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Abstract The spanwise undulated cylinder geometry inspired by seal whiskers has been shown to alter shedding frequency and reduce fluid forces significantly compared to smooth cylindrical geometry. Prior research has parameterized the whisker-inspired geometry and demonstrated the relevance of geometric variations on force reduction properties. Among the geometric parameters, undulation wavelength was identified as a significant contributor to forcing changes. To analyze the effect of undulation wavelength, a thorough investigation isolating changes in wavelength is performed to expand upon previous research that parameterized whisker-inspired geometry and the relevance of geometric variations on the force reduction properties. A set of five whisker-inspired models of varying wavelength are computationally simulated at a Reynolds number of 250 and compared with an equivalent aspect ratio smooth elliptical cylinder. Above a critical non-dimensional value, the undulation wavelength reduces the amplitude and frequency of vortex shedding accompanied by a reduction in oscillating lift force. Frequency shedding is tied to the creation of wavelength-dependent vortex structures which vary across the whisker span. These vortices produce distinct shedding modes in which the frequency and phase of downstream structures interact to decrease the oscillating lift forces on the whisker model with particular effectiveness around the wavelength values typically found in nature. The culmination of these location-based modes produces a complex and spanwise-dependent lift frequency spectra at those wavelengths exhibiting maximum force reduction. Understanding the mechanisms of unsteady force reduction and the relationship between undulation wavelength and frequency spectra is critical for the application of this geometry to vibration tuning and passive flow control for vortex-induced vibration (VIV) reduction. 

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 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. 

This work investigates the response of forces from fluid flow around seal whisker inspired cylinder geometry at swept back angles. The unique, undulated surface of seal whiskers has been shown to reduce drag and oscillating lift in comparison to smooth cylinders of equivalent dimensions. As seals swim through the water, their whisker orientation with respect to the freestream is constantly changing due to body position, but also the ability to manipulate the position of their whiskers while sensing. Though the effects of orientation and geometry parameters such as varied angle of attack, changes to undulation wavelength, amplitude, and aspect ratio have been investigated in previous literature, there is little research dedicated to characterizing the response of undulated cylinder geometry at sweep angles. In this paper, direct numerical simulation of incompressible flow over a highly resolved whisker surface is used to simulate flow structures and forces over whisker-inspired cylinders at a range of sweep angles from 0 to 60 degrees. It is observed that the decrease in forces in comparison to circular cylinders is still present at all swept angles tested. Root-mean-squared lift coefficient displays a 51.9 to 93.8% reduction, whereas drag displays a 12.9 to 39.1% reduction. When compared to forces on a streamlined elliptical cylinder, sweep angles of 0 to 30 degrees result in a force reduction advantage for the undulated cylinder geometry. Beyond this range at sweep angles of 45 and 60, drag and lift coefficients closely mirror those of the streamlined ellipse and undulated geometry offers no improvement.