322 million tons/yearPublication Open Access Potential of biomass fuel conservation in selected Asian countries(Pergamon, 1999-07-01) Bhattacharya, S. C; Attalage, R. A; Leon, M Augustus; Amur, G. Q; Salam, P Abdul; Thanawat, CThe potential of savings in the biomass consumed for energy in seven Asian countries—China, India, Pakistan, Nepal, Philippines, Sri Lanka and Vietnam—is estimated, if the centuries-old traditional methods of biomass use are reconsidered and an efficient, rational use is implemented. The present pattern and share of biomass consumption of different traditional biomass energy devices are recorded. The efficiency of traditional technologies and that of improved ones—technologies which are practically applicable or already in use somewhere else—are compared and the potential of biomass savings is calculated. The total biomass saving potential in all seven countries together has been estimated at 322 million tons/yearPublication Open Access Vertical vibrations of a rigid disk embedded in a poroelastic medium(John Wiley & Sons, Ltd., 1999-12-25) Zeng, X; Rajapakse, R. K. N. Dhis paper considers the steady-state vertical vibrations of a rigid circular disk embedded at a finite depth below the free surface of a poroelastic medium. Biot's elastodynamic theory for porous media is used in the analysis. General solutions for axisymmetric poroelastic fields are obtained by using Hankel integral transforms. Analytical solutions for influence functions corresponding to four types of buried axisymmetric excitations are derived. The embedded disk problem is fomulated in terms of a set of coupled integral equations for unknown traction and pore pressure jumps across the disk. The kernel functions of the integral equations are the influence functions corresponding to buried vertical, radial and pore pressure ring loads. The system of integral equations is solved numerically by discretizing the disk into several concentric annular rings. Selected numerical solutions for displacements, vertical stress and pore pressure due to a buried fully flexible disk (uniform pressure) are also presented. The vertical compliances of a rigid disk are examined for different depths of embedment, poroelastic materials and hydraulic boundary conditions. Solutions for traction and pore pressure jumps are also examined. The present results are useful in the study of dynamic response of embedded foundations and anchors in poroelastic soils.Publication Embargo Exact stiffness method for quasi-statics of a multi-layered poroelastic medium(Pergamon, 1995-06-01) Senjuntichai, T; Rajapakse, R. K. N. DA method is presented to study the three-dimensional quasi-static response of a multi-layered poroelastic half-space with compressible constituents. The system under consideration consists of N layers of different thickness and material properties overlying a homogeneous half-space. Fourier expansion, Laplace transforms and Hankel transforms with respect to the circumferential, time and radial coordinates, respectively, are used in the formulation. Laplace-Hankel transforms of displacements and pore pressure at layer interfaces are considered as the basic unknowns. Exact stiffness matrices describing the relationship between generalized displacement and force vectors of a finite layer and a half-space are derived explicitly in the transform space. The global stiffness matrix of a layered system is assembled by considering the continuity of tractions and fluid flow at layer interfaces. The time histories of displacements, stresses and pore pressure are obtained by solving the stiffness equation system for discrete values of Laplace and Hankel transform parameters, and using numerical quadrature schemes for Laplace and Hankel transform inversions. Selected numerical results for different layered systems are presented to portray the influence of layering and poroelastic material properties. The advantage of the present method is that for an N-layered system, it yields a numerically stable symmetric stiffness matrix of order 4N × 4N when compared to the unsymmetric and numerically unstable coefficient matrix of order 8N × 8N associated with the conventional method based on the determination of layer arbitrary coefficients.