A Novel Acoustic Feedback Mechanism Originating From Within the Laminar Separation Bubble

Abstract

A computational aeroacoustic analysis of tonal noise generated by a laminar separation bubble (LSB) in close proximity to the airfoil trailing edge (TE) is completed through the study of the SD 7037 airfoil at the Reynolds number, Re, of 41000. Analysis of the boundary layer transition uncovered a new feedback mechanism originating from within the suction side LSB caused by the lift-off and subsequent touchdown of the Kelvin-Helmholtz (K-H) rolls. The numerical methods used are incompressible wall-resolved Large Eddy Simulation (LES) and the Ffowcs-Williams and Hawkings (FW-H) acoustic analogy, and results are validated against aerodynamic and aeroacoustic experiments. The simulation accurately predicts amplified tonal noise at 1 deg angle of attack and the tonal noise source is identified as the K-H rolls shedding past the airfoil TE. The LSB feedback mechanism is found to operate independently from the TE acoustic feedback typically discussed for tonal noise amplification. An estimation of the interference between the TE acoustic feedback and the LSB feedback predicts constructive interference which is consistent with the further amplification of the tonal noise seen in the validating acoustic measurements. The analysis shows that K-H rolls in the LSB are both the source of the tonal noise generated at the TE and a mechanism of feedback originating from within the LSB.