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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. 

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.