Unsteady psp measurements of the shock dynamics on a transonic laminar airfoil

Unsteady Pressure Sensitive Paint is applied to measure and investigate the shock dynamics on a transonic laminar airfoil which exhibits a rapid shock oscillation at a normalized frequency, based on chord and freestream velocity, close to one. The main motivation is to assess the capability of the PSP technique to capture the pressure fluctuations imposed at the upper surface of the wing by the oscillating shock wave, in the light of the recent developments performed at ONERA of the unsteady Anodized-Aluminum PSP method. Beyond frequency resolution, it is found that three main difficulties arise when applying PSP for the present unsteady laminar flow. The first concerns the roughness of the PSP layer, which must remain low to prevent unwanted transition of the laminar boundary layer. The second deals with the correction of illumination as compressibility and wing deformations may affect it. The third is about the correction of the temperature sensitivity of the PSP, which is mandatory in the present high speed flow situation. These three questions are carefully addressed. In addition, measurements for the case when the boundary layer at the upper surface of the airfoil is made fully turbulent by forced tripping are carried out for reference. In this case the dynamics is much slower, with a normalized frequency close to 0.05. The PSP pressure fields offer a rich database to analyze the flow features. In particular the spatial sampling provided by the PSP helps to qualify the two-dimensional nature of the shock movements and to question the wave propagation process implied by these unsteadiness.