Both experiments and numerical simulations have provided evidence that an initially fully developed two-dimensional boundary layer, subjected to a sudden spanwise forcing, exhibits a decrease of turbulent quantities such as the Reynolds shear stress, turbulent kinetic energy and turbulent friction drag. In past experiments and investigations, such forcing has traditionally been in the form of spanwise wall oscillations, spanwise travelling Lorentz forcing, superimposed spanwise pressure gradients and spanwise travelling waves of an inplane flexible wall. The aim of this work is to take the idea a step further and develop an active surface which locally executes the motions described above making such a system more readily deployable. Two surfaces were developed: both executing in-plane local oscillations with amplitude close to or larger than the mean streak spacing in a turbulent flow, but based on two different technologies, electroactive polymers in the dielectric form of actuation and electromagnetic motor forcing. The effect of these two surfaces was confined to wall-normal heights on the order of the linear sublayer of the turbulent boundary layer, and frequency and wavelength similar to those reported in literature. Extensive hot-wire measurements, some PIV measurements and direct measurement of friction drag using a bespoke drag balance are presented for the systematic variation of the relevant parameters for turbulent friction drag reduction. Electroactive polymers (EAP) are able to undergo relatively large deflections at high frequencies. Developments in the field of EAP such as static and dynamic characterisation of the EAP membranes in use in this work, development of robust electrodes and their characterisation, in-house manufacturing of thin silicone membranes and post-processing of pre-built silicone membranes are presented. Numerical studies of the optimum pre-strain values and of the optimum electrode to passive portions width ratios are presented. Actuator development techniques including EAP membrane pre-stretch in a bespoke jig, EAP membrane pre-conditioning to go past the Mullins' effect, electrode preparation procedure and deposition, and frame preparation are presented. Actuator characterisation results including analysis of multi-flash photographs and laser profilometer scans for in-plane and out-plane deflections at different frequencies are also presented.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:535605 |
| Date | January 2011 |
| Creators | Gouder, Kevin |
| Contributors | Morrison, Jonathan |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/6911 |
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