Research

Our research is in applied fluid dynamics and focuses on those conditions where the forces on a body immersed in a fluid are due to the formation of vortical flow structures. This often occurs in nature, where natural evolution has led to optimal solutions for complex problems. Hence, we seek inspiration from the fluid mechanics of plants and animals to develop new technology. Vortices, for instance, are exploited by natural flyers to fly stably and efficiently in the turbulent wind. Similarly, the forces on very thin surfaces, such as the wings of small drones and the sails of a yacht, are dominated by vortex flow. 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.

COVID-19 Airflow Research

COVID-19 airflow research

The SARS-CoV-2 virus (coronavirus) is primarily transmitted through virus-laden fluid particles ejected from the mouth of infected people. Our research aims to gain new fundamental understanding on the aerodynamics of respiratory droplets and to provide physical evidence on the effectiveness of risk mitigation strategies.

For example, in Cummins et al (2020) we investigated the aerodynamics of droplets through the Maxey-Riley equation and we show that droplets with minimum horizontal range are from a few μm to a few hundred μm. The result that such droplets have a very short range could have important implications for the interpretation of existing data on droplet dispersion.

In Viola et al (2020) we use a background oriented schlieren technique to investigate the air flow ejected by a person while breathing and coughing, the aerosol dispersion and the effectiveness of face covering in mitigating virus transmission risk. We found that all face covers without an outlet valve reduce the front flow through jet by more than 2/3rd. Surgical and hand-made masks, and face shields, generate several leakage jets, including intense backward and downwards jets that may present major hazards.

In Bandiera et al (2020), we study the effects of face coverings on mitigating dispersion of large respiratory tract droplet that land within seconds. These droplets are thought to be the main route to transmission of SARS-CoV-2 virus. We quantify the number of droplets in flight using laser sheet illumination and UV- light for those that had landed at table height at up to 2 m. We find that wearing a face covering decreases the number of projected droplets by > 1000-fold. We estimate that a person standing 2 m from someone coughing without a mask is exposed to over 10,000 times more respiratory droplets than from someone standing 0.5 m away wearing a basic single layer mask. Our results indicate that face coverings show consistent efficacy at blocking respiratory droplets and thus provide an opportunity to moderate social distancing policies.

 

Yacht Engineering

Yacht Engineering

Our research in yacht engineering aims to develop new fundamental knowledge on the aerodynamics of sails to improve the performance of competitive yachts. Sails are very thin wings. Differently from conventional wings, such as the wings of an airplane, the flow separates at the leading edge forming complex tridimensional vortical flow structures. Hence, while wings are typically designed to prevent flow separation and enable the air to flow smoothly along the wing, the flow around the sails is separated and the aerodynamic force is mostly provided by the vortices in the flow (Arredondo-Galeana and Viola, 2018). See for example this 16-min keynote presentation on how sails generate forces or the companion paper Viola et al (2020).

Read more about our Yacht Engineering Research.

 

Biomechanics

Biomechanic Dandelion flight

The fluid mechanics principles that allow a passenger jet to lift off the ground are not applicable to the flight of small flyers. The reason for this is scaling: human flight requires very large Reynolds numbers, while small plants have comparatively small Reynolds numbers. At this small scale, there are a variety of modes of flight available to plants: from parachuting to gilding and autorotation.

Our group studies the aerodynamics of small plumed fruit that utilise a new mode of flight. If a dandelion fruit, for example, is picked up by the breeze, it can be carried over hundreds of miles. Incredibly, the filament structures of these seeds are mostly empty space, making this an extremely efficient mode of transport (Cummins et al, 2018). Moreover, the fruit can become more or less streamlined depending on the environmental conditions; in this way, they behave as a smart technology (Seale et al 2018). We are uncovering the novel engineering principles of these fruit, using a combination of numerical, analytical, and physical modelling. Our group has built a specialised wind tunnel, which we use alongside particle image velocimetry and high-speed imaging to visualise and measure the flow around these fruit.

 

Tidal Energy

Tidal Energy Research

Very strong currents flow in some parts of the ocean, thousands of time more powerful than a strong wind. The power in these currents is completely renewable and virtually unlimited. In Europe, for example, highly energetic tidal sites include the north of Scotland, the straights of Messina, the Dardanelles Strait, the coasts of Brittany and Normandy. The first MW-scale arrays of tidal turbines is currently being installed in Scotland - yet our understanding of the tidal flow remains marginal. More importantly, it is still unclear which is the most efficient technology for energy harvesting. Our research aims to understand the unsteady hydrodynamics of tidal energy harvesters and to design more durable and efficient technology (Want et al 2018, Scarlett et al 2019, Scarlett and Viola 2020). Inspired by the extraordinary abilities of natural swimmers, we study morphing technology that can mitigate load fluctuations and new-concept array design inspired by the synergetic fluid dynamics interactions in fish shoals.

Photograph of a model-scale tidal turbine developed at the Institute for Energy System, courtesy of Dr G. Payne.