Unsteady pressures and forces during transonic buffeting of a supercritical airfoil

Abstract

Introduction S airfoils have been found to have favorable buffet characteristics compared with conventional airfoils as observed from investigations in the National Aeronautical Establishment High Reynolds number 2D test facility ' The Bauer Garabedian Kprn No 1 airfoil was the first supercritical airfoil to be investigated at NAE and its buffet onset boundary was reported by Kacprzynski 2 His results were found to be in good agreement with theoretical predictions by Redeker and Proksch 3 using Thomas buffet onset criterion The method used for determining experimental buffet onset was to observe the analog signal from a force element of one of the sidewall balances supporting the model and to note the flow conditions when oscillations first appear Other common methods used in determining buffet onset are to detect the divergence of the trailing edge pressure or the break in the lift curve slope These experimental techniques are rather straightforward requiring little sophistication in instrumentation In order to gain a better insight into the aerodynamic aspects of buffet, not just buffet onset, detailed investigations of surface pressure fluctuations are necessary Such an in vestigation was carried out at NAE with the BGK No 1 airfoil in the Mach number range 0 5 to 0 8 at chord Reynolds number 15 to 21 x 10 Both constant incidence and step pause incidence runs were performed The constant incidence runs were of sufficiently long duration that spectral correlation, arid coherence functions for force and pressure data could be obtained. The step pause runs were processed to give only rms values of the force and pressure fluctuations Reference 5 describes the experimental arrangements and procedure for data processing together with detailed statistical data In this Note only some representative results are presented