Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Classification of vortex patterns of oscillating foils in side-by-side
  configurations   The unsteady hydrodynamics of two in-phase pitching foils arranged in
side-by-side (parallel) configurations is examined for a range of Strouhal
number and separation distance. Three distinct vortex patterns are identified
in the Strohual number-separation distance phase maps, which include separated
wake, merged wake, and transitional-merged wake. Furthermore, a novel model is
introduced based on fundamental flow variables including velocity, location,
and circulation of dipole structures to quantitatively distinguish vortex
patterns in the wake. The physical mechanism of wake merging process is also
elucidated. When an oscillating foil experiences the jet deflection phenomenon,
secondary structures shed from the primary street traverse in the other
direction by making an angle with its parent vortex street. For parallel foils,
secondary structures from the vortex street of the lower foil interact with the
primary vortex street of the upper foil under certain kinematic conditions.
This interaction triggers the wake merging process by influencing circulation
of coherent structures in the upper part of the wake. It is unveiled that
merging of the wakes leads to enhancements in propulsive efficiency by
increasing thrust generation without a significant alteration in power
requirements. These are attributed to the formation of a high-momentum jet by
the merged vortex street, which possesses significantly larger circulation due
to the amalgamation of the vortices, and major alterations in the evolution of
leading edge vortices. Thus, flow physics that are thoroughly explored here are
crucial in providing novel insights for future development of flow control
techniques for efficient designs of bio-inspired underwater propulsors.

22
The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th
11
The categories of this paper are: hep-lat
15
The categories of this paper are: nucl-ex
24
The categories of this paper are: dg-ga
13
The categories of this paper are: quant-ph
1
The categories of this paper are: cs
9
The categories of this paper are: nlin
27
The categories of this paper are: comp-gas
17
The categories of this paper are: q-fin
19
The categories of this paper are: eess
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
28
The categories of this paper are: alg-geom
2
The categories of this paper are: math
37
The categories of this paper are: supr-con