Experimental and Numerical Investigations on Control Methods of Cavity Tone by Blowing Jet in an Upstream Boundary Layer

Paper #:
  • 2017-01-1786

Published:
  • 2017-06-05
Abstract:
Intense tonal sound often radiates from flows around a cavity such as a sunroof or various gaps between parts of automobiles, and this sound is referred to as cavity tone. This cavity tone is due to fluid-acoustic interactions in cavity flows, where a vortex impinging at the downstream wall generates an acoustic wave and the acoustic wave propagating in the upstream direction induces another vortex near the upstream edge. The control of the cavity tone by blowing jets into the oncoming boundary layer is focused on. Moreover, the effects of the spanwise pitch of the jets on the flow and acoustic fields were investigated. To clarify the control effects on the cavity flow and tone, both low-noise wind tunnel experiments and direct aeroacoustic simulations based on the compressible Navier-Stokes equations were performed. Main experiments and computations were performed at the freestream Mach number of 0.09, where the oncoming boundary layer is laminar. The ratio of the boundary layer thickness to the cavity length is δ/L = 0.06, where the cavity length is L = 20 mm. The spanwise pitch of the PA or jets was changed in the range of p/L = 0.1 to 1.25. The measured results present that the reduction level of cavity tone with the effective arrangement of the pitch of p/L = 0.5 is more than 20 dB at the jet velocity of 10 % of the freestream velocity. The predicted results without the control show that the vortices with a spanwisely high coherence are shed in the cavity flow and become intense acoustic sources for the cavity tone. In the controlled flow, longitudinal vortices are introduced into the upstream boundary layer by the jets. Measured spanwise distributions of coherent output power (COP) of the acoustic sources are clarified to become non-uniform and lower.
Also in:
  • SAE International Journal of Passenger Cars - Mechanical Systems - V126-6EJ
  • SAE International Journal of Passenger Cars - Mechanical Systems - V126-6
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