Experimental study of open pipe noise radiation with inflow conditions

Noise reflection and transmission at the outlet of an open pipe are required to estimate tailpipe noise of ICE exhaust and intake circuits. At both ends of these systems, wave is interacting with a stationary outflow or inflow. Without flow, incident wave is nearly totally reflected in phase opposition at duct outlet and far field radiation is nearly monopolar. In the outflow case, interaction of wave with turbulent eddies has been extensively studied recently. Published data is scarce on inflow conditions; in consequence experimental investigations on this topic are still needed. To that end, the reflection coefficient at the outlet of a baffled and unbaffled open pipe is measured using a two microphones technique and radiation is evaluated by measuring the transfer function between the internal and external pressures. Tests are carried out for Helmholtz numbers up to 1.5 and Mach numbers up to 0.15. In presence of inflow, interaction between wave and flow turbulence inside the pipe, renders measurement noisy. Placing microphones further from the inlet; reduces this issue but phase measurements at low Strouhal numbers are still inaccurate. Results show that energy reflection is not affected by the inflow and outside radiation remains monopolar. In this case, a simple analitycal model for reflection and radiation can be used.