Adaptive structures for the control of cellular separation
Creators: name: Garland, Michael
Publisher: name: Imperial College London
Year: 2016

description: This work describes the research undertaken on the development of adaptive structures to reduce turbulent boundary layer separation from a wing. Separation control is a safety critical function that is currently filled by the application of static vortex generators to the wings on most modern aircraft. These devices generate vorticity which produces a downstream mixing effect, energising the boundary layer and postponing separation. The mixing of the boundary layer also increases the drag of the aircraft, reducing efficiency. As static devices, the mixing effect is also permanent, regardless of the current likelihood of separation. Adaptive structures allow the development of beneficial geometry from the body’s surface without the use of breaks or mechanisms in the structure surface. This allows geometry modification without sources of parasitic drag or turbulent transition. The first subject of this work is the development of an adaptive surface to provide the desired momentum transfer through the boundary layer when required, and which can be retracted when not needed, reducing drag and increasing efficiency. Adaptive structures inhabit a complex design space due to the coupling between bending and in-plane stretching of the surface. In previous morphing studies, design optimisation has frequently been used to identify the ideal design parameters. Initially, the design methodology is developed on a test case transferring momentum within a zero-pressure gradient boundary layer. The resulting geometry is then tested experimentally and the structural and fluidic response is found to compare well to simulations. Once the design approach is validated, it must be applied to an efficient location on an aerofoil. The second area of research is therefore the complex, three-dimensional, separation from a 2D aerofoil. This is investigated experimentally with both mean and time-dependent data. The naturally occurring, three-dimensional and spanwise periodic topology of the separated flow, termed a `stall cell', is investigated to determine a suitable location for the application of targeted control at a critical point. Fourier analysis and Proper Orthogonal Decomposition are applied to the time-dependent data gathered to extract coherent, periodic, fluctuations in the separated flow field. The variation of the relative strengths of these features, distinct in frequency, is isolated to regions within the stall cell. Knowledge of the flow field gained during this work is applied to stall cell reduction and a single vortex generator is applied to the wing upstream of an identified critical point within the flow field. The separated area is seen to reduce significantly with this actuation. The design methodology developed previously is applied to the initially curved surface of an aerofoil. The final structure is manufactured and tested experimentally and found to be effective in reducing the separation extent. The control is found to be less effective than the static vortex generators. However, unlike the static device, the adaptive version is fully elastic, in both deployment and reaction, and thus shows none of the detrimental effects associated with traditional devices.