Understanding turbulent free-surface vortex flows using a Taylor-Couette flow analogy.

Area of Science:

• Fluid Dynamics
• Turbulence Research
• Vortex Dynamics

Background:

• Free-surface vortices are crucial for understanding natural and technological rotating flows.
• Despite extensive study, their turbulent, three-dimensional flow fields remain incompletely understood.
• The Taylor-Couette flow system, exhibiting Taylor vortices, is a well-characterized related phenomenon.

Purpose of the Study:

• To investigate the presence and characteristics of secondary vortices in turbulent free-surface vortices.
• To explore the analogy between free-surface vortex dynamics and the established Taylor-Couette flow system.
• To analytically determine the conditions for instability in wall-bounded free-surface vortices.

Main Methods:

• Utilized 2D ultrasonic Doppler velocity profiling for flow observation.
• Employed laser-induced fluorescence dye for enhanced visualization.
• Performed three-dimensional numerical simulations for validation.

Main Results:

• Identified distinguishable, time-dependent "Taylor-like" vortices in the secondary flow field of free-surface vortices.
• Confirmed the existence of these vortices through complementary experimental and numerical methods.
• Analytically demonstrated that free-surface vortices can become unstable via centrifugal forces, akin to Taylor-Couette flow.

Conclusions:

• The free-surface vortex exhibits "Taylor-like" vortex structures, suggesting a strong analogy with Taylor-Couette flow.
• Rayleigh's stability criterion confirms the potential for instability driven by centrifugal forces.
• This analogy allows for the application of established Taylor-Couette flow knowledge to better understand free-surface vortex stability and structure.