Application of Temperature-Sensitive Paint for transition detection on an NLF forward-swept wing under harmonic pitch oscillation at flight-relevant Mach and Reynolds numbers

Abstract

To reduce the fuel consumption and emissions of next generation commercial aircraft, the highly promising natural laminar flow (NLF) technology is being implemented more and more frequently. By maintaining a laminar flow over the largest possible area of both wings through either passive or active measures, the overall aircraft friction drag over these surfaces can be significantly reduced, which results in a distinct increase in the aerodynamic performance of the aircraft. To raise the Technology Readiness Level of this laminar technology in aircraft design, an NLF forward-swept wing fuselage-belly-fairing configuration was designed in the KoPa 33 research initiative ECOWING. In the LuFo VI-2 successor project ULTIMATE, joint wind tunnel experiments in the European Transonic Windtunnel and Computational Fluid Dynamics simulations have been performed on this specific configuration. The various point-wise (e.g. unsteady pressure sensors, static pressure taps, temperature sensors, and accelerometers) and optical measurement techniques that were applied in the wind tunnel tests included also time-resolved cryogenic Temperature-Sensitive Paint (cryoTSP). This latter optical measurement technique was implemented to obtain in particular the location and dynamic motion of the boundary-layer transition on the upper (hence, suction) surface of the forward-swept wing model. The first test campaign, i.e. the performance test, demonstrated a clear influence of variations in flight relevant Mach and Reynolds numbers – both under on-design and off-design conditions – on the spanwise transition distribution, the resultant amount of laminarity over the wing, and the boundary layer transition mechanisms for a wide range of static pitch angles and during various quasi-steady pitch-sweeps at selected Mach and Reynolds numbers. In the current paper, the emphasis is placed on the TSP results of the subsequent dynamic test, in which the focus was shifted to the aerodynamic behavior of the NLF forward-swept wing while forced to perform a harmonic pitch oscillation using a newly-designed pitch oscillator. Besides isolated and combined variations in the Mach and Reynolds number, the influences of the mean angle, amplitude, and reduced frequency of the pitch oscillations were captured successfully by TSP. The analysis of the data using the “D0-method”, an alternative to the “DIT-method”, namely revealed distinct changes in both the location and dynamics of the transition line during pitching limit-cycle oscillations, induced by a separate altering of the value of each of the test parameters. Moreover, it was demonstrated that absolute motions of the transition location as small as O(10−4) m could accurately be resolved by the TSP technique.