On The Effects Of Perturbed Channel Flow On Thermal Field

Abstract

The direct numerical simulation of a fully developed turbulent flow with heat transfer and
the direct numerical simulation of a perturbed turbulent channel flow with heat transfer, has been performed.
Buoyancy effects were neglected, thus the temperature was considered as a passive scalar. For
the first calculation, for non-perturbed flow, the isoflux, constant temperature, and uniform energy source
boundary conditions has been used for the thermal field. Mean and turbulence values of velocity and temperature
fields are compared with data from the literature for the isoflux case. The second calculation
for the perturbed channel flow simulation with heat transfer, has been performed to investigate velocity
and temperature fields dissimilarity. For this second calculation the uniform energy source case for the
thermal field was used. Perturbations were applied into the flow locally by blowing from a span-wise
slot at the lower wall, and suction from a similar slot at the upper wall. In this first work on perturbed
turbulent channel flow, no developing calculation was used, rather than very small values of the transpiration
velocity and slot width has been used in conjunction with a long periodic computational domain.
The main results from this study show that the skin-friction and the Stanton number suffer clear changes
owing to local blowing or suction. While local blowing yields a decreases of the skin-friction and the
Stanton number, local suction increases these coefficients. Qualitatively the effects on both coefficients
are similar for every perturbation. The local extremes, however, for the skin-friction are smaller than
those for St. And also the region of velocity field affected by the perturbation is larger than for the
temperature field. The budgets for the axial mean velocity and for the mean temperature show that the
main source of dissimilarity for blowing is the mean pressure gradient and the convection terms. Mean
pressure gradient make mean velocity to change in the wall region in a slight smoother way than mean
temperature. This small differences in the variation of mean velocity and temperature yields dissimilarities
in the turbulence production in the budget of the second moment of the fluctuations of axial velocity
and temperature. The perturbed mean flow transfers energy to turbulence spreading out its effect into a
larger region than those affected in the thermal field. The responsible for these differences in the mean
flow and mean thermal field is the non-local effect of the mean pressure gradient on convection terms
of the mean momentum equation. For the fluctuations of axial velocity and temperature fields the main
causes of dissimilarity are the small differences in the behavior of the gradient of the mean values, which
yields dissimilarity mainly in the turbulence production term.