Elastomer Surface Pressure Sensor and its Intergration to a 'Smart' surface for Active Flow Control

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The no-slip condition between the surface of an aircraft and the air in which it flies is responsible not only for drag, but for the lift as well. An obvious potentially huge benefit to a carbon-based economy is the reduction in fuel consumption of aircraft, road vehicles and ships. The technological goal of doing this, together with many others, constitutes an important area of research called flow control and the same techniques used to reduce drag may also be used to help control aircraft by improving their stability and manoeuvrability, that is producing additional lift or thrust when it is needed. This is often done by delaying separation, that is, by preventing stall that could otherwise occur when an aircraft is flying slowly or when it is manoeuvring near the limits of its flight envelope. Another way of doing this could be by introducing small perturbations into the jet that provides the thrust so making it deflect or break up more quickly. Such control of an aircraft would be especially useful if it were unmanned. What we wish to do here is to take advantage of recent developments in materials called polymers (the most often used polymer is PVC or plastic) some of which possess piezoelectric properties. By piezoelectric, we mean that the polymer can be made to expand or compress by applying a voltage or charge, and correspondingly, produce charge when the polymer is compressed. These are also called electroactive polymers or EAPS. If the polymer has the right properties and is designed optimally, it can be used in a great many applications such as the in the body as an artificial muscle. Obviously the most important 'muscle' in the body is the heart which is of course involved in the flow of blood around the body.Our specific application is to take the idea of a dimple on a golf ball and use it as an actuator, as the basis of changing the properties of the boundary layer flow around more-or-less any type of body. Dimples are, in fact, very efficient vortex generators and we have started using dimples that are made of EAP so that the dimple consists of a diaphragm that pops up and down either in a cyclic (or harmonic) fashion, or can be made to do so when required, a so-called ondemand vortex generator. In this case, we would wish the dimple to produce a single vortex of known strength for as long or as short as we would wish, and this requires an understanding of the basic fluid behaviour so that a model may be implemented. This means we have to sense the properties of the boundary layer and we can do this by taking advantage of the piezoelectric behaviour of the EAP. Then the polymer not only has an array of dimples for controlling the boundary layer, but it also has an array of pressure sensors so that the surface pressure signal may used to control the dimples. We can even develop a 'smat all-polymer skin that is made up of separate EAP layers where individual layers can be designed specifically to sense the forces of the skin, or to be actuated as an on-demand dimple actuator. Then we would be able to sense the pressure on the surface and actuate at the same position. Initially, we hope to control boundary layer by open-loop control only. In this case, the measured pressure is only used to diagnose the effect of the dimples. However, much more complicated (and potentially much more beneficial) is closed-loop control, in which the measured pressures are used to determine when and where the dimples should be actuated. This would require a control model, that is a clear expectation of how we would wish the flow to be. Each model would be very dependent on a great many conditions, not least the type of flow.