Control of boundary layer streaks induced by free-stream turbulence using a novel velocity-pressure control framework.

In this proposal we aim to develop a new framework for active control of spatially developing flows and apply it to stabilise the streaky structures developing in incompressible and compressible boundary layers in the presence of high free-stream turbulence. This flow scenario is very common over aircraft wings and turbine blades. To the best of our knowledge, this is the first time that active control of such flows is attempted worldwide. The current approach, which is based on the wall-normal velocity and vorticity formulation to derive a state-space system suitable for controller design, has several limitations. The new control framework will use instead the primitive variables, velocity and pressure. This formulation offers greater flexibility and, most importantly, makes the incorporation of the effect of free-stream turbulence straightforward. This choice of variables however introduces new challenges from the control perspective because the standard optimal control algorithms can not be applied. In the attached case of support, the limitations of the current approach are explained, the benefits of the new formulation are highlighted and the challenges that must be dealt with for the successful control of streaks are analysed. This work offers a unique opportunity to put the UK in a leading position in the new and rapidly developing area of active flow control.

The project will focus on the control of streaks occurring in incompressible and compressible boundary layers in the presence of free-stream turbulence. Many industrial flow systems involve an external free-stream flow disturbed by vortical fluctuations impinging on a boundary layer. If such disturbances are sufficiently energetic, the boundary layer will become turbulent over a short distance. Examples include boundary layers over aircraft wings and fuselage, flows in engine intakes, in jet nozzles and over turbine blades (where the laminar streaks dominate about 50-70% of the blade length). Compressibility effects should be taken into accout for some of these applications. These streaky structures are thought to be precursors of by-pass transition. The main aim of the project is to develop controllers capable of attenuating the growth of the laminar streaks, and, in turn, of delaying the onset of transition. This would result in an extension of the laminar region and therefore in drag reduction. Control of drag represents one of the major challenges in modern fluid engineering. For example, skin-friction drag is the major source of energy loss for aircraft vehicles. It constitutes about half of the total drag, while the remaining portion is due to lift-induced drag and to pressure losses due to separation. The main direct consequence of drag reduction is the improvement of the energy performance of the vehicle. It can lead to lower economic cost due to reduced fuel consumption, reduction of vehicle weight, increase of the number of passengers, and an improved impact on the environment due to the decrease of air pollution, with a beneficial influence on the global warming problem and more generally on the quality of life. For example, the estimated consequences of a 1% drag reduction in commercial aircrafts are 0.2% drop of flight costs, a cut of 1.6 tons of weight or an increase of 10 passengers. A reduction of turbulence intensity will further limit aerodynamic noise, a source of nuisance in densely inhabited areas. Research in flow control for applications in the aeronautical industry has been recently revived by the objective set by the Advisory Council for Aeronautics Research in Europe (ACARE) of reducing fuel consumption and CO2 by 50% and NOx by 80% per passenger kilometre by 2020. This makes the present project very timely.