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Research Papers: Fundamental Issues and Canonical Flows

On the Impulse Produced by Chordwise Flexible Pitching Foils in a Quiescent Fluid

[+] Author and Article Information
Francisco J. Huera-Huarte

Department of Mechanical Engineering,
Universitat Rovira i Virgili,
Tarragona 43007, Spain
e-mail: francisco.huera@urv.cat

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received June 8, 2017; final manuscript received September 28, 2017; published online December 4, 2017. Assoc. Editor: Pierre E. Sullivan.

J. Fluids Eng 140(4), 041206 (Dec 04, 2017) (10 pages) Paper No: FE-17-1333; doi: 10.1115/1.4038168 History: Received June 08, 2017; Revised September 28, 2017

In this paper, a parametric study showing the impulsive performance of foils with different flexural stiffness pitching in a quiescent fluid is presented. A wide range of Reynolds numbers (different imposed kinematics) and foil rigidities is covered, depicting how flexibility effects on impulse are more important at the largest Reynolds numbers. The impulsive performance of the system is derived from direct thrust force measurements. Passive flexibility alters vortex strength and formation in the wake of the pitching foil. These changes in the wake formation can be used to explain the differences in the measured impulses. The wake dynamics is studied after quantitative analysis of particle image velocimetry data, and it is linked to the momentum transfer generated by the foil.

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References

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Figures

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Fig. 1

Schematic (top view) of the experimental setup

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Fig. 2

Schematic (side view) of the experimental setup

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Fig. 3

Temporal evolution of the angular position of the shaft (θ), thrust coefficient (CT), and impulse (I) for a typical test conducted. The foil in this case was the infinitely rigid one or reference one, rotating at 277 deg/s (Re = 30,600) with a stroke swept angle of 90 deg.

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Fig. 4

Dimensionless vorticity (ωzc/U) in the wake of the rigid foil (EI0) for the same case of Fig. 3. The sequence starts in the upper left corner to the right. The 12 DPIV instantaneous vorticity snapshots cover approximately three strokes times or 3T. The angular speed imposed in this example case was 277 deg/s (Re = 30,600) with a stroke covering a swept angle of 90 deg.

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Fig. 5

Impulse at t* = 5 (I) as a function of swept angle (2θ0) and angular speed (Ω) or Re) for all the foils investigated withdifferent flexural stiffness. EI symbols appear detailed in Table 1, EI0 is for the rigid foil, and EI1 to EI5 are for the flexible foils with increasing flexibility.

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Fig. 6

Temporal evolution of the impulse for swept angles of 60 deg and 90 deg, for all the cases with the largest Re in Fig. 5. Same symbol convention for stiffness is used as in Fig. 5.

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Fig. 7

Angular position of the actuating shaft (θ), thrust coefficient (CT) and impulse (I) for three experiments with similar imposed kinematics, but different rigidity. Solid dots are for the reference case with EI0 (infinitely rigid), hollow circles for the case with flexural stiffness EI1 and triangles is for the case with EI5 (the most flexible one).

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Fig. 8

Deformation of the foil for the three experiments (from Fig. 7) with similar kinematics and different rigidity. Reference case or infinitely rigid, flexural stiffness EI1 and finally EI5 or the most flexible one.

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Fig. 9

Temporal evolution of dimensionless vorticity in the wake of the flexible foil with highest flexibility (EI5, ▿ in the previous figures)

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Fig. 10

Temporal evolution of dimensionless vorticity in the wake of the flexible foil with the smallest flexibility (EI1, ○ in the previous figures)

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Fig. 11

Evolution of the characteristic size (Δy/c) and the inclination (α) of the vortex structure in the wake of the foils (EI5—▿, EI1—∘, and EI0—•)

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Fig. 12

Evolution of vortex strength (Γ*) in the wake of the foils (EI5—▿, EI1—∘, and EI0—•)

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