Stall cell formation over a post-stall airfoil: effects of active perturbations using plasma actuators

• Published in: Experiments in fluids 2018-09, Vol.59 (9), p.1-16, Article 132
• Main Authors: Esfahani, Ata, Webb, Nathan, Samimy, Mo
• Format: Article
• Language: English
• Subjects: Aerodynamics; Aspect ratio; Dielectric barrier discharge; Engineering; Engineering Fluid Dynamics; Engineering Thermodynamics; Flow visualization; Fluid dynamics; Fluid flow; Fluid- and Aerodynamics; Heat and Mass Transfer; Laminar flow; Particle image velocimetry; Plasma actuators; Research Article; Reynolds number; Stability; Stability analysis; Stalling; Strouhal number; Upstream; Velocity measurement
• ISSN: 0723-4864; 1432-1114
• DOI: 10.1007/s00348-018-2588-y

Stall cells have been observed over stalled airfoils by many researchers and there have been extensive discussions in the literature on their formation mechanism. They have been known to be unsteady and sensitive to even the smallest upstream perturbations, and therefore, can be made spatially steady by fixing the upstream perturbation location. In the current experimental work, flow with a Reynolds number of 5·10 5 over a stalled VR-7 airfoil with an aspect ratio 3 was perturbed using a single nanosecond dielectric barrier discharge (NS-DBD) plasma actuator positioned near the leading edge and covering the span of the airfoil. Surface oil flow visualization and stereo particle image velocimetry were employed to investigate spanwise non-uniformities on the airfoil surface as well as in the flow. The results show a gradual appearance of stall cells by increasing the perturbation Strouhal number to approximately 3 times the natural shedding Strouhal number of St n  = 0.6 (St e  ~ 3St n ). The stall cells become well-defined and eventually saturated with no further changes with increasing perturbation Strouhal number beyond 10St n . The instability responsible for the stall cells is self-sustained as the stall cells persist after the perturbations are stopped. The results of the current work are discussed and interpreted using recent modal and non-modal linear global instability analyses, even though they are carried out in much lower Reynolds number laminar stalled flows over airfoils.