Oscillating bodies placed in a uniform flow field can create beautiful trailing vortices. In this image, a plate with an aspect ratio of 0.54 (span to chord) is placed in a uniform flow field and pitched at a double amplitude-to-span ratio of 0.31. This wake appears fairly organized, though not all wakes take this form. The secret lies in the Strouhal number, which describes the oscillating nature of the body/wake in relation to the flow surrounding it. In this particular case, the Strouhal number is 0.23. As the Strouhal number increases, the flow bifurcates, forming multiple rows of vortices. The structure of trailing vortices could be related to the thrust produced by the flapping plate or foil and how efficiently the thrust is produced. However, this relationship is still being explored: does the structure of the vortices somehow affect the propulsion efficiency, or is the structure merely a result of efficient propulsion? Finned aquatic animals swim at a variety of Strouhal numbers and exhibit different wake structures. Studying the fluid dynamics surrounding their fin behaviors could help inspire better designs on engineered aquatic vehicles.[1]
Fluorescent dyes were introduced along the base of the flapping plate to help visualize this flow pattern. The colors of the dyes were chosen well to create contrast in the image.
Image attribution: J. H. J. Buchholz and A. J. Smits, “Wake of a low aspect ratio pitching plate,” Physics of Fluids, vol. 17, no. 9, p. 091102, Aug. 2005, doi: 10.1063/1.1942512.
Link: https://doi.org/10.1063/1.1942512
[1] A. J. Smits, “Undulatory and oscillatory swimming,” Journal of Fluid Mechanics, vol. 874, p. P1, Sept. 2019, doi: 10.1017/jfm.2019.284.
8 Comments. Leave new
First Prize. I really enjoy how much movement is captured in these still images, and how the contrasting dye adds both an artistic flair and emphasizes the flow being visualized.
Third Prize: The vorticity in this image look so magical. I really like the multi colored florescent dyes used to show the flow. I have seen flows before with a single color so I am interested to know how they used multiple in this set up.
Third Prize: This is a fantastic use of dye techniques, and the science behind it is super interesting. I especially like how many layers you can see in the vortices.
Third Prize: I really like how science and art come together to create this image.
Second prize! I appreciate the art within the science in this image. The structures of the wakes are captured by the dye in a very beautiful way.
Second Prize: I really appreciate the way that this image illustrates some of the most fundamental low-speed flow phenomena. The fluorescent dye makes the mechanisms behind the vortex structures feel much more approachable, given the different colors highlight the interactions between the vortices.
First Prize: I enjoy both the scientific application and artistic style of this image. I think that the dyes used create beautiful color combinations and do an excellent job of showing the wake.
Third Prize: I really like this image not only because of how well the dye was able to capture the structure of the wake, but also because the structures of the wakes themselves are very defined and have a visually compelling layout.