On the modular modelling for dynamical simulation with application to fluid systems

Abstract: This licentiate thesis highlights some topics on modular modelling for dynamical simulation with application to fluid systems. The results are based on experience from the development of the fuel cell component simulation environment NFCCPP. The general application is cross-enterprise simulation of technical systems. There are four main topics: component definition including selection of interfaces, lumped modelling of fluid components, the use of dynamical equations to reduce simulation time in large systems and methods of to protect the intellectual property (IP) of a component. An overview of different dynamical fluid simulation tools such as HOPSAN, MATLAB/Simulink and Easy5 is presented. Special focus is on interfaces, where different approaches for representing interfaces are presented using an illustrative example. Selecting interfaces is however not a separated task from how to set up and solve the underlying equations, which also is shown. Equations to model a lumped component are derived, to get a mathematical background to what problems there are to solve. These equations are derived especially to be applicable in block model software simulation tools such as MATLAB/Simulink. The equations are also compared with the bond-graph approach of representing dynamical systems. A twinscrew compressor is modelled in MATLAB/Simulink as an implementation of these equations. A method to decrease the simulation time in dynamical fluid system is also presented. The technique is to add virtual mass in the force equation to get a slower acceleration of the fluid. Using this slower response, it is possible to use larger time-steps when integrating the equations and thus the total simulation time can be reduced. The error introduced using this method is a modelling error in the time domain, and it is comparable with using unit transmission lines (UTL:s), as does HOPSAN. The protection of the intellectual property (IP) of a component model is presented. The concept of clamping is thoroughly explained, as it often is overlooked in conventional IPprotection. Three concepts for code protection are presented: “Centralised simulation with remote user control”, “Localised simulation with simulation-time model usage control” and “Parallel distributed simulation”. The NFCCPP implementation of the concept “Localised simulation with simulation-time model usage control” is presented in more detail.