Implementation of drop-based oil flow interferometry to estimate wall shear stress in three-dimensional flows

Abstract

This thesis introduces a simplified drop based oil-film interferometry (OFI) approach using silicone oil droplets to measure wall shear stress in three-dimensional flows. Wall shear stress is a critical parameter in fluid dynamics, significantly affecting aerodynamic drag and turbulent flow behavior. Accurate measurement for wall shear stress is essential for validating computational fluid dynamics simulations and improving engineering designs. Traditional techniques such as Pitot static tubes, Preston tubes and force balance methods have limitations due to their point-based measurement, intrusive nature of measurements and they become extremely complex to implement on curved surfaces. To overcome these challenges researchers have explored other advanced measurement techniques such as microelectromechanical sensors (MEMs), liquid crystal coatings, and oil-film based techniques. Among these oil-film based techniques have been extensively studied in the past and are proven to be accurate. Additionally implementing oil-film based techniques is much easier and are significantly cheaper than the other mentioned. Although oil-film based techniques are becoming increasingly popular to measure skin friction, it has its own challenges. One of the major challenges is the knowing the direction of surface streamlines on complex three-dimensional surfaces. Many researchers have in the past have tackled this problem, some have had a dedicated separate runs to acquire streamline data before implementing the oil-film based techniques, a few have suggested doping the oil with tracer particles in order to get the streamline direction in a single run. However all these methods require intense post processing methods in order to quantify streamline data, and can quickly increase the complexity of the method. We introduce a simplified version of the OFI technique using silicone oil droplets. By applying small oil droplets, the local streamline direction is determined via droplet trajectories, eliminating the need for separate flow visualization runs. Interferogram pattern formed due to thinning oil films are then analyzed with an area-averaging technique, providing a more comprehensive use of the interferogram data to compute wall shear stress. Experiments at Reynolds numbers of 250,000 and 650,000 are performed in the Virginia Tech's Stability Wind Tunnel. The method is validated using laser doppler velocimetry (LDV) measurements and with the classical oil flow visualization technique (OFV). At low Reynolds number the drop-based OFI measurements and LDV measurements demonstrate agreement within 4% in magnitude and 3% wall shear stress direction. The local streamline directions predicted by drop-based OFI method agree with OFV for both the Reynolds numbers. A comparative study between experiments and CFD predictions using k-omega SST model is also presented which show similar trends of wall shear stress across the entire hill geometry. At low Reynolds number the mean deviation between experiments and CFD deviated was 4%, and at high Reynolds number the mean deviation between experiments and CFD was 3%.