Analysis of the onset and evolution of a dynamic stall vortex on a periodic plunging aerofoil

The onset and evolution of the dynamic stall vortex (DSV) are analysed by means of large eddy simulations of an SD7003 aerofoil undergoing periodic plunging motion in a transitional Reynolds number flow (Re = 6 x 10(4)). Interactions between upstream propagating Kelvin-Helmholtz instabilities and a shear layer formed at the leading edge trigger flow separation. The former appear to be related to acoustic waves scattered at the trailing edge due to initial vortex shedding. Two freestream Mach numbers (M-infinity = 0.1 and 0.4) are employed to examine the flow differences due to compressibility variations. The existence of a common timing for the acoustic perturbations in both flows suggests a possible Mach number invariance for the birth of the Kelvin-Helmholtz instability. Increasing compressibility, however, induces earlier spanwise fluctuations, higher flow three-dimensionality and a weaker and more diffuse DSV, which is formed further downstream of the leading edge and has lower residency time. In order to better characterize the onset of the DSV, two empirical criteria are assessed: the leading edge suction parameter and the chord-normal shear layer height. Results demonstrate a higher robustness of the latter with respect to Mach number variations. Modal decomposition, performed with both the classical dynamic mode decomposition (DMD) and its multi-resolution variant (mrDMD), highlights key trends and demonstrates the capacity of the mrDMD to extract physically meaningful flow structures related to the stall onset. Such detailed characterization of the shear layer can be used for a systematic exploration of flow control strategies for unsteady aerofoils.

Automated summary

The onset and evolution of the dynamic stall vortex are analyzed using large eddy simulations. The results show that the Kelvin-Helmholtz instability interacts with the shear layer at the leading edge, triggering flow separation. Comparisons between different Mach numbers reveal that increased compressibility leads to weaker and more diffuse vortices. The chord-normal shear layer height is found to be a more robust criterion for characterizing the onset of the dynamic stall vortex. Modal decomposition methods are used to extract physically meaningful flow structures related to the stall onset.