Preliminary assessment of tunnel wall interference in the NDA cryogenic wind tunnel

Abstract

As the second stage of a preliminary cryogenic airfoil testing program, the test section interference of the National Defense Academy cryogenic tunnel was evaluated. A R4 airfoil model, which has the chord length of 12 cm and the aspect ratio of 0.5, was tested in the range of Mach number 0.5 to 0.75, and that of Reynolds number 7 x 106 to about 1.1 x 10 7. The experimental results were corrected with empirical wall interference correction methods, the Barnwell-Sewall method for sidewall boundary layers, and the Blackwell method for the top and bottom walls. This preliminary evaluation showed that the sidewall boundary layers dominate the tunnel wall interference of the present cryogenic tunnel, and there may be some possibility of utilizing the tunnel for performing two-dimensional airfoil tests if more precise wall interference parameters are obtained. Nomenclature b = tunnel width Cp = pressure coefficient Cp = equivalent pressure coefficient c = chord length of the airfoil cn = section normal force coefficient H = shape factor M - local Mach number Mc = corrected freestream Mach number ME = averaged experimental Mach number at the wall over the airfoil MT = averaged theoretical Mach number at the wall over the airfoil Mx = tunnel freestream Mach number M^, = equivalent Mach number a = angle of attack AMB = blockage Mach number correction AMy = sequential Mach number correction AMw = sidewall Mach number correction 8* = sidewall displacement thickness Subscripts ambient condition