Oscillating Foil Research Articles

Effect of Trailing Edge Cylindrical Spoiler Structure on the Energy Harvesting Performance of the Oscillating Foil

This study explores a novel flow control approach for enhancing the energy harvesting performance of an oscillating foil, which is a promising technology for harvesting renewable energy from wind and ocean currents. This approach involves placing a cylindrical spoiler structure at the trailing edge of the foil and investigating the effects of its diameter and spacing on the energy harvesting efficiency. The results show that the spoiler cylinders can significantly increase the energy harvesting efficiency of the foil by up to 19.26% compared to the case without them. The optimal configuration for the spoiler cylinders is found to be 11.0% of c in diameter, 0.5% of c in transverse spacing, and 0 in longitudinal spacing. Moreover, this flow control method is proven to be highly effective under a wide range of motion kinetic parameters. The findings of this study provide technical guidance for the design and implementation of oscillating foil energy harvesting devices, highlighting their practical engineering value.

A Novel and More Efficient Oscillating Foil for Wave-Driven Unmanned Surface Vehicles.

In the wave-driven unmanned surface vehicles (WUSVs), oscillating-foils are the most straightforward and widely used wave energy conversion mechanism, like the wave glider. However, WUSVs usually sail slowly compared with other types of USVs. Improving the thrust of the oscillating foil to increase its speed can help WUSVs improve their maneuverability and shorten the completion of ocean missions. This paper proposed a novel method to enhance oscillating foils’ thrust force using asymmetric cross-section shape and asymmetric oscillating motion. The thrust enhancement effect is verified by CFD simulation and pool experiment. The experimental results show that the asymmetric wing can enhance the propulsive force by at least 13.75%. The speed enhancement of WUSVs brought by this enhanced thrust is at least 7.6%, which has also been verified by simulation and sea experiment. The asymmetric foil only needs to make low-cost modifications on the traditional rigid symmetric foil to achieve the desired thrust enhancement effect.

Passive Flexibility Effect on Oscillating Foil Energy Harvester

It is well known that structural flexibility enhances the performance of flapping foil propellers. There is, however, much less knowledge about the effect of deformability on the flow energy extraction capacity of flapping foils. Following recent work on an oscillating foil energy harvesting device with prescribed foil deformations (Liu et al., “A Bio-Inspired Study on Tidal Energy Extraction with Flexible Flapping Wings,” Bioinspiration and Biomimetics, Vol. 8, No. 3, 2013, Paper 036011), fully coupled dynamics of a flapping foil energy harvester with a passively deformable foil is investigated. Toward this end, the dynamics of a foil with a realistic internal structure (containing a rigid leading edge and a flexible trailing edge with a stiffener) in an energy harvesting regime through a fluid–structure interaction scheme is computationally studied. To examine the effect of different levels of flexibility, various materials (ranging from metals such as copper to virtual materials with arbitrary elasticity and density) for the stiffener have been tested. With the virtual materials, the effects of Young’s modulus and density ratio have been studied. The simulation results show that flexibility around the trailing edge could enhance the overall energy extraction performance. For example, with a copper stiffener, an increase of 32.2% in efficiency can be reached at high reduced frequency. The performance enhancement is achieved mostly in cases with low Young’s modulus and density ratio. A possible underlying mechanism is that the specific foil deformations in these cases encourage the generation and shedding of vortices from the foil leading edge, which is known to be beneficial to flow energy extraction.

Research on energy extraction characteristics of an adaptive deformation oscillating-wing

Oscillating foil machines represent a type of flow energy harvesters which perform pitching and plunging motions simultaneously to harness the energy from incoming stream. In this paper, a new adaptive deformation oscillating wing was proposed and the theoretical performance of such a concept was studied here through unsteady two-dimensional simulations using an in-house developed computational fluid dynamics code. During operation, the proposed oscillating foil whose initial shape is symmetric can be deformed into a cambered foil, which aims to produce large lift force. Our numerical results suggest that the power efficiency of the proposed oscillating foil can be about 16.1% higher than the conventional oscillating foil without deformation. In addition, the effects of the maximum bending displacement and effective angle of attack on the efficiency of proposed oscillating foil were also discussed in this work.

Optimal Operating Parameters for an Oscillating Foil Turbine at Reynolds Number 500,000

The use of oscillating foils to develop turbines to extract energy from an oncoming flow has recently been found competitive with rotor blade designs. Moreover, the rectangular extraction plane of the oscillating-foil turbine presents clear advantages in the field of hydrokinetic turbines. The present study, based on two-dimensional unsteady Reynolds-averaged Navier–Stokes numerical simulations, focuses on the range of operating parameters maximizing power extracted by a single oscillating foil for small-scale application Reynolds number. The power-extraction efficiency is mapped in parametric spaces associated with different motion amplitudes. Efficiency maxima reach about 43% in all the parametric spaces considered, but for different values of pitching amplitude and frequency. It is found that the best parametric zones in all cases exhibit similar characteristics with regards to the effective angle of attack. Indeed, they are all associated with a maximum effective angle of attack of about 33 deg and a maximum (normalized) rate of change of the latter of 0.55. It is further observed that, contrary to laminar results previously published, the occurrence of leading-edge vortex shedding is not necessarily observed in optimal cases at these higher Reynolds number conditions.

Unsteady panel method for flapping foil

A computer program based on a potential-flow panel method was developed to evaluate the hydrodynamic forces acting on a harmonically heaving and pitching two-dimensional rigid foil. A new formulation of the unsteady Kutta condition, postulating a finite pressure difference at the trailing edge of the oscillating foil, is proposed and implemented in the numerical procedure. A comparison with published experimental data [Anderson JM, Streitlien K, Barrett DS, Triantafyllou MS. Oscillating foils of high propulsive efficiency. J Fluid Mech 1998;360(1):41–72; Read DA, Hover FS, Triantafyllou MS. Forces on oscillating foils for propulsion and maneuvering. J Fluids Struct 2003;17(1):163–83; Schouveiler L, Hover FS, Triantafyllou MS. Performance of flapping foil propulsion. J Fluids Struct 2005;20(7):949–59] showed good agreement with the computational results. The analysis of the experimental results indicates clearly that robust computational procedures are necessary for reliable data processing in terms of calibration, noise removal, correction factors, etc. The proposed two-dimensional approach to the problem is currently being developed as a three-dimensional procedure that will address the unsteady flow effects such as inertia-related ones that are currently omitted.

Evaluation of an Oscillating Foil Device Using Energy Expenditure

Evaluation of an Oscillating Foil Device Using Energy Expenditure

1820 振動翼式タービンに関する数値的試作(S17-1 風力発電(1),21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

1820 振動翼式タービンに関する数値的試作(S17-1 風力発電(1),21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Characteristics of Vortical Jet Structures of a Hydrofoil

Oscillating foil propulsion, the engineering application of fish-like movement of a hydrofoil, has received in recent decades as a possible competitor for propellers. The oscillating foil produces an effective angle of attack, resulting in a normal force vector with thrust and lift components, and it can be expected to be a new highly effective propulsion system. We have explored propulsion hydrodynamics as a concept in wake flow pattern. The present study has been examined various conditions such as oscillating frequencies and amplitudes in NACA0010 profile. Flow visualizations showed that high thrust was associated with the generation of moderately strong vortices, which subsequently combine with trailing-edge vorticity leading to the formation of a reverse Karman vortex street. Vortex generation was inherent to jet production and playeda fundamental role in the wake dynamics. And it was shown that the strong thrust coefficient obtained as the Strouhal number was larger.

Dynamics of a Three-Dimensional Oscillating Foil Near the Free Surface

We investigate the free-surface effects on an underwater three-dimensional foil undergoing constant translation and periodic oscillation using a hybrid computational method. In this approach, the fluid motion around the foil is modeled via boundary-integral formulation, and the shed vorticity is represented as a shear layer originated from the trailing edge, while the free-surface waves are resolved by a spectral algorithm. We apply this method to investigate the hydrodynamic forces and free-surface wave signatures of two canonical problems of oscillating foils with forward speed: a horizontal foil undergoing heaving motion and a vertical foil in sway motion. In the horizontal-foil case, we find that the solution has a strong dependence on the unsteady parameter. Near the critical value, the mean thrust and the propulsive efficiency of the foil are significantly reduced, and the free-surface wave field is dominated by the unsteady waves produced by the oscillatory motion rather than the steady Kelvin waves due to forward motion. The surface waves induced by the wake vortices appear as a sequence of ringlike patterns along the track. In contrast, for the swaying vertical foil, the hydrodynamic forces vary smoothly near τ c , while the wake-induced waves exhibit the characteristic meandering feature.

Chaotic Flow Generated by an Oscillating Foil.

Chaotic Flow Generated by an Oscillating Foil.

2104 弾性振動翼による推進の数値解析

2104 弾性振動翼による推進の数値解析

Study on Propulsion by Partially Elastic Oscillating Foil (2 nd Report)

In a previous paper, the authors analyzed partially elastic oscillating foils by a linearized theory and idenfied its various characteristics.In this paper, the authors propose here a nonlinear theory using singularity distribution method. In this theory, Hess & Smith's method is extended in order to treat unsteady foil motion and wake distortion is taken into account by discrete vortices. In the case of partially elastic oscillating foil, the solution is obtained by satisfying also the equation of motion which can be derived from the equation of elastic curve.The results obtained by the present theory was found to be in good agreement with experimental ones using a large amplitude rigid oscillating foil. From this fact, it can be said that this calculation method is effective for quantitative analyses of oscillating foil propulsors.Some simple oscillating foil propulsors were designed for practical application to ships. The results showed that adequate thrust force can be achieved and efficiencies as high as 65% can be envisaged in case of elastic foils.

Study on Propulsion by Partially Elastic Oscillating Foil (1st Report)

Existing theoretical investigations on oscillating foil propulsion have rarely treated a foil which has chordwise flexibility. However, an experimental investigation showed that propulsion performance of a partially elastic oscillating foil Is better than that of a rigid one.In this paper, the authors propose a theory for partially elastic oscillating foils, where the deformation of the elastic part is treated by incorporating the equation of elastic curve in the small amplitude linearized oscillating foil theory. Using this theory, theoretical analysis of propulsion performance were made in detail for both elastic and rigid foils.The following results were obtained.(1) It was shown that propulsion performance of an oscillating foil is decided by three parameters of foil motion, namely, reduced frequency, phase difference between heaving and pitching motion and modified heaving to pitching amplitude ratio.(2) Under the same conditions of motion, compared to a rigid foil, an elastic foil produces less thrust but at a higher efficiency.(3) To obtain the same thrust, an elastic foil is more efficient compared to a rigid one.(4) To produce a given thrust at the highest efficiency, the conditions of motion, namely, heaving and pitching, is not so much affected by the variation in reduced frequency in case of an elastic foil compared to that of a rigid one. This fact is desirable from practical operating conditions.