A comparative shear-sensing system is improved, manufactured, and field-tested to simultaneously measure the friction drags of two surface samples attached side-by-side on a variety of objects placed in fluid flows. The main goal of this system is to quantify the comparative advantage of one surface to another by letting the two surfaces experience temporally and spacially the same flow even if it varies continually and unpredictably, such as on the submerged hull of a boat traveling on the sea. The system is designed to have a compact size, low-profile shape, and high-resolution sensing so that it is adaptable to a wide range of fluid flows and test environments. The shear sensing system is composed of (1) a sensor plate including two floating plates each suspended by an identical set of beam springs, (2) an encoder plate including two high-resolution optical encoders to measure the displacements of the two floating plates, (3) a camera system including two endoscopes hidden in a streamline-profiled house to observe the sample surface under water during the measurement, and (4) a holder plate fixed to the test object and mounted with the sensor-encoder plate assembly as well as the optional camera system. The two 4 cm x 7 cm surface samples are attached side-by-side on the two floating (i.e., suspended) plates and flush with their surrounding surface in fluid flow. The optical encoders are placed on the other side of the floating plates from the fluid flow and connected to PC through cables. The underwater camera system allows the operator to view and video-record the sample surface throughout flow tests. The shear-sensing system has been tested successfully in both hydrodynamic (in a water tunnel and under a boat) and aerodynamic (in a wind tunnel) flows. Furthermore, low-cost fabrication of the sensor plate is demonstrated by machining stacks of as many as ten plates in a single cutting operation. Lastly, we explore 3D printing of the sensor plate as well as a transparent sensor plate to assess more options in the future.
