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1. Quasi-steady aerodynamic theory under-predicts glide performance in flying snakes

   https://doi.org/10.1242/jeb.247989  

   Yeaton, Isaac J; Ross, Shane D; Socha, John J (October 2024, Journal of Experimental Biology)

   Flying snakes (genus Chrysopelea) glide without the use of wings. Instead, they splay their ribs and undulate through the air. A snake's ability to glide depends on how well its morphing wing-body produces lift and drag forces. However, previous kinematics experiments under-resolved the body, making it impossible to estimate the aerodynamic load on the animal or to quantify the different wing configurations throughout the glide. Here, we present new kinematic analyses of a previous glide experiment, and use the results to test a theoretical model of flying snake aerodynamics using previously measured lift and drag coefficients to estimate the aerodynamic forces. This analysis is enabled by new measurements of the center of mass motion based on experimental data. We found that quasi-steady aerodynamic theory under-predicts lift by 35% and over-predicts drag by 40%. We also quantified the relative spacing of the body as the snake translates through the air. In steep glides, the body is generally not positioned to experience tandem effects from wake interaction during the glide. These results suggest that unsteady 3D effects, with appreciable force enhancement, are important for snake flight. Future work can use the kinematics data presented herein to form test conditions for physical modeling, as well as computational studies to understand unsteady fluid dynamics effects on snake flight. 

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2. Advanced Wind Tunnel Boundary Simulation for Kevlar Wall Aeroacoustic Wind Tunnels

   Szoke, Mate; Devenport, William; Borgoltz, Aurelien; Roy, Christopher; Lowe, Todd (May 2021, In Advanced Wind Tunnel Boundary Simulation II)

   This study presents the first 3D two-way coupled fluid structure interaction (FSI) simulation of a hybrid anechoic wind tunnel (HAWT) test section with modeling all important effects, such as turbulence, Kevlar wall porosity and deflection, and reveals for the first time the complete 3D flow structure associated with a lifting model placed into a HAWT. The Kevlar deflections are captured using finite element analysis (FEA) with shell elements operated under a membrane condition. Three-dimensional RANS CFD simulations are used to resolve the flow field. Aerodynamic experimental results are available and are compared against the FSI results. Quantitatively, the pressure coefficients on the airfoil are in good agreement with experimental results. The lift coefficient was slightly underpredicted while the drag was overpredicted by the CFD simulations. The flow structure downstream of the airfoil showed good agreement with the experiments, particularly over the wind tunnel walls where the Kevlar windows interact with the flow field. A discrepancy between previous experimental observations and juncture flow-induced vortices at the ends of the airfoil is found to stem from the limited ability of turbulence models. The qualitative behavior of the flow, including airfoil pressures and cross-sectional flow structure is well captured in the CFD. From the structural side, the behavior of the Kevlar windows and the flow developing over them is closely related to the aerodynamic pressure field induced by the airfoil. The Kevlar displacement and the transpiration velocity across the material is dominated by flow blockage effects, generated aerodynamic lift, and the wake of the airfoil. The airfoil wake increases the Kevlar window displacement, which was previously not resolved by two-dimensional panel-method simulations. The static pressure distribution over the Kevlar windows is symmetrical about the tunnel mid-height, confirming a dominantly two-dimensional flow field. 

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3. Wind Tunnel Study of Wake-induced Aerodynamics of Parallel Stay-Cables and Power Conductor Cables in a Yawed Flow

   https://doi.org/https://doi.org/10.12989/was.2020.30.6.617  

   Jafari, M.; Sarkar, P.P. (January 2020, Wind and Structures)

   null (Ed.)

   Wake-induced aerodynamics of yawed circular cylinders with smooth and grooved surfaces in a tandem arrangement was studied. This pair of cylinders represent sections of stay-cables with smooth surfaces and high-voltage power conductors with grooved surfaces that are vulnerable to flow-induced structural failure. The study provides some insight for a better understanding of wake-induced loads and galloping problem of bundled cables. All experiments in this study were conducted using a pair of stationary section models of circular cylinders in a wind tunnel subjected to uniform and smooth flow. The aerodynamic force coefficients and vortex-shedding frequency of the downstream model were extracted from the surface pressure distribution. For measurement, polished aluminum tubes were used as smooth cables; and hollow tubes with a helically grooved surface were used as power conductors. The aerodynamic properties of the downstream model were captured at wind speeds of about 6-23 m/s (Reynolds number of 5×10^4 to 2.67×10^5 for smooth cable and 2×10^4 to 1.01×10^5 for grooved cable) and yaw angles ranging from 0º to 45º while the upstream model was fixed at various spacing between the two model cylinders. The results showed that the Strouhal number of yawed cable is less than the non-yawed case at a given Reynolds number, and its value is smaller than the Strouhal number of a single cable. Additionally, compared to the single smooth cable, it was observed that there was a reduction of drag coefficient of the downstream model, but no change in a drag coefficient of the downstream grooved case in the range of Reynolds number in this study. 

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4. Effects of body shape on aerodynamic performance and wake structures in snake-like gliding

   Gong, Y. (January 2022, AIAA)

   Flying snakes are the only snakes on Earth capable of aerial gliding, taking advantage of fluid dynamic principles to leap from point to point among the trees. During their gliding, the locomotion of aerial undulation is observed. We hypothesize that this locomotion and its associated unsteady vortex dynamics are critical to their aerodynamic performance. However, there is a lack of detailed three-dimensional flow field information around the snake body in gliding due to the difficulties in experimental flow visualizations of live animals. In this study, a computation fluid dynamics (CFD) study has been conducted to study the fluid dynamics of a snake-like gliding. A mathematical equation describing the horizontal undulation motion was applied for constructing snake-like 3D computational models and a series of flow simulations were conducted. An immersed-boundary-method (IBM)-based direct numerical simulation (DNS) flow solver along with adaptive mesh refinement (AMR) was used in the simulation. Specifically, different head positions, corresponding to different horizontal wave shapes and their effect on aerodynamic performance, flow field and wake structures behind the body will be studied. In addition, the dynamic undulating motion is introduced in the model and a CFD simulation is also conducted. Results from this study are expected to bring a step stone to understanding snake-inspired locomotion. 

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5. Flow Interactions Between Low Aspect Ratio Hydrofoils in In-line and Staggered Arrangements

   https://doi.org/10.3390/biomimetics5020013  

   Kurt, Melike; Eslam Panah, Azar; Moored, Keith W. (June 2020, Biomimetics)

   Many species of fish gather in dense collectives or schools where there are significant flow interactions from their shed wakes. Commonly, these swimmers shed a classic reverse von Kármán wake, however, schooling eels produce a bifurcated wake topology with two vortex rings shed per oscillation cycle. To examine the schooling interactions of a hydrofoil with a bifurcated wake topology, we present tomographic particle image velocimetry (tomo PIV) measurements of the flow interactions and direct force measurements of the performance of two low-aspect-ratio hydrofoils ( A R = 0.5 ) in an in-line and a staggered arrangement. Surprisingly, when the leader and follower are interacting in either arrangement there are only minor alterations to the flowfields beyond the superposition of the flowfields produced by the isolated leader and follower. Motivated by this finding, Garrick’s linear theory, a linear unsteady hydrofoil theory based on a potential flow assumption, was adapted to predict the lift and thrust performance of the follower. Here, the follower hydrofoil interacting with the leader’s wake is considered as the superposition of an isolated pitching foil with a time-varying cross-stream velocity derived from the wake flow measurements of the isolated leader. Linear theory predictions accurately capture the time-averaged lift force and some of the major peaks in thrust derived from the follower interacting with the leader’s wake in a staggered arrangement. The thrust peaks that are not predicted by linear theory are likely driven by spatial variations in the flowfield acting on the follower or nonlinear flow interactions; neither of which are accounted for in the simple theory. This suggests that unsteady potential flow theory that does account for spatial variations in the flowfield acting on a hydrofoil can provide a relatively simple framework to understand and model the flow interactions that occur in schooling fish. Additionally, schooling eels can derive thrust and efficiency increases of 63-80% in either a in-line or a staggered arrangement where the follower is between two branched momentum jets or with one momentum jet branch directly impinging on it, respectively. 

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