Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session A01: Focus Session: Fluids Next: Soft Body Slamming Fluids I |
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Chair: Christine Gilbert, Virginia Tech Room: Sagamore 1234567 |
Sunday, November 20, 2022 8:00AM - 8:13AM |
A01.00001: Mechanics of jumping by slamming against water Invited Speaker: Ho-Young Kim Semiaquatic arthropods, such as water striders and fishing spiders, can jump on water by rapidly pushing their legs against water surface. Typical-sized water striders (e.g. Aquarius remigis) adjust their leg speed not to break the water surface so that they can fully utilize the surface tension of water. However, it was recently found that large water striders (e.g. Gigantometra gigas) as well as fishing spiders break the water surface in downstroke of their middle legs, implying that water drag can also launch the small animals into the air. Here we construct a mathematical model to predict the trajectory of water jumping arthropods whether they rely on surface tension or drag of water, and compare the results with the data from high-speed visualization of those animal jumps. In particular, we pay attention to the role of air bubbles formed around superhydrophobic hairs of middle legs of large water striders that boost the water drag. |
Sunday, November 20, 2022 8:13AM - 8:26AM |
A01.00002: Soft sphere water impact Invited Speaker: Jesse L Belden Highly compliant elastic spheres possess remarkable water skipping capabilities. Furthermore, when impacting normal to a free-surface, elastomeric spheres deform significantly and begin to vibrate producing unique nested cavities. In this talk, we reveal the coupling between water impact and material response, and discuss the interesting body dynamics that result. The most remarkable of these behaviors is the ability for elastic spheres to skip hundreds of times over a great distance on the water surface. We show that for certain values of a dimensionless ratio of material shear modulus to fluid inertia forces, a stable deformation mode is sustained in the sphere, thus enabling this `water-walking' behavior. This same dimensionless ratio characterizes the water entry of soft spheres perpendicular to the free-surface as well. We show how the material response alters the traditional cavity behavior and force characteristics normally observed for rigid sphere water entry. |
Sunday, November 20, 2022 8:26AM - 8:39AM |
A01.00003: Water Entry of a Flexible Wedge with Open and Closed Boundary Conditions Christine Gilbert, M Javad Javaherian, John Gilbert Wedge water entry can be used to study slamming on high-speed craft, seaplane landing, and biological flows such as birds diving into the water. The focus of this talk is on experimental measurements and reduced order modeling from two projects of differing applications: (1) slamming of small craft, where the wedge boundary conditions are closed on all edges, and (2) biologically inspired water entry, where the wedge boundary conditions are opened on all edges but one. For both applications, the flexural rigidity is low such that there are fluid-structure interactions. The difference in boundary conditions strongly affects the amount of deformation seen in the bottom plates of the wedge. Therefore, the flexural rigidity alone is not enough to determine the extent of the fluid-structure interaction. In this talk, the effect of boundary conditions will be discussed and its influence on the hydroelasticity factor, R, will be discussed based on experimental findings backed with theoretical analysis. |
Sunday, November 20, 2022 8:39AM - 8:52AM |
A01.00004: Froude number and flexibility effects on partially-submerged bio-inspired propulsors Rafael T Burger, Peyton I TenEyck, Leah R Mendelson Jumping archer fish are capable of sustained propulsion while partially-submerged and crossing the water-air interface. Inspired by such behavior, we characterize free surface interactions, total thrust production, and propulsive performance of a partially-submerged, flexible, finite-span heaving plate over a range of depths and propulsor stiffnesses. Using high-speed imaging, we determine how the plate bends in response to stroke dynamics, including the Froude number. Using multi-axis force and torque measurements, we characterize how the kinematics and resultant plate shape correlate with thrust and lateral loads. We examine the wake patterns using PIV and identify changes in vortex shedding patterns, wake strength, and the level of free surface interaction. By testing over a range of depths, we gain insight into how propulsive performance, including net thrust production and efficiency, evolves during water exit. We additionally identify cases where free surface effects are significant and cases where partially-submerged propulsion resembles fully-submerged behaviors. |
Sunday, November 20, 2022 8:52AM - 9:05AM |
A01.00005: How foxes dive into snow Jisoo Yuk, Anupam Pandey, Leena Park, William E Bemis, Sunghwan Jung Some mammals plunge-dive or dig out snow to catch prey hidden beneath the snow. Among them, arctic foxes and red foxes are known to be great hunters that catch small animals by snow diving. Here, we investigate the morphological characteristics of snow-diving foxes and the dynamics of snow diving to understand how they penetrate snow. First, we scanned dry skulls of a series of species in Felidae (cats and allies) and Canidae (dogs and allies, including foxes) to analyze key geometrical features such as snout length and width. In comparison with the bobcat and puma in the Felidae, the fox's snout is noticeably longer and narrower, thereby having a higher curvature. Next, we evaluated the benefits and dynamics of the structural differences between cats and foxes when diving into snow. To measure the impact force, we dropped 3D-printed skulls of cats and foxes into a container filled with snow. We also tested artificially snout-shortened 3D-printed models of foxes. When the snout is reduced in length by 25%, it generates twice the impulse compared to the original fox snout. Similarly, the bobcat generates a greater force during the impact phase. These results imply that the fox’s sharp and long snout helps to quickly approach prey when diving through snow with less impact force. |
Sunday, November 20, 2022 9:05AM - 9:18AM |
A01.00006: Water entry of flexible impactors John T Antolik, Jesse L Belden, Nathan B Speirs, Daniel Harris The problem of high-speed water entry is fundamental to a number of engineering and naval applications and has been the subject of extensive study over the past century. Despite such advances, the role of impactor compliance has received very little attention, and may play a role in biological divers such as sea birds. We present experiments on the normal impact of a two degree-of-freedom flexible body onto a pool of water. An axisymmetric nose is connected to the impactor body via a linear spring and damper. We report direct impact force measurements over a range of spring stiffnesses, damping coefficients, and impact velocities. Notably, we observe a reduction in peak impact force for the flexible system as compared to an equivalent rigid impactor. Other qualitative observations and preliminary modeling results will be discussed. |
Sunday, November 20, 2022 9:18AM - 9:31AM |
A01.00007: How Olympic Divers Manipulate the Air Cavity to Reduce Splash Elizabeth A Gregorio, Elias Balaras, Megan C. Leftwich Olympic divers achieve significantly higher scores when they perform a dive that appears splash-less. To accomplish this, they perform a "rip" entry, characterized by seething bubbles and the sound of tearing paper. This is technique involves performing an underwater maneuver, such as the pike save, in which the diver rolls forward at the shoulders and hips after entering the water. While this technique is widespread in competitive diving, there is little research on how their manipulation of the air-water interface leads to a smaller splash. In this study, we develop a simplified single jointed diver model and drive it into a pool of water. The impact is recorded with a high-speed camera and analyzed for underwater kinematics, air cavity development, and splash production. These images are used to investigate how rolling after impact manipulates the air cavity. The model is modified to understand how changing the depth of roll initiation affects the underwater air cavity dynamics. The results of these experiments will be used to validate computational fluid dynamics simulations to further understand the surface dynamics. |
Sunday, November 20, 2022 9:31AM - 9:44AM |
A01.00008: Rubber Popper Slams and Nucleates a Toroidal Cavitation Bubble Sharon Wang, Akihito Kiyama, Sunghwan Jung Cavitation bubbles occur everywhere, not only in the fluid machinery but also in animal activities. In this present study, we employ the rubber popper to study cavitation upon object impact. The rubber popper can achieve fast deformation (up to 8 m/s in our case) even underwater, as it impacts a glass substrate. Three high-speed cameras allow us to visualize the dynamics of the bubble and the 3D- deformation characteristics of poppers simultaneously. We observed that cavitation occurs on the popper surface before it hits the glass substrate, forming an annular shape of the popper, and rebound multiple times. We measured the lifetime of the bubble as well as found the radius of the cavitation bubble at its peak. The impacting speed of the popper and its shape change are identified. Experimental parameters are the popper size, location, and fluid types. It was found that the higher the popper location, the larger the velocity for both popper sizes, where the small popper had an overall larger velocity at similar heights to the medium popper. Cavitation bubble lifetime was positively correlated with the bubble radius. We also discuss the possible future work and applications such as the surface cleaning process. |
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