Study of mechanical disturbances effects on the laminar separation bubble by means of infrared thermography

Abstract

The aim of this paper is to show the possibility to increase small wind turbine performances using a Micro Electro Mechanical System (M.E.M.S.) placed inside the blade. In particular its effects on the Laminar Separation Bubble (LSB) phenomenon are presented. This is a local boundary layer separation, that may occur on aerodynamic bodies operating at low Reynolds numbers as the root blade sections of small horizontal axis wind turbine rotors. Its presence induces an aerodynamic efficiency drop-off due to a drag increase and a lift decrease. Tests are performed on an airfoil designed for the root section of a small wind turbine having a 10 kW nominal power. A M.E.M.S. was placed internally the wing section to produce mechanical disturbances directly inside the boundary layer; M.E.M.S.’ lower face is clamped to the wing structure, while the upper one is glued to a PVC movable strip aligned with the airfoil upper surface. M.E.M.S. was electrically supplied at different amplitudes and frequencies in order to vary vibration modes. Preliminary measurements, performed by using a pressure distribution analysis, were carried out to qualitatively locate the LSB presence. Laminar separation, transition and turbulent reattachment points positions are quantitatively detected by means of a thermographic approach: the heated thin foil technique is used to observe temperature distribution on the airfoil surface and a numerical energy balance approach was employed to evaluate the local convective heat transfer coefficient h. Afterward the local Stanton number distribution was calculated and a first and second derivatives analysis allowed to localize the LSB characteristic points positions. Results showed a very effective M.E.M.S. action for frequencies about 175 Hz and a Reynolds number of 10 5. A lift increases about 50%, was showed with a consequent increase of the wind turbine C p values.