Transonic similarity theory applied to a supercritical airfoil in heavy gas

The use of a high-molecular-weight test gas to increase the Reynolds number range of transonic wind tunnels is explored experimentally. Modifications to a transonic wind tunnel for heavy-gas operation are described, and the real-gas properties of the example heavy gas (sulfur hexafluoride) are discussed. Sulfur hexafluoride is shown to increase the test Reynolds number by a factor of more than 2 over air at the same stagnation conditions and test section Mach number. Experimental and computational pressure distributions on an advanced supercritical airfoil at Mach numbers of 0.7 and 0.72 in both sulfur hexafluoride and nitrogen are presented. Transonic similarity theory is shown to be successful in transforming the heavy-gas results to equivalent nitrogen (air) results, provided the correct definition of gamma is used and viscous effects are not dominant. When strong shocks are present on the airfoil upper surface, transonic similarity theory is shown to be less successful in the shock-boundary-layer interaction region, in agreement with computational predictions.