Development of Shear Stress Measurement Technique and Plotting Surface Streak Lines using Oil Technique
thesisposted on 2021-06-25, 17:46 authored by Syed ShoiabSyed Shoiab
Wall shear stress is an important parameter in accessing the aerodynamics performance of an object. With the race to optimize the performance of an aerodynamic body, multiple shear stress measurements have been developed such as OFI(Oil Film Interferometry), MEMS (Microelectromechanical Sensors), Hotwire, Preston tube, Hot films, etc. where most of these techniques are expensive or intrusive. In the experimental measurement techniques, the complexities and intrusiveness often playa major role in selecting a measurement technique.
Hence the developed shear stress measurement technique is simple yet effective to obtain accurate shear stress results using oil. The secondary challenge faced in the aerodynamic experiments is to obtain a flow visualization. The developed shear stress measurement technique is expanded to obtain the qualitative and quantitative flow development data around the ROI. The developed methodology relies on the thin film assumption where the droplet is sufficiently thin such that the gravity, pressure, and surface tension forces acting on the droplet are not significant, and the droplet is deformed only under the significant shear stress forces. This technique assumes constant shear stress along the deformed droplet, at regions of high shear gradient this technique can be employed using smaller droplets applied at the constant shear regions. The viscosity of the oil used must be chosen accordingly to the test conditions to obtain a constant thinning rate for a substantial amount of time. Finally, the droplet deformation is tracked using a camera and the thinning rate of the droplet is computed using an image processing tool developed.
The obtained results are compared with the theoretical results for a flat plate and with the CFD results for the SACOC testing. The technique was tested in a range of test conditions with a low subsonic of 20m/s to a compressible flow of Mach 0.3 and Mach 0.5. Uncertainty estimation is performed to recognize the confidence level of the developed technique.
Developed measurement technique clubs two different measurements, shear stress, and qualitative and quantitative flow viz, acquiring the data for both in a single test, thereby reducing the repeatability of experiments, resources, and computational power. Moreover, the resources required for this technique are easily available at a low cost and is very simple to implement.