No abstract

Fig. 2 Random error transmission ratio as a function of D/L Fig. 3 Vorticity bias error (%) as a function of spatial resolution

This paper presents the first implementation of a novel class of dynamic time-resolved direct skin friction measurements sensor based on active ionic polymer transducers. These ionic polymer sensors have the advantage that they contain no moving parts, perform a direct measurement of shear, and can be mounted directly to the surface of an existing vessel with no modification. During the present effort we characterize the accuracy of the sensors and validate their dynamic measurement response. Using an oscillating Stokes layer calibration procedure we demonstrate measurement accuracy in fluctuating shear on the order of 4.92% over a range of stresses of +/- 3 Pa and signal-to-noise-ratio on the order of 60 dB. The frequency response of the sensor is over 10 kHz however due to experimental limitations we were not able to calibrate for frequencies higher than 140 Hz. These sensors have been shown to be insensitive to vibration or pressure. Also, an automatic change of impedance compensation approach is proposed that allows in-situ recalibration of the sensors and accounts for environmental effects such as changes of temperature on the sensors performance. The results demonstrate the potential for using ionic polymer sensors to perform accurate, high frequency measurements of shear in turbulent boundary layers.

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.

No abstract