Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H14: Free Surface Flows: Water Entrance and Splashes |
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Chair: Christine Ikeda, Virginia Polytechnic Institute and State University Room: C125-126 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H14.00001: On the Effect of Structural Response on the Hydrodynamic Loading of a Free-Falling Wedge Christine Ikeda, Brandon Taravella, Carolyn Judge High-speed planing craft are subjected to repeated slamming events in waves that can be very extreme depending on the wave topography, impact angle of the ship, forward speed of the ship, encounter angle, and height out of the water. The current work examines this fluid-structure interaction problem through the use of wedge drop experiments and a theoretical prediction. The experimental program consisted of two $20^\circ$ deadrise angle wedges dropped from a range of heights, $0.15 \le H \le 0.6$~m, while pressures and accelerations of the slam were measured. The first wedge had a rigid bottom, and the second wedge had a flexible bottom. Both experiments are compared with a non-linear boundary value flat cylinder theory in order to determine the effects of flexibility on the hydrodynamic pressure. The code assumes a rigid structure, therefore, the results between the code and the first experiment are in good agreement. The second experiment shows pressure magnitudes that are lower than the predictions due to the energy required to deform the structure. [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H14.00002: Crossing the boundary: experimental investigation of water entry conditions of V-shaped wedges Tingben Xiao, Daniel Yohann, Lionel Vincent, Sunghwan Jung, Eva Kanso Seabirds that plunge-dive at high speeds exhibit remarkable abilities to withstand and mitigate impact forces. To minimize these forces, diving birds streamline their shape at impact, entering water with their sharp beak first. Here, we investigate the impact forces on rigid V-shaped wedges crossing the air-water interface at high Weber numbers. We vary the impact velocity $V$ by adjusting the height from which the wedge is dropped. Both a high-speed camera and a force transducer are used to characterize the impact. We found that the splash base and air cavity show little dependence on the impact velocity when rescaling by inertial time $d/V$, where $d$ is the breadth of the wedge. The peak impact force occurs at time $t_p$ smaller than the submersion time $t_s$ such that the ratio $t_p/t_s$ is almost constant for all wedges and impact velocities $V$. We also found that the maximum impact force, like drag force, scales as $AV^2$, where $A$ is the cross-sectional area of the wedge. We then propose analytical models of the impact force and splash dynamics. The theoretical predictions agree well with our experimental results. We conclude by commenting on the relevance of these results to understanding the mechanics of diving seabirds. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H14.00003: Bubble baths: just splashing around? Wesley Robinson, Nathan Speirs, Saberul Islam Sharker, Randy Hurd, BJ Williams, Tadd Truscott Soap Bubbles on the water surface would seem to be an intuitive means for splash suppression, but their presence appears to be a double edged sword. We present on the water entry of hydrophilic spheres where the liquid surface is augmented by the presence of a bubble layer, similar to a bubble bath. While the presence of a bubble layer can diminish splashing upon impact at low Weber numbers, it also induces cavity formation at speeds below the critical velocity. The formation of a cavity generally results in larger Worthington jets and thus, larger amounts of ejected liquid. Bubble layers induce cavity formation by wetting the sphere prior to liquid impact, causing them to form cavities similar to those created by hydrophobic spheres. Droplets present on a pre-wetted sphere disrupt the flow of the advancing liquid during entry, pushing it away from the impacting body to form an entrained air cavity. This phenomena was noted by Worthington with pre-wetted stone marbles, and suggests that the application of a bubble layer is generally ineffective as a means of splash suppression. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H14.00004: Make water entry great again! Randy Hurd, Jesse Belden, Michael Jandron, Tate Fanning, Tadd Truscott Upon free surface impact, silicone rubber spheres deform significantly and begin to vibrate producing unique nested cavities. We show that sphere deformation and cavity formation can be characterized by material shear modulus, density and impact velocity. Additionally, material vibration scales with sphere diameter and material wave speed. Applying a modified diameter, which reflects deformation, effectively collapses experimental pinch-off data with Froude number. A scaling argument shows that a deformable sphere loses energy proportional to the vibrational period of the sphere in the first stages of impact. The effective force coefficient of a deformable sphere through impact is nearly identical to a rigid sphere with the same solid-liquid density ratio. The scaling predicts how the cavity and projectile dynamics of a deformable sphere differs from a rigid counterpart. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H14.00005: Stable, streamlined and helical cavity formation by the impact of Leidenfrost spheres Mohammad Mansoor, Ivan Vakarelski, Jeremy Marston, Tadd Truscott, Sigurdur Thoroddsen This work reports results from an experimental study on the formation of stable-streamlined and helical cavity wakes following the free-surface impact of Leidenfrost spheres. The Leidenfrost effect encapsulates the sphere by a vapor layer to prevent any physical contact with the surrounding liquid. This phenomenon is essential for the pacification of acoustic rippling along the cavity interface to result in a stable-streamlined cavity wake. Such a streamlined configuration experiences drag coefficients an order of magnitude lower than those acting on room temperature spheres. A striking observation is the formation of helical cavities which occur for impact Reynolds numbers ${Re}_{0}\ge 1.4\times {10}^{5}$ and are characterized by multiple interfacial ridges, stemming from and rotating synchronously about an evident contact line around the sphere equator. This helical configuration has $40-55\thinspace \% $ smaller overall force coefficients than those obtained in the formation of stable cavity wakes. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H14.00006: Aerodynamic shapes of two-dimensional splashes Lionel Vincent, Tingben Xiao, Daniel Yohann, Eva Kanso We investigate experimentally the long-term evolution of a splash induced by the water entry of 90-degree wedge of breadth $d$ and length $L$ such that $L \gg d$. We find that for large speed entry speed $V$, the splash shows both concave and convex curvature, as opposed to a single concave curvature observed for low speed. This peculiarity is found to be the result of a kink generated by the initial dynamics, the growth of which is favored by aerodynamics efforts $\rho V^2$, where $\rho$ designates the density of air, and inhibited by surface tension effects scaling as $\sigma / d$. The transition between simply-curved splash and doubly-curved splash is found to happen for Weber number $\rho V^2 d / \sigma \simeq 1$. Doubly-curved splash sheets undergo significant stretch during their life span, altering the breaking up process. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H14.00007: An Experimental Study of Droplets Produced by a Plunging Breakers Martin Erinin, Dan Wang, David Towle, Xinan Liu, James Duncan In this study, the production of droplets by a mechanically generated plunging breaking water wave is investigated in a wave tank. The breaker, with an amplitude of 0.070 m, is generated repeatedly with a programmable wave maker by using a dispersively focused wave packet (average frequency 1.15 Hz). The profile histories of the breaking wave crests along the center plane of the tank are measured using cinematic laser-induced fluorescence. The droplets are measured using a cinematic digital in-line holographic system positioned at 30 locations along a horizontal plane that is 1 cm above the maximum wave crest height. This measurement plane covers the entire region in the tank where the wave breaks. The holographic system is used to obtain the droplet diameters (d, for d \textgreater 100 microns) and the three components of the droplet velocities. From these measurements and counting only the droplets that are moving up, the spatio-temporal distribution of droplet generation by the breaking wave is obtained. The main features of the droplet generation are correlated with the features and phases of the breaking process. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H14.00008: Flow-induced oscillations of a floating moored cylinder Daniel Carlson, Yahya Modarres-Sadeghi An experimental study of flow-induced oscillations of a floating model spar buoy was conducted. The model spar consisted of a floating uniform cylinder moored in a water tunnel test section, and free to oscillate about its mooring attachment point near the center of mass. For the bare cylinder, counter-clockwise (CCW) figure-eight trajectories approaching A*$=$1 in amplitude were observed at the lower part of the spar for a reduced velocity range of U*$=$4-11, while its upper part experienced clockwise (CW) orbits. It was hypothesized that the portion of the spar undergoing CCW figure eights is the portion within which the flow excites the structure. By adding helical strakes to the portion of the cylinder with CCW figure eights, the response amplitude was significantly reduced, while adding strakes to portions with clockwise orbital motion had a minimal influence on the amplitude of response. \newline [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H14.00009: Critical Steady Surface Waves of Idea Fluid over a Bump with Surface Tension Jeongwhan Choi, Sangwon Lee, Joonkyoung Kim, Sungim Whang The paper deals with steady forced surface waves propagating on a two-dimensional incompressible and inviscid fluid with a small bump placed on a rigid flat bottom. If the surface tension coefficient T on the free surface is not zero and the wave is moving with a constant speed C, the wave motion is determined by two non-dimensional constants, F =√gh and τ = T/(ρgh2), where g is the gravity constant and h is the height of the fluid at infinity. It has been known that F = 1 and τ = 1/3 are the critical values of F and τ , respectively. In the critical case F = 1 + λ1ϵ2 and τ = 1/3 + τ1ϵ with ϵ > 0 a small parameter, a time-dependent forced Kawahara (F-Kawahara)equation is derived to model the wave propagation on the free surface and the steady F-Kawahara equation is studied both theoretically and merically. It is shown that the steady F-Kawahara equation has many different kinds of one and multi-hump solutions when τ1 and λ1 vary. In particular, for a fixed τ1, there is a λ0 < 0 such that if λ1 < λ0, two one-hump steady solutions can be obtained, one with small amplitude and the other with large amplitude. By using the unsteady F-Kawahara equation, it appears that the small one-hump solution is stable while the large one is nstable. In addition, two-hump solutions are unstable. [Preview Abstract] |
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