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Flows in Complex Systems

Optimal Tandem Configuration for Oscillating-Foils Hydrokinetic Turbine

[+] Author and Article Information
Thomas Kinsey

 Laboratoire de Mécanique des Fluides Numérique, Department of Mechanical Engineering, Laval University, Quebec City, QC, G1V 0A6, Canadathomas.kinsey.1@ulaval.ca

Guy Dumas

 Laboratoire de Mécanique des Fluides Numérique, Department of Mechanical Engineering, Laval University, Quebec City, QC, G1V 0A6, Canadagdumas@gmc.ulaval.ca

J. Fluids Eng 134(3), 031103 (Mar 23, 2012) (11 pages) doi:10.1115/1.4005423 History: Received April 15, 2011; Revised October 25, 2011; Published March 21, 2012; Online March 23, 2012

A numerical investigation based on 2D URANS simulations is performed in order to seek an optimal spatial configuration for two oscillating foils within a hydrokinetic turbine. The objective of the study is to maximize the power extraction efficiency of the turbine. Tandem spatial configurations are considered because in such arrangement both hydrofoils are sharing the same flow window, which allows the turbine to reach higher efficiencies. The relative positioning of the downstream foil oscillating in the wake shed by the upstream hydrofoil is seen to be critical. Indeed, favorable interactions between the downstream foil and the wake vortices may lead to unexpectedly high power-extraction efficiencies (up to 64%), while unfavorable interactions may cause the downstream foil to contribute negatively to the total power extracted. A global phase shift parameter is introduced to characterize the tandem configuration. This parameter combines the inter-foil spacing and motion phase-shift into a single term. It is found useful to predict additional favorable configurations based on known results for cases with similar upstream-foil wake behavior. A comparison with experimental data is provided. Numerical predictions are seen to overpredict the power extraction performance in some cases. This is likely due to the broken 2D coherence of vortices in the 3D reality which affects the vortex-induced velocities and the subsequent foil-wake interactions.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Oscillating-foils hydrokinetic turbine from Laval University [1]

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Figure 2

Spatial configuration nomenclature

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Figure 3

Heaving and pitching motions (φ=90deg)

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Figure 4

Schematic of streamtubes around two actuator disks, (a) and (b), in a tandem configuration. In a real (viscous) fluid, the upstream foil’s wake is re-energized through the action of turbulence, shed vortices, and viscous diffusion, which all tend to transfer flow momentum from the free stream into the wake.

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Figure 5

Schematics illustrating two different scenarios of global phase shift Φ. T is the oscillation period.

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Figure 6

2D mesh details and boundary conditions for the dual-hydrofoil turbine in tandem configuration. Total number of cells: 181,000.

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Figure 7

Example of a tandem configuration characterized by a weak interaction of the downstream foil with wake vortices. (a) Flowfield snapshot showing isolines of vorticity (dashed for negative vorticity) and velocity vectors colored by the ratio of local dynamic pressure over the free stream dynamic pressure (|V|/U∞)2. (b) Instantaneous vertical force coefficient (CY) for both foils whose product with the instantaneous heaving velocity Vy represents the main contribution Py to the total power extracted. The curve for the downstream foil has been phase-shifted in order to allow direct comparison. Both curves exhibit very similar shape — Case 5 in Table 1.

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Figure 8

Classification of the downstream foil interaction with wake vortices. Four configurations are depicted for the hydrofoil in its upstroke motion. Cases v1 and v2 lead to an increased instantaneous effective dynamic pressure (q∞) on the hydrofoil through the vortex-induced velocity, which has a favorable impact on power extracted. Cases v3 and v4 show vortex-induced velocity contributing negatively to the available dynamic pressure, hindering power extraction performance. VIV = vortex-induced velocity.

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Figure 9

Example of an efficient tandem configuration characterized by a strong positive interaction of the downstream foil with wake vortices. (a) Flowfield snapshot showing vorticity and velocity vectors. (b) Instantaneous vertical force coefficient (CY) for both foils (see caption Fig. 7). The downstream foil reaches higher vertical force values due to the added dynamic pressure provided by the wake vortices — Case 3 in Table 1.

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Figure 10

Example of a non-efficient tandem configuration characterized by a strong negative interaction of the downstream foil with wake vortices. (a) Flowfield snapshot showing vorticity and velocity vectors. (b) Instantaneous vertical force coefficient (CY) for both foils (see caption Fig. 7). The downstream foil experiences dynamic stall causing important fluctuations in its vertical force curve — Case 20 in Table 1.

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Figure 11

For fixed motion amplitudes, decreasing the reduced frequency implies greater effective angles of attack and occurrence of dynamic-stall vortex shedding. Velocity vectors colored by dynamic pressure ratio (|V|/U∞)2 and isolines of vorticity (dashed for negative vorticity) are shown at t/T = 0 and 0.75 — Cases 13, 16, and 19 in Table 1.

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Figure 12

Example of vortex-induced downwash provoking dynamic stall on the downstream hydrofoil. Velocity vectors colored by dynamic pressure ratio (|V|/U∞)2 and isolines of vorticity (dashed for negative vorticity) — Case 19 in Table 1.

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Figure 13

Comparison of power-extraction efficiencies for a single-foil versus a tandem-foil turbine, including separate contributions from upstream and downstream foil—θ0  = 75 deg, Lx=5.4c, φ1-2=-180deg, and H0=c

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Figure 14

Power extraction efficiency over reduced frequency for tandem cases with fixed inter-foil spacing (Lx=5.4c) versus cases with constant global phase shift (Φ1-2≈90deg)—θ0=70deg, and H0=c

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Figure 15

Similitude in foil-wake interactions between two different tandem configurations sharing the same global phase shift, frequency, and motion amplitudes. Flow fields present isolines of vorticity (dashed for negative vorticity) and velocity vectors colored by the ratio of local dynamic pressure over the free stream dynamic pressure. Φ1-2≈90deg, θ0=70deg, and H0=c.

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Figure 16

Similitude in performance between two different tandem configurations sharing the same global phase shift, frequency, and motion amplitudes. Instantaneous power (a) and vertical force coefficient together with heaving velocity (b) are plotted in the relative time frame of each downstream foil to allow direct comparison. Φ1-2≈90deg, θ0=70deg, and H0=c.

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Figure 17

Similitude in performance between two different tandem configurations sharing the same global phase shift, frequency, and motion amplitudes. Instantaneous power (a) and vertical force coefficient together with heaving velocity (b) are plotted for each upstream foil. Φ1-2≈90deg, θ0=70deg, and H0=c.

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Figure 18

Comparison of power-extraction efficiency between 2D and 3D tandem numerical simulations against the hydrodynamic efficiency from experiments partly adapted from Ref. [17]

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