Aerodynamicists reveal link between fish scales and aircraft drag

Date:
October 7, 2020
Source:
City University London
Summary:
A new research study has revealed that fish scale arrays generate
a streaky base flow on the surface of the animal which yields
important clues into reducing drag - the aerodynamic force that
opposes an aircraft's motion through the air - by more than
25 percent.



FULL STORY ==========================================================================
The team's findings have been published in Nature: Scientific Reports: "Transition delay using biomimetic fish scale arrays," and in the Journal
of Experimental Biology: "Streak formation in flow over biomimetic fish
scale arrays."

========================================================================== Reducing drag means faster aircraft speeds and less fuel consumption --
an important area of study for aerodynamicists such as Professor Bruecker, City's Royal Academy of Engineering Research Chair in Nature-Inspired
Sensing and Flow Control for Sustainable Transport, and City's Sir
Richard Oliver BAE Systems Chair for Aeronautical Engineering.

Through their biomimetic study, Professor Bruecker's team has discovered
that the fish-scale array produces a zig-zag motion of fluid in
overlapping regions of the surface of the fish, which in turn causes
periodic velocity modulation and a streaky flow that can eliminate Tollmien-Schlichting wave induced transition to reduce skin friction
drag by more than 25 percent.

An examination of oil flow visualisation using computational fluid
dynamics (CFD) on sea bass and common carp enabled the authors to come up
with a working hypothesis: "Computation Fluid Dynamics (CFD) was used to
study the flow pattern over the surface and revealed a hitherto unknown
effect of the scales as a mechanism to generate a regular pattern of
parallel streamwise velocity streaks in the boundary layer. To prove their existence also on the real fish skin, oil flow visualisation was done
on sea bass and common carp, which indeed confirmed their presence in a
regular manner along their real body, with the same arrangement relative
to the scale array as observed along the biomimetic surface. These
results let the authors hypothesize about a possible mechanism for
transition delay, inspired by various previous fundamental transition
studies, where streaky structures generated by cylindrical roughness
elements or vortex generator arrays have shown a delay of transition."
Using the specially equipped laminar water tunnel at the University of Stuttgart in Germany, Professor Bruecker and Professor Rist (University
of Stuttgart) have tested the hypothesis of a transition (drag) delay
by experimenting with a smooth flat plate and a flat plate covered with biomimetic fish scale arrays.

Their surprising research outcome runs counter to the common belief
that roughness promotes by-pass transition. Instead, the scales largely increase the stability of the base flow similar to an array of vortex generators.

A technical realisation of such patterns on aerodynamic surfaces will
pave the way towards the drastic reduction in fuel consumption and future zero-emission flight.


========================================================================== Story Source: Materials provided by City_University_London. Original
written by John Stevenson. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Muthukumar Muthuramalingam, Dominik K. Puckert, Ulrich Rist,
Christoph
Bruecker. Transition delay using biomimetic fish
scale arrays. Scientific Reports, 2020; 10 (1) DOI:
10.1038/s41598-020-71434-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201007123025.htm

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