Velocity and Temperature Natural Dissimilarity in a Turbulent Channel Flow

Abstract

Natural dissimilarity or de correlation of axial velocity and
temperature fluctuations, in a tur bulent channel flow, is studied
using direct numerical simulation, DNS. Buoyancy effects were
neglected, thus the temperature was considered as a passive scalar. A
uniform energy source case for the thermal field has been
used. Results for molecular Pr or Sc numbers equal to 1.0 and 0.71 are
presented. More evidences of the strong correlation of axial velocity
and temperature in the wall layer are shown, like as the similar
patter of the skin friction and streamwise vorticity correlation, with
that between wall heat flux and streamwise vorticity correlation. The
importance of the most energetic events on the dissimi larity between
the axial velocity and temperature fluctuations is examined using
conditional probability. It is shown that although the most energetic
events are responsible of the strongest instantaneous dis
similarities, their contribution to the mean dissimilarity is less
than a half in the whole channel. As a complement to many previous
results in the literature analyzing fluctuations of longitudinal
velocity and temperature in frequency domain, spectral density
functions is used in order to study dissimilarity. The results
presented here include new variables, as the spectra of the
fluctuations of axial velocity and temperature difference, and the
spectra of the fluctuations of the pressure field. Spectral density
functions at different distances from the wall show, that the main
cause of dissimilarity between axial velocity and temperature
fluctuations is the shift toward higher frequencies of temperature in
comparison to any velocity components, and specially to axial
velocity, in the viscous, buffer, and beginning of the logarithmic
region. However, in contrast with this situation next to the wall,
there is a general tendency to spectral convergence at the center of
the channel. Based on the spectra of the fluctuations of the pressure
field, it appears that one can conclude that such actions next to the
wall and at the center region are driven by the pressure field. It is
speculated, however, that the commented convergence at the center
region can be greater for higher Reynolds numbers than that used in
the present work.