This thesis aims at getting a better understanding of the properties, scalings and
similarities of turbulent axisymmetric wakes, as well as possible applications that
arise from the information learnt.
Over the last 60 years, axisymmetric wakes have been generated using axisymmetric
bodies, such as disks, spheres and bodies of revolution, and key parameters
such as the drag coefficient, shedding frequency and similarity and scaling of the
wake width and velocity deficit have been documented and verified by numerous
experimental and numerical studies. However, in this thesis the aim is to use asymmetric
wake generators to generate the axisymmetric wakes and see if this has any
effect on the results. These asymmetric wake generators are made up of a square
plate and a number of fractal plates, where the perimeter of the plates can be
increased by as much as 16 times that of the square. As well as increasing the
perimeter, the irregularity, or fractal dimension, is also increased.
It is found that the drag coefficient of the fractal plates is increased to beyond the
values observed for regular polygons and a theory is presented that could explain this
possible change in the drag coefficient, whereby the drag coefficient is the product
of the volume of the wake and the dissipation of the turbulent kinetic energy within
the wake. Wake profiles were taken over a moderate downstream distance of up to
50l, where l is the characteristic length of the plates, defined as the square root
of the frontal area. Using the measured integral width of the wake directly, it was
found that the volume of the wake decreased with increasing fractal dimension and
iteration. Using these values, the similarity and scaling of the wake was carried out
and a new high local Reynolds number scaling for turbulent axisymmetric wakes was discovered and for which the data from the fractal plates fit very well. The
intensity of the vortex shedding is also shown to decrease with increasing perimeter
and fractal dimension and it is found that the rate at which these vortices are shed is
the same for all plates if the characteristic length is used to normalise the frequency.
It is also discussed how the decrease in the energy of the vortex shedding is linked
to the volume of the wake.
Finally, the use of fractal geometries to manipulate the wake to reduce noise is
also investigated, with emphasis placed on various aspects of an aircrafts wing.