AEROMEMS II - Enhancing the performance of high lift systems
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Outline: High lift devices are secondary assemblies that increase the aerodynamic lift and drag forces acting on aircraft primary aerofoil surfaces during landing and take-off manoeuvres. This particular research project has the aim of optimising the flow of air around aerofoil trailing edge flaps.
An important feature of airflow past an aerofoil or flap is that a thin layer of air immediately adjacent to the surface is slowed down in overcoming viscous forces due to the presence of the surface. This layer is known as the boundary layer.
When the momentum of air within the boundary layer becomes insufficient to allow the flow to proceed into regions of increasing pressure, the boundary layer will no longer continue to follow the surface profile. Hence separation occurs.
The lift and drag coefficients for a flap are functions of both Reynolds number and the angle of attack. As the angle of attack is increased, the lift coefficient increases smoothly until a maximum value is reached. Increasing the angle of attack beyond this point will produce a sharp drop in lift. The flap is said to have stalled when lift drops in this manner.
A stall situation comes about when flow separation occurs over a major proportion of the upper surface of the flap.
The mixing process, caused by the random velocities present within a turbulent boundary layer, has the effect of distributing kinetic energy throughout the layer. Therefore a turbulent boundary layer will withstand a more adverse pressure gradient than a laminar one. This is a property that can be exploited in aerofoil and flap design, in order to delay flow separation as long as possible.
Objectives: The fundamental work associated with this research project will look at the interaction of synthetic jets with cross-flow boundary layers. The aim is to assess the effectiveness of synthetic jets in revitalising reduced momentum within boundary layers, and hence delay separation.
Previous Analysis:Flow visualisations from previous wind tunnel experiments utilising synthetic jets show the creation of vortex filaments travelling axially within the boundary layer flow.
Plan:(i) To ascertain the delay in separation which can achieved over an idealised flap profile using a single array of synthetic jets. The synthetic jets will be situated perpendicular to the direction of the oncoming airflow.
(ii) To enable the specification of synthetic jet configurations to achieve optimum results:
(a) Size of jet orifice.
(b) Shape of orifice, i.e. circular, rectangular, etc.
(c) Lateral spacing between the jets.
(d) Axial positioning measured from the leading edge of flap.
(e) Volumetric flow rates through jets.
(f) Frequency of operation, for periodic operation.