Session G02: Focus Session: Fluids Next: Soft Body Slamming Fluids II

Air cushioning during disc impact on a water surface

We study the influence of air in the early phase of a slamming solid-liquid impact, in the geometry of a horizontal disc approaching a free liquid surface. To determine the role played by the intermediate air layer, we measure the free-surface deformation by means of an in-house developed profilometric technique based on total internal reflection, using an experimental setup in which a linear motor impacts a disc with a well-controlled velocity onto a water surface. Simultaneously, the force on the disc and the pressure in the air-water phase below it are measured, using strain gauge and pressure sensors. The experiments reveal the entrapment of an air pocket. They are compared to simulations using a two-phase boundary integral method in order to understand the cushioning effect of the entrapped air layer on the load that is experienced by the disc.   

Author not Attending

In nature, many animals dive into water at high speeds; e.g. humans dive from cliffs, gannets plunge, and aquatic animals porpoise and breach. For humans, extreme sports such as cliff diving or high diving provide excitement, but can be close to the limit of bodily injuries. For animals, high dives can provide opportunities to capture prey, escape from predators, or communicate providing a benefit to the diver despite the potential risk of injury. In this study, we demonstrate how similarity in the morphology of diving fronts unifies the slamming force across diving animals. For humans, the evolution of this slamming force depends on the body position at impact, i.e. whether it is a feet-first, head-first or hand-first dive. By measuring a time-averaged impulse on human models, we are able to estimate the unsteady hydrodynamic force that an average human body may experience during the impact phase of a dive, and evaluate whether the force can cause muscle or bone injuries. Thus this study sheds light on a fluid mechanics based protocol for safe high diving and an evolutionary driver for animal morphology.

 

    

A deformable bubble passing through an interface between two miscible liquids

We conducted experiments to understand interfacial phenomena when a deformable bubble passes through an interface between two miscible liquids. We used tap water and water-glycerine mixtures at the upper (liquid 1; μ₁ = 1 mPa·s, ρ₁ = 998 kg/m³) and lower (liquid 2; μ₂ = 1.3 - 900 mPa·s, ρ₂ = 1021 - 1257 kg/m³) liquids, respectively, and the cap bubble (R_cap = 12 - 35 mm) was considered. Varying the viscosity ratio (μ₂/μ₁) and R_cap, we measure how the interface deforms. When μ₂/μ₁ = 1.3 - 5, wake fragmentation and path instability happened so fast that the interface of the miscible liquids generated by the bubble collapsed instantly. When R_cap ≤ 18 mm, the interface collapsed due to bubble path instability at μ₂/μ₁ = 10 - 40 and small interfacial disturbance occurred at μ₂/μ₁ > 40. When R_cap ≥ 23 mm, the Kelvin-Helmholtz (KH) and azimuthal instability were observed simultaneously at μ₂/μ₁ = 10 - 210, and the KH instability was not clearly observed at μ₂/μ₁ > 210. Based on these, we suggested an instability regime according to μ₂/μ₁ and interfacial Eötvös number. We also found that these interfacial phenomena were related to bubble deformation characterized by the large-amplitude waves.   

Impact Dynamics of Suspension Droplets on Liquid Films

Droplet impact on surface is ubiquitous in many industrial applications, such as inkjet printing and spray coating. Rich phenomenon and impact outcomes including bouncing, merging, and splashing have been observed due to the intrinsic complexity stemming from the interplay of surface tension, viscous effect and inertial effect. Impact dynamics of Newtonian droplet on solid and liquid surfaces have been studied extensively. Non-Newtonian effect add another layer of complexity which gives rise to more intricate behaviors. In this talk, I will present our experimental study of suspension droplets impacting on liquid films. Richer phenomenon compared to that with Newtonian liquids have been observed and discussed in terms of a regime map in the Weber number-volume fraction space. Scaling analysis will be discussed to understand the controlling mechanism for the interaction between the suspension behavior and the complex flow field induced by droplet impact.   

Author not Attending

We study water entry of steel spheres in the presence of an ultrasoft, thin elastic membrane of thickness of a few hundred microns floating on a free surface. The presence of a thin elastic layer of negligible inertia establishes an air-water interface that can sustain a strain-dependent surface tension. As the sphere impacts the free surface, we observe the formation of a dynamically elongating and propagating deformation front surrounding the impacting body (which elastically stores the energy in the membrane material), and an outer zone where radial wrinkles gradually develop before the membrane eventually gets pulled down by the descending sphere. During the first stage of impact and elongation, the outer zone beyond the deformation front of the membrane is unperturbed by the impact and penetration of the sphere. Following this stage, the stretched membrane relaxes and recoils while pivoting on the sphere. We observe regimes when the ultrathin floating film is able to elastically absorb all of the kinetic energy of the impactor and dissipate it into the surrounding fluid motion. We map out the water entry behavior over a range of dimensionless groups including the Weber number and Froude number as well as the membrane parameter, such as the elasto-inertial number.