Department of Aeronautics, Imperial College London
Doi
Abstract
Most turbulence studies up to the present are based on statistical modeling, however,
the spatio-temporal flow structure of the turbulence is still largely unexplored. Tur-
bulence has been established to have a multi-scale instantaneous streamline structure
which influences the energy spectrum and other properties such as dissipation and
mixing.
In an attempt to further understand the fundamental nature of turbulence and its
consequences for efficient mixing, a new class of flows, so called “turbulent-like”, is in-
troduced and its spatio-temporal structure of the flows characterised. These flows are
generated in the laboratory using a shallow layer of brine and controlled by multi-scale
electromagnetic forces resulting from a combination of electric current and a magnetic
field created by a fractal permanent magnet distribution. These flows are laminar, yet
turbulent-like, in that they have multi-scale streamline topology in the shape of “cat’s
eyes” within “cat’s eyes” (or 8’s within 8’s) similar to the known schematic streamline
structure of two-dimensional turbulence. Unsteadiness is introduced to the flows by
means of time-dependent electrical current.
Particle Tracking Velocimetry (PTV) measurements are performed. The technique
developed provides highly resolved Eulerian velocity fields in space and time. The
analysis focuses on the impact of the forcing frequency, mean intensity and amplitude
on various Eulerian and Lagrangian properties of the flows e.g. energy spectrum and
fluid element dispersion statistics. Other statistics such as the integral length and time
scales are also extracted to characterise the unsteady multi-scale flows.
The research outcome provides the analysis of laboratory generated unsteady multi-
scale flows which are a tool for the controlled study of complex flow properties related
to turbulence and mixing with potential applications as efficient mixers as well as in
geophysical, environmental and industrial fields