Speaker
Description
A variety of experimental techniques has been developed to analyse the aerodynamics of immersed bodies, classified based on their measurement principles. Specifically, phenomena like transition and separation have been observed through different experimental techniques, such as oil flow visualization, the use of pressure and temperature sensitive paint, Particle Image Velocimetry and Infrared thermography. The latter has gained widespread attention due to its non-intrusive nature and has been established as a valuable tool for thermo-fluid-dynamics research. As a matter of fact, separating–reattaching flows exhibit pronounced variations in near-wall flow behaviour that lead to substantial changes in the convective heat transfer coefficient [1]. When the surface temperature exceeds that of the surrounding air, these local variations in convective heat transfer manifest as measurable surface temperature gradients, which can be directly associated with the evolution of the underlying flow field [2].
All the experiments are carried out in a subsonic open-circuit wind tunnel with a rectangular test section of $300$ mm × $400$ mm, with a contraction ratio of $10$ and appropriate screens put at the entrance of the inlet nozzle to ensure a low turbulence intensity level ($0.1 \%$) in the test section.
The tested models are symmetric NACA airfoils, for which the angle of attack is varied, while the Reynolds number is set to $Re_c={10}^5$. A FLIR X6980-HS Infrared camera is used to acquire the temperature maps in each investigated condition. In order to obtain sufficient thermal contrast, the airfoil models are uniformly heated by halogen lamps. Moreover, estimates of the Stanton number are inferred by applying the thin film sensor model to the airfoils. Two-component planar PIV is used to provide benchmark estimates of the mean locations of separation, transition, and reattachment.
A good agreement was found between the estimation of the locations of separation and transition obtained through Infrared thermography and from PIV measurements. Most of all, the results demonstrate the capability of surface temperature measurements to identify and quantify the location of a laminar separation bubble, which is a predominant flow field feature for airfoils at low Reynolds number.
References
[1] Spalart, P. R., and Strelets, M. K., “Mechanisms of Transition and Heat Transfer in a Separation Bubble,” Journal of Fluid Mechanics,Vol. 403, Jan. 2000, pp. 329–349.
[2] Gartenberg, E., and Roberts, A. S. J., “Twenty-Five Years of Aerodynamic Research with IR Imaging,” Journal of Aircraft, Vol. 29, No. 2, 1992, pp. 161–171. 10.2514/3.46140
