Biomimicry is becoming an increasingly popular discipline. In the field of fluid dynamics, designs inspired by nature have outperformed conventional ones in terms of reduced drag, flow separation and improved stability. Recent work done on biomimetic airfoils demonstrate this fact. In this article, I review research on the aerodynamic and hydrodynamic designs of 3 species and their applications. More research needs to be done on biomimetic optimization of airfoils to improve efficiency and conserve energy. Such designs could be applied to improve turbines and transportation systems. This could lead to better implementation of renewable energy and reduce the damage caused by fossil fuels.
In 2015, fossil fuels accounted for 80.04% of the total energy usage in the world. In 2014, 78% emissions were greenhouse emissions, out of which 32% was from coal. Carbon emissions increase global temperature and decrease rainfall which causes drastic changes in the growth of food crops. The rise in temperature can have dramatic effects on sea levels, causing the Arctic ice to melt and cause floods in other areas.However two types of renewable energy sources-wind and hydroelectric are responsible for approximately 10 grams of CO2E/kWh each on a life-cycle basis compared to 1000 grams of CO2E/kWh. Both utilize the energy of fluids to produce electricity using turbines. A key parameters deciding turbine efficiency are drag, lift and flow separation. This is in part determined by the geometry of the blade. In nature, aerial and aquatic creatures have acquired streamlined shapes through millions of years of evolution. Scientists in the past have taken inspiration from these designs to improve aerodynamic performance. One such example is the Shinkansen bullet train of Japan. Using the streamlined shaped of a Kingfisher beak, Japanese engineers modified the bullet train to reduce the noise it produced while emerging out of a tunnel.
This practice of taking inspiration from nature to create or modify existing systems is called Biomimicry. In this article, I review the research done on the morphology of some animals and how they improve aerodynamic performance and hydrodynamic performance. Lastly, we explore the implications of such research on the environment.
According to Beem and Triantafyllou, “Behavioral experiments with harbor seals have demonstrated their outstanding ability to detect and track hydrodynamic signatures left by swimming animals and moving objects. Despite having their auditory and visual sensory cues blocked, the seals successfully follow the paths of bodies, which had swum ahead of them by 30 seconds or longer. The seals’ vibrissae (whiskers) are reported to be sensitive enough to detect the minute water movements left in these hydrodynamic trails”. Hanke showed that using these whiskers on blades reduced unsteady forces by 90% and drag by 40% [Hanke et al]. CT scans and optical microscope measurements showed that the elliptical cross section of the whiskers were inclined at 5-7° to the flow direction.
Vikram Shyam et al. (2015) studied whiskers collected from Harbor Seals, Elephant Seals and California Sea Lions. Using 3D scans, optical microscope and CT scans, they measured various parameters such as pitch of undulations, angle of inclination on the whiskers. With the help of this data they created a blade with undulations on the leading edge that matched the undulations of the whiskers. A Variable Speed power turbine was taken and swept through sinusoidal profiles resulting in the Seal Blade. The numerical investigations were conducted using Glenn-HT (a CFD code developed at NASA). For the experiments they used the SW-2 facility at NASA. The test section had a modular for PIV measurements. And was operated in the Mach 0.1 – Mach 0.7 range.
Drag reductions of upto 50% were measured. Possible applications include drag reduction in airframe, landing gear, etc.
Other examples of drag reduction in nature are- Sharks. Shark skin contains denticle like structures from which a material was manufactured which showed a drag reduction of 10% compared to corresponding smooth surfaces.
The wings of a dragonfly is not smooth, it contains corrugated structures which gives them stability
Masatoshi Tamai, Zhijian Wang, Ganesh Rajagopalan, Hui Hu, 2007, investigated the flow behavior around an airfoil shaped like a corrugated wind of a dragonfly and compared it with a flat plate and a streamlined airfoil. The streamlined airfoil had a maximum thickness of 17% of the chord length. The flat plate and the corrugated dragonfly airfoil were made of wood plates and 4 mm.
Particle Image Velocimetry (PIV) flow measurements were taken around the airfoil nose at Re = 3.4×104 in a closed-circuit low-speed wind tunnel at the Aerospace Engineering Department of Iowa State University. The cross sectional area of the wind tunnel was 1 × 1 ft. The tunnel was fitted with a contraction section and a cooling system to provide uniform low turbulent incoming flow.
“Illumination was provided by a double-pulsed Nd: YAG laser”. The flow velocity at the inlet was set to 5.0 m/s. A 1376 x 1040 pixel CCD camera was used for acquiring images.
It was found that at 10 degree angle of attack unsteady vortices shed from the corners of the corrugated cross section, and the laminar boundary layer starting from the leading edge transitioned to turbulent as it approached the first corner. These acted as “turbulators”. Unlike the streamlined airfoil and flat plate, in the corrugated airfoil the turbulence transition occurred without laminar flow separation. The vortexes were trapped in the valleys of the corrugated airfoil. They interacted with high-speed flow streams outside the valleys to pump high speed fluid near wall regions. This enabled the boundary layer to withstand the adverse pressure gradient and avoid flow separation and stall.
These airfoils could improve Micro Air Vehicle design for surveillance and military use due to their enhanced stability. “ The aerodynamic design principles applicable to traditional, “macro-scale” aircraft may not be used for MAVs anymore due to the significant difference in chord Reynolds numbers. As a result, airfoil shape and wing planform designs that are optimized for traditional, “macro-scale” aircraft are found to degrade significantly when used for MAVs” [as cited in Masatoshi p2]. Therefore, this study has accelerated the development of ultra- light vehicles using the design principles in dragonfly’s wings.
Chen Rao, Teruaki Ikeda, Toshiyuki Nakata and Hao Liu, 2017, studied the serrated wings of owls. Due to these serrations they are capable of silent flight which aids them while catching prey. They can suppress sound to a frequency below 2 kHz during gliding or flapping [as cited in Chen Rao, p01]
First, CFD models of two single feather wing models having infinite wingspan with and without leading edge serrations were created. The serrations in the model were 3 mm in length and had a width and spacing of 0.5 mm. In the study, Large-Eddy Simulation was used in a low speed wind tunnel of 1 × 1 × 2 m at an airflow speed of 3.0 m s −1. Illumination was provided by a pulsed laser of wavelength 532 nm. Simulations were conducted in ANSYS CFX.
For the clean wing model, a free shear layer was formed at the separation point, where the Kelvin –Helmholtz (KH) instability occurred. This caused oscillations in the shear layer creating vortex shedding which finally resulted in transition to turbulence. Whereas, in the serrated wing model, the leading edge vortex broke into small eddies, scattering the instability and suppressing the vortex shedding resulting in increased stability of suction flow. It was also found that the serrated wing model showed a reduction in force production at AoA 15 ° while significantly suppressing the aerodynamic noise production.
The humpback whale flippers possess bumps or ‘tubercles’ on its leading edge which also suppress noise. Inspired by this, an airfoil with tubercles was created and tested in a low speed wind tunnel. It showed significant suppression of tonal noise.
Applications are: noise suppression in wind turbines, aircraft and drones.
SIMILARITIES BETWEEN NATURAL AND ARTIFICIAL SYSTEMS
Natural structures such as trees and the body structure of animals have developed through evolution but the artificial systems have been developed by man. As such it would seem obvious there might some differences in the modus operandi of both systems. However, the recent developments in the field of Biomimicry show the contrary.
“In nature the spine and ribs work in conjunction with one another to provide support and protection. This idea seemed plausible for the buildings as well. The ribs provide support for the roof and create enclosure, in the form of a building. While designing the famous tower, Maurice Koechlin, assistant to the architect of the Eiffel tower, was inspired by the femur, the lightest and the strongest bone of the human body”[ S. Arslan, A. G. Sorguc et.al 2004]. Other examples include the roof of the Stuttgart Airport Passenger Terminal (1996) in Germany designed by Meinhard von Gerkan. This design was inspired by the branching structure of trees.
The Sailfish skin is another interesting example of superior design by nature. They are one of the fastest marine animals alongside sharks. The skin of a Sailfish contains V shaped protrusions on it. Sagong in 2013 found that it produced a 5% reduction in skin friction drag. [Sagong et.al]
The desert saguaro cactus has a complex surface of geometry of cavities and spines. Sharon Talley and Godfrey Mungal in 2002, studied this geometry and its aerodynamic performance. Compared to smooth cylinders, cylinders with cavities showed a drag reduction. [Sharon Talley, Godfrey Mungal, 2002]
The biological adaptations of various animals to move through fluids efficiently were studied. Due to the unique morphology of these animals, the airfoils inspired by them performed better than conventional airfoils. Further research needs to be done on other species so that we can improve the efficiency of turbines, aircraft and watercraft. Such sustainable designs would reduce the dependence on fossil fuels and reduce the negative impact on environment. In times of increasing pollution levels, we need to look to nature for meeting our energy needs.