Determination of the Critical Cross Flow N-factor for the Low-Speed Wind Tunnel Braunschweig (DNW-NWB)

In order to determine the critical cross flow N-factor for DNW-NWB wind tunnel, transition experiments are conducted on a 3D-wing model. Infrared thermography is used to detect spanwise the transition line at various angles of attack and Reynolds numbers. The obtained transition lines are then prescribed in RANS flow simulations of the experiment. Calculated boundary layer data of laminar regions serves directly as input for a subsequent analysis of boundary layer stability. Employing local linear stability analysis, critical N-factors are attained at the transition location for each inviscid streamline. A 2-N-factor method is used for individual treatment of Tollmien-Schlichting and cross flow instability. From the broad variety of observed transition scenarios, cross flow dominated cases are filtered. Those are used to calculate an uncertainty weighted mean value of the critical cross flow N-factor for NWB facility. The results show N_CFcrit to be in the range of 7.4–9.3. Remaining scatter and a noticeable dependency of N_CFcrit to the surface side of the model are likely to be related to methodological shortcomings of the e^N method.