Airfoil validation using coupled Navier-Stokes and eN transition prediction methods

Navier-Stokes airfoil computations coupled to e(N) transition prediction are feasible, provided the limiting N factor is known beforehand. In the present study a procedure is outlined to validate free-transition airfoil experiments in wind tunnels for which the limiting N factor is not known a priori. The approach does not rely on mesh adaption procedures to obtain adequate laminar viscous layer data from Navier-Stokes computations for the stability analysis. To the contrary, the laminar viscous layer is computed by a boundary-layer method applying as input the pressure distribution from Navier-Stokes computations on initial meshes. Two measurement campaigns are validated: the NLF(l)-0416 laminar airfoil in the low-speed NASA Langley Low-Turbulence Pressure Tunnel LTPT and the NACA 64(2)A015 airfoil in the NASA Ames 12-Foot Pressure Tunnel. The free-transition measurements in both tunnels include pressure distributions and transition locations and, in supplement, for the NLF(l)-0416 laminar airfoil lift and drag measurements. The computational results document the validity of the present approach, the existence of a constant limiting N factor for a specific wind tunnel, and an excellent agreement with the experimental findings.