Shear Stress Sensor Using Shape Memory Films

There is a need for a low profile, simple, accurate, localized, responsive sensor to measure shear stress in fluid flow. The unusual properties of the shape memory alloy Ni2MnGa suggest it could make a very sensitive sensor. In recent years,single-crystal films of this alloy have been grown by Molecular Beam Epitaxy. Once released from their substrate, the films have exhibited shape memory behavior. We pursue a concept for the sensor that based on the expectation that, when sheared, themartensite in an unconstrained thin-film of single-crystal Ni2MnGa will respond in a way that induces a measurable change to its electrical resistivity. We propose to design, fabricate, and test this sensor. The proposed shear stress sensor fabricatedfrom shape memory film has the potential of overcoming existing problems plaguing skin friction measurements in turbulence. These devices would be simple to use and easily positioned at hundreds of locations on a surface. Apart from applications forLarge Eddy Simulation and accurate measurements of skin friction, such a sensor would open the way to measure full field velocity-field/wall shear stress correlations which could be implemented in active control schemes for turbulence. Theoretical modelssuggest that up to 50% skin friction reductions are possible if these schemes could be implemented successfully. The aircraft industry in the U.S. alone would save, in fuel costs, an estimated $250 million annually for every 2% reduction in turbulent skinfriction.