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Recent regulatory requirements have introduced, for the first time, catalyst exhaust systems with closed loop air/fuel control into the severe environment of stern-drive and inboard-powered pleasure marine vessels. These engines often maintain consistently high power levels due to vessel drag. Sea water used to cool the engine and exhaust is corrosive, and the engine experiences high g-loads when the planing vessel is used in wavy sea conditions. Engineers must face these challenges in order to develop a durable, efficient, clean-operating, and affordable marine engine. Computational fluid dynamics (CFD) has become a key tool to drive the design optimization of catalyst exhaust systems for marine applicatons. CFD models are used to simulate the unsteady exhaust gas flow of a fired engine. In particular, CFD is used to develop an exhaust system which will promote efficiency, low emissions, and robust closed-loop air fuel control.

Quarter-wave resonators are commonly used as acoustic silencers in automotive air induction systems. Similar closed side branches can also be formed in the idle air bypass, exhaust gas recirculation, and positive crankcase ventilation systems of engines. The presence of a mean flow across these side branches can lead to an interaction between the mean flow and the acoustic resonances of the side branch. At discrete flow conditions, this coupling between the flow and acoustic fields may produce high amplitude acoustic pressure pulsations. For the quarter-wave resonator, this interaction can turn the silencer into a noise generator, while for systems where a valve is located at the closed end of the side branch the large pressure pulsations can cause the valve to fail. This phenomenon is not limited to automotive applications, and also occurs in natural gas pipelines, aircraft, and numerous other internal and external flows.