Numerical study of an acoustic field effect on boundary layer transition mechanism

Abstract

The laminar-to-turbulent transition mechanism is fundamentally important for the concepts aiming at reducing CO2 emissions and noise pollution. This research is focused on understanding the acoustic excitation effect on the transition mechanism with a laminar separation bubble. The transition over the suction side of the NACA 0012 airfoil at an angle of attack of 2 degrees and Reynolds number of 2 × 105 with zero free-stream turbulence intensity is considered. The investigation is performed based on three flow cases obtained for a smooth and wavy surface modified airfoils using the large-eddy simulation method. The acoustic excitation is controlled by keeping or removing a naturally occurring acoustic feedback loop by introducing surface waviness on the airfoil’s pressure side. The flow solution for the standard NACA 0012 airfoil provides the acoustic excitation case, whereas the airfoil with the wavy surface on the pressure side provides the case without acoustic excitation. The results confirm that a shorter separation bubble and earlier laminar-to-turbulent transition can be triggered in response to the acoustic excitation. Furthermore, the coupling of the acoustic field and hydrodynamic instabilities is investigated. Finally, it is shown by referring to experimental data that the Kelvin-Helmholtz instabilities inside the separation bubble are amplified by the acoustic excitation.