From bluff-body flows to performance enhancement of tidal turbines

The first part of the talk will focus on my doctoral research, which aims to understand the mechanism of unsteady flow and thrust generation in the flow field of an oscillating cylinder filament system through detailed flow visualization and particle image velocimetry measurements. Fluid structure interaction is ubiquitous in nature. For example, fish swim and birds fly by making use of an intricately coupled interaction between their body and the surrounding flow field. In addition, the flexibility in their wings and tails aid in enhancing thrust and maximizing propulsive efficiency. Some marine animals like tadpoles have a circular head that resembles a bluff body. In addition, they have a flexible tail. They propel themselves forward by oscillating both their head and tail. In this work we approximately model the motion of a ‘tethered tadpole’ by studying the flow past a rotationally oscillating cylinder with an attached flexible filament. The streamwise force and power are estimated through control volume analysis using an improved expression which considers the streamwise and transverse velocity fluctuations in the wake. These terms become important in a flow field where asymmetric wakes are observed. An attached filament significantly modifies the flow past an oscillating cylinder-filament system from a Bénard Kármán vortex street to a reverse Bénard Kármán vortex street, albeit over a certain range of Strouhal number ~ 0.25 - 0.5, encountered in nature, in flapping flight and in the flow past pitching airfoils. The transition from a Kármán vortex street to a reverse Kármán vortex street precedes the drag-to-thrust transition. Most of the momentum and energy addition in the flow field happens near the filament tip, which indicates that filament oscillation is responsible for thrust generation. Maximum thrust is generated at the time instants when vortices are shed in the wake from the filament tip.

The second part of the talk will focus on my postdoctoral research at The University of Edinburgh. The aim of the project is to investigate the effectiveness of passively pitching blades on tidal turbine performance. Tidal energy is a reliable and sustainable source of renewable energy that can contribute towards energy security. Tidal turbine blades experience large unsteady load fluctuations due to flow unsteadiness, turbulence, yaw and shear of the oncoming flow, which may lead to structural fatigue. In order to operate over a wide range of conditions, many turbines adopt an active collective pitch control system, where the blades pitch simultaneously, to ensure that the turbine can self-start when the flow velocity is slow, cap the maximum power to protect the generator when the flow velocity is high, and stop the turbine in case of emergency. However, active pitch control systems require complex pitch mechanisms and gear boxes which can increase the need for frequent maintenance, operational costs and potential for failure. Here, we present a passive pitch system, where a torque is applied at the blade root, by means of a mechanical spring such that the blade pitch varies in response to the hydrodynamic moment. In this talk, we demonstrate through experiments that for any flow velocity, there is a combination of spring stiffness and preload that allow a passive pitch blade to generate the same mean power and thrust than a fixed pitch blade. When the turbine is operated at higher tip speed ratios (λ) than the design condition, the power remains about constant while the thrust and the thrust fluctuations decrease significantly (up to 40% for a 20% increase in λ). If λ is further increased, the power decreases at higher rate than a fixed pitch turbine. Therefore, the angular velocity of a passive pitch blade can be increased to cap the maximum power when the flow velocity is higher than rated conditions. These results demonstrate that passive pitch can potentially replace active pitch without compromising performance and potentially increasing reliability.