Navier-Stokes Computations of Laminar Airfoils using e* Transition Prediction

Kurzfassung

The DoAL3 and NLF(1)-0416 laminar airfoils are investigated applying a Reynolds averaged Navier-Stokes code coupled to e* transition prediction including computations of the transition zone between fully laminar and fully turbulent flow. The DoAL3 and NLF(1)-0416 laminar airfoils are tested in the transonic tunnel TWB and the DLR Braunschweig and in the low speed NASA Langley Low-Turbulence Pressure Tunnel LTPT, respectively. The free transition measurements include pressure, lift and drag data; for the low speed NLF(1)-0416 laminar airfoil the transition locations are measured in supplement. The limiting N-factor for the TWB is determined beforehand, contrary to the LTPT tunnel, where the limiting N-factor is a priori not known. Based on the transition location measurements on the NLF(1)-0416 laminar airfoil the limiting N-factor for the LTPT tunnel is determined computationally. The computations are executed in an iterative manner applying the Navier-Stokes code, a finite difference boundary layer method for laminar flow which uses the pressure distribution evaluated by the Navier-Stokes method as input and a linear stability method. The boundary layer method is mainly envolved to control the quality of the Navier-Stokes laminar viscous layer results. For the start the transition location is fixed right upstream of laminar separation on both airfoil surfaces in the Navier-Stokes computation. The N-factors of unstable Tollmien-Schlichting waves are then evaluated by the linear stability code. the limiting N-factor determines the new transition location for the subsequent Navier-Stokes run. Usually three iterations produce converged solutions. It is shown, that the computed pressure distributions, the lift and drag polars for both airfoils agree fairly well with the experimental results.