Investigation of the Unsteady Behaviour of a Laminar Separation Bubble Using Infrared Thermography
A. G. Guerra (1,2), C. Mertens (1), J. Little (2), B. W. van Oudheusden (1)
(1) Faculty of Aerospace Engineering, Delft University of Technology, The Netherlands
(2) Dept. of Aerospace and Mechanical Engineering, The University of Arizona, United States
The laminar separation bubble (LSB) that forms on the suction side of a modified NACA 64_3 − 618 airfoil at a chordbased Reynolds number of Re = 200000 is studied using wind tunnel experiments. First, the LSB is characterized over a range of static angles of attack, using surface pressure measurements and infrared thermography (IT). For the conditions tested, excellent agreement between the techniques is obtained, showing an upstream shift of the bubble with increasing angle of attack. For the study of steady LSBs, the infrared approach is superior, given its higher spatial
resolution and experimental simplicity. The complexity is then increased to study the influence of aerodynamic unsteadiness on the bubble. For this purpose, a pitching-type motion is imposed on the wind tunnel model, testing reduced frequencies up to k = 0.25. While surface pressure measurements are not affected by the change in experimental conditions, the infrared approach is limited by the thermal response of the surface. To overcome this limitation, an extension of the recently proposed differential infrared thermography (DIT) method is considered. With this method, the unsteady behavior of the LSB can be partially detected. Both experimental techniques indicate a hysteresis in bubble location between the pitch up and pitch down parts of the motion, caused by the effect of the
aerodynamic unsteadiness on the adverse pressure gradient. However, the DIT measurements indicate a larger hysteresis, which is attributed to the thermal response time of the model surface.