Validation of Acoustic Simulations and Investigation of Aeroacoustic Flow Effects for Marine Exhaust Components using Finite Element Analysis

Abstract

This work presents two methodologies for numerical acoustic finite element simulations of exhaust components. The methodologies are used to create a case study for a patented mixing silencer component. First, a finite element method -based linear acoustics model for transmission loss calculations in exhaust components is validated. The results are compared to zero-flow measurements of the component, and the model is reported to achieve results that can be considered exact for engineering decision-making and dimensioning in cases where flow is negligible. Second, the work also establishes a state-of-the-art complementary model, based on methods validated for other geometries: A linearised Navier-Stokes model combining computational fluid dynamics and the finite element method to incorporate the effects of a mean flow on the transmission loss of a component. This allows for evaluation of whether flow effects need to be considered in the design. The coupling is one-way, meaning that the fluid solution affects the acoustics, but not the other way around. The main simulations are done on the patented mixing silencer geometry using Comsol 6.4. The silencer is a reactive quarter-wave chamber, which has been combined with another component for space-efficiency in marine applications. Computational fluid dynamics simulations are done using OpenFOAM v13. The results indicate that exhaust flow causes a meaningful decrease in resonant frequency, where maximal transmission loss occurs. Thus, the flow conditions need to be considered in the design and tuning of large-scale exhaust components. Meaningful results are also obtained on the oscillatory nature of the flow through the component during the CFD studies. The created models allow for a multitude of future studies to better understand, characterise and simulate various components from a design perspective.