Reynolds number effect on drag reduction in pipe flows by a transverse wall oscillation
arxiv(2023)
摘要
Direct numerical simulations of turbulent pipe flow with transverse wall
oscillation (WWO) and with no transverse wall oscillation (NWO) are carried out
at friction Reynolds numbers Reτ = 170, 360, and 720. The period and
amplitude of the oscillation are selected to achieve high drag reduction in
this Reynolds number range, and the effect of increasing Reynolds number on the
amount of drag reduction achievable is analyzed. Of a particular interest in
this study is the identification of the scales of motion most affected by drag
reduction at different Reynolds numbers. To answer this question, both
one-dimensional and two-dimensional spectra of different statistical quantities
are analyzed with and without transverse wall oscillation. The effect of wall
oscillation is found to suppress the intermediate- and large-scale motions in
the buffer layer of the flow, while large-scale and very-large-scale motions in
the log layer and the wake region are enhanced. While suppression of the
near-wall turbulence promotes drag reduction, enhancement of the large-scale
motions in the log and the wake region is found to oppose drag reduction. Since
higher Reynolds number flows support development of a growing range of
large-scale structures, it is suggested that their prevalence in the energy
spectra combined with their negative effect on drag reduction account for a
reduced effectiveness of wall oscillation as a drag reduction mechanism with
increasing Reynolds numbers.
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