Intermittent Flow Configurations on Stalled Profiles

The laminar separation, transition and turbulent reattachment over a two dimensional NACA 0012 airfoil are investigated at low Reynolds number (Re of order of 105) in a low speed wind tunnel. 2D LDA measurements are performed and instability of the flow field around the profile has been observed at angle of attack of 10 degrees. In such case the flow field is characterised by strong unsteady coherent structures which are generated by the flow separation and reattachment over the profile. Classical statistical estimators like the mean value and the standard deviation are not anymore valid for describing the flow field due to the intermittent nature of the unsteadiness. A different approach to the flow field analysis looking at the probability density function has been used for pointing out the most probable flow conditions over the profile. This approach allowed a better characterisation of the laminar bubble separation and turbulent flow reattachment. The instability phenomena experimentally observed have been analysed by a numerical simulation using a commercial code, obtaining satisfactory results.