The influence of turbulence on the laminar-turbulent transition in the wind tunnel and during free flight

Abstract

During active convection high inflow turbulence is present in the lower earth atmosphere. This is relevant for airfoils with long run of laminar boundary layer since inflow turbulence effects the transition process and location. Therefore, research on the turbulence influence is an important topic in the field of natural laminar flow airfoils during the last decades. Free flight measurements have been used along with intensive wind tunnel investigations to study this effect. Whereas the comparison of free flight measurement data and wind tunnel results is already challenging for steady inflow conditions it is even more complex for higher inflow turbulence. This is mainly caused by the different spectral amplitude distributions for the inflow fluctuations in wind tunnels and in free flight. In the earth atmosphere, large scale fluctuations are always coupled with small scale turbulences. In the wind tunnel the artificial reproduction of large scale fluctuations is difficult due to the geometrically restrictions. However, the present study aims to compare free flight and wind tunnel test results for different inflow turbulences levels using a method which takes the different spectral amplitude distributions into account. Hotwire measurement techniques are employed to measure the time dependent inflow turbulences together with surface mounted hot-film sensors, which are used to capture integral and spectral boundary layer quantities. The latter ones show a dominant Tollmien–Schlichting wave transition process in the wind tunnel and during free flight. Integral values are in good agreement for the suction side of the airfoil but show some systematic differences between wind tunnel and free flight measurements for the pressure side.