Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optimal counter-current exchange networks   We present a general analysis of exchange devices linking their efficiency to
the geometry of the exchange surface and supply network. For certain parameter
ranges, we show that the optimal exchanger consists of densely packed pipes
which can span a thin sheet of large area (an `active layer'), which may be
crumpled into a fractal surface and supplied with a fractal network of pipes.
We derive the efficiencies of such exchangers, showing the potential for
significant gains compared to regular exchangers (where the active layer is
flat), using parameters relevant for biological systems.

22
The categories of this paper are: chao-dyn
33
The categories of this paper are: plasm-ph
6
The categories of this paper are: astro-ph
31
The categories of this paper are: ao-sci
23
The categories of this paper are: solv-int
35
The categories of this paper are: mtrl-th
15
The categories of this paper are: nucl-ex
2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
7
The categories of this paper are: hep-th
24
The categories of this paper are: dg-ga
16
The categories of this paper are: stat
30
The categories of this paper are: q-alg
8
The categories of this paper are: hep-ex
1
The categories of this paper are: cs
18
The categories of this paper are: econ