Numerical study on using sulfur hexafluoride as a wind tunnel test gas

A NUMERICAL study is presented that investigates effects of using sulfur hexafluoride, SF6, as a substitute for air. Inviscid results for airfoils indicate that for transonic cases the shock location calculated for SF6 is vastly different from that in air and moves progressively forward on the airfoil as the freestream pressure is increased and real gas effects become more pronounced. Application of a simple Mach number scalling procedure results in good correlation between SF6 and air even for pressures at which nonideal gas effects are significant. Computations for subsonic turbulent flows over a NACA 0012 airfoil show that the maximum angle of attack at which steady lift can be obtained is different between air and SF6. In addition, for SF6, this angle of attack depends on the freestream conditions. Improved agreement with air can be achieved by altering the freestream Mach number according to the inviscid scaling procedure. Transonic results show that even with Mach number scaling, the shock location and skin friction values calculated between air and SF6 are in disagreement. This is attributed to the limitations of the scaling procedure and to a thinner boundary layer for SF6.