More Like this

1. Effect of Nonuniform Flexibility on Hydrodynamic Performance of Pitching Propulsors

   https://doi.org/10.1115/1.4041976  

   Zeyghami, Samane; Moored, Keith W. (April 2019, Journal of Fluids Engineering)

   Many aquatic animals propel themselves efficiently through the water by oscillating flexible fins. These fins are, however, not homogeneously flexible, but instead their flexural stiffness varies along their chord and span. Here, we develop a simple model of these functionally graded materials where the chordwise flexibility of the foil is modeled by one or two torsional springs along the chord line. The torsional spring structural model is then strongly coupled to a boundary element fluid model to simulate the fluid–structure interactions. We show that the effective flexibility of the combined fluid–structure system scales with the ratio of the added mass forces acting on the passive portion of the foil and the elastic forces defined by the torsional spring hinge. Importantly, by considering this new scaling of the effective flexibility, the propulsive performance is then detailed for a foil with a flexible hinge that is actively pitching about its leading edge. The scaling allows for the resonance frequency of the fluid–structure system and the bending pattern of the propulsor to be independently varied by altering the effective flexibility and the location of a single torsional spring along the chord, respectively. It is shown that increasing the flexion ratio, by moving the spring away from the leading edge, leads to enhanced propulsive efficiency, but compromises the thrust production. Proper combination of two flexible hinges, however, can result in a gain in both the thrust production and propulsive efficiency. 

   more » « less
2. WAKE STRUCTURES AND EFFECT OF HYDROFOIL SHAPES IN EFFICIENT FLAPPING PROPULSION

   Kelly, J. (August 2021, ASME Fluids Engineering Division Summer Meeting)

   In this work, direct numerical simulation (DNS) is used to investigate how airfoil shape affects wake structure and performance during a pitching-heaving motion. First, a classshape transformation (CST) method is used to generate airfoil shapes. CST coefficients are then varied in a parametric study to create geometries that are simulated in a pitching and heaving motion via an immersed boundary method-based numerical solver. The results show that most coefficients have little effect on the propulsive efficiency, but the second coefficient does have a very large effect. Looking at the CST basis functions shows that the effect of this coefficient is concentrated near the 25% mark of the foils chord length. By observing the thrust force and hydrodynamic power through a period of motion it is shown that the effect of the foil shape change is realized near the middle of each flapping motion. Through further inspection of the wake structures, we conclude that this is due to the leading-edge vortex attaching better to the foil shapes with a larger thickness around 25% of the chord length. This is verified by the pressure contours, which show a lower pressure along the leading edge of the better performing foils. The more favorable pressure gradient generated allows for higher efficiency motion. 

   more » « less
3. Experimental Study of Inertia-based passive Deformation of a Heaving and Pitching Airfoil Operating in the Energy Harvesting Regime

   Siala, F.F.; Fard, K.; Liburdy, J.A. (October 2019, Physics of fluids)

   The effects of passive, inertia-induced surface deformation at the leading and trailing edges of an oscillating airfoil energy harvester are investigated experimentally at reduced frequencies of k = f c=U¥ = 0.10, 0.14 and 0.18. Wind tunnel experiments are conducted using phase-resolved, two-component particle image velocimetry to understand the underlying flow physics, as well as to obtain force and pitching moment estimates using the vortex-impulse theory. Results are obtained for leading and trailing edge deformation separately. It is shown that both forms of deformation may alter the leading edge vortex inception and detachment time scales, as well as the growth rate of the circulation. In addition, surface deformation may also trigger the generation of secondary vortical structures, and suppress the formation of trailing edge vortices. The total energy harvesting efficiency is decomposed into contributions of heaving and pitching motions. Relative to the rigid airfoil, the deforming leading and trailing edge segments are shown to increase the energy harvesting efficiency by approximately 17% and 25%, respectively. However, both the deforming leading and trailing edge airfoils operate most efficiently at k = 0:18, whereas the peak efficiency of the rigid airfoil occurs at k = 0:14. It is shown that the deforming leading and trailing edge airfoils enhance the heaving contribution to the total efficiency at k = 0:18 and the negative contribution of the pitching motion at high reduced frequencies can be alleviated by using a deforming trailing edge. 

   more » « less
4. Scaling laws for the propulsive performance of three-dimensional pitching propulsors

   https://doi.org/10.1017/jfm.2019.334  

   Ayancik, Fatma; Zhong, Qiang; Quinn, Daniel B.; Brandes, Aaron; Bart-Smith, Hilary; Moored, Keith W. (July 2019, Journal of Fluid Mechanics)

   Scaling laws for the thrust production and energetics of self-propelled or fixed-velocity three-dimensional rigid propulsors undergoing pitching motions are presented. The scaling relations extend the two-dimensional scaling laws presented in Moored & Quinn ( AIAA J. , 2018, pp. 1–15) by accounting for the added mass of a finite-span propulsor, the downwash/upwash effects from the trailing vortex system of a propulsor and the elliptical topology of shedding trailing-edge vortices. The novel three-dimensional scaling laws are validated with self-propelled inviscid simulations and fixed-velocity experiments over a range of reduced frequencies, Strouhal numbers and aspect ratios relevant to bio-inspired propulsion. The scaling laws elucidate the dominant flow physics behind the thrust production and energetics of pitching bio-propulsors, and they provide guidance for the design of bio-inspired propulsive systems. 

   more » « less
5. Effect of Actively-Controlled Trailing Edge Flap Upon Airfoil energy Harvesting

   Strahl, J. (April 2022, ASME Fluids Division Summer Meeting)

   The application of a flapping foil with prescribed trailing edge motion to energy harvesting in a low reduced frequency (k = fc/U∞) regime was experimentally studied. The effects of the phase and amplitude of the applied trailing edge motion upon time-variant power extraction capability have been measured and are interpreted. On these bases, an optimized motion profile is developed. The airfoil design used was NACA0015 in profile with a chord length of c = 150mm, the pitching axis located at the 1/3 chord position, and an actively-controlled trailing edge flap hinged at the 2/3 chord location. The pitching and heaving amplitudes are θ0 = 70◦ and h0 = 0.6c respectively, with a phase delay of 90◦. Although the aspect ratio was 2, end plates were used to minimize 3-dimensional effects and simulate a 2-dimensional airfoil. Data were collected in a low-speed wind tunnel with turbulence intensities below 2%. The Reynolds number (Rec = U∞c/ν) range was 27, 000 ≤ Rec ≤ 60, 000 with a corresponding reduced frequency range of 0.04 ≤ k ≤ 0.10. The proposed trailing edge motion profile offers a measured maximum increase of 25.6% in cycle-averaged heaving power coefficient over a rigid foil operating under the same conditions. Results indicate that smaller trailing edge amplitudes offer greater improvements, and demonstrate that the influence of trailing edge motion can be more pronounced at low reduced frequencies. 

   more » « less