Multicolor dye-based flow structure visualization for seal-whisker geometry characterized by computer vision.

Pinniped vibrissae possess a unique and complex three-dimensional
topography, which has beneficial fluid flow characteristics such as substantial
reductions in drag, lift, and vortex induced vibration. To understand and leverage
these effects, the downstream vortex dynamics must be studied. Dye visualization
is a traditional qualitative method of capturing these downstream effects,
specifically in comparative biological investigations where complex equipment
can be prohibitive. High-fidelity numerical simulations or experimental particle
image velocimetry (PIV) are commonplace for quantitative high-resolution flow
measurements, but are computationally expensive, require costly equipment,
and can have limited measurement windows. This study establishes a method
for extracting quantitative data from standard dye visualization experiments
on seal whisker geometries by leveraging novel but intuitive computer vision
techniques, which maintain simplicity and an advantageous large experimental
viewing window while automating the extraction of vortex frequency, position,
and advection. Results are compared to direct numerical simulation (DNS) data
for comparable geometries. Power spectra and Strouhal numbers show consistent
behavior between methods for a Reynolds number of 500, with minima at the
canonical geometry wavelength of 3.43 and a peak frequency of 0.2 for a Reynolds
number of 250. The vortex tracking reveals a clear increase in velocity from
roll-up to 3.5 whisker diameters downstream, with a strong overlap with the
DNS data but shows steady results beyond the limited DNS window. This
investigation provides insight into a valuable bio-inspired engineering model while
advancing an analytical methodology that can readily be applied to a broad range
of comparative biological studies.