A study of swept and unswept normal shock wave/turbulent boundary layer interaction and control by piezoelectric flap actuation

摘要

The interaction of a shock wave with a boundary layer is a classic viscous/inviscid interaction problem that occurs over a wide range of high speed aerodynamic flows.For example, on transonic wings, in supersonic air intakes, in propelling nozzles at offdesign conditions and on deflected controls at supersonic/transonic speeds, to name a few. The transonic interaction takes place at Mach numbers typically between 1.1 and 1.5. On an aerofoil, its existence can cause problems that range from a mild increase in section drag to flow separation and buffeting. In the absence of separation the drag increase is predominantly due to wave drag, caused by a rise in entropy through the interaction.The control of the turbulent interaction as applied to a transonic aerofoil is addressed in this thesis. However, the work can equally be applied to the control of interaction for numerous other occurrences where a shock meets a turbulent boundary layer. It is assumed that, for both swept normal shock and unswept normal shock interactions, as long as the Mach number normal to the shock is the same, then the interaction, and therefore its control, should be the same.Numerous schemes have been suggested to control such interaction. However, they have generally been marred by the drag reduction obtained being negated by the additional drag due to the power requirements, for example the pumping power in the case of mass transfer and the drag of the devices in the case of vortex generators. A system of piezoelectrically controlled flaps is presented for the control of theinteraction. The flaps would aeroelastically deflect due to the pressure differencecreated by the pressure rise across the shock and by piezoelectrically induced strains.The amount of deflection, and hence the mass flow through the plenum chamber, wouldcontrol the interaction. It is proposed that the flaps will delay separation of theboundary layer whilst reducing wave drag and overcome the disadvantages of previouscontrol methods. Active control can be utilised to optimise the effects of the boundarylayer shock wave interaction as it would allow the ability to control the position of thecontrol region around the original shock position, mass transfer rate and distribution.A number of design options were considered for the integration of the piezoelectricceramic into the flap structure. These included the use of unimorphs, bimorphs andpolymorphs, with the latter capable of being directly employed as the flap. Unimorphs,with an aluminium substrate, produce less deflection than bimorphs and multimorphs.However, they can withstand and overcome the pressure loads associated with SBLIcontrol.For the current experiments, it was found that near optimal control of the swept andunswept shock wave boundary layer interactions was attained with flap deflectionsbetween 1mm and 3mm. However, to obtain the deflection required for optimalperformance in a full scale situation, a more powerful piezoelectric actuator material isrequired than currently available.A theoretical model is developed to predict the effect of unimorph flap deflection on thedisplacement thickness growth angles, the leading shock angle and the triple pointheight. It is shown that optimal deflection for SBLI control is a trade-off betweenreducing the total pressure losses, which is implied with increasing the triple pointheight, and minimising the frictional losses.