Welcome back. This week, I hope to continue our break from the regular hydrodynamic stability program and share a project I worked on last fall. This work involved water channel dye visualization experiments to reveal the locations of flow separation and reattachment on the cambered NACA 65(1)-412 airfoil at transitional Reynolds numbers Re ∈ [104, 105]. These experiments fit into a broader project across multiple universities that aimed to control flow separation on low Reynolds number airfoils in order to increase lift and reduce drag. Currently, the project is in its fifth year, and will likely continue for several more years as more control techniques are tested. So without further ado, let’s review the project:
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My second project in the wind tunnel was to design a series of experiments that could give precise, qualitative visualizations of flow separation around a NACA 65 airfoil. These experiments would take place in a water channel, and would evolve in a series of steps. To lay the groundwork for these experiments, I designed a 3D printed model of a NACA 65 with a network of tubing inside of it that would allow me to eject dye out the leading edge. The network consisted of one 5 mm diameter pipe that ran across the span of the wing, and 11 smaller holes that extended from the main pipe to the leading edge. These 11 smaller holes would allow me to release dye from multiple spanwise locations at once and compare flow patterns at varying locations from the water channel end walls.
Once the wing was printed and sanded to a smooth finish, I spent a week researching the proper dye to use in the experiments. This was a nuanced task because the water channel had a green laser that could illuminate flows very nicely, and it was imperative to use a dye that could reflect this laser light properly. After some deliberation, I decided to use Rhodamine 6G dye, which had a dull red-yellow color in normal lighting, but illuminated spectacularly under the green laser light. With the wing and dye chosen, we had to create a test rig that could secure the wing to the water channel. To construct this support structure, we used a CNC machine to carefully modify two large acrylic sheets to fit around the airfoil. Once the wing was between the acrylic sheets, it could be lowered into the water channel and remain firmly in place at all anticipated flow speeds.
After three months of preparation, these supplies were finally ready, and we could collect data. The initial images successfully demonstrated the formation of a Laminar Separation Bubble behind the leading edge at angle of attack 8º. However, these images were not as crisp as we wanted. We therefore decided to order a special camera filter that could accentuate the laser light and block out unwanted noise. Upon conducting a test at ? = 4º with the new camera filter, we were pleased to find images that met our desired quality standards.
For the remainder of the Fall 2023 semester, our goal was to continue collecting more data at ? ∈ [0º, 12º] and Re ∈ [104, 105]. Once these images are collected, we would be able to analyze how the angle and position of the flow separation line changed as ? and Re changed, and compare these findings with finite-time Lyapunov exponent fields and flow control strategies.
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