Analysis of resonance phenomena caused by obstacles in HVAC exhaust nozzles using a combined CFD-CAA approach

摘要

In this paper we employ a validated CFD-CAA method for analyzing peaks in the sound spectra of HVAC exhaust nozzles. By analyzing the hydrodynamic pressure and the scaling of the sound spectra we can show that the peaks in the sound spectra are not correlated to the frequency distribution of the flow spectra. In fact, when looking at the spatial distribution of the Fourier coefficient for the acoustic pressure p_A and the hydrodynamic pressure p_H at the peak frequencies, a pure acoustic resonance at these peak frequencies can be observed. As the underlying hybrid method is based on one way coupling only and produces accurate results these resonances can not be caused by a feedback mechanism between acoustic field and flow field. This fact is demonstrated by pure acoustic simulations where the sound field is exited with a singular broadband noise peak upstream the nozzle. This simulation method can reproduce the observed resonances showing that these peaks are in fact trapped modes around obstacles in the HVAC exhaust nozzle. We can show that observed peaks are related to trapped modes caused by obstacles in the HVAC duct and that these trapped modes can exist around many kinds of obstacles in realistic HVAC ducts. Furthermore we present a possible explanation for an effective energy transfer from the flow to the trapped modes.