A long separation bubble usually starts far behind the leading edge and modifies the pressure distribution on the upper side of the airfoil. This type of bubble is associated with a pressure drag increase and a loss in lift. A short bubble is just behind the leading edge, does not alter macroscopically the surface pressure distribution, and changes only slightly the lift coefficient but at high angle of attack an abrupt stall occurs because of the bubble burst. In this case the lift decreases immediately and a very high pressure drag is observed.
The LSB phenomenon occurs when a laminar boundary layer separates in presence of an adverse pressure gradient. If a transition occurs in the separated shear layer a turbulent reattachment could be reached, the separated region is named ‘bubble’. This phenomenon may induce a body drag coefficient increase. It occurs, for instance, in sailplanes wing tips and wind turbine blades when these operate at low Reynolds number.
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Schematical overview of the principle of the laminar separation bubble
The Experiment
Two small rectangular wings were used in wind tunnel measurements; the first was equipped with pressure taps to carry out standard aerodynamical measurements, and the other has been coated by a thin metal sheet to be used in thermographic investigations with the ThermaCAM SC 3000, through an infrared window placed on the wind tunnel section. An electric supply has been provided at the metal coating to achieve a uniform heat flux boundary condition by Joule effect.
The thermal images obtained by the ThermaCAM SC 3000 showed a localized warmer zone on the wing upper surface; a comparison with the pressure distribution results and the smoke flow visualizations confirmed that this warmer zone corresponded to the bubble position, confirming that the thermographic investigation is able to visualize boundary layer separation phenomena.
The bubble location and dimensions have been studied for many angles of attack, and for different Reynolds numbers; the bubble position seems strongly influenced by the incidence angle, a reduction of the overall dimensions is shown increasing the angle and the Reynolds number.
From left to right: Ing. F. D’Amico, Prof. R. Ricci, Prof. G. Cesini, Ing. S. Montelpare, Ing. E. Silvi
Why infrared thermography?
The methods normally used to investigate the presence of a Laminar Separation Bubble are the load balance, the pressure coefficient analysis and the smoke and oil visualization technique. The choice to use infrared thermography is due to the fact that it is a non-intrusive measurement technique and gives a real time visualisation of the phenomena.
According to Professors Ricci and Cesini, the University of Ancona opted for a FLIR Systems ThermaCAM SC 3000 infrared camera system because of its extremely high thermal sensitivity of 0.02°C making it possible to see the slightest temperature differences. Furthermore, combined with the ThermaCAM Researcher HS package, the SC 3000 allows in-depth analysis of real-time images at 750 Hz.
If you would like more information on the research please contact:
Prof. Renato Ricci - Faculty of Architecture- University “G. D’Annunzio”of Chieti- viale Pindaro n.42 I-65127 Pescara (Italy) Email: r.ricci@unich.it
Prof. Gianni Cesini - Faculty of Engineering - University of Ancona - via Brecce Bianche I-60100 Ancona (Italy) Email g.cesini@unian.it
