Ignazio Maria Viola's Fluid Dynamics Research Laboratory
Our research is in applied fluid dynamics and focuses on the viscous flow around immersed bodies. Examples of applications include the behaviour of the air around the sail of yacht, and the water around the blade of a tidal turbine. We are particularly interested in those conditions where the nature of the onset flow is turbulent and the forces on the body are due to the formation of vortical flow structures. This occurs, for instance, on yacht sails where the sharp leading edge leads to the formation of a large vortex which contributes significantly to the yacht thrust; it occurs on tidal turbine blades, where periodic variations of the current due to the waves lead to the formation of large vortical structures and, in turn, to large load fluctuations and fatigue failure; it occurs on a microscopic scale on the closest layer of flow to the hull of a ship, where the roughness of the surface leads to microscopic vortices which increase the friction resistance. Our research aims to understand and, when possible control, the formation, stability and interaction of these vortices in order to improve performance, efficiency and survivability of different engineering systems.
Research on yacht engineering allows supporting the nautical industry and promoting high-quality green and sport values. Offshore sailing has become one of the sports allowing the highest revenue boost to local economies. For instance, the direct and indirect financial impacts of the America’s Cup on the hosting region are only 20% lower than the impact of the FIFA World Cup. Our research in yacht engineering is internationally leading and aims to develop new fundamental knowledge on the aerodynamics of sails and to develop new technology to improve the performance of competitive yachts.
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From yacht sails to tidal turbines
The flow field experienced by yacht sails is remarkably similar to the one experienced by tidal turbines. The velocity experienced by a sailing yacht is the combination of the atmospheric boundary layer and the boat velocity, resulting in a velocity profile that increases and rotates from the foot to the head of the sail. Similarly, the flow experienced by a tidal turbine blade is the combination of the tidal stream and the blade rotation, resulting in a velocity profile that increases and rotates from the root to the tip of the blade. Sails are approximately 15 times bigger than tidal blades but this is balanced by the kinematic viscosity of the water, which is 15 times larger than the air, resulting in the same Reynolds number, which is the non-dimensional number that controls the equation of the fluid motion. Also, both yacht sails and tidal turbines experience very large flow fluctuations, due to turbulence and waves. These observations and the recent understanding of how vortex flow can be exploited to improve sail performances as well as tidal turbine blades are the motivation that led our group to apply our specialist expertise on high performance sailing yachts to tidal turbine blades.
Europe requires to increase energy generation from renewable sources in order to meet its 2050 policy objective of reducing greenhouse gas emissions to 80 – 95% below 1990 levels. Tidal energy may be converted by the next generation of renewable energy technology and Europe has some of the best sites in the world for its extraction: including the north of Scotland, the straights of Messina, the Dardanelles Strait, the coasts of Brittany and Normandy.
One of the main technological goals that must be addressed in order to enable the development of the tidal industry is to increase blade structural reliability, through enhanced fatigue performance and durability in normal operating conditions and enhanced survivability in extreme conditions. The combination of high-speed tidal streams and large waves, which induce underwater periodic flow variations, can lead to extreme peak loads on the blades and to dynamic failure. In normal operating conditions, waves and turbulence lead to millions of cycles of load fluctuations per year that can cause early fatigue failures. The current reliability of axial tidal turbine blades has been estimated in one failure every two years, while the expected lifetime of a turbine should be 20 years. For comparison, on wind turbines, the blade failure rate is only one in every ten years. Therefore, there is an urgent need to develop new blade technology to increase reliability.
Development of wing and sail technologies for aviation and high-performance yachts has resulted in surfaces that change shape continuously to improve performance, loading and safety. Based on the group’s experience in the aerodynamics of Americas Cup yachts, our research aims to explore the extent to which the accumulated knowledge, understanding and practice in high-performance yacht design could be integrated into tidal turbine blade design to improve their performance and operation.
Boundary layer flow control
The friction between the flow and the body is the main form or resistance for all streamlined bodies that do not generate lift such as, for instance, ship hulls. The friction resistance is affected by microscopic flow features in the very first layer of fluid near the body. For instance, the surface roughness leads to the generation of microscopic vortices which dissipate energy and lead to higher friction than on a smooth surface. Coatings with different surface finishes leads to differences of more than 1% in the resistance of a large ship, resulting in differences of millions of pounds per year in fuel consumption. On the other hand, the friction can be decreased if the flow fluctuations in the turbulent flow are decreased.
For instance, the use of carefully designed compliant surfaces can allow decreasing the friction by 10%. The use of compliant walls is extremely promising for a number of reasons: the potential drag reduction increases with the level of turbulence of the boundary layer; it has the additional effect of suppressing vibrations and flow-induced noise; it is a passive flow control means and therefore it is resilient to the hostile marine environment. Our research aims to understand the underlying mechanisms of friction resistance and to develop underpinning flow control technology for drag reduction.