Surging and plunging oscillations of an airfoil at low Reynolds number

Abstract

We investigate the forces and unsteady flow structures associated with harmonic oscillations of an airfoil in the streamwise (surging) and transverse (plunging) directions in two-dimensional simulations at low Reynolds number. For the surging case, we show that there are specific frequencies where the wake instability synchronizes with the unsteady motion of the airfoil, leading to significant changes in the mean forces. Resonant behaviour of the time-averaged forces is observed near the vortex shedding frequency and its subharmonic; the behaviour is reminiscent of the dynamics of the generic nonlinear oscillator known as the Arnol'd tongue or the resonance horn. Below the wake instability frequency, there are two regimes where the fluctuating forces are amplified and attenuated, respectively. A detailed study of the flow structures associated with leading-edge vortex (LEV) growth and detachment are used to relate this behaviour with the LEV acting either in phase with the quasi-steady component of the forces for the amplification case, or out of phase for the attenuation case. Comparisons with wind tunnel measurements show that phenomenologically similar dynamics occur at higher Reynolds number. Finally, we show that qualitatively similar phenomena occur during both surging and plunging.

Analysis of Flow Timescales on a Periodically Pitching/Surging Airfoil

“…Choi et al [19] investigated surging and plunging airfoils computationally at Re 500 and experimentally at Re 5.7 × 10 4 . In surging experiments, they found attenuation or amplification of the unsteady forces, depending on the reduced frequency k of the motion.…”

Dynamic stall on a pitching and surging airfoil

“…Using a 'quick pitch' motion, they were able to reduce the TEV circulation by at least 60 %. Choi et al (2015) performed computation on surging and plunging flat plates. They showed attenuation or amplification of the unsteady component of the forces depending on reduced frequency due to the dynamics of the leading edge vortex.…”

Feedback control of unstable flows: a direct modelling approach using the Eigensystem Realisation Algorithm

“…and used in many studies (e.g. Taira & Colonius 2007;Colonius & Taira 2008;Ahuja & Rowley 2010;Joe et al 2010;Joe, Colonius & MacMynowski 2011;Ma et al 2011;Tu & Rowley 2012;Brunton et al 2013Brunton et al , 2014Choi, Colonius & Williams 2015;Flinois & Colonius 2015) so only an overview of the formulation of this code is given here. The reader is referred to the cited studies and the references therein for further details.…”