Biomimetic applications of penguins’ geometric features: feasibility and performance analysis
Creators: name: Masud, Mahadi
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401299
subject: Fluid mechanics and thermal engineering not elsewhere classified
classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)
classificationCode: 401706
subject: Numerical modelling and mechanical characterisation

description: Penguins are the utmost specialised wing-propelled divers as their flapping action is accountable for the entire underwater propulsion. However, the propulsion mechanisms of penguins are yet to be fully understood, and this is mainly due to the fact that their specific features are still largely unknown. Penguins swim in a way similar to flying, and they cruise for long periods by suddenly switching to fast steep dives. Moreover, their flippers demonstrate multimodal functionality. Multimodality of flipper action means that flapping performance is dominant in certain periods and, in that case, Strouhal number (St=  (f A)/U, where, f is the wing-beat frequency in Hz, A is the flapping amplitude in m, and U is the swimming speed in m/s) is important, and there are also periods when the steady (non-flapping) mode is dominant, when the flipper performs like a fixed foil; therefore, those two modes are subject to analysis. Propulsive performance of swimming and flying animals is often evaluated by using dimensionless numbers, such as the Strouhal, Reynolds and Swimming numbers. They have been shown to provide an understanding of locomotion efficiency, using relatively simple approaches and avoiding overly complex computational models. Specifically, it has been reported that efficient propulsion is more likely to occur when St values – estimated from propulsive frequencies and amplitudes – are within a relatively narrow range, depending on the corresponding Reynolds number (Re=(ρ_w UL)/μ, where, ρ_w is the density of seawater in kg/m^3, U denotes the swimming speed in m/s, L is the height of the little penguin in m, and μ is the dynamic viscosity of water kg/m·s), broadly expressing the fluid resistance to the animal motion. The Strouhal number for seven species of penguins was estimated, and by comparing these values with those for alcids – seabirds that can also fly – it was found that penguins swim at least as efficiently as alcids. Some researchers argued that the St value is not sufficient to ensure higher propulsive efficiency because St estimates are based on assumptions that might be incorrect; therefore, to evaluate the reliability of the St estimate, the Reynolds and Swimming numbers (Sw=(ρω_p AL)/μ=2π St Re, where ω_p is the animal propulsive frequency in rad/s) were also calculated. Those estimated values precisely designate that, as anticipated, penguins employ efficient propulsion mechanisms.<br><br>

Furthermore, the energy cost and the cost of transport of penguin dives were computed from the free-ranging dive data, on the basis of an existing biomechanical model. It was found that the numerically obtained energy cost from free-ranging dive data is not far from the minimum cost predicted by the model. Moreover, the hydrodynamic forces (i.e., drag and buoyancy force) during penguin dive was estimated, and the estimated buoyancy force at lower depth proves why penguin swims upwards to the water surface capitalising on the high buoyancy force while their wings act as a fixed foil. Therefore, it strongly suggests that a more comprehensive understanding of penguin swimming behaviour would not only be useful from a biological point of view, for example, to gather information on the bird energy expenditure but could also be instrumental for developing penguin-inspired man-made devices. <br><br>

In the process of assessing the second part of the multimodal functionality, i.e. (fixed foil characteristics of penguin wing), the aerodynamic performance analysis of a little penguin wing was conducted at low Reynolds numbers (i.e., Re= 5*10^4, 7.5*10^4, and 1*10^5) and moderate Reynolds numbers (i.e., Re=2*10^5, Re=4.8*10^5) to assess its feasibility for use in different scale unmanned aerial vehicles (UAVs). The results strongly suggest that penguin wing shape performs in a superior manner at higher angles of attack (AOA) in comparison to the traditional NACA profile, as for all the specified low mean chord-based Re stall occurs at 16 deg AOA, whereas for moderate mean chord-based Re stall occurs at 21 deg. For the set of low Re, maximum lift coefficient of 0.83 was obtained at 15 deg AOA and Re= 1*10^5, while maximum lift-to-drag ratio (≅6.04) was obtained for the same Re but at 10 deg AOA, at which point the highest efficiency is expected. For the second set of Re, i.e., moderate Re, the maximum lift-to-drag ratio of 6.7 was also obtained at 10 deg AOA. At Re=4.8*10^5 , and 15, 20 and 25 deg AOA, penguin wings provide 29.11%, 51.46% and 24.02% greater lift compared to the reported lift for baseline NACA0012 aerofoil, respectively, and, at the same conditions, penguin wings generate 8.04%, 5% and 20.27% lesser drag compared to the reported drag for NACA0012 aerofoil, respectively. Moreover, penguin wings, even at comparatively lower Re offer higher vorticity (≅ 5.94*10^6 s^(-1)) values over the surface than the baseline aerofoil and modified aerofoil.<br><br>

For the scaled-up 2D wing, at both Reynolds numbers, i.e., Re=2*10^5and 4.8*10^5  up to C*5, the drag and lift coefficient values are approximately similar. Whereas for C*10, the estimated drag and lift coefficient is higher than all the other 2D scaled-up wings. Whereas for the scaled-up 3D wing, both the scaled-up wings (W2 and W3) and actual size wing (W1), stall occurs beyond 15 deg AOA. Furthermore, the W1, i.e., the actual size wing, offers slightly better performance in terms of lift generation and lift-to-drag ratio.<br><br>

Finally, although the results of the current study strongly indicate that penguin wings (actual and scaled-up wings) act as a flow separation control device; however, all the analysis was done excluding the main body of UAVs. Therefore, in future, the whole system should construct, and after performing a wide range of experimental and numerical analyses, it is possible to reach a firm conclusion.