Spontaneous axis-switching swirling in transonic core-annular flow of highly differentiated fluids

Abstract Forcing fluids to interact at high speeds creates a multitude of instabilities. Prior studies considered the consequences of these instabilities for core-annular flow (CAF) or atomization systems. However, the existence of spontaneous axis-switching swirling (SASS) modalities in CAF have never been explored. We computationally demonstrate this new behavior for a CAF of steam surrounded by a cold non-Newtonian fuel. Wave pile-up created 3-D fuel surface disturbances which eventually became thin slender lobes with radial modal deformation. Those slender lobes were subject to further instabilities and resulted in oscillatory tangential velocities that were a large fraction of the core steam velocity and orthogonal to the axially flowing base fuel wave. Since these tangential velocities were not externally imposed by atomizer geometry or boundary conditions, it was concluded that they are self-generating. Frequency analysis revealed that SASS tended to operate at the base wave frequency. We additionally explored numerical uncertainty with our previously validated computational tool and found that SASS behavior can be achieved on a coarse, limited mesh at relatively low simulation times.