Abstract
In this paper, direct numerical simulations are performed for low-speed flows past a NACA0012 aerofoil at high incidence angles. The aim is to investigate the significance of quadrupole noise generated due to separated shear layers, in comparison to dipole noise emanating from the aerofoil surface. The two different noise components (dipole and quadrupole) are calculated by using the Ffowcs Williams & Hawkings method in two different approaches: one with a solid surface and another with a permeable surface. The quadrupole noise is then estimated approximately by taking the relative difference between the two. The current study provides detailed comparisons between the quadrupole and dipole noise components at various observer locations in a wide range of frequencies. The comparisons are also made in terms of Mach number scaling, which differs significantly from theoretical predictions and changes rapidly with frequency. Additionally, pre-, near- and full-stall conditions are cross-examined, which reveals significant differences in the quadrupole contributions, including changes in the major source locations and frequencies. It is found that the inclusion of the quadrupole sources gives rise to the predicted noise power level at all frequencies (varying between 2 and 10 dB for an observer above the aerofoil) compared to the dipole-only case. The quadrupole contribution is far from negligible even at the low subsonic speeds (Mach 0.3 and 0.4) when aerofoil stall occurs.
Highlights
Studies on aerofoil trailing edge (TE) noise generated by a turbulent boundary layer scattered at the TE have been well established since the 1970s
For a free-stream Mach number of M∞ = 0.4 and α = 15◦ angle of attack, quadrupole noise was dominant for a wide range of frequencies
The largest differences between the quadrupole and dipole noise are found in the upper half-plane, which was consistent with the separation and transition occurring only on the aerofoil suction side under the chosen flow conditions
Summary
Studies on aerofoil trailing edge (TE) noise generated by a turbulent boundary layer scattered at the TE have been well established since the 1970s. Various studies have been attempted to identify some of the key differences in the noise source mechanisms between the pre- and post-stall conditions These mainly focused on dipole noise at low Mach numbers. Mayer et al (2019) and (Zang et al 2020) identified an increase in low-frequency energy across the full aerofoil surface as the angle of attack is increased They related this to pressure-velocity coherence calculations which indicated that the entire separated region upstream of the TE might have contributed to the radiated sound. Perhaps the most significant is the possible contribution from quadrupole sources This has received virtually no attention since Wolf et al (2012), mainly due to studies limited to very low Mach numbers.
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