Laminar-Turbulent Transition on a DU89-134/14 Airfoil at Low Reynolds Number for use at High Altitude

Oil flow visualisations, infrared thermography and pressure measurements are used to investigate the flow topology around a DU89-134/14 airfoil at chord-based Reynolds numbers Re_c = 2.5 × 10⁵ and Re_c = 5 × 10⁵ for angles of attack α = 1^◦ and α = 5^◦. The general flow topology was investigated via the oil flow visualisations, while mean separation, transition and reattachment locations were determined by infrared thermography and pressure measurements. The experimental measurements are corroborated with 3D Reynolds-Averaged Navier-Stokes (RANS) simulations using the transitional (γ− Re_θ ) SST turbulence model. Two flow regimes over the suction side of the DU89-134/14 airfoil are detected. One occurs for Re_c = 2.5 × 10⁵ and α = 1^◦, where the laminar flow separates, transitions to turbulence, and does not reattach. In contrast, flow reattachment is detected with the consequent formation of a laminar separation bubble for the other observed cases. There is excellent agreement between oil flow visualizations and infrared thermography measurements in predicting separation and reattachment locations. Pressure measurements show good agreement with infrared thermography in predicting separation, transition, and reattachment locations, although erroneous interpretations are possible if there is a lack of a clear pressure plateau characteristic of flow separation. 3D RANS numerical simulations show very good performance for Re_c = 5 × 10⁵, whilst for Re_c = 2.5 × 10⁵ the performance is acceptable before the onset of transition, after which they are not capable of predicting flow features accurately.