Engineering Computations

Comparing displacement ventilation and mixing ventilation as HVAC strategies through CFD : Table of Contents

Nuno Serra, Viriato Semiao — Sat Oct 24 08:00:20 BST 2009

Abstract:
Purpose – The purpose of this paper is to compare two different ventilation strategies, displacement and mixing, in heat, ventilating and air conditioning (HVAC) systems with recourse to computational fluid dynamics (CFD). Design/methodology/approach – The flow and the heat and mass transfer are numerically predicted inside an air-conditioned room with a desk and an occupant for the cooling and heating periods in moderate climate regions, like Mediterranean countries. Focus is placed on energy efficiency, thermal comfort and internal air quality (IAQ), evaluated from the simulations of the three-dimensional, turbulent, non-isothermal and buoyant flow of moist air. Findings – For the cooling period, displacement exhibits higher energy and ventilation efficiencies promoting simultaneously better comfort for the occupant. For the heating period, mixing performs better due to the short-circuit phenomenon occurring with the displacement flow. Overall, mixing behaves better for air-conditioning of typical office rooms in Mediterranean-climate countries, where heating and cooling climatization modes have to be alternated according to the season. Research limitations/implications – Room, desk and occupant are designed as parallelepipeds. No experimental work is performed but models used are previously validated by other authors against experimental data. Practical implications – The results indicate a short-circuit flow phenomenon that must be avoided when designing HVAC systems. Originality/value – Use of grilles layout typical for the cooling period to study the air-conditioning of a typical office room during the heating period, incorporating in the model a transport equation for the moisture. IAQ is simulated together with the flow, the heat and the comfort conditions: velocities, temperature, predicted mean vote (PMV), predicted percentage of dissatisfied (PPD), draught rating (DR), PPD due to air quality (PDQ) and air moisture content are calculated simultaneously.

A heuristic solution to the university timetabling problem : Table of Contents

Aderemi O. Adewumi, Babatunde A. Sawyerr, M. Montaz Ali — Sat Oct 24 08:00:20 BST 2009

Abstract:
Purpose – The purpose of this paper is to consider the problem of university lecture timetabling. Timetabling deals with the problem of placing certain resources into a limited number of time slots, subject to given constraints, in order to satisfy a set of stated objectives to the highest possible extent. It is a well-known and established NP-hard problem. University timetabling is a major administrative activity especially in the third world universities. Solving the problem requires dynamic heuristics with predictable performance especially as the number of courses increases without corresponding increase in needed resources. Design/methodology/approach – A genetic algorithm metaheuristic is designed to handle a real-life case study. Given the present structure of the case study, a modular approach to the design of the timetable schedules is adopted. The approach considers timetable in a bottom-up fashion at the various levels of department, faculty or entire university. Simulation study is conducted using the open source Java IDE, Eclipse® 3.0 in a window XP/vista environment running on a processor of 1.12?GHz. Findings – Using the data sub-set from the case study, simulation experiments are conducted based on the proposed method and obtained promising results. Research limitations/implications – Given the modular approach, the timetable system can easily be adapted to other various levels in the institution. Originality/value – With reference to the case study, this is believed to be the first application of metaheuristics to a timetabling problem. The sensitivity analysis of the algorithm parameters is very valuable in guiding actual application development for the problem.

The forward and inverse problems in magnetic induction tomography of low conductivity structures : Table of Contents

Ryszard Palka, Stanislaw Gratkowski, Krzysztof Stawicki, Piotr Baniukiewicz — Sat Oct 24 08:00:20 BST 2009

Abstract:
Purpose – The purpose of this paper is to develop a magnetic induction tomography (MIT) system as well as the conductivity reconstruction algorithms (inverse problem). Design/methodology/approach – In order to define and verify the solution of the inverse problem, the forward problem is formulated using mathematical model of the system. The forward problem is solved using the finite element method. The optimization of the excitation unit is based on the numerical solutions of the direct problem. All the dimensions and shape of the excitation system are optimized in order to focus the main part of the magnetic field in the vicinity of the receiver. Finally, two formulations of the inverse problem are discussed: based on the inversion of the Biot-Savart law and based on the artificial neural networks (ANNs). Findings – The formulation of the forward problem of the considered MIT system is given. The construction of the exciter unit that focuses the main part of the magnetic field in the vicinity of the receiver is proposed. Two formulations of the inverse problem are discussed. First, using the inversion of the Biot-Savart law and second, using the ANN. The neural networks seem to be a promising tool for reconstructing the MIT images. Originality/value – The paper demonstrates a real-life MIT system whose performance is satisfactorily predicted by mathematical models. The original design of the exciter is shown. The new approach to the inverse problem in MIT, the use of the ANN, is presented.

The natural neighbour radial point interpolation method: dynamic applications : Table of Contents

L.M.J.S. Dinis, R.M. Natal Jorge, J. Belinha — Sat Oct 24 08:00:20 BST 2009

Abstract:
Purpose – The purpose of this paper is to extend the natural neighbour radial point interpolation method (NNRPIM) to the dynamic analysis (free vibrations and forced vibrations) of two-dimensional, three-dimensional and bending plate problems. Design/methodology/approach – The NNRPIM shape-function construction is briefly presented, as are the dynamic equations and the mode superposition method is used in the forced vibration analysis. Several benchmark examples of two-dimensional and plate bending problems are solved and compared with the three-dimensional NNRPIM formulation. The obtained results are compared with the available exact solutions and the finite element method (FEM) solutions. Findings – The developed NNRPIM approach is a good alternative to the FEM for the solution of dynamic problems, once the obtained results with the EFGM shows a high similarity with the obtained FEM results and for the majority of the studied examples the NNRPIM results are more close to the exact solution results. Research limitations/implications – Comparing the FEM and the NNRPIM, the computational cost of the NNRPIM is higher. Originality/value – The paper demonstrates extension of the NNRPIM to the dynamic analysis of two-dimensional, three-dimensional and bending plate problems. The elimination of the shear-locking phenomenon in the NNRPIM plate bending formulation. The various solved examples prove a high convergence rate and accuracy of the NNRPIM.

Pressure pulsation prediction by 3D turbulent unsteady flow simulation through whole flow passage of Kaplan turbine : Table of Contents

Shuhong Liu, Jianqiang Mai, Jie Shao, Yulin Wu — Sat Oct 24 08:00:20 BST 2009

Abstract:
Purpose – The purpose of this paper is to predict pressure pulsation in Kaplan hydraulic turbines. Design/methodology/approach – State of the art numerical simulation techniques are employed to simulate three-dimensional flows in the whole flow passage of a Kaplan turbine so that pressure pulsations can be computed in both time domain and frequency domain. Numerical results are verified by experiments carried out on the most advanced experimental platform in China. Findings – It is found that the proposed numerical model is a viable tool for prediction of pressure pulsations. The simulation shows that the model turbine and prototype turbine have the same pressure pulsation frequencies and rotating frequencies and the same transmission patterns under similar operation conditions. However, there is no similarity for the amplitude of the pressure pulsation between the model turbine and the prototype turbine. Therefore pressure pulsations in a prototype turbine cannot be obtained by scaling the experimental results of the model turbine using a similarity relationship. Practical implications – The findings will be very valuable for the design of hydraulic turbines and large-scale hydraulic power stations. Originality/value – The proposed numerical method provides a viable tool for hydraulic turbine and power station designers to predict the pressure pulsations in prototype turbines. It is a useful tool to help improve the performance of hydraulic turbines. The findings made in the numerical simulation have been verified by experiments, which is also a valuable reference for hydraulic turbine designers.

Boundary element analysis of mixed-mode stress intensity factors in an anisotropic cuboid with an inclined surface crack : Table of Contents

Chia-Hau Chen, Chao-Shi Chen, Ernian Pan, Han-Chou Tseng, Pao-Shan Yu — Sat Oct 24 08:00:20 BST 2009

Abstract:
Purpose – The purpose of this paper is to present special nine-node quadrilateral elements to discretize the un-cracked boundary and the inclined surface crack in a transversely isotropic cuboid under a uniform vertical traction along its top and bottom surfaces by a three-dimensional (3D) boundary element method (BEM) formulation. The mixed-mode stress intensity factors (SIFs), KI, KII and KIII, are calculated. Design/methodology/approach – A 3D dual-BEM or single-domain BEM is employed to solve the fracture problems in a linear anisotropic elastic cuboid. The transversely isotropic plane has an arbitrary orientation, and the crack surface is along an inclined plane. The mixed 3D SIFs are evaluated by using the asymptotical relation between the SIFs and the relative crack opening displacements. Findings – Numerical results show clearly the influence of the material and crack orientations on the mixed-mode SIFs. For comparison, the mode-I SIF when a horizontal rectangular crack is embedded entirely within the cuboid is calculated also. It is observed that the SIF values along the crack front are larger when the crack is closer to the surface of the cuboid than those when the crack is far away from the surface. Research limitations/implications – The FORTRAN program developed is limited to regular surface cracks which can be discretized by the quadrilateral shape function; it is not very efficient and suitable for irregular crack shapes. Practical implications – The evaluation of the 3D mixed-mode SIFs in the transversely isotropic material may have direct practical applications. The SIFs have been used in engineering design to obtain the safety factor of the elastic structures. Originality/value – This is the first time that the special nine-node quadrilateral shape function has been applied to the boundary containing the crack mouth. The numerical method developed can be applied to the SIF calculation in a finite transversely isotropic cuboid within an inclined surface crack. The computational approach and the results of SIFs are of great value for the modeling and design of anisotropic elastic structures.

Accuracy, reliability and validity of finite element analysis in metal forming: a user's perspective : Table of Contents

A.E. Tekkaya, P.A.F. Martins — Sat Oct 24 08:00:20 BST 2009

Abstract:
Purpose – The purpose of this paper is to provide industrial, education and academic users of computer programs a basic overview of finite elements in metal forming that will enable them to recognize the pitfalls of the existing formulations, identify the possible sources of errors and understand the routes for validating their numerical results. Design/methodology/approach – The methodology draws from the fundamentals of the finite elements, plasticity and material science to aspects of computer implementation, modelling, accuracy, reliability and validation. The approach is illustrated and enriched with selected examples obtained from research and industrial metal forming applications. Findings – The presentation is a step towards diminishing the gap being formed between developers of the finite element computer programs and the users having the know-how on the metal forming technology. It is shown that there are easy and efficient ways of refreshing and upgrading the knowledge and skills of the users without resorting to complicated theoretical and numerical topics that go beyond their knowledge and most often are lectured out of metal forming context. Originality/value – The overall content of the paper is enhancement of previous work in the field of sheet and bulk metal forming, and from experience in lecturing these topics to students in graduate and post-graduate courses and to specialists of metal forming from industry.

Numerical simulation of cutterhead and soil interaction in slurry shield tunneling : Table of Contents

Jianqi Shen, Xianlong Jin, Yun Li, Jiyun Wang — Sat Oct 24 08:00:20 BST 2009

Abstract:
Purpose – This paper aims to provide a 3D finite element (FE) model for dynamic simulation of cutterhead and soil interaction in slurry shield tunneling. Design/methodology/approach – Dynamic numerical simulation of excavation process is realized by combined use of submodeling method and arbitrary Lagrangian Eulerian (ALE) approach. The model size reduction, soil mesh refinement and stress state initialization are fulfilled by submodeling. The large soil deformations, failures and flows are handled by ALE approach. Computation time is reduced by parallel domain decomposition with recursive coordinate bisection method. Validation of the proposed approach is achieved by comparing the numerical results with monitored data from the model test for Yangtze River tunneling project. Findings – The proposed approach proves to be an effective technique to simulate the cutterhead and soil interaction dynamically in tunnel excavation. Comparative study on the effect of mesh density indicates the requirement of relative mesh refinement. Exploration of the parallel computing performance points out the best decomposed domain for the simulation. Parametric study on the effect of rotary speed and investigation on soil properties presents the significant factors for torque. Practical implications – The proposed numerical model can help in the development process of reduced-scale model test, as well as design and selection of slurry shield machines. Originality/value – The originality comes from the need to evaluate the excavation performance of slurry shield machine in tunneling project. This contribution provides a 3D numerical approach, which takes into account the stress state in soil and dynamic contact effects between soil and cutterhead. In this work, large deformation in soil is handled. Besides, soil failures and flows are captured.