Noise Induced by Weak Shock Waves in Automobile Exhaust Systems (Effects of Viscosity and Back Pressure)

Abstract

Paper reports on the effects of viscosity and back pressure on shock wave formation in an automobile exhaust pipe. In the present experiment, a periodic wave generator was designed and built for investigating the transition of compression waves into shock waves. Pipes with various diameters were tested to observe compression waves developing into shock waves under the influence of viscosity. A numerical simulation was also conducted using the TVD (Total Variation Diminishing) finite difference scheme applied to the Navier-Stokes equations. It was found that the viscosity hardly affects the transition of compression waves into shock waves as long as the pipe has a practical size suitable to automobile exhaust systems. The back pressure in the exhaust pipe is more important than the viscosity for the transition of compression waves into shock waves.

Subsequently, the mechanism of the noise generation at the exhaust outlet was discussed. From the aeroacoustic analysis, one infers that the magnitude of sound pressure emanated from the open end is proportional to the gradient of pressure variation in the exhaust pipe.