textWe study laboratory-produced fluid turbulence under the influence of rapid rotation.
Three-dimensional (3D) turbulence is generated by strong pumping through sources and
sinks at the bottom of a rotating tank (48.4 cm high, 39.4 cm diameter) filled with water.
This flow evolves toward quasi-two-dimensional (quasi-2D) turbulence with increasing
height in the tank. Digital Particle Image Velocimetry (DPIV) measurements were taken
using tracer particles illuminated by a horizontal laser light-sheet. The quasi-2D flow near
the top of the tank contains many long-lived coherent vortices and jets with a wide range of
sizes near the top of the tank. The vertical vorticity field exhibits complex dynamics such
as vortex birth, merger, scattering, and destruction.
Measurements using two synchronized cameras and vertically separated light sheets
revealed coherent structures that were columnar and extended vertically throughout the
tank. The effect of rotation greatly increased the vertical correlation of the flow, even for
small rotation rates. A gradual decay in the correlation of increasingly vertically separated
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planes was observed, rather than a sharp transition.
The discrete wavelet packet transform (DWPT) and discrete wavelet transform
(DWT) were used to extract and study the dynamics of the localized coherent structures
near the top of our tank. We separated the flow into a low-entropy “coherent” and a highentropy
“incoherent” component by thresholding the coefficients of the DWPT and DWT
of the vorticity field. Similar thresholdings using the Fourier transform and JPEG compression,
the Okubo-Weiss criterion and Proper Orthogonal Decomposition (POD) were
also tested. We found that the DWPT and DWT yield similar results and are much more
efficient at representing the total flow than other methods. Only about 3% of the largeamplitude
coefficients of the DWPT and DWT were necessary to represent the coherent
component and preserve the vorticity probability density function, transport properties, and
spatial and temporal correlations of the measured fields. The remaining small amplitude
coefficients represent the incoherent component, which has near Gaussian vorticity PDF,
contains no coherent structures, rapidly loses correlation in time, and does not contribute
significantly to the transport properties of the flow. This suggests that one can faithfully
describe and simulate such turbulent flow using a relatively small number of wavelet or
wavelet packet modes.Physic
