Thermographic Analysis of Mechanical Disturbances Effects on Laminar Separation Bubble

The Laminar Bubble is a local boundary layer separation phenomenon that may occur on aerodynamic bodies operating at low Reynolds numbers. The effect of acoustic disturbances on the laminar bubble behaviour was illustrated in a previous work (Qirt 2006); in this paper are presented the results obtained by means of a M.E.M.S placed inside the tested airfoil. The experimental analyses are carried out in the wind tunnel of the “Universita Politecnica delle Marche” - Department of Energetic. The IR visualizations are obtained by means of the “heated thin foil technique”. The temperature distribution is post-processed by using a Matlab code that allows to obtain the local Stanton number distribution over the airfoil surface. This analysis is performed by varying the airfoil angle of attack and the operating Reynolds number. The results show a laminar bubble reduction by using the M.E.M.S. disturbance effect; the reduction is related to the disturbances frequency and to the bubble position on the airfoil surface. The adverse pressure gradient, related to the airfoil angle of attack, and the local Reynolds number (based on the thickness displacement) are the main parameters that influence the wave disturbance development inside the boundary layer. For the tested angles of attack and Reynolds number the bubble presence is not avoided but there is a marked longitudinal extension reduction: this is clearly observed in the thermographic results. The increasing interest for renewable energies is driving many industries to design small wind turbines having a nominal power rate of some dozen kW. Southern Europe Lands like Italy normally show low annual average wind speeds and so frequently the wind turbines operate at low velocities. The low tangential velocities at the blades root and the low wind incoming velocities may induce a low operating Reynolds number for airfoils used in blade root. Moreover these airfoils normally have a large maximum thickness due to structural design requirement and so in conclusion they are frequently subjected to the laminar separation bubble presence. This phenomenon is a local boundary layer detachment that induces a varied surface pressure distribution: i.e. a constant pressure area. The main problems related to a laminar bubble presence are an airfoil efficiency decrease, that is reflected in a reduced energetic production, and a possible cyclic bubble appearance, that is reflected in periodic pressure oscillations dangerous for the blades fatigue life. This paper describe the use of infrared thermography for a quantitative evaluation of mechanical disturbances effects on the Laminar Bubble presence. The disturbances are supplied by means of a M.E.M.S. installed under the airfoil extrados surface; a sinusoidal power supply is used to obtain an alternative vertical displacement having controlled amplitudes and frequencies. The airfoil is coated with a thin aluminium foil 20 micron thick and a constant AC electric current is supplied to produce a uniform volumetric heat flux boundary condition. The temperature distribution related to the local convective heat transfer coefficient is revealed by the IR thermography and the inverse heat transfer problem is resolved to recognize the fluid dynamic field: so the laminar bubble behaviour.