Aerofoil dipole noise due to flow separation and stall at a low Reynolds number

Abstract

Aerofoil self-noise produced by flow separation and stall is relatively little understood regarding the underlying generation mechanisms. The focus of this work is to provide an improved level of understanding particularly with regard to the dipole noise sources utilising a high-fidelity direct numerical simulation. A NACA0012 aerofoil is considered under three different flow conditions at a Reynolds number Re ∞ = 50, 000 and a Mach number M ∞ = 0.4. These include: a pre-stall condition with a laminar separation bubble (α = 5 •), near-stall (α = 10 •), and fully stalled (α = 15 •). The noise radiation in the far-field is significantly increased at low frequencies for the full-stall case for all observer directions which is consistent with previous experimental observations. The dominant source regions for each configuration are identified for low, medium and high frequencies, separately. A number of key findings are made concerning the source characteristics in full-stall case which differ considerably from the lower angle of attack cases. It is found that the location of the dominant sources changes more significantly with frequency for the full-stall case. Additionally, for medium to high frequencies the maximum acoustic source amplitude is weaker for the full-stall case, despite comparable levels observed in the far-field. This seemingly contradictory observation highlights the importance of phase variations in the wall pressure fluctuations. For the frequencies considered in this paper it is shown that the full-stall case usually produces a relatively more in-phase source distribution, resulting in a more efficient radiation despite the lower amplitude levels. The important flow structures which are responsible for the dipole sources are also identified through analysis of the pressure field at isolated frequencies. It is found that for low frequencies coherent structures in the shear layer are responsible for the scattering of the wall pressure fluctuations at the TE, which agrees with previous findings in the literature. However, at medium and high frequencies the shear layer structures are found to be relatively weak in the proximity of the TE. This indicates that the noise may be generated through other means, for example scattering of fluctuating pressure induced by vortices shed from the TE.

Quadrupole noise generated from a low-speed aerofoil in near- and full-stall conditions

“…In previous work by the authors, the dipole noise was investigated under the same flow conditions (Turner & Kim 2020a). Although it was not the focus of the previous study, there was convincing evidence of significant quadrupole sources, particularly for nearand full-stall conditions.…”

“…In this section, the full-stall case is compared to pre-stall (α = 5 • ) and near-stall (α = 10 • ) conditions. The two additional datasets are carried out based on M ∞ = 0.4 with L z = 0.2L c for the pre-stall case, and L z = L c for the near-stall case (Turner & Kim 2020a). Figure 17 compares the divergence of velocity and local Mach number fields for the three incidence angles.…”

“…2017; Lacagnina et al. 2019; Mayer, Zang & Azarpeyvand 2019) and numerical (Turner & Kim 2020 a , b ) approaches. More specifically, Moreau et al.…”

“…They suggested that noise generation was due to shear layer instabilities, which create a wall pressure footprint that is scattered by the TE. A recent numerical study by Turner & Kim (2020 a ) provided more detailed information of the role of separated shear layers in the generation of stall noise by analysing the frequency filtered pressure field. It was found that for low frequencies, coherent structures in the shear layer dominate, whereas at mid-to-high frequencies, turbulent structures in the wake near the TE are stronger.…”

Experimental characterization of the noise generated by an airfoil oscillating above stall

“…Instabilities and coherent structures in the detached shear layer and shear layer flapping could induce unsteady hydrodynamic pressure on the airfoil surface that can be scattered at the trailing edge and radiate to the far-field. Direct Numerical Simulations (DNS) coupled with a Ffowcs Williams and Hawkings solver were used to study the noise emitted by a stalled NACA0012 at low Reynolds number [7]. The simulations confirm that shear layer vortices are convected on the suction side of the airfoil and induce strong pressure fluctuations that are scattered at the trailing edge as a dipole pulse.…”