Session Q15: Free-Surface Flows: Interaction with Structures

The impact of a flexible plate on a calm water surface: the effect of plate stiffness

The oblique impact of an elastic rectangular plate (length 108 cm, width 41 cm and thickness 0.66 cm) on a quiescent water surface is studied experimentally. The plate is installed via pinned supports to a dynamometer system, which is connected to a carriage capable of combined horizontal and vertical velocity components, U and V, respectively. The two short edges of the plate are held horizontal, while the angle between the plate's two long edges and the still water surface (called the pitch angle), U and V are varied. These variables are chosen to create a set of experimental conditions in which U/V is held constant while the dimensionless plate stiffness ratio (the ratio of the hydrodynamic pressure force to the plate length times its plate stiffness) is varied. It is found that at large pitch angles, the curves of dimensionless force and moment vs dimensionless time nearly collapse to a single curve, while at a small pitch angle, the curves are strongly affected by the stiffness ratio. This latter case indicates a strong fluid-structure interaction. Work on the effect of longitudinally varying plate stiffness ratio is also underway.   

The impact of a flexible plate on a calm water surface: the effect of pitch and submergence time

The oblique impact of an elastic rectangular plate (length 108 cm, width 41 cm and thickness 0.66 cm) on a quiescent water surface is studied experimentally. The plate is installed via pinned supports to a dynamometer system, which is connected to a carriage capable of combined horizontal and vertical velocity components, U and V, respectively. The two short edges of the plate are held horizontal, while the angle between the plate's two long edges and the still water surface (called the pitch angle, alpha), U and V are varied. These variables are chosen to create a set of experimental conditions that maintain a constant plate stiffness ratio in the regime of strong fluid-structure interaction while varying alpha and the submergence time ratio (the time of the plate's water entry divided by the plate's first order natural period). As alpha is decreased, the submergence time ratio has a larger effect on the dimensionless force, moment and deflection. At small alpha, the spray root breaks up before reaching the plate's leading edge. The kinetic energy of the plate in the pinned support reference frame provides further insights into the strength of the fluid-structure interaction.   

Impact force alleviation effect of surface pattern on disk falling onto water surface

Impact force during water entry is a limiting factor in the design of air-water amphibious systems. We report mitigation of the impact force by patterning a macroscale mesh of square holes on a flat disk falling onto water. The air trapped in the square holes cushions the impact; the air pressure slowly rises then falls, lengthening the impact duration and relieving the peak impact force. While the size and falling speed of the disk determine the total impulse, the alleviation effect is affected by the total volume of holes but not the falling speed and pattern geometry. Theoretical analysis agrees with our experimental results.

    

Cavity dynamics after the injection of a microfluidic jet onto capillary bridges

The impact of solid and liquid objects onto liquids and soft solids results on the creation and expansion of an air cavity inside the impacted objects. In this paper we study the impact of microfluidic jets generated by thermocavitation processes on a capillary bridge between two parallel planar walls. Different capillary bridge types were studied, Newtonian liquids, viscoelastic liquids and agarose gels. We model the critical impact velocity for a jet to traverse a capillary bridge type. We show different types of cavity collapse, depending on the Weber number and the capillary bridge properties. We conclude that the type of collapse determines the number and size of entrained bubbles. Furthermore, we study the effects of wettability on the adhesion forces and contact line dissipation, upon cavity collapse. for hydrophobic walls a Worthington jet is energetically favourable. In contrast, for hydrophilic walls, the contact line dissipation suppresses the Worthington jet formation. Our results provide strategies for preventing bubble entrapment and give an estimation of the cavity dynamics for needle-free injection applications among other applications.   

SurferBot: a wave-propelled aquatic vibrobot

Nature has evolved a vast array of strategies for propulsion at the air-fluid interface. Inspired by a survival mechanism initiated by the honeybee (Apis mellifera) trapped on the surface of water, we present the SurferBot: a centimeter-scale vibrating robotic device that self-propels on a fluid surface. This low-cost and easily assembled device is capable of rectilinear motion thanks to forces arising from a wave-generated, unbalanced momentum flux, achieving speeds on the order of centimeters per second. In addition to a detailed description of the fluid mechanics underpinning the SurferBot propulsion, other modes of SurferBot locomotion will be presented. Ongoing and future applications of the SurferBot will be discussed including robotics, fluid mechanics pedagogy, and fundamental explorations of active and driven particles at fluid interfaces.   

Soft Fluidic Couple Oscillators

Coupled oscillators are a classical problem in physics, where it is well known that efficient transfer of energy occurs between interacting systems. Here we numerically study the dynamics of a fluidic coupled oscillator composed of two interacting free-surface water waves separated by a soft, compliant membrane. We vary the system control parameters, namely the membranes' stiffness and the water depth of the two wave chambers, to observe different regimes of coupling between the interacting wave tanks. We model the compliant membrane dynamics using an unsteady Young-Laplace type equation with a deformation dependent membrane tension, and an added-mass coupling between the oscillating wave modes.   

Free surface effects on a rotating cylinder

Flow around a circular cylinder is a canonical bluff body flow which has widespread fundamental importance in fluid mechanics. Many studies have focused on the vortex shedding from static or rotating cylinders subjected to a uniform free stream and usually neglect end effects.  Studies considering end effects have primarily focused on the bottom wall whereas much less attention has been paid to cylinder-free surface interactions characterised by a run-up and a depression of the free surface.  We present new results which investigate the free surface effects on a flow around a static and rotating cylinder with 900<Re<5000, mounted vertically in an open water channel. At each ????, we vary the rotational frequency of the cylinder using a stepper motor. Particle image velocimetry measurements are conducted over the midplane downstream of the cylinder in the streamwise-wall-normal direction to capture the flow field from the free surface (y/d = 0) to the free stream region (y/d = -12.5). Additional measurements are also performed over the cross-sectional planes near the free surface and in the free stream region. Over the range of rotation rates investigated, we observe different shedding mechanisms in the free stream and free surface regions which will be presented in the meeting.   

Drag on a partially immersed sphere at the capillary scale

Characterization of the drag on bodies in a background flow represents one of the most timeless and practical applications of fluid mechanics. For the case of objects partially immersed at a free surface, most prior work focuses on large scales where the effects of surface tension are negligible. In the present work, we use a custom benchtop flume to investigate the drag force on centimetric objects in steady flow constrained at the air-water interface. Surface profile reconstructions and particle image velocimetry measurements are presented in conjunction with direct force measurements. The experimental measurements are compared directly to numerical simulations performed using a variational multi-scale, mixed interface-capturing/interface-tracking formulation. In general, we observe that the drag force steadily increases with submergence, reaching a peak value which significantly exceeds the fully submerged case. Implications for the design of surface microrobots and other applications will be discussed.   

Thin film flow over slippery substrates

We present a theoretical and experimental study of the free-surface flow of viscous fluids over a slippery, structured substrate. Much like classical hydrophobic surfaces, the substrate consists of a periodic sequence of two-dimensional, fluid-filled cavities. The shear stress exerted by the overlying viscous fluid drives a return flow within each cavity. This induces slip at the interface between the two fluids, which can be characterised in terms of a slip length proportional to the ratio of the viscosities of the two fluids. A series of experiments of a thin film of viscous fluid over a structured, fluid-saturated substrate confirms that the slip length is proportional to the viscosity ratio.