Experimental Investigation of a Laminar Separation Bubble Subjected to Wing Structural Motion

Abstract

Unmanned aerial vehicles have proliferated in the last few decades, with applications that include military, commercial and recreational. Their size and typical flight velocities are characterized by moderate Reynolds numbers O(10^4-10^5). For such conditions, boundary layers can remain laminar and therefore are highly susceptible to separation and the generation of laminar separation bubbles (LSBs), when compared to more conventional aircraft. Extensive research has been done to study the influence of Reynolds number, angle of attack, sweep angle or freestream turbulence level on the nature of LSBs. However, the study of LSBs subject to unsteadiness is rather limited. This is especially relevant for small aircraft that typically fly in gusty environments such as cities. The problem is aggravated by the recent shift toward composite manufacturing, which allows more efficient high-aspect-ratio configurations, but that deform considerably more when subjected to unsteady loads.
The LSB that forms on the suction side of a modified NACA 64_3-618 airfoil at a chord-based Reynolds number of Re = 200k is studied in a series of wind tunnel experiments conducted at The University of Arizona. Three different flow measurement techniques are considered in the experiments to identify the bubble: surface pressure measurements, Particle Image Velocimetry and Infrared Thermography. The capabilities of the three techniques are first explored in a static characterization of the LSB over a range of angles of attack. For the conditions tested, excellent agreement between the techniques is obtained, showing an upstream shift of the bubble with increasing incidence. For the study of static 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, two different types of structural motion are imposed on the wind tunnel model. A first experiment considers a pitching-type motion, with reduced frequencies up to k = 0.25. While surface pressure measurements and PIV are not heavily affected by the change in experimental conditions, the infrared approach becomes 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. Even so, the unsteady behaviour of the bubble can be only partially detected with this method. All-three techniques considered 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.
The second type of structural motion studied consists of a sinusoidal plunge, with an amplitude of h = 6% of the airfoil chord and a reduced frequency of k = 0.67. The surface pressure measurements and PIV still capture a hysteresis in bubble location along the cycle, expressed in terms of the effective angle of attack induced by the plunging motion. However, due to the increased frequency of the motion, the thermal response of the surface reduces and the infrared approach fails to detect the unsteady bubble.