Convection Velocities of Passive Scalar and Velocity Fluctuations in Wall-Bounded Turbulence

Taylor’s hypothesis of frozen turbulence is applied to many experimental use cases to
convert time series into spatial field data. To do so, the concept of a convection velocity
is needed, which characterises the velocity at which coherent turbulent structures move
within the considered flow. This thesis uses a definition of convection velocity from the
literature to analyse the convection behaviour of different quantities in wall-bounded
turbulence. Therefore, a deeper understanding of the underlying physical mechanisms is
gained, which can be used to apply Taylor’s hypothesis more sophisticated. The considered
quantities are structures of all three velocity components as well as passive scalar structures
at three different Prandtl numbers. Direct numerical simulations of periodic channels
at three different Reynolds numbers are conducted to generate data. To validate these
data, typical turbulence statistics are retraced, which show good agreement with results
from the literature. Next, a transport equation analysis is conducted, revealing where
and why Taylor’s hypothesis might not be applicable. The results of this analysis are in
... mehr
good agreement with data from the literature in the case of turbulent velocity structures.
Therefore, trustworthy insights can be gained from the remaining analyses on the behaviour
of passive scalar structures. In the following step, the convection velocities of all structures
considered are inspected regarding their dependency on the wall distance. To validate
these results, known statistics from the literature are used. In addition, new findings
regarding the influence of the Prandtl and Reynolds numbers, different-sized structures,
and physical driving mechanisms are made. Finally, a spectral analysis of the convection
velocity distribution in the Fourier space is performed. The insights resulting from this
provide fundamental information on the physical behaviour, which was shown by the
previous analyses.