Turbulent and Non-Turbulent Interfaces in Low Mach Number Airfoil Flows
This thesis reports the first successful identification and investigation of turbulent/non-turbulent interface (TNTI) in airfoil flows – all the previous TNTI studies were confined to canonical flows such as wakes, jets or flat-plate boundary layers. In the present study, airfoil TNTIs were detected by the Fuzzy Cluster Method and confirmed to be physical a posteriori by the distinctive quasi-step jump behaviour in conditionally-averaged statistics along traverses normal to the TNTIs. To investigate the kinematics of airfoil TNTIs, the NACA-0012 airfoil flow at 2-deg angle of attack (AoA), chord-based Reynolds number Re_c=100,000 and Mach number Ma=0.5 was simulated. We compared the TNTI statistics in four flow regions: transitional and turbulent boundary layers, upper and lower wakes. Airfoil curvature parameters are noticeably affected by the transitional state of the flow; there are only minor differences between the turbulent-boundary-layer TNTI and the turbulent-wake TNTI. Downstream of transition, local entrainment is more pronounced on relatively flat TNTI surfaces. To investigate the dynamics of airfoil TNTIs, three DNS cases were considered: the cylinder flow at diameter-based Reynolds number Re_D=10,000 and Ma=0.2 with a symmetric wake and strong vortex shedding, the NACA-0012 flow at 2-deg AoA with a slightly asymmetric wake and weak vortex shedding, and the NACA-0012 flow at 10-deg AoA, Re_c=60,000 and Ma=0.3 featuring a highly asymmetric wake and strong vortex shedding. We investigated four subzones of the instantaneous TNTIs: leading edge (LE), trailing edge (TE), bulge and trough. The zonal conditionally-sampled invariants of the velocity gradient tensor, enstrophy production and orientation of local structures suggest that wake TNTI properties depend more heavily on the degree of vortex shedding and relatively less on the degree of wake asymmetry. Random relative orientation between the vorticity vector and the TNTI normal is observed in the trough, which casts doubts on the notion of vortex structure confinement across the entire TNTI. The turbulent flow near the TE is found to be effective in enstrophy production, whereas the turbulent flow near the LE is the least effective. Overall, the present work expanded the TNTI research field into the domain of aeronautical flow.
URI for this recordhttp://hdl.handle.net/1974/28778
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