2018/07/23 by Eltayeb ElJack, Julio Soria, ElJack, Eltayeb +1
Engineering · #Fluid Dynamics and Turbulent Flows #Fluid Dynamics and Vibration Analysis #Particle Dynamics in Fluid Flows
paper · pdf · doi:10.48550/arxiv.1807.08681
The present investigation shows that a triad of three vortices, two\nco-rotating vortices (TCV) and a secondary vortex lies beneath them and\ncounter-rotating with them, is behind the quasi-periodic self-sustained\nbursting and reformation of the laminar separation bubble (LSB) and its\nassociated low-frequency flow oscillation (LFO). The upstream vortex of the TCV\n(UV) is driven by the gradient of the oscillating-velocity across the laminar\nportion of the separated shear layer and is faithfully aligned with it. The\nsecondary vortex acts as a roller support that facilitates the rotation and\norientation of the TCV. A global oscillation in the flow-field around the\naerofoil is observed in all of the investigated angles of attack, including at\nzero angle of attack. The flow switches between an attached-phase against an\nadverse pressure gradient (APG) and a separated-phase despite a favourable\npressure gradient (FPG) in a periodic manner with some disturbed cycles. When\nthe direction of the oscillating-flow is clockwise, it adds momentum to the\nboundary layer and helps it to remain attached against the APG and vice versa.\nThe transition location along the separated shear layer moves upstream when the\noscillating-flow rotates in the clockwise direction causing early-transition\nand vice versa. The best description of the mechanism is that of a whirligig.\nWhen the oscillating-flow rotates in the clockwise direction, the UV whirls in\nthe clockwise direction and store energy until it is saturated, then the\nprocess is reversed. The present investigation paves the way for formulating a\ntime-accurate physics-based model of the LFO and stall prediction, and opens\nthe door for control of its undesirable effects.\n