The onset of dynamic stall at a high, transitional Reynolds number

Abstract

Dynamic stall due to a ramp-type pitching motion is investigated on the NACA 0012 airfoil at chord Reynolds number of $Re_{c}=1.0\times 10^{6}$ through the use of wall-resolved large-eddy simulation. Emphasis is placed on the unsteady boundary-layer interactions that develop as the airfoil approaches stall. At this Reynolds number it is shown that turbulent separation moves upstream across much of the airfoil suction surface. When turbulent separation reaches the leading-edge separation bubble, a bursting event is initiated leading to a strong coherent leading-edge vortex structure. This vortex wraps up the turbulent shear layer to form a large dynamic stall vortex. The use of large-eddy simulation elucidates the roll of the laminar separation bubble in defining the onset of the dynamic stall process. Comparisons are made to identical simulations at lower Reynolds numbers of $Re_{c}=0.2\times 10^{6}$ and $0.5\times 10^{6}$. This comparison demonstrates trends in the boundary-layer mechanics that explain the sensitivity of the dynamic stall process to Reynolds number.

Unsteady Flow Physics of Airfoil Dynamic Stall

“…This transient pressure feature has also been observed in the simulations of Benton and Visbal. [21,22] Finally, the overall fluctuation energy at moderate and high frequencies is observed, from Fig. 3.18 a), to diminish rapidly for angles of attack beyond the suction peak.…”

“…Gross [20], who also used LES to study the flow behavior about a Sirkosky SCC-A09 airfoil at Re c = 0.2×10 6 , for complex motion profiles, which included non-inertial effects from the centrifugal and Coriolis forces. The criticality of the LSB to dynamic stall onset was investigated even further in the recent numerical simulations of Benton and Visbal [21,22] involving an NACA 0012 geometry subjected to a constant-pitch rate maneuver at Re c = 1.0 × 10 6 . These authors demonstrated that, for the given flow conditions, the LSB breakdown was initiated by a direct interaction with the turbulent separation that starts from the airfoil trailing edge and propagates upstream with increasing angles of attack.…”

“…For the current investigation, a pitch rate of Ω + = 0.05 was used, since it has been studied in the past using high-fidelity simulations. [18,19,[21][22][23][24] The experiments were designed to capture the dynamic stall process across a range of angles of attack between α = −5 • and α = 30…”

“…The model was installed vertically inside the test section, with a small gap at the tunnel floor and the ceiling in order to facilitate the dynamic motion of the airfoil. The NACA 0012 geometry was selected since it has been used extensively in the past for dynamic stall investigations [18,19,21,22]. The airfoil model was assembled from three different sections, as shown inFig 2.2.…”

Darrieus vertical-axis water turbines: deformation and force measurements on bioinspired highly flexible blade profiles

“…The loads on a VAWT can successfully be reproduced with numerical methods like large-or detached-eddy simulations, but underlying flow dynamics such as the onset of leading-edge vortices or the occurrence of the vortex shedding are not always reproducible with satisfying accuracy (Ferreira et al 2007). In parallel, experimental research is carried out on dynamic stall and its influence on VAWT, such as in Buchner et al (2017) or Miller et al (2018), or with a focus on simplified models like pitching plates or profiles (Benton and Visbal 2019) in order to provide deeper insights in the underlying mechanism and to develop control strategies for technical applications.…”