Experimental validation of XFOIL and Star CCM+ airfoil performance predictions

Abstract

In recent decades, the surge in computational capabilities has made CFD systems indispensable in engineering applications ranging from aircraft and vehicle design to the aerodynamic analysis of advanced aerodynamic devices. Parallel to these simulations, the aerodynamic predictions from XFOIL calculations using the vortex panel method have offered a rapid and robust approach for airfoil design and optimisation, enabling engineers to fine-tune performance based on the predicted aerodynamic behaviour of complex designs. However, because the results of CFD and XFOIL predictions can differ depending on specific airfoil flow conditions, experimental validation remains essential to ensure that computational models reliably mirror real-world aerodynamic predictions.

In this research, two distinct airfoils were investigated: the ST1 airfoil serving as the baseline, which is geometrically similar to the profiles used in Schempp-Hirth Discus-2 sailplanes (Braun (2021)), and the newly developed OPT110, which was designed to offer improved aerodynamic performance over a broad operating speed range. A comparative analysis of lift and drag characteristics predicted by Star CCM+ and XFOIL revealed substantial discrepancies. Star CCM+ predicted that the OPT110 exhibited lower drag at lift coefficients below 0.12, while the ST1 demonstrated lower drag at lift coefficients above 0.12, gradually converging toward that of the OPT110 as lift increased. In contrast, XFOIL predictions consistently showed lower drag for the OPT110 relative to the ST1 at lift coefficients above 0.4 and below 0.24. These discrepancies highlighted the relevance of further experimental investigation.

Attempts to measure the comparative performance of these two airfoils were conducted at the CSIR. While these wind tunnel tests produced ambiguous results, the insights gained were instrumental in developing a vehicle-mounted airfoil testing system. The custom platform enables testing of the computational predictions under environmental conditions which closely correlate with the conditions present in conventional low-turbulence twodimensional wind tunnel testing. The data acquired from the aerodynamic test platform allowed for an extensive comparison between simulated and experimental airfoil performance. The experimental measurements using the aerodynamic test platform confirmed the XFOIL trends, indicating that the OPT110 maintained lower drag compared to the ST1 at lift coefficients above 0.58 and below 0.2.

The detailed comparison of the polar curves demonstrated that the predictions from XFOIL more accurately reflect the real-world aerodynamic behaviour of both the OPT110 and ST1 airfoils when compared to Star CCM+. The findings in this study highlight the importance of experimental validation in the validation of aerodynamic performance, especially with aerodynamically sensitive geometries such as sailplane wing profiles.