Proposal for aerodynamic and acoustic analysis of a barn owl biomimetic airfoil

1. Running head: PROPOSAL FOR AERODYNAMIC AND ACOUSTIC ANALYSIS 1
Proposal for Aerodynamic and Acoustic Analysis of a Barn Owl Biomimetic Airfoil
Salman K. Rahmani
Middle Tennessee State University
Author’s Note:
If any questions or concerns arise regarding the information of this article, please contact Salman
Rahmani at (615)-351-1114 of sr4h@mtmail.mtsu.edu

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Introduction
For years, the human race has turned to nature for inspiration on how to better understand
the world in which we live. In particular, aerospace engineers frequently examine birds to gain a
fundamental idea on how to improve the characteristics of flight. With the development of the
Unmanned Aerial Vehicle (UAV), helicopter, and large passenger aircraft, a rising problem in
the aerospace community is the amount of noise that these crafts’ propellers produce. This
problem is of substantial stature in the sense that it is a disturbance to residential neighborhoods,
hampers stealth operations for specialized combat units, and proves the fact that there is still a lot
to learn about the physics of flight.
In this project, the undergraduate researcher will attempt to analyze sound properties of
an aircraft propeller that has been retrofitted with modifications inspired by barn-owls, also
known as Tyto Alba (Peregrine, n.d). These modifications include leading edge ‘combs’ and
trailing edge ‘fringes’ which can be seen in Fig 1. In addition to providing data and results on
sound, the researcher will also provide an aerodynamic analysis in hopes of providing a more
immersive understanding of the results.
Background
Due to the increasing magnitude of the issue, engineers have spent extensive hours
examining the main productive factors of sound on propellers and wings. For example, in an
extensive study conducted by NASA, Jack Marte and Donald Kurtz discovered that the main
propagation of sound on rotating airfoils was catalyzed by vortices shedding off of the trailing
edge, and airfoil tip (Marte, 1970).

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In addition to analyzing the main causes of sound on airfoils, engineers have also began
to observe barn-owls due to their impeccable ability to achieve near-silent flight. As a result of
this research, three main characteristics of owls have been noted by engineers to have the most
significant impact of sound reduction during flight, the three characteristics include: the leading
edge combs, the trailing edge fringes, and the velvety upper surface of the wing (Cohen, 2013).
Further studies have been conducted on the three characteristics to try and understand their
contribution to noise reduction. One such study is Rao et al’s examination of the Owl-Inspired
leading-edge serrations and their impact on sound suppression. Rao et al found that, by only
computationally testing the leading edge combs, the sound produced by the wings were reduced
when compared against a wing without the combs (Rao et al, 2017).
In addition to computational methods, analysts have also used other methodologies, such
as experimental methods, to try and investigate the effects these wing modifications have on
silent flight. One such methodology, carried out by Thomas Geyer, Ennes Sarradj, and Christoph
Fritzsche, consisted of measuring data by having barn owl conduct ‘flyovers’ of sound
acquisition systems. Geyer et al arrived at the conclusion that major sound reductions occurred
due to the owls’ wings. However, the modifications that implicated the sound reduction could
not be pinpointed due to the lack of sensitivity of the pressure spectrometer that was being
utilized for the experiment (Geyer et al, 2014). Another study that attempted to examine the
characteristics of barn owl modifications is Emma Gist’s experimental work in outfitting an
airfoil with a velvet-like upper surface to dampen rotational sound levels. Gist showed that in an
anechoic chamber, an airfoil with a velvet-covered upper surface outperformed a standard airfoil
in regards to sound suppression (Gist 2017). Braxton Harter also showed that in an anechoic
chamber, the sound produced by an airfoil can be decreased utilizing owl-inspired modifications.

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Harter displayed that through the use of trailing edge serrations, the amount of disturbances
created by a propeller can be reduced at the expense of thrust efficiency (Harter 2018).
Purpose
The purpose in carrying out this examination is to shed some light on how owl-like
modifications orchestrate sound reduction on translating propellers. The researcher will not only
conduct a sound analysis of the modifications, but will also carry out an aerodynamic analysis to
determine how the two disciplines interact with one another. To reiterate, the modifications
under question throughout this study will be that of the leading edge combs, as well as the
trailing edge fringes. This project is vital for two main reasons. The first being that most of the
research in regards to silent flight thus far has been experimental, which is costly and produces
data skewness due to lack of equipment calibration/sensitivity. This study will eradicate those
issues by being conducted computationally. The second reason that this study is of paramount
importance is that most examinations have only focused on one modification at a time. This
poses an issue because due to air’s chaotic nature, an introduction of any sort of new variable
greatly influences the behavior of the air particles and thus, the sound properties. With this being
said, just because a result was determined for a single modification does not mean the same
result will hold if multiple modifications are examined in unison. This study will address that
complication by observing sound and aerodynamic properties of multiple modifications at once.
This project varies from my previous URECA-funded project in the sense that I am
entering a new field of study, acoustics. My previous research was based on classifying the
aerodynamic properties of class-8 vehicles without regards to sound. This project however,
exposes me to new methodologies of how scientists draw inspiration from biomimicry, a
viewpoint that is critical in today’s engineering community. By entering this new field of study, I

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will be introduced to theories and ideas that I never would have been exposed to had I remained
only in aerodynamics-based research.
Methods
The method that will be utilized throughout the course of this year-long study will be that
of Computational Fluid Dynamics (CFD). First, the rendering of the computer model will be
created within a Computer-Aided Design (CAD) package known as Inventor and then imported
to the CFD software. The CFD software is licensed under the name of ANSYS-Fluent and has
the ability to measure various physical properties of fluid flows such as sound, pressure, velocity,
etc. Parameters will be entered into ANSYS to represent real scenarios to try and produce the
most genuine results possible in regards to sound and aerodynamics. After each simulation has
concluded, the data (sound, turbulence, etc) will be harnessed and stored in a Microsoft Excel
file which will then be thoroughly analyzed at the end of the research period. Consequently, a
detailed representation of the findings will then be reported in the form of an article and
presented to the university research office. All testing and analysis is to be conducted in the
MTSU Aerospace Research Lab.

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Timeline
The timeframe in which my involvement will span will be September 7th, 2017 to May 9th, 2018.
All of the following dates within the following timeline are approximations.
Sep 7th, 2017 – Sep 30th 2017
-Methodology research on how to optimize CFD study in regards to propeller-based
bodies
Oct 1st 2017 – Oct 31st, 2017
-Rendering and modeling of modified trailing edge propeller (MTEP)
-Rendering and modeling of modified leading edge propeller (MLEP)
November 1st, 2017 – November 30th, 2017
-Rendering and modeling of dual modification (leading and trailing edge) propeller
(DMP)
December 1st, 2017 – December 31st, 2017
- Test and optimize computational method to yield best results while reducing
computation time
January 1st, 2018 – January 31st, 2018
-Conduct testing of MTEP
February 1st, 2018 – February 28th, 2018
-Harness data from MTEP tests

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-Conduct testing of MLEP
March 1st, 2018 – March 31st, 2018
-Harness data from MLEP tests
-Begin testing of DMP
April 1st, 2018 – April 30th, 2018
-Harness data from DMP tests
-Search for simulation inconsistences and correct if necessary
- Analysis of results from all simulations
-Prepare final report
May 1st, 2018 – May 9th, 2018
-Touch up final report and submit
Collaboration with Mentor
Throughout the course of my research, Dr. Callender will lead the project as research
supervisor while I will be listed as the undergraduate researcher. He will provide me with
guidance along the way in case I encounter any serious issues pertaining to the research. We will
have weekly conferences in order to minimize error and increase efficiency as we proceed.

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Figure 1. (Barn owl comb and fringes, provided by The City Birder)

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References
Cohen, J. J. (2013, October). Silent Stalkers. Super Science, 25(2), 1-19.
Fund, T. P. (n.d.). Barn Owl. Retrieved September 05, 2017, from
https://www.peregrinefund.org/explore-raptors-species/Barn_Owl
Geyer, T., Sarradj, E., & Fritzsche, C. (2014). Measuring owl flight noise. Inter.Noise 2014, 1-
16.
Gist, E. (2017). Noise Reduction for Small Aircraft Propellers. 1-15. Retrieved September 4,
2017.
Harter, B. N. (2017). Leading Edge Modification. 1-5. Retrieved September 4, 2018.
Marte, J. E., & Kurtz, D. W. (1970). A Review of Aerodynamic Noise From Propellers, Rotors,
and Lift Fans. 1-58. Retrieved September 4, 2017.
Rao, C., Ikeda, T., Nakata, T., & Liu, H. (2017). Owl-inspired leading-edge serrations play a
crucial role in aerodynamic force production and sound suppresion . Bioinspiration
& Biomimetics, 1-13. Retrieved September 4, 2017.
The City Birder. (2014, March 13). Retrieved September 05, 2017, from
http://citybirder.blogspot.com/2014/03/raptor-on-raptor-violence.html