Reducing tonal sound from a cascade of flat plates by varying the thickness ratio of neighboring plates

To mitigate the tonal noise from air flow around a cascade of flat plates, such as the grills in front of automobile engines, the thickness ratio of neighboring plates was varied in numerical simulations. Direct aeroacoustic simulations were based on compressible Navier-Stokes equations. For the baseline case with all plates at the thickness ratio of 1.0, the tonal sound is radiated because of the acoustic resonance occurring between the plates. In a controlled flow with plates alternating between large and small thicknesses, the width of the wake behind each plate varies between the neighboring plates. This indicates that the flow around the cascade of flat plates becomes biased. Simulated sound-pressure spectra show that the intensity of tonal sound at the fundamental frequency is reduced by the large thickness ratio between neighboring plates. Also, a secondary intensity peak appears at a frequency higher than the fundamental frequency. This peak appears because vortex shedding occurs at this higher frequency in the wakes behind the thin plates, whereas it occurs at the fundamental frequency in the wakes behind the thick plates. To elucidate the flow and sound related to the tonal sound at the fundamental frequency, the flow fields were phase-averaged with reference to the vortex shedding from the thick plates. With all plates of the same size, two-dimensional large-scale vortices are formed in the wakes of all the plates. Moreover, vortex shedding occurs in the anti-phase mode between the neighboring plates, which reinforces acoustic resonance. Meanwhile, for controlled flow with the plates of alternating thickness, the vortices lose their structure and spread into finer-scale vortices. The synchronization of vortex shedding between the neighboring plates is suppressed, which weakens the tonal sound from acoustic resonance.