Piezo-electric foils as a means of sensing unsteady surface forces

The experimental determination of steady and unsteady surface forces is an elementary problem in experimental fluid dynamics, e.g., in experimental aerodynamics. Up to now, unsteady forces such as pressure or shear fluctuations have been detected by means of special plug-in probes (e.g., miniature pressure transducers). An alternative and attractive technique of monitoring unsteady surface forces has become possible through the development of piezoelectric foils. With this novel type of sensor, which simply can be glued onto a surface, the piezoelectric effect of polarized plastic foils is used to register time-dependent pressure or shear loads. First of all, the paper concentrates on the fundamentals of this new measuring technique. Furthermore, some practical applications in experimental aerodynamics are outlined.List of symbols A area - c chord - C capacity - D pipe diameter - d _3i piezoelectric constant, i = 1–3 - f frequency - h foil thickness - p pressure - p pressure fluctuation - Q electric charge - R impedance - t time - u flow velocity - U voltage - x streamwise coordinate - Re Reynolds number - angle of attack - friction factor - aspect ratio - sweep angle - _w wall shear stress - wall shear stress fluctuation Indices i direction of mechanical stress 1, 2: in direction of foil plane 3: normal to foil plane - ll lower limit - RMS root-mean-square - w wall condition - free stream condition A version of this paper was presented at the 6th Symposium on Turbulent Shear Flows, Toulouse, France, September 1987