Direct shear stress measurement device for use in atmospheric boundary layers

Surface shear stress is an important quantity that characterizes the nature of a turbulent boundary layer. The problem of directly measuring the amount of shear stress acting on a surface has been previously studied. This thesis focuses on the design and fabrication of a 127 mm diameter floating disk plate. The small design affords many advantages including ease of implementation, reduced cost, portability, a higher expected frequency response, and the ability to deploy large arrays of these units to achieve spatial distribution and capture heterogenous effects. The shear stress sensor consists of a circular disk, with three springs attached to a housing each at 120o apart. The disk is floating on a pool of liquid that provides damping. A vertical cavity surface emitting laser (VCSEL) is placed at the center of the disk and shines onto the surface of a 2-D position sensitive detector (PSD) that measures the (x; y) position of the disk centroid. With the use of several models, that were developed to obtain key parameters, the known disk centroid position is converted to force using the system transfer function. A physical prototype was built using the obtained parameters, and the fabrication and assembly are discussed, along with the static calibration results. These results correlate very well with the mathematical design to within 1.5% of the average value.