The leading-edge vortex of yacht sails

In the present work we experimentally verify, for the first time, that a stable Leading-Edge Vortex (LEV) can be formed on an asymmetric spinnaker, which is a high-lift sail used by yachts to sail downwind. We tested a rigid sail in isolation in a water flume at a Reynolds number of ca. 104. The flow field was measured with Particle Image Velocimetry (PIV) over horizontal cross sections. We found that on the leeward side of the sail (the suction side), the flow separates at the leading edge reattaching further downstream and forming a stable LEV. The LEV grows in diameter from the root to the tip of the sail, where it merges with the tip vortex. We detected the LEV using the γ criterion, and we verified its stability over time. The lift contribution provided by the LEV was computed solving a complex potential model of each sail section. This analysis indicated that the LEV provides more than 10% of the total sail’s lift. These findings suggest that the maximum lift of low-aspect-ratio wings with a sharp leading edge, such as spinnakers, can be enhanced by promoting the formation of a stable LEV

Floating offshore vertical axis wind turbines : opportunities, challenges and way forward

The offshore wind sector is expanding to deep water locations through floating platforms. This poses challenges to horizontal axis wind turbines (HAWTs) due to the ever growing size of blades and floating support structures. As such, maintaining the structural integrity and reducing the levelised cost of energy (LCoE) of floating HAWTs seems increasingly difficult. An alternative to these challenges could be found in floating offshore vertical axis wind turbines (VAWTs). It is known that VAWTs have certain advantages over HAWTs, and in fact, some small-scale developers have successfully commercialised their onshore prototypes. In contrast, it remains unknown whether VAWTs can offer an advantage for deep water floating offshore wind farms. Therefore, here we present a multi-criteria review of different aspects of VAWTs to address this question. It is found that wind farm power density and reliability could be decisive factors to make VAWTs a feasible alternative for deep water floating arrays. Finally, we propose a way forward based on the findings of this review

Passively morphing blades for load alleviation of tidal turbines

Tidal turbines are exposed to time variable loading that can lead to premature failure [1,2]. The use of passive unsteady load mitigation technology, such as bend-twist coupling, is typically limited to low frequency fluctuations and is not suitable to large blades, due to structural rigidity requirements. Active control systems, such as actuated flaps, can respond to higher frequencies than whole-blade passive devices due to their smaller size [3]. However, active systems may reduce turbine reliability. Hence there is a need to develop reliable technology that mitigates unsteady loading in a varied range of frequencies, in order to prolong the fatigue life of tidal turbines. Analytically, it is possible to cancel the unsteady loading of a tidal turbine that rotates through the ocean shear layer with a fully chordwise highly flexible blade. Here, we demonstrate that under attached flow conditions, when a blade is rigid near the leading-edge and flexible near the trailing-edge, the unsteady load mitigation is proportional to the ratio of the flexible length to the full chord of the blade. We verify this relationship experimentally with a blade that has a passively morphing trailing-edge. The morphing trailing-edge extends 25% of the chord of the blade and it allows unsteady load mitigation of up to 25%, without any variation in the mean load -thus there is no penalty in terms of power extraction. In separated flow conditions, when the length-scale of vortical structures is similar to that of the flexible part of the blade, the load mitigation is about 15%. Hence, chordwise morphing blades alleviate loads in variable flow conditions and can contribute to tidal turbine survivability in a reliable way. [1] G.T.Scarlett, B.Sellar, T.van den Bremer,I.M.Viola, Unsteady hydrodynamics of a full-scale tidal turbine operating in large wave conditions, Renewable Energy, 143 (2019), 199-213. [2] G.T.Scarlett,I.M.Viola, Unsteady hydrodynamics of tidal turbineblades, Renewable Energy, 146 (2020), 843-855. [3] A. Young, J. Farman, R. Miller, Load alleviation technology for extending life in tidal turbines, Proceedings of 2nd International Conference on Renewable Energies Offshore, RENEW 2016, 521-530

Force generation mechanisms of downwind sails

The force generation mechanisms of a yacht sail are discussed with the aid of force and flow measurements on a model scale spinnaker. The velocity and vorticity fields are measured on five horizontal sections with particle image velocimetry. By comparing the forces measured with a balance to those computed from the vorticity field, we demonstrate how the force generation can be interpreted by the production and stretching of vortex rings. We consider vortex rings to be continuously generated and shed from the perimeter of the sail. The intersection of their vertical legs with horizontal planes are leading and trailing edge vortices. The sail force is due to the rate of change of the impulse of the vortex rings. Consequently, we show that the force can be computed from the time-averaged vorticity field using the Kutta-Joukowski lift formula, or from the strength and relative velocity of the leading and trailing edge vortices, or from the vorticity flux at the perimeter of the sail. The drag is estimated with Filon's drag formula. These results confirm experimentally the theoretical work of Viola et al. [1] and pave the way to the development of design methodologies that improve sail performance by manipulating the local vorticity field

A low cost oscillating membrane for underwater applications at low Reynolds numbers

Active surface morphing is a nonintrusive flow control technique that can delay separation in laminar and turbulent boundary layers. Most of the experimental studies of such control strategy have been carried out in wind tunnels at low Reynolds numbers with costly actuators. In contrast, the implementation of such a control strategy at low cost for an underwater environment remains vastly unexplored. This paper explores active surface morphing at low cost and at low Reynolds for underwater applications. We do this with a 3D printed foil submerged in a water tunnel. The suction surface of the foil is covered with a magnetoelastic membrane. The membrane is actuated via two electromagnets that are positioned inside of the foil. Three actuation frequencies (slow, intermediate, fast) are tested and the deformation of the membrane is measured with an optosensor. We show that lift increases by 1%, whilst drag decreases by 6% at a Strouhal number of 0.3, i.e., at the fast actuation case. We demonstrate that surface actuation is applicable to the marine environment through an off the shelf approach, and that this method is more economical than existing active surface morphing technologies. Since the actuation mechanism is not energy intensive, it is envisioned that it could be applied to marine energy devices, boat appendages, and autonomous underwater vehicles

Hydrodynamic characteristics of remora's symbiotic relationships

Symbiotic relationships have developed through natural evolution which can provide advantages to parties in terms of survival. For example, that of the remora fish attached to the body of a shark to compensate for their poor swimming ability. From the remora's perspective, this could be associated to an increased hydrodynamic efficiency in swimming and this needs to be investigated. To understand the remora's swimming strategy in the attachment state, a systematic study has been conducted using the commercial Computational Fluid Dynamics CFD software, STAR-CCM+ to analyse and compare the resistance characteristics of the remora in attached swimming conditions. Two fundamental questions are addressed: what is the effect of the developed boundary layer flow and the effect of the adverse pressure gradient on the remora's hydrodynamic characteristics? By researching the hydrodynamic characteristics of the remora on varying attachment locations, the remora's unique behaviours could be applied to autonomous underwater vehicles (AUVs), which currently cannot perform docking and recovery without asking the mother vehicle to come for a halt

Morphing blades for unsteady load alleviation of wind and tidal turbines

A critical challenge for wind and tidal turbines is managing highly unsteady inflow due to the flow shear profile, turbulence and, for tidal devices, waves. This causes large variations in the magnitude and direction of hydrodynamic loading both temporally and spatially, across the rotor swept area. The fluctuating loads cause vibrations, which are transmitted to the rest of the turbine, causing fatigue and reducing reliability. Load variations also cause generator power output to fluctuate, reducing power quality and necessitating the use of expensive power electronics. Offshore wind and tidal turbines are particularly affected. Offshore wind turbines are larger so experience greater temporal and spatial flow variation across the rotor and have more flexible blades, which increases aeroelastic coupling. Tidal turbines operate in a harsher environment than wind turbines; seawater is more than 800 times denser than air, wave loading adds further instability to the flow and cavitation may occur during stall. To successfully alleviate unsteady loads, the response bandwidth must be twice that of the disturbance frequency. Wind turbulence occurs up to approximately 6 Hz, necessitating a bandwidth of 12 Hz. Traditional, full-blade pitch actuators only operate up to about 0.25 Hz and are typically underpowered, further limiting their effectiveness. Generator reaction torque, another traditional control parameter, has high bandwidth but is limited by its inherent lack of spatial discrimination. Existing active control methods using small, low inertia surfaces, such as trailing edge flaps, are effective at unsteady load mitigation. However, they require power, electronics, hinges, bearings and mechanisms that are susceptible to debris and biofouling. Additional complexity poses a risk to reliability and increases O&M, a major driver for LCOE offshore, thereby discouraging the use of active control. Existing passive control methods predominantly rely on aeroelastic tailoring, such as bend-twist coupling, which respond too slowly to mitigate turbulence and only significantly affect loads on the outer section of the blade. We propose a novel passive load-control system that is capable of turbulence rejection and is equally applicable to wind and tidal turbines, as well as aircraft. Our novel morphing blade has a flexible, variable geometry trailing edge that extends along the entire blade span. Its relatively lower inertia enables mitigation of high frequency load variations, previously only achievable through active control. Additionally, its spanwise tailoring allows loads to be cancelled along the entire blade, even at the root, producing a cleaner wake and improving We previously showed that the tailored morphing blade completely counteracts load fluctuations along the entire blade span. This prevents flow separation, lowering the time-averaged thrust, which simultaneously increases power output and quality. In the current work, we experimentally test a prototype morphing blade for a tidal turbine in a combined wave-current flume and compare it to a solid blade. Both blades use the NACA 0012 profile, with the morphing blade having a sprung trailing edge flap. Tests are conducted on 2D blade sections that span the width of the flume. For these preliminary investigations, the blade kinematics are prescribed to mimic the deterministic hydrodynamic load oscillation experienced by a horizontal axis blade rotating through the flow shear profile in a real tidal current channel. Thus, the blade section is oscillated in pure heave in a uniform freestream flow. Hydrodynamic force and PIV analyses provide insight into the fluid-structure interaction and vortex dynamics of the novel blade

Smart blades with actively controlled compliant skin

As new materials are developed to make wind turbine blades lighter and recycle friendly, new opportunities emerge to design smart materials that tackle fundamental fluid dynamic problems, such as flow separation. To this aim, we introduce a technology tailored to prevent flow separation through an actively compliant skin on the suction side of a blade cross-section. The skin oscillates to enhance mixing in the boundary layer and prevent flow separation. The concept is demonstrated with a NACA 0025 3D printed blade section whose suction side is covered with an off the shelf polymer magnetoelastic membrane. This type of membrane provides the required structural stiffness for the prototype, as opposed to low stiffness attempted membrane combinations of silicon rubber with dispersed iron particles. The off the shelf approach proved also to be economically affordable and time efficient to manufacture the blade. Figure 1a shows the blade subject to membrane slow actuation and figure 1b the membrane subject to fast actuation. The membrane is actuated via two electromagnets positioned inside of the blade. Hence, the assembly avoids moving joints and ensure that the electronics remain isolated from the environment. The empty space inside of the blade is filled with foam to support external pressure and help the membrane keep its shape. The membrane extends about three quarters of the span of the blade and is glued to the assembly by means of a thin layer of silicon rubber. The control of the membrane is implemented with an Arduino Uno microcontroller and the power electronics with an L298N motor driver. Two pulse width modulation (PWM) duty cycles are used to control the voltage fed into the electromagnets and hence the displacement of the membrane. A time delay between the two duty cycles controls the speed of the actuation. The deformation of the membrane was measured at three actuation speeds in air and water with an optosensor positioned inside the body of the blade. The performance of the membrane was satisfactory and similar in both environments. Load cell measurements are carried with 6-axis load cell and sampled at 1000 Hz in a water flume. The blade is positioned at a fixed angle of attack and the membrane actuated at three different speeds. The flume is 9 m long, 0.4 m wide and 0.9 m high with a flat, horizontal bed. The mean water depth is set to 0.5 m. Results show that the lift force increases by about 1% with increasing membrane actuation frequency, whilst drag reduces by about 7%. Power spectrum density of the force signals shows frequency peaks at the corresponding actuation frequency of the membrane. Future and ongoing work includes evaluating the impact of such technology in the torque and thrust outputs of a turbine roto

A novel energy harvesting mechanism and its design methodology for underwater gliders using thermal buoyancy engines

Underwater gliders are becoming popular in ocean exploration. However, the main development limitation of underwater gliders is still around energy. This paper proposes a new-type energy harvesting mechanism and explores its design methodology for the gliders using thermal buoyancy engines. With the temperature difference in the ocean, the thermal buoyancy engine changes the buoyancy of the glider and drives the glider to ascend and descend through the water and drive a turbine behind to harvest energy. Based on this harvesting mechanism, firstly, a new-type thermal engine with high ballast capacity is developed with patent applied. Secondly, a dedicated turbine design and optimization method based on modified Blade Element Momentum (BEM) theory has been developed to maximize the energy harvesting capability