Vortex trajectory hysteresis above self-formed vortex ripples

Abstract

Particle image velocimetry (PIV) using fluorescent tracer particles has been used to measure the velocity field of the fluid above self-formed vortex ripples in an oscillatory flow with suspended sediment. Measured velocities were used to find distributions of phase-averaged velocity and vorticity. Using the distributions, discrete vortices were then identified and tracked. Vortices that originated in the wakes of ripples were carried by the flow to adjacent ripples where they recombined with vortices being generated during the next half-cycle of the flow. Two possible states were observed for similar flow conditions, where vortex migration was either 0.9 or 1.5 ripple wavelengths before recombination. Evidence indicates that this may be the result of two possible ripple wavelengths: one that is approximately twice the flow amplitude, and one that is approximately 1.4 times the flow amplitude. The 2-D circulation strength of discrete vortices, defined as the integral of vorticity over the area of the vortex, was numerically estimated from PIV measurements. Maximum dimensionless values of circulation tend to increase as the Reynolds number of the oscillatory flow increases and to decrease as the mobility parameter (a dimensionless number that quantifies the sediment transport capacity of the oscillatory flow) increases. This is interpreted as an effect of enhanced dissipation of vorticity due to the presence of increasing concentrations of entrained sediment as the mobility number increases.