Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Drag reduction in pipe flow by optimal forcing   In most settings, from international pipelines to home water supplies, the
drag caused by turbulence raises pumping costs many times higher than if the
flow were laminar. Drag reduction has therefore long been an aim of high
priority. In order to achieve this end, any drag reduction method must modify
the turbulent mean flow. Motivated by minimization of the input energy this
requires, linearly optimal forcing functions are examined. It is shown that the
forcing mode leading to the greatest response of the flow is always of m=1
azimuthal symmetry. Little evidence is seen of the second peak at large m (wall
modes) found in analogous optimal growth calculations, which may have
implications for control strategies. The model's prediction of large response
of the large length-scale modes is verified in full direct numerical simulation
of turbulence ($Re=5300$, $Re_\tau\approx 180$). Further, drag reduction of
over 12% is found for finite amplitude forcing of the largest scale mode, m=1.
Significantly, the forcing energy required is very small, being less than 2% of
that by the through pressure, resulting in a net energy saving of over 10%.

37
The categories of this paper are: supr-con
29
The categories of this paper are: funct-an
10
The categories of this paper are: q-bio
4
The categories of this paper are: cond-mat
33
The categories of this paper are: plasm-ph
6
The categories of this paper are: astro-ph
28
The categories of this paper are: alg-geom
12
The categories of this paper are: nucl-th
21
The categories of this paper are: adap-org
34
The categories of this paper are: chem-ph
26
The categories of this paper are: bayes-an
30
The categories of this paper are: q-alg
35
The categories of this paper are: mtrl-th
31
The categories of this paper are: ao-sci
19
The categories of this paper are: eess
36
The categories of this paper are: cmp-lg