Gas flow across a wet screen: analogy to a relief valve with hysteresis

The flow of gas through a wet fine-mesh screen is analysed in terms of the capillary forces of the liquid wetting the screen and the pressure difference across the screen thickness driving the gas flow. Several different types of time-dependent flow are shown to be possible. The most interesting type is one in which the pressure difference opens small channels in the liquid, which are then closed rapidly by the wetting action of the liquid. The opening and closing exhibit hysteresis, and the flow is highly oscillatory.     

Representation and analysis of transfer lines with machines that have different processing rates

A transfer line is a tandem production system, i.e. a series of machines separated by buffers. Material flows from outside the system to the first machine, then to the first buffer, then to the second machine, the second buffer, and so forth. In some earlier models, buffers are finite, machines are unreliable, and the times that parts spend being processed at machines are equal at all machines. In this paper, a method is provided to extend a decomposition method to large systems in which machines are allowed to take different lengths of time performing operations on parts. Numerical and simulation results are provided.     

Computer-assisted Modelling and Analysis of Linear Programming Problems: Towards a Unified Framework

A framework for model formulation and analysis to support operationsand management of large-scale linear programs is developed fromthe combined capabilities of CAMPS and ANALYZE. Both the systemsare reviewed briefly and the interface which integrates thetwo systems is then described. The model formulation, matrixgeneration, and model management capability of CAMPS and thecomplementary model and solution analysis capability of ANALYZEare presented within a unified framework. Relevant generic functionsare highlighted, and an example is presented in detail to illustratethe level of integration achieved in the current prototype system.Some new results on discourse models and model management supportare given in a framework designed to move toward an ‘intelligent’system for linear programming modelling and analysis.     

Model simplification and digital design of multivariable sampled-data control systems via a dominant-data matching method

A dominant-data matching method is developed for model simplification and design of digital multivariable sampled-data control systems. A mixed method combining dominant-data matching and the dominant-pole technique is also derived for determining a stable reduced-degree multivariable digital controller. A real semiactive terminal homing missile system is used as an illustrative example.     

Variational inequality model for the simulation of gas transmission networks

The steady-state simulation of a gas transmission network involves the solution of a system of nonlinear algebraic equations subject to a set of inequality constraints. The purpose of this paper is to consider how the problem can be cast into a variational inequality framework using the dual extremum principles described by Noble and Sewell in 1972. The reliability of the model is tested on two networks supplied by the British Gas Corporation.     

Modelling of uncertainty in discrete dynamical systems

Dynamic systems that are not Gaussian, stationary and linear are difficult to model by full probabilistic analysis. Sufficient information for practical application can often be obtained by second moment analysis, described in the paper. Alternatively, second moment analysis can be performed using point distributions. Two new methods in this class, one exact for linear systems and one approximate, are described. Examples show the application and illustrate the accuracy.     

Modelling of discontinuities through dipole distribution

An indirect boundary element method using dipole distribution is employed in order to model discontinuities inside the flow region. The problem of flow under a dam is treated with a sheet-pile in its foundation. The discontinuity across the sheet-pile is demonstrated, a general boundary element procedure for a mixed problem is outlined and the coefficients of the linear system are given in analytical form. Very good agreement with existing analytical results is obtained.     

Evaluation of pressure levels in pipelines due to solar heat gains

A detailed numerical model based on a lumped capacity analysis, is developed for assessing potential pressure increases in pipelines due to solar energy gains. Test computations for gasoline and water show that for operational conditions of no pipeline flow, pressures reach levels for which structural damage may occur.     

Modelling considerations in optimum design of reinforced structures

The paper provides some potentially useful modelling schemes for reducing computational cost in designing reinforced structures. The schemes are based on the concepts of element and cross-section idealizations while maintaining the characteristics of the load path. An example of a double layer reinforced panel is given and it is shown how these schemes can lead to increased flexibility and reduced cost in obtaining minimum weight designs.     

Re-entrant lines

Traditionally, manufacturing systems have mainly been treated as either job shops or flow shops. In job shops, parts may arrive with random routes, with each route having a low volume. In flow shops, the routes are fixed and acyclic, as in assembly lines. With the advent of semiconductor manufacturing plants, and more recently, thin film lines, this dichotomy needs to be expanded to consider another class of systems, which we call re-entrant lines. The distinguishing feature of these manufacturing systems is that parts visit some machines more than once at different stages of processing.Scheduling problems arise because several parts at different stages of processing may be in contention with each other for service at the same machine. There may be uncertainties in the form of random service or set-up times, as well as random machine failures and repairs. The goal of scheduling is to improve performance measures such as mean sojourn time in the system, which is also known as the mean cycle-time, or the variance of the cycle-time.In this paper we provide a tutorial account of some recent results in this field. We describe several scheduling policies of interest, and provide some results concerning their stability and performance. Several open problems are suggested.The research reported here has been partially supported by the National Science Foundation under Grant No. ECS-90-25007, and the Joint Services Electronics Program under Contract No. N00014-84-C-0149.     

Optimisation of multi-position machines and transfer lines

A new optimisation problem for design of multi-position machines and automatic transfer lines is considered. To reduce the number of pieces of equipment, machining operations are grouped into blocks. The operations of the same block are performed simultaneously by one piece of equipment (multi-spindle head). At the studied design stage, constraints related to the design of blocks and workstations, as well as precedence constraints for operations are known. The problem consists in an optimal grouping of the operations into blocks minimizing the total number of blocks and workstations while reaching a given cycle time (productivity). A constrained shortest path algorithm is developed and tested.     

Modelling of characteristic delays in multivariable control systems wcbsl

Pure time delays in multivariable control systems place severe restrictions on achievable feedback performance. This paper considers an approach to modelling distributed time-delay systems using discrete convolution. The basis for convolution algebra is briefly outlined and the new concepts of characteristic pattern and vector delays are introduced. A process control example is given that illustrates the concepts and shows typical results obtained using WCBSL (Windows Convolution-Based Simulation Language)     

Modelling of laminate structures in elasto-plasticity with a mixed finite element

The main advantage of the mixed finite element (displacements-stresses) is that, because of its continuity, it gives a good approximate stress field, without needing a high degree of interpolation in displacements. This degree of approximation is essential for elastic-plastic computations. However, the total continuity of stresses is too strong in a laminate structure along the interfaces. We show a method of achieving the correct level of continuity without losing the advantage of a good approximation. Some examples of laminate plane structures with plastic areas along the interfaces are given.     

Optimal control of a magnetohydrodynamic viscous heat-conducting gas flow

The control problem for a one-dimensional flow of a polytropic viscous heat-conducting perfect gas through an interval is considered. The density of external currents is taken as the control. The existence of an optimal control function is proved. Necessary optimality conditions are derived. The compactness of the set of solutions is established.     

Numerical Investigation of Cavitation Interacting with Pressure Wave

A computational fluid dynamics solver based on homogeneous cavitation model is employed to compute the two-phase cavitating flow. The model treats the two-phase regime as the homogeneous mixture of liquid and vapour which are locally assumed to be under both kinetic and thermodynamic equilibrium. As our focus is on pressure wave formation, propagation and its impact on cavitation bubble, the compressibility effects of liquid water have to be accounted for and hence the flow is considered to be compressible. The cavitating flow disturbed by the introduced pressure wave is simulated to investigate the unsteady features of cavitation due to the external perturbations. It is observed that the cavity becomes unstable, locally experiencing deformation or collapse, which depends on the shock wave intensity and freestream flow speed.