Vortex dynamics of transitional airfoil flows over finite and periodic wings with laminar separation bubbles using the lattice-Boltzmann method

The flow over a NACA0012 airfoil at a moderate Reynolds number Re = 50,000 and angle of attack of 3 degrees is investigated using the lattice-Boltzmann method (LBM). The LBM solutions are computed for finite and spanwise-periodic wings in direct numerical simulation (DNS) mode, i.e., without subgrid-scale and wall models. A validation is performed against a Navier-Stokes wall-resolved large eddy simulation for the periodic span, and good agreement is achieved between the different approaches, showing that the LBM can provide accurate solutions of boundary layers under transitional regime, but with a significant computational cost reduction. For the periodic wing, a laminar separation bubble (LSB) forms over the suction side of the airfoil, leading to intermittent vortex shedding that impacts transition to turbulence and the generation of strong spanwise-coherent vortices. Different shedding patterns are observed including the advection of single vortical structures and pairing of two vortices, which may or may not break into finer turbulent scales. For the finite wing, tip effects are investigated on the laminar separation bubble and it is observed that the pressure fluctuation signal from the trailing edge is less intermitent compared to the periodic wing. An analysis of the separation bubbles over both wing configurations shows that frequency modulation by bubble breathing impacts the pressure fluctuation signal on the trailing edge, which in turn is responsible for the airfoil aeroacoustics. This research advances in the comprehension of the LSB behavior in transitional airfoil flows, impacting the unsteady aerodynamics and aeroacoustics of blades and propellers.