Kinematics of the Turbulent and Nonturbulent Interfaces in a Subsonic Airfoil Flow

Abstract

The subject of turbulent and nonturbulent interfaces (TNTIs) has been extensively studied using idealized free-shear flows and zero-pressure-gradient flat-plate boundary layers. However, it remains to be addressed whether a TNTI can be quantitively identified in complex aerodynamic flows where separation bubbles, transition, turbulent boundary-layer separation and an asymmetric wake coexist in a complex spatially developing fashion. Here, we report a direct numerical simulation study at and at a low Reynolds number past a NACA-0012 airfoil at a angle of attack. The threshold-free fuzzy cluster method is used for TNTI identification, and it is corroborated by a joint probability density function-based method. The TNTIs detected are confirmed to be physical a posteriori by the distinctive quasi-step jump behavior in conditionally averaged statistics along traverses normal to the interfaces. The possible connection between the TNTI curvature and local entrainment is also investigated. Airfoil TNTI curvature parameters are found to be noticeably affected by the transitional state of the flow; at the same time, there are only minor differences between the TNTIs in the boundary-layer region and in the wake region. Conditionally sampled results suggest that there is little propensity for local entrainment to occur on either the leading or trailing edge of the TNTIs. Downstream of transition, local entrainment is more pronounced on relatively flat TNTI surfaces for both the airfoil wake and boundary layer.