Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Effect of axially varying sandpaper roughness on bubbly drag reduction
  in Taylor-Couette turbulence   We experimentally investigate the influence of alternating rough and smooth
walls on bubbly drag reduction (DR). We apply rough sandpaper bands of width
$s$ between $48.4\,mm$ and $148.5\,mm$, and roughness height $k = 695\,{\mu}m$,
around the smooth inner cylinder (IC) of the Twente Turbulent Taylor-Couette
facility. Between sandpaper bands, the IC is left uncovered over similar width
$s$, resulting in alternating rough and smooth bands, a constant pattern in
axial direction. We measure the DR in water that originates from introducing
air bubbles to the fluid at (shear) Reynolds numbers $\textit{Re}_s$ ranging
from $0.5 \times 10^6$ to $1.8 \times 10^6$. Results are compared to bubbly DR
measurements with a completely smooth IC and an IC that is completely covered
with sandpaper of the same roughness $k$. The outer cylinder is left smooth for
all variations. Results are also compared to bubbly DR measurements where a
smooth outer cylinder is rotating in opposite direction to the smooth IC. This
counter rotation induces secondary flow structures that are very similar to
those observed when the IC is composed of alternating rough and smooth bands.
For the measurements with roughness, the bubbly DR is found to initially
increase more strongly with $\textit{Re}_s$, before levelling off to reach a
value that no longer depends on $\textit{Re}_s$. This is attributed to a more
even axial distribution of the air bubbles, resulting from the increased
turbulence intensity of the flow compared to flow over a completely smooth wall
at the same $\textit{Re}_s$. The air bubbles are seen to accumulate at the
rough wall sections in the flow. Here, locally, the drag is largest and so the
drag reducing effect of the bubbles is felt strongest. Therefore, a larger
maximum value of bubbly DR is found for the alternating rough and smooth walls
compared to the completely rough wall.

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The categories of this paper are: quant-ph
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The categories of this paper are: math
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The categories of this paper are: mtrl-th
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The categories of this paper are: nucl-ex
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The categories of this paper are: alg-geom
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The categories of this paper are: chao-dyn
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The categories of this paper are: patt-sol
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The categories of this paper are: math-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: cond-mat
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The categories of this paper are: gr-qc
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The categories of this paper are: bayes-an
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The categories of this paper are: q-bio
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The categories of this paper are: eess
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The categories of this paper are: solv-int
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The categories of this paper are: comp-gas