Plasma-actuator controlled turbulent jets

Aeronautics and air transport is a vital sector of our society and economy. Aviation currently accounts for about 2% of human-induced CO2 emissions with more than 3.12 billion passengers and 48 million tons of freight worldwide last year with an average of more than 100,000 flights every day. Worldwide traffic is predicted to grow at a rate of 4% to 5% per year for the next 30 years. It simply means that more than 16 billion passengers and 25 million flights are expected in 2050. Aviation will have to find ways to meet the growing demand for air transport whilst reducing its environmental impact, specifically the level of noise and of carbon emissions. Innovative solutions are also needed to deal with fuel consumption so that aviation does not become increasingly dependent on more and more expensive energy sources. It is clear that it requires a significant step change in the technologies of future aircraft.

In recent years, the development of devices known as plasma actuators has advanced the promise of controlling flows in new ways that increase lift, reduce drag and improve aerodynamic efficiencies, advances that may lead to safer, more efficient and quieter aircraft. Dielectric barrier discharge (DBD) plasma actuators consist of two electrodes, one exposed to the ambient fluid and the other covered by a dielectric material. When an A.C. voltage is applied between the two electrodes the ambient fluid over the covered electrode ionizes. This ionized fluid is called the plasma and results
in a body force vector which exchanges momentum with the ambient, neutrally charged, fluid.

For this project, high-resolution simulations will be carried out on the most powerful supercomputers in Europe in order to demonstrate the potential of DBD plasma actuators for the control of turbulent jets. The problem of jet noise pollution has become more severe in the past few decades due to the ever increasing number of flights, the tightening of environmental impact regulations, and the development of urban/residential areas in close proximity to airports. The scientific objective of the present project is to advance our understanding of aeroacoustic mechanisms up to the point where we can
propose targeted plasma control strategies for free shear flows to tackle the problem of jet noise pollution. This research project is a first step in the development of new technologies based on plasma actuators in the aeronautic sector not only for noise reduction purposes but also potentially for mixing enhancement and for a better efficiency of jet engines.

As of today, active flow control technologies have not been implemented in commercial aircraft. The large number of parameters (location of the actuator, orientation, size, relative placement of the embedded and exposed electrodes, applied voltage, frequency) affecting the performance of plasma actuators makes their development, testing and optimisation a very complicated task. Experimental approaches require numerous high-cost and time consuming trial-and-error iterations. Computational Fluid Dynamics (CFD) can complement ideally experiments with the potential to investigate in
detail plasma-actuator controlled turbulent flows.

By 2032 it estimated that worldwide more than 29,000 new large civil airliners, 24,000 business jets, 5,800 regional aircraft and 40,000 helicopters will be required to deal with the constant increase of the worldwide traffic. For instance, more than 16 billion passengers per year worldwide are expected in 2050. Aviation will have to find ways to meet this growing demand whilst reducing its environmental impact, specifically the level of noise and of carbon emission. It is clear that it requires a significant step change in the technologies of future aircraft. The UK is directly concerned by this challenge as it the second national aerospace industry in the world, with a 17% global market share for a turnover of more than £20 billion every year, sustaining directly and indirectly more than 200,000 jobs.

This application proposes to investigate the potential for noise reduction of a very promising active flow control strategy based on plasma actuators.
Plasma actuators can be used in a wide range of engineering applications such as viscous drag reduction and boundary layer separation control in low-speed flows, shock wave modification and wave drag reduction in supersonic and transonic flows, and supersonic boundary layer transition control. The idea here is to use them to manipulate a turbulent round jet in order to either reduce the acoustic sources or to minimise their propagation.
The number of active flow control devices that have successfully transitioned from a laboratory prototype to a real-world aeronautical application is still relatively small and the proposed Computational Fluid Dynamics project is offering a great opportunity to explore active control strategies as a complement to experiments. Another important aspect of this project is related to the identification of acoustic noise in a turbulent flow. To develop efficient noise reduction strategy, it is necessary to understand where the noise is coming from. The main problem in identifying the acoustic sources is that they often get eclipsed by the much larger turbulent fluctuations and become inaccessible if evaluated directly. This project is aiming to propose a general framework for the identification of acoustic sources in turbulent flows based on indirect evaluation of the acoustic sources and their propagations.

The Department of Aeronautics at Imperial College has agreed to provide a DTA student to work on this project. This student and the PDRA funded by this grant will become experienced scientists in High Performance Computing and in the manipulation of turbulent flows. They will both attend High Performance Computing training sessions organised by the UK supercomputing service ARCHER. They will both collaborate with the team of Professor Choi from the University of Nottingham as it is the UK experimental leading research group for active flow control strategies using plasma actuators. Finally, they will also interact with the PhD students from the Centre for Doctoral Training in Fluid Mechanics across the Scales at Imperial College to develop their lecture skills.

Plasma actuators are one of the most promising technologies to manipulate a turbulent jet with strong potential benefits in mixing efficiency for combustion, propulsion efficiency and noise reduction. However, a thorough scientific understanding of this new active control solution is required before it can be implemented at an industrial scale. Only very few research groups in the UK are working on active plasma control solutions. This research project will hopefully encourage other research groups to start using plasma actuator in their research as it can be applied to a wide range of flow configurations. It will also hopefully arouse curiosity and interest from the many aerospace industries in the UK looking for innovative active flow control solutions.