Abstract
Fluid-structure-interactions (FSI) has applications in diverse areas and has been a research focus for several decades. FSI is used in studying aerodynamics and can be observed from a flag flapping in the wind, or a parachute deploying. The nonlinear coupling between a structure and fluid flow makes computational and theoretical studies challenging. One of the critical fluid dynamic parameters of FSI research is skin-friction coefficient (Cf ), a dimensionless parameter used to determine viscous drag on a structure. An accurate estimation of Cf can provide critical information regarding FSI physics and aid in developing robust computational and theoretical models. Furthermore, traditional experimental approaches to measure Cf are intrusive and can alter the fluid flow field and structure properties. One such traditional method being a Stanton tube, where the tube is placed close to the structure surface to collect pressure differences, in turn obstructing the flow near the structure. Another method being a Micro Electrical Mechanical System (MEMS) sensor, a reliable method that requires contact with the surface, influencing the behavior of the structure. This study aims to design and develop an optical-based Cf measurement technique (i.e., Oil-Film-Interferometry (OFI)) that is non-intrusive and can be used to estimate values on a very flexible and transparent membrane. For this purpose, an in-house OFI system was designed by creating a 3-D printed model that negates pressure gradients at the leading edge of a flat plate. The OFI system was used to perform experiments on a flat plate model in the wind tunnel facility at the University of Idaho’s Experimental Fluids and Aerodynamics Laboratory (EFAL). In-plane image analysis of oil fringe patterns were achieved by process of photogrammetry, which mapped a 3-D space to a 2-D image plane. After image correction, a Windowed Fourier Transform (WFT) approach was used to analyze fringe spacing that led to determination of the local oil height that is used to iteratively solve for Cf . The OFI technique successfully measured Cf on a flat plate model and yielded a deviation within 6% from theoretical results. After a successful demonstration in the wind tunnel, the in-house OFI system was designed to measure skin friction on a flexible membrane model using the open jet facility in EFAL. The OFI results on the flexible membrane yielded a deviation within 8% of theoretical Cf results over a flat plate. This study successfully demonstrated the application of the OFI technique to measure Cf on flexible transparent structures. Performing OFI to measure the skin friction coefficient on a flexible membrane has provided a new approach to accurately characterize wall shear stresses on a flexible structure, setting the foundation for future work in which OFI measurements will be taken on a flexible membrane with various modes of oscillation.