Session X04: Free-Surface Flows: Interaction with Physical Structures (10:45am - 11:30am CST)

Oscillatory response of a gel to wave-induced velocity gradients in the context of marine oil spill remediation

The use of gellants as an oil-spill remediation technique is challenged by a lack of academic literature on the interactions between marine surface gels and their environments. This study adopts an experimental perspective to investigate the stretching of a gel in wave-induced spatial velocity gradients, the product of orbital particle motion in linear waves. Analytical models were developed to describe the response of a viscoelastic gel to such a forcing for Kelvin-Voigt and Maxwell materials. Bottom-of-tank experiments were designed to isolate wave-induced velocity gradient stretching from bending effects. The experiments were conducted in the 4.7 meter wave tank at the University of British Columbia using gelatin as a model gel. Applying monochromatic, linear waves to the system resulted in an observable oscillatory strain in the gel; and, for the longer gels, breakage. Analytical models under-predicted strain by a constant factor. Incorporating this factor shows a close match between data and model results. Breakage occurred where peak stress across the gel's cross-section exceeded a threshold value. This threshold stress (5 Pa) was considerably less than the oscillatory yield stress of the material (35 Pa), indicating that breakage was due to fatigue.   

Electrostatically-Assisted Direct Ink Writing for Additive Manufacturing

While nozzle-based printing is already arguably versatile, such sub-categories as Direct-Ink-Writing (DIW) are difficult to be used to print materials on rough surfaces. Recently electrohydrodynamic (EHD) elements added new features in droplet positioning but also revealed limitations in the achievable build height due to the need for a grounded substrate or embedded electrode. Here, we introduce an additional electrode added to the printhead generating an electric field between the above-mentioned electrode and printing nozzle. The resulting Coulomb force pulls the extruded ink in the direction of printing allowing faster translational speed, thinner trace widths, and improved deposition on rough surfaces without a decrease in build height. We also developed the electrohydrodynamic theory of the proposed DIW processes. The integration of the electrode to the printhead allowed successful prints at the machine's maximum speed of 500 mm/s for a documented situation in which DIW previously failed in existing literature (also, on rough surfaces where printing was impossible before). Along with new design opportunities, these results unlock speed restriction within nozzle-based printing while significantly expanding versatility and substrate choices.   

Experimental and numerical studies of wave-rigid body interaction: Wave attenuation properties of a constrained floating breakwater

Interactions between surface gravity waves and a mounted rigid body are complex, as waves may reflect, refract, or even overtop the body. Studies of these phenomena are critically important in determining and improving the safety and efficiency of offshore structures. Here the motions of a floating breakwater held by wires to the seabed are studied through numerical and experimental approaches. A model of the floating breakwater with a scale of 1:20 is tested in a water channel with wave maker. Wave properties, current velocity and the constraint force of the floating breakwater are measured in the laboratory. Computationally, numerical methods utilizing the volume of fluid (VOF), six degree of freedom (6DOF) and fluid-structure interaction (FSI) schemes are used to simulate the motions of the waves and the floating breakwater. The performance of the floating breakwater in terms of wave attenuation is assessed by varying the wavelengths, wave amplitudes, and current velocities both computationally and experimentally. The motions of floating breakwater are also analyzed. Agreements between the experimental and numerical tests are encouraging.   

Splash Production of Single Jointed Divers

Olympic divers achieve a no splash entry by pulling apart their arms and rolling their bodies after hitting the surface, performing what is called a ``rip dive.'' While the practice is widespread in competitive diving, there is no data on how the surrounding fluid is manipulated to minimize the splash. To better understand this complex phenomenon, we create an experimental model consisting of thin, single-jointed objects falling into a pool of water. A spring pin is inserted at the joint to investigate how the roll changes the splash formation. The creation of an air cavity during the roll is inspected to understand how the fluid is manipulated. In addition, the length ratio between the upper and lower joints are varied to better understand the movement. The water entry of these objects is recorded using high speed video to qualitatively analyze the splash and an accelerometer is attached to estimate force upon entry. These results will begin to illuminate how competitive divers are able to achieve minimal splash upon water entry and later be used to validate computational fluid dynamics simulations.   

Water Entry of Flexible Panels: Pressure-Load Estimations from Contact Line Measurements and Theory

Wedge water entry is often studied to predict the slamming pressure as a preliminary tool for the design of the high-speed craft. Flexible-bottom panels are of current interest due to lower weight and more widespread use of composite materials in hulls. Since the existing analytical models mostly focus on rigid panels, attention has been drawn to experimental investigations on flexible panels. Mounting pressure sensors to measure the impact loads in experiments for plates with low flexural rigidity increases the panel flexural rigidity. A new technique is presented to estimate the pressure load on the plate using the location of contact line and the existing theoretical models. A set of experiments are conducted for different panels and the contact line is measured with high-speed photography. Modifications have been made to the theoretical models to account for a free-fall entry, consistent with the current experimental setup. The experimental contact line location is then used as an input for these models to estimate the pressure and kinematics of the panel. To validate this method, derived pressure results are compared with experimental measurements for panels with low to moderate flexural rigidities.   

On the Vertical Impact of a Highly-Flexible Cantilevered Plate on a Free Surface

Taking inspiration from nature and biology, the structural response and reconfiguration of a flexible structure in a flow can decrease drag if the structure becomes streamlined with the flow direction. Much research on passive reconfiguration of structures such as seagrasses or leaves on trees have been conducted in a single fluid, either in air or water. The current project is on both passive and active control of flexible plates interacting with a free surface interface between air and water. This talk is on a subset of the larger project and is on passive reconfiguration of a highly-flexible set of cantilevered plates, forming a V-shape, vertically impacting a free surface. Impact loading occurs as the plate passes through the free surface. While a seemingly simply problem, the large deformations of the plate are nonlinear and cause significant changes in the free surface elevation and pressure loads on the flexible body. In this talk, preliminary experimental measurements will be discussed and interpreted in light of theory.   

Levitating a cylinder on a thin viscous film

We demonstrate that it is possible to levitate a circular cylinder placed horizontally on a vertical belt covered in a thin layer of oil by moving the belt upwards at a specific speed. The cylinder rotates and is balanced at a fixed location on the belt. Levitation occurs solely through viscous lubrication effects. We present the results of an experimental and numerical study of this fluid-structure interaction.   

Consecutive sphere water entry and impact force reduction: post hoc ergo propter hoc!

An object impacting a quiescent water surface experiences severe deceleration that can be very high in the first few moments of surface penetration. This sudden impact acceleration is potentially catastrophic to the impacting object, and can even be fatal for a person jumping from a far enough height. We propose a way of reducing this high-water entry impact force by employing a consecutive two-sphere water entry system. Two vertically separated spheres free-fall onto a water pool, with the first sphere creating a subsurface air cavity during impact and the second sphere falling through different stages of this cavity depending on the varied separation distance between the spheres. The cavity-sphere interactions lead to reduced impact force for the trailing sphere, which in turn can be characterized into five different two-sphere entry modes using a dimensionless time known as the `Matryoshka' number. We report up to \textasciitilde 78{\%} impact force reduction compared to a quiescent single sphere drop, and reveal how different cavity dynamics contribute to variations in force reduction for consecutive two-sphere water entry.~   

Double sphere water entry: side by side

Objects with a rough surface texture can create underwater cylindrical air cavities in the wake of water entry. These cavities greatly influence the descent trajectory, structural integrity, and drag force of the body. These properties along with different cavity characteristics have been studied at length for the case of a single sphere. An interesting extension to these studies is the simultaneous water entry of multiple roughened objects next to one another. We performed a series of experiments using two hydrophobic spheres at varied horizontal distances from each other, dropped in free-fall from a height range of 5 - 170 cm onto a water pool, and studied the evolution of the cavities that are created. In general, a simultaneous two-sphere impact on water introduces interactive flow effects, greatly influencing parameters such as cavity pinch-off time, cavity diameter, and the Worthington jet. We also report on a critical non-dimensional horizontal distance of influence beyond which the cavities formed by the two spheres have no effect on one another.   

Surface-seal changes with impact speed

Surface-seal is one of the most violent and complex fluid phenomena that occurs when a sphere enters the water. Surface-seal is associated with the formation of a thin splash sheet that moves up and outward at first, then inward to close above the cavity. This closure event is followed by the dramatic cavity pull-away from the free surface, with the cavity remaining but still attached to the sphere. A scaling law for the timescales of the surface-seal has been proposed by Gilbarg & Anderson (1948) for the lower-speed water entry and verified by several researches for entry speeds up to 30 m/s of the sphere entry speed. In contrast, for faster speeds, the theoretical work done by Lee et al. (1997) predicted that the scaling law breaks down, yet there is still no experimental verification. Here, we show our experimental measurements on the timescales associated with surface-seal at relatively faster speeds (up to 128 m/s). Our results confirm that the conventional scaling law does break down when the sphere speed reaches a threshold value (around 90 m/s in our case) and suggests clues for the future theoretical developments.   

The Fluid-Structure Interaction during the Impact of Flexible Plates on a Water Surface

The controlled oblique impact of three flexible rectangular plates on a quiescent water surface is studied experimentally. The plates are made of 6061 aluminum and have the same lengths ($L=108$~cm) and widths ($W=41$~cm) but different thicknesses ($h=6.35$, 7.94 and 12.7~mm). During testing, each plate is installed on a dual-axis carriage via a mounting structure that allows each end of the plate (width $W$) to rotate about a transverse axis located slightly above the plate. The pitch angle, $\alpha$, of the undeformed plates is set to 10$^\circ$, leading edge up, in all cases. The horizontal and vertical components of the carriage velocity, $U$ and $W$, respectively, are held constant during the impact time, $T_i$, taken is the time interval between the trailing and leading edge of the plate passing through the still water level, $T_i = L\sin\alpha/W$. The total force and moment on the plate, out-of-plane deflection, in-plane strain and water surface profiles around the plate are measured. The roles of the component of the plate velocity normal to its undeformed surface, the direction of the plate velocity given by $U/W$, and the ratio of $T_i$ to the natural period of the plate are explored.   

Material exchange via shear instabilities above flexible aquatic vegetation

Aquatic vegetation is typically flexible and streamlined, which allows it to passively reconfigure to reduce the fluid load. By altering the hydrodynamic conditions, submerged vegetation can affect the transport of sediment, nutrients, dissolved oxygen, and planktonic fauna in aquatic systems. Flows through submerged canopies can develop instabilities; when the velocity shear at the top of the plant canopy exceeds a threshold, waves develop in the flow and evolve into vortices. These vortices induce a synchronous waving of the vegetation, known as monami. Modeling this phenomenon is challenging because of the two-way coupling between vegetation and flow. We develop a multiphase model in which the vegetation is represented by buoyant, flexible stems of fixed length that deform and change the canopy height, which is accounted for by conformal mapping. This approach reproduces the monami dynamics in an open channel and is used to investigate how the instability affects tracer exchanges between the vegetation and freestream. The transport dependence is studied as a function of the stem flexibility, canopy to free-surface height ratio, and flow Reynolds number, from both Eulerian and Lagrangian perspectives.   

CFD Study of Extreme Ship Responses in Seaways by Designed Wave Trail

The assessment of extreme responses of ships in seaways is an important subject in the ship design. Linear prediction tools (e.g. the frequency domain and linear spectral analytical methods) are often used to predict the ship responses in waves. However, these tools are not reliable when it comes to severe operational conditions resulting in extreme motions. The need for nonlinear solver is imminent to enhance the accuracy of ship response predictions but computations are very time-consuming and practically impossible to search through all wave environments to assess the possible extreme responses. This study aims at designing a wave trail based on a few high-fidelity CFD simulations to predict extreme responses of ships in an optimal way. The studies focus on heave and pitch responses of semi-captive KCS (KRISO Container-Ship) advancing at Fr$=$0.26 in irregular head waves. Studies are also conducted in calm water and regular waves to validate the results against available experimental data. Simulations are achieved using our in-house CFD solver (CFDFoam) which has been built around OpenFOAM but is designed for ship hydrodynamic applications.