Local and global stability of airfoil flows at low reynolds number

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Local and global stability of airfoil flows at low reynolds number

Jones, L.E. / Sandberg, R.D. / Sandham, N.D.

A range of compressible airfoil flows have been investigated in terms of their local and global stability characteristics. All of the airfoil flows exhibited separated regions which roll-up into vortex shedding. The separated legions are convectively unstable, and the convective instability growth rates increase with airfoil incidence. All of the airfoil flows, including a NACA-0006 geometry and higher Reynolds number flow, have been found to be absolutely unstable via a three-dimensional instability of the vortex shedding. Growth rates of the absolute instability mechanism are a similar order of magnitude to those of convective instability within the separated shear layer, and the absolute instability mechanism is responsible for the self-sustained transition to turbulence observed in three-dimensional simulations in the absence of boundary layer tripping or forcing. The time-averaged flowfields of several cases were also found to be globally unstable via an acoustic feedback instability. The feedback instability was found to occur only above a certain onset incidence and at M = 0.4. Growth rates ot the feedback instability are low in comparison to convective instability, however it is likely that the mechanism plays a role in frequency selection of the vortex shedding, and potentially for the generation of particular tones in three-dimensional flows.