・ The fixed geometry source which is modeled as a subsystem which accepts gas pressure and maintains constant volume and linear position, regardless of the gas pressure applied.
・ The geometry sink (constant pressure source) which is modeled as a subsystem which maintains constant pressure regardless the volume and volume change rate .
・ The geometrical energy loss which calculates the energy loss during the conversion process of gas energy to useful mechanical energy. The energy loss is calculated using a pressure drop. The heat produced due the pressure drop is dissipated to heat group elements, allowing correct energy calculations.
・ The pressure combiner which sums the gas pressures of two or more female geometry interfaces to a single equivalent one.
2.2.4 Heat transfer group
In order to model the heat transfer taking place in a physical system, the system is divided to smaller parts (called lumps), which may be considered to have uniform temperature. This type of analysis is called the lumped-heat-capacity method [6]. Thus the heat transfer of a system can be calculated by breaking down the system to lumps of uniform temperature each. By increasing the number of the lumps (in which the system has been divided) better accuracy can be obtained. Since the accuracy of the heat transfer model is proportional to the number of the lumps used, the desired level of accuracy can be obtained based on the simulation needs for all the system or for each part of it. Furthermore, using data about the temperature distribution of the physical system and simulation experience, an improved choice on the number and boundary of the lumps can be made. The heat transfer elements introduced here are:
・ The heat capacitor which models a finite amount of mass having uniform temperature (lump) which accumulates heat.
・ The conduction heat flux controller which calculates the heat passing between heat capacitors and/or heat sinks using conduction heat transfer (using the Fourier law).
・ The convection heat flux controller which calculates the heat passing between heat capacitors and/or heat sinks using convection heat transfer (using the Newton law) .
・ The radiation heat flux controller which calculates the heat passing between heat capacitors and/or heat sinks using radiation heat transfer (using the Steffan-Boltzman law).
・ The heat source element which models an idealized source of heat where the heat passed to a heat capacitor is constant and invariant of the properties of the heat capacitor.
・ The heat sink element which models an idealized heat capacitor which acts as a system of infinite mass, thus remaining insensitive of the amount of heat flux entering the system, thus maintaining constant temperature and thermal properties.
・ The heat combiner which sums together the heat flux from two or more heat flux controllers and/or heat sources to an equivalent heat passing to a heat capacitor or heat sink. This enables a heat capacitor or heat sink to be connected with more than one heat flux controller or heat source.
Typically a heat flux controller is placed between two capacitors controlling the amount of heat passed between them. The geometry data required for the calculation of the heat flux, are included in the heat flux controller model (namely the contact area and radiation shape factor) while the physical and thermal properties of the two materials exchanging heat, are provided through the two female heat transfer interfaces the element is equipped. The amount of heat passed, is calculated using a heat transfer model which uses the thermodynamic properties of the two capacitors namely the material's temperature T, thermal conductivity k, radiation emissivity ε, Reynolds number Re and Prandtl number Pr. The heat capacitor uses then the total heat accumulated to calculate the new temperature using the material's mass m and the specific heat at constant pressure cp
2.2.5 Ship group
In order to model and study the complete ship (the engine coupled with the propeller and the hull) models of the hull and propeller are required. The inclusion of the propeller and hull models, enables the study of the interactions between them and the engine and reveals how the performance of each subsystem affects the general response of the system. Studying the complete system also enables the optimization of the it as unit, making possible to estimate the overall system efficiency and find ways to increase it. The ship elements introduced here are:
・ The hull which is modeled using the hydrodynamic derivatives which, is adequate for studying the dynamic behavior of the hull for surface vessels moving in calm water. The ship movements modeled are those interesting for surface vessels moving in calm water which are surge X, sway Y, yaw N and roll K.
・ The propeller which is modeled using a quasi steady state model based on the dimensionless thrust KT and torque KQ diagrams versus the advance coefficient J. Although this method is not very accurate, since does not effectively takes into account the altered flow of the water due to the hull presence and motion, several corrections are introduced in order to circumvent this.
・ The force-torque source element which models externally applied forces to the hull such as wind resistance and tug towing forces, enabling the study of more complex and realistic scenarios.
・ The force-torque sink element which models an idealized situation where the hull can keep constant speed regardless of the forces and torques applied. This element is very useful for studying the behavior of the propeller and force - torque elements under controlled conditions.
・ The force-torque combiner which sums the forces and torques applied from two or more elements to a equivalent single one.
