Explorative methods for turbulent flow control

Basso, Alessio (2018) Explorative methods for turbulent flow control. MPhil thesis, University of Nottingham.

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Turbulent skin-friction is a matter of importance for the aerospace sector, where the increase in fuel costs is of concern. In view of this consideration, the aircraft aerodynamic efficiency can be increased by reducing turbulent drag. In fact, turbulent drag can up to 45% of the total drag force encountered in subsonic commercial transportation. Four different concepts have been relevant over the last few decades to achieve turbulent skin-friction reduction:

- Fluid suction;

- Tangential gas injections;

- Particle additives applied to main flow stream;

- Compliant walls.

However, all the above methods result in a delay of the turbulence transition point, rather than a disruption of the turbulence self-sustenance mechanism.

Outer coherent motions, often identified in so-called hairpin vortices, are profoundly correlated with the dynamics of turbulence. Therefore, their actuation is considered as a valid approach to interrupt the near-wall cycle and reduce the associated friction drag. On the basis of the above considerations, the project presented in this dissertation proposes an innovative flow actuator for active control of turbulent skin friction for subsonic regimes. The key features for this flow actuator are:

- A real-time identification of hairpin eddies directly from the near-wall region. This idea is crucial when considering direct intervention of hairpin eddies as the key method for turbulence control;

- Disruption of coherent motions. Actuators should have a fast response to disrupt hairpins within their persistence time interval;

- Response adjustment in real time. This is to effectively reduce turbulent drag by continuously monitoring the near-wall signal.

In addition, in the specific case of flow control on aircraft wings, the flow actuator should not modify the surface smoothness and aerodynamic.

Therefore, the present work has explored two different methodologies which can be combined together to achieve turbulent skin-friction reduction:

- An approach making use of image processing and pattern recognition to identify outer coherent motions directly from the near-wall region. This method is interesting, especially for flow control on aircraft wings where hairpin footprints can be read at the wall without affecting the wing aerodynamic shape;

- The use of laser energy deposition (LED) to induce a local remote flow perturbation. Laser energy deposition is convenient, especially because of its fast response. Moreover, since no mechanical components are required, the dynamic response of the boundary layer can be shorter than conventional actuators.

In the perspective of producing persistent perturbations which can interfere with hairpin vortices, laser energy deposition is used here to to trigger the ignition of flammable bubbles and the sublimation of carbon-dioxide particles. Particle image velocimetry (PIV) in quiescent air has been performed to measure the effects induced by laser energy deposition. Ignition of bubbles filled with flamable mixtures is found interesting in this work due to the fast and intense jets produced.

Developments presented in this dissertation provide the ground work to develop an active-feedback actuator which can monitor and disrupt the turbulence near-wall cycle, by manipulating outer coherent motions. This technology is helpful to reduce turbulent skin friction on aircraft wings in subsonic transportation.

Item Type: Thesis (University of Nottingham only) (MPhil)
Supervisors: Choi, Kwing-So
Zanchetta, Pericle
Morvan, Herve P.
Keywords: Turbulence; Airplanes, Wings; Actuators; Drag (Aerodynamics)
Subjects: T Technology > TJ Mechanical engineering and machinery > TJ212 Control engineering systems. Automatic machinery
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 49715
Depositing User: Basso, Alessio
Date Deposited: 13 Jul 2018 04:40
Last Modified: 08 May 2020 08:30
URI: https://eprints.nottingham.ac.uk/id/eprint/49715

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