The use of active ionic polymers in dynamic skin friction measurements

Abstract

A class of wall shear stress sensors has been developed. The potential of ionic polymer membrane transducers for measuring skin friction in liquid flows is demonstrated. Ionic polymer transducers are thin polymer membranes that exhibit high sensitivity to mechanical strain, and have been shown to demonstrate sensitivities two orders of magnitude higher in charge-sensing mode than piezoelectric polymers such as PVDF. Thus, they are as sensitive to mechanical strain as piezoelectric ceramics (i.e. PZT) but have the high compliance and durability of a polymer. The application of active ionic polymers in delivering easy to implement, accurate, dynamic measurements of skin friction in harsh environments promises significant advantages over current technologies. In particular, a robust technique for measuring wall shear stress is needed to assess the effectiveness of new friction-reducing techniques, including the use of lubricants and micro-bubble injection within the viscous sublayer. Conventional technologies have been unable to provide sufficiently accurate measurements over a large range of fluid velocity fluctuation scales. Moreover, their implementation can be complicated in the case of non-flush mounting sensors, and their applicability is often limited to forgiving environments. An initial feasibility test was designed with the objective of replicating classic theoretical and experimental skin friction coefficient results for a sharp edge flat plate boundary layer. An ionic polymer and a piezoelectric film (PVDF) were evaluated for Reynolds numbers ranging from the laminar flow regime to fully turbulent flow. The PVDF sensor displayed no discernable response to wall shear. The ionic polymer sensor, however, showed significant response to wall shear and strong correlation with the Reynolds number. In addition, a Stokes oscillating plate apparatus was designed for calibration and testing of the ionic polymer sensor.