Finite Element Based Method for Investigation of Exterior Acoustics

Abstract

A challenge with renewable energy is that the power source is time varying. To be able to supply renewable power at all hours, the company Azelio is developing a Thermal Energy Storing (TES) system. During the development, multiple design changes are made at a rapid pace. Therefore, it is important to utilize numerical tools to save time and money. Azelio’s TES system is Stirling engine based. The free mass loads in the engine causes vibrations, which make sound propagate from the system. The sound level is mostly not an issue since the system will be located in uninhabited areas, but it can be of importance if the system would be serviced during operation. That is why a numerical method for determining the exterior sound level would be of interest. The purpose is, thereby, to develop a generic method that can be used to predict exterior sound. The method will be developed for the current TES system, but should be possible to use when evaluating new designs. A numerical method for predicting the exterior far field sound level located in a free field was developed using the finite element method in Ansys Mechanical APDL. The exterior acoustics problem was solved using a one-way coupling from structure to fluid. The structural vibrations were computed through mode-superposition in a harmonic analysis, in which the system was assumed to be linear and time-invariant. The vibrations were then used to generate waves in a fluid full harmonic analysis. To validate the numerical model of the system, an experimental modal analysis was performed on the current TES system, but no measurements during operation were made. Due to the complexity of the system and time restriction, the results from the experiment lacked in correlation with the results from the finite element analysis. However, this does not say anything about the developed sound predicting method. One conclusion that can be drawn is that it is difficult to model and do experiments on a complex system. The developed method is generic and can be applied to any vibrating system. It can be used to compute the sound pressure level in a plane or a point, at various engine speeds. The sound level in a point can be compared to measurements, while the plane contour plot can be used in the development process to identify parts that radiate a lot of sound. The method was developed for a complex system and should be tested on a simpler system. To validate the method, various tests should be performed.