Boundary layer control within the flow control group invlves both the development of novel actuators and fundamental research into the properties of turbulent and transitional boundary layers.
At high Reynolds numbers, typical of a passenger aircraft, the long streawmise coherent motions in the outer part of a turbulent boundary layer exert considerable influence in the near-wall region. Understanding this inner/outer interaction is important and we tackle this problem by using a novel experimental design.
The experiment consists of grid-generated turbulence passing first over a moving ground plane in a wind tunnel so as to create a shear-less boundary layer, which is then rapidly sheared as it encounters a stationary floor further downstream. The velocity measurements carried out show that, for a certain streamwise extent, the pre-multiplied spectra of the streamwise velocity fluctuations, which display a bi-modal shape, resemble those in an equilibrium turbulent boundary layer.
We also carry out measurement of turbulent pressure fluctuations, both on the floor and inside the boundary layer. The in-flow static pressure measurements are carried out using a specially-designed needle probe that can be traversed within the boundary layer. These measurements show a high degree of correlation of pressure fluctuations at the edge of the boundary layer with those at the wall, supporting the view that pressure can act as an effective agent in coupling the inner and outer turbulent motions.
Detailed characterization of the pressure field is currently underway. We believe that the understanding gained from this work, especially with regard to the role of pressure, can help us devising more effective flow control strategies. One such strategy, using a linear passivity-based control, is presently being explored in a turbulent channel-flow setting and the numerical simulations show that the controller operates primarily on the ‘linear’ or ‘rapid’ part of the field of pressure fluctuations.
Academic staff
Professor Jonathan Morrison
Dr Eric Kerrigan
Dr Matthew Santer