Fiber-flow interaction in the near field of a coaxial round jet

The instantaneous interaction (one-way coupling) between inertial fibers and flow field in a coaxial, round water jet was investigated using planar, time-resolved particle image velocimetry at a ratio between outer and inner jet bulk velocities of ru=2.5. Nylon fibers with aspect ratio of 31 were issued from the inner jet and tracked across the field of view in the downstream direction. Results indicated that translational and rotational fiber motion strongly depended on the instantaneous flow field. Close to the nozzle exit, fibers lagged the flow and were preferentially aligned with the streamwise direction. Farther downstream, mean fiber velocities exceeded those of the mean flow whereas preferential alignment was lost. Individual fiber tracks showed an abrupt change ("transition") in velocity and orientation when interacting with the internal shear layer at different streamwise locations. Abrupt jumps in fiber velocity, orientation, rotation rates, as well as translational and rotational slip were revealed by conditionally sampling the tracks according to transition location. This was done for two groups of fibers. For fibers in the first group, transition occurred farther downstream and was less pronounced compared to those in the second group. The latter were entrained by strong toroidal vortices that rotated and propelled fibers into the fast-moving outer jet ("sling effect"). Magnitudes of rotational slip strongly increased at transition, reaching plateau values farther downstream. Local translational and rotational Stokes numbers based on fiber motion and flow field itself illustrate the complex fiber-flow interaction in this nonhomogeneous flow.