Exploring Airfoil Tonal Noise Reduction with Elastic Panel Using Perturbation Evolution Method

Irsalan Arif, Di Wu, Garret C. Y. Lam, Randolph C. K. Leung

Research output: Journal article publicationJournal articleAcademic researchpeer-review

Abstract

This Note reports an exploratory study of the feasibility of airfoil tonal reduction by means of an elastic panel flush mounted on an airfoil suction surface. The airfoil takes a NACA 0012 profile at an angle of attack of 5 deg with a chord-based Reynolds number of 5 × 10^4 and a Mach number of 0.4. The panel is expected to absorb the energy of boundary-layer instabilities convecting with airfoil flow by means of its own flow-induced vibration. As such, the flow fluctuation responsible for scattering at the airfoil trailing edge as noise and the subsequent aeroacoustic feedback loop that underlies the sustained tonal noise radiation are weakened. A perturbation evolution method is adopted for the feasibility study of panel design due to its lower computational resource requirement than full direct aeroacoustic simulation. The base flow for the perturbation evolution method is obtained from averaging the time-stationary solutions of the accompanying direct aeroacoustic simulation of the same flow with a fully rigid airfoil. To allow the elastic panel to set into continuous flow-induced vibration, the analysis is implemented with a broadband excitation instead of a discrete Gaussian pulse. The nonlinear equation is solved by the conservation element and solution-element method, whereas the panel dynamic equation is solved by the standard finite difference method. A monolithic coupling scheme is invoked for its capability of accurately resolving the nonlinear interaction between boundary-layer instability and flow-induced panel vibration. The perturbation evolution method is applied with various panel structural parameters and panel locations, and the resulting potential for reducing airfoil tonal noise is studied. Generally, all elastic panel designs yield varying levels of tonal noise reduction but maintain more or less the same directivity as the rigid airfoil. It implies that the existence of an elastic panel does not alter the nature of the aeroacoustic feedback loop but only modifies its effectiveness at reducing noise. It is found that a panel located just ahead of the sharp growth of natural boundary-layer instability within the airfoil separation bubble provides the strongest reduction of instabilities that are responsible for scattering into noise at the airfoil trailing edge, and hence provides the most noise reduction among all cases studied. A panel located at the plateau in the boundary-layer instability amplitude or in the proximity of the airfoil trailing edge gives a much lower noise reduction. In addition, for a given panel location, higher noise reduction is achieved when the structural parameters of the panel are tuned in such a way that its fluid-loaded natural frequency is coincident with the dominant frequency of the flow fluctuation passing over it. A resonant panel in the best location is able to yield an almost uniform azimuthal noise reduction of around 2.1 dB, whereas a nonresonant panel at the worst location gives only a 0.5 dB noise reduction. Based on the results of the study, installation of a flush-mounted elastic panel is proven to be a feasible method for airfoil tonal noise reduction. Furthermore, the adopted perturbation evolution method appears to be a viable tool supporting quick panel design iterations to search for optimal noise reduction because it takes only around 10% computational time of the corresponding DAS calculation. The time saving can then be spent on DAS calculation for the optimal panel design for gaining better understanding of the relevant physics of noise reduction.
Original languageEnglish
Pages (from-to)1-11
Number of pages11
JournalAIAA Journal
Volume58
Issue number9
DOIs
Publication statusPublished - 3 Aug 2020

Keywords

  • Airfoil noise
  • Flow-induced structural instability Aeroacoustic-structural interaction
  • Direct aeroacoustic simulation
  • Flow-induced vibration
  • Noise control

ASJC Scopus subject areas

  • Aerospace Engineering
  • Acoustics and Ultrasonics
  • Mechanical Engineering
  • Computational Mechanics

Cite this