Multi-Rate Simulation Techniques for Electric Ship Design

The power and propulsion system for an electric ship is a complex combination of electrical, mechanical, thermal and fluid dynamic components. Effective analysis of its behavior requires detailed computer simulations. Simulations of the complete system can be computationally very demanding, and execution times can be particularly problematical when multi-run optimization studies or real-time simulations are required. One approach to reducing the computational load in these simulations is to partition the system into a number of subsystems with different dynamic ranges. This allows different parts of the system to be solved with different time steps, thus avoiding the use of a small time step that is dictated by the fastest components, to simulate slow components that could be solved satisfactorily with longer time steps. The result is a simulation with many fewer individual calculations and therefore faster execution. This approach is referred to as multi-rate simulation. Although it offers more efficient execution, it also raises additional questions regarding the accuracy and stability of the simulation. Multi-rate simulation has been investigated using the example of an unmanned underwater vehicle (UUV) in a joint study by California State University, Chico, the University of South Carolina, and the University of Glasgow. The simulation language ESL promises to be a valuable aid in evaluating multi-rate techniques.