Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session Q20: Free Surface Flows: Fluid-Solid InteractionsFree Surface FSI
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Chair: Daniel Harris, Brown University Room: 704 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q20.00001: Near-Zero-Drag Objects S. T. Thoroddsen, I. U. Vakarelski, E. Klaseboer, A. Jetly, M. M. Mansoor, A. A. Aguirre-Pablo, D. Y. C. Chan The quest to reduce aerodynamic drag on blunt objects is driven by the need to reduce propulsive energy. Solid objects moving in an ideal fluid experience no drag. This prediction, known as the D'Alembert's paradox, is resolved by the no-slip boundary condition on the solid surface, which promotes boundary-layer separation and form drag. Here we report objects which minimizes both the form and viscous drag within a liquid, by encasing a free-falling solid sphere inside a streamlined gas cavity. The cavity-shape self-adjusts to the streamlined potential-flow solution satisfying the Bernoulli equation on the free surface, when taking into account the hydrostatic pressure gradient. The tear-drop-shaped gas cavity is originally formed around the sphere as it impacts a pool surface in a deep tank, providing that the sphere is heated above the Leidenfrost temperature. By assuming zero form-drag we can predict the separation point of the free surface from the solid. This sphere-in-cavity structure typically has a drag coefficient less than 10\% that of a solid object with the same shape. This should represent the smallest possible drag. The fall velocity is uniquely predicted by sphere density and cavity volume, with larger cavities fall faster. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q20.00002: Water entry of cylindrical bodies with various aspect ratios Nayoung Kim, Hyungmin Park We experimentally investigate the water entry of cylindrical bodies with different aspect ratio (1.0-8.0), focusing on the deformation of free surface and resulting phenomena over and under the surface. The experiment is performed using a high-speed imaging (upto 10000 fps) and PIV. The head and tail of bodies are hemispherical and the nose part is additionally roughened with a sandpaper to see the effect of roughness as well. The release height is also adjusted to change the impact velocity at the free surface (Reynolds number is order of $10^5$). For smooth surface (without cavity formation), a thin liquid film rises up the body after impacting, gathers at the pole and forms a jet over the free surfaces. The jet is created in the form of a thick and thin jet. The thin jet is produced by a water film riding up the surface of an object, and a thick jet is produced by rising water from underwater as the object sinks. However, as the aspect ratio increases, the liquid film does not fully ride up the body and cannot close, so there is an empty space below the free surface. With roughness (with cavity), the liquid film is detached from the body and splash/dome is formed above the free surface. The splash height and its collapsing time decrease with increasing the aspect ratio. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q20.00003: Numerical simulation of flow past an elliptic cylinder at different angles of attack near a free surface using Level-set method Rahul Subburaj, Prof. Vengadesan S Flow past an elliptic cylinder near a free surface will be simulated using Level-set method and Immersed boundary method. The interaction of wake behind the elliptic cylinder and the free surface will be investigated by varying parameters such as aspect ratio, angle of attack, submergence depth and Froude number. The huge umbrella of parameters covers the cases of horizontal elliptic cylinder, circular cylinder and flat plate that is already present in the literature. The fluid solver uses level-set method with one-fluid approach for tracking the interface and immersed boundary method to simulate rigid bodies. Interesting phenomena like vortex-shedding suppression, free surface deformation by vortices, etc., are expected to be found for these set of parameters. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q20.00004: Crossing the boundary: numerical investigation of water entry conditions Dionysios Angelidis, Fotis Sotiropoulos Several engineering and scientific applications involve water impact problems. To accurately capture the dynamics of the cavity formation and the water ejected as a body hits the water, the formidable range of temporal and spatial scales should accurately be resolved with affordable computational cost. We have enhanced the potential of the two-phase flow version of the immersed-boundary adaptive mesh refinement flow solver, developed by our group, to perform high-fidelity two-phase flow calculations on locally refined grids. We employ a level-set method and tackle the computational challenges arise during the explicit solution of a mass-conserving reinitialization equation. In contrast to conventional approaches, we propose a convergence criterion which enables the number of iterations to be self-adjusted based on the values of the distance function. The efficiency of our method is demonstrated by performing two-phase flow calculations including the high-speed water entry of a V-shaped wedge. Our results are found to be in good agreement with experimental measurements and enable us to gain insight into the instability that arises on the onset of the closure phase of the cavity. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q20.00005: Water walking - an evolution of water surface skipping Randy Hurd, Jesse Belden, Michael Jandron, Allan Bower, Sean Holekamp, Tadd Truscott Previous work has shown that elastomeric spheres skip more easily than disk-shaped stones. This is due to increased lift stemming from sphere deformation, which provides an increased cross-sectional area and favorable attack angle upon impact. We extend lift models developed for individual impacts to long-range multiple impact events and compare the estimates to experimental results, which show good agreement. Additionally, a surprising new mode of skipping is observed that resembles water-walking, wherein a quickly rotating sphere produces small successive impacts allowing it to move parallel to the water surface. The dynamics of this new multiple skip behavior are rationalized analytically and tested experimentally. [Preview Abstract] |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q20.00006: The harbingers of water entry Nathan Speirs, Jesse Belden, George Badlissi, Zhao Pan, Sean Holekamp, Tadd Truscott The impact dynamics of solid objects on a flat, quiescent pool has been studied for over a century. We investigate sphere impact on a water surface which has been altered by a preceding jet. The jet of water creates a large subsurface cavity prior to impact of the sphere, which follows directly in its path. This method of ``free-surface preparation'' suppresses cavity formation at sphere impact velocities for which it would otherwise be expected. Additionally, it greatly diminishes the large initial impact force, and alters the forces after impact. We show reduction in the maximum impact force by 50-75{\%} from the quiescent pool cases. [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q20.00007: Impact of a Hydrophobic Sphere onto a Bath Daniel M. Harris, John Edmonds, Carlos A. Galeano-Rios, Paul A. Milewski Small hydrophobic particles impacting a water surface can rebound completely from the interface (Lee \& Kim, \emph{Langmuir}, 2008). In the present work, we focus on the bouncing dynamics of millimetric hydrophobic spheres impacting the surface of a quiescent water bath. Particular attention is given to the dependence of the normal coefficient of restitution and contact time on the impact velocity and the radius and density of the sphere. Our experimental observations are compared to the predictions of a fluid model derived from linearized Navier-Stokes under the assumption of a high Reynolds number regime (Galeano-Rios et al., \emph{JFM}, in press). In the model, the motions of the sphere and the fluid interface are found by imposing the natural geometric and kinematic compatibility conditions. Future directions will be discussed. [Preview Abstract] |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q20.00008: Flexibility Considerations on the Hydrodynamic Loading on a Vertical Wedge Drop Zhongshu Ren, Zhaoyuan Wang, Carolyn Judge, Fred Stern, Christine Ikeda High-speed craft operating at in waves frequently become airborne and slam into the water surface. This fluid-structure interaction problem is important to understand in order to increase the operating envelope of these craft. The goals of the current work are to investigate both the hydrodynamic loads and the resulting structural response on a planing hull. A V-shaped wedge is dropped vertically into calm water. The hydrodynamic pressure is measured using pressure sensors at discrete points on the hull. Two hulls are studied: one is rigid and one is flexible. Predictions of the hydrodynamic loading are made using Wagner’s theory, Vorus’s theory, and simulations in CFDShip Iowa. These predictions assume the structure is completely rigid. These predictions of the pressure coefficient match well with the rigid hull, as expected. The spray root is tracked in the rigid experimental set and compared with the theoretical and computational models. The pressure coefficient measured on the flexible hull shows discrepancies with the predictions due to the fluid-structure interaction. These discrepancies are quantified and interpreted in light of the structural flexibility. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q20.00009: Numerical study of interactions among air, water, and rigid/flexible solid bodies Sida He, Zixuan Yang, Lian Shen We develop a numerical method for simulating the fluid-structure interactions (FSI) among air, water, and rigid/flexible solid bodies. The influence of solid bodies on fluid flows is captured by an immersed boundary method. The air and water are simulated as a coherent system, with a coupled level-set and volume-of-fluid method implemented to capture the interface between air and water. Six degrees of freedom are considered for the motion of rigid bodies. A finite element method based on thin-shell theory is utilized to simulate the arbitrarily large deformation of flexible plates. Both strong coupling and loose coupling have been applied based on the density ratio of structure and fluid. We validate our code using benchmark cases with one-phase fluids, including the vortex-induced vibration of a rigid circular cylinder and the vibration of cantilever mounted behind a square cylinder caused by vortex shedding. We also test the accuracy of the code for simulating FSI problems with two-phase fluids in the context of barge floating at fluid interface and violent breaking waves impinging onto rigid/flexible plates. It is found that our simulation results are in good agreement with laboratory and simulation results reported in literature. [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q20.00010: Interfacial interactions in the wake of elliptic cylinders Prashant Khandelwal, S. Vengadesan The interfacial interaction between two fluids has always been an interesting field to investigate, for the discontinuity treatment and the tracking of the interface between two fluids, a lot of methods have emerged. We have developed a code for Level-Set Method with Immersed Boundary Method using one-fluid approach. We validate the code for level set for Rayleigh-Taylor instability and Rising bubble problem. We investigate the influence of the interface on the flow structures in the wake of a solid body.Flow over rigid bodies with wall effects has been studied but the interesting part comes when there is interface is in the vicinity of the body. The energy transfer between two different fluids with different properties causes waves on the interface which tightly binds fluids. The rigid elliptic cylinder is located in one of the fluids and is very near to the interface and we investigate forces and effects of the other fluid for incoming velocity profiles and different orientation and movements of the elliptic cylinder in 2-d. We also look at the interaction of multiple elliptic cylinders with the interface and we investigate the forces on the bodies as well. [Preview Abstract] |
Tuesday, November 21, 2017 3:00PM - 3:13PM |
Q20.00011: Diffraction of waves past two vertical thin plates on the free surface: A comparison of theory and experiment Dong Min Shin, Yeunwoo Cho Diffraction of waves past two vertical thin plates on the free surface is studied theoretically and experimentally. A particular attention is paid to the wave motions depending on the relationship between the wavelength ($\lambda )$ and the width (b) between the two plates for a given draft (d) and water depth (h). For d/h$=$0.19, at resonance modes when b/$\lambda =$0.245 (first), 0.695 (second), 1.11 (third), 1.55 (fourth), etc., the overall transmission features the maximum with no reflection. In the first mode, the water column between the plates moves up and down with no wave motions. In the second mode, it shows the fundamental standing wave motion. In the remaining modes, it shows another standing wave motions with relatively higher frequencies. As d/h increases (0.1--0.4), the resonance points move to values b/$\lambda =$0, 0.5, 1, 1.5, etc., and, at those resonance points, the peaks of reflection and transmission coefficients become more sharp and narrow. The loss of energy of incoming waves is also observed at every transmission in the two plate system, and, in particular, more energy loss near a resonant frequency. In addition, it is found that energy is lost mainly due to the transmission process not the reflection process. [Preview Abstract] |
Tuesday, November 21, 2017 3:13PM - 3:26PM |
Q20.00012: Spray formation during the vertical impact of a flat plate on a quiescent water surface An Wang, James H. Duncan Spay formation during the impact of a rigid flat plate (122~cm by 38~cm) on a quiescent water surface is studied experimentally. The plate is mounted on a carriage that is driven by an electric servo motor that can slam the plate vertically into the water surface under feedback-controlled motions at various speeds. The long edges of the plate are kept horizontal and the short edges are set at various angles (roll angles) with respect to the quiescent water surface. A laser light sheet is created in a vertical plane at the middle of the long edges of the plate. The evolution of the spray within the light sheet is measured with a cinematic laser induced fluorescence technique. Two types of spray are found with nonzero roll angles. The first type is a cloud of high-speed droplets and ligaments that are generated when the plate's leading edge impacts the free surface. The second type is a thin water sheet that is connected to the trailing edge of the plate via a crater and is formed after the trailing edge moves below the local water level. In a reference frame moving with the plate, the profiles of the crater collapse when scaled with a power law function of time. The characteristics of the two types of spray are found to be affected by both the roll angle and the impact velocity. [Preview Abstract] |
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