Research On Fluid-structure Coupling Dynamics With Dynamic Wetting

Fluid-solid coupling with dynamic wetting is commonly seen in daily lives,nature and industrial applications,and thus it is of scientific significance to investigate such flow problems.In this thesis,we numerically investigate three flow phenomena:the entrapment of particles at the gas-liquid interface,the interaction between water waves and objects in a confined pool,and the jumping mechanisms of water striders.The results are summarized as follows:(1)We interpret the role of wettability in the process of entrapping the particles by the interface.Upon impact onto a liquid pool,a millimeter-sized hydrophobic sphere is seen to follow one of the three regimes:sinking,bouncing or being entrapped at the interface,which depends on its impact speed,diameter,density,and wettability.Especially,the wettability affects the magnitude of the capillary force exerted by the interface at the contact line,and consequently influences two important processes after impact:interface penetration and sphere rebounding.Two scaling models have been proposed to interpret the critical conditions of the entrapment of spheres at the interface,and they were shown to be in good agreerement with the numerical results.We also found that the penetration depth for a rebounding sphere at certain impact speed followed hmax/D～sin-2(θ/2)Weλρ,while the deepest distance that a sphere can reach without piercing the interface followed hmax/D～sin2(θ/2)λρ-1.(2)We reveal the mechanism how the impact-induced water waves affect the fate of objects in a confined pool.We find that when a hydrophobic sphere impacting onto a confined pool at a certain speed,it does not submerge in a wide pool but does in a narrow pool,and that the reflection of the impact-induced gravitycapillary waves from the pool boundary is responsible for this phenomenon.If defining the maximum pool width that can affect the fate of the impact sphere as Sc,we show that Sc is mainly dependent on the sphere diameter,no matter whether the surface waves are the capillary or gravity waves.For S<Sc,two important pool sizes(Sw,1 and Sw,2,and Sw,2≥Sw,1)are identified at the same impact speed,and the sphere submersion takes place at Sw,1≤S≤Sw,2.Based on the flow features identified in simulations,a scaling law is proposed to correlate the Weber number and Bond number with Sw.The theoretical prediction is shown to agree well with the numerical results.(3)We identify the jumping principle of water striders through numerical simulations and theoretical analysis.We establish the 2D jumping model of water striders for numerical simulations.According to the different motion of water striders,we identify three types of motion:piercing,skating and jumping.In particular,we obtain the motion equation of the water strider’ s leg and get its analytical solution.Accordingly,we deduce the jumping speed and the required time as a function of the length and sweeping speed of the leg.The theoretical prediction is in good agreement with the numerical results.Finally,we theoretically obtain the critical condition for the water strider to pierce the water surface:ΩM*1/2～L-1,which is also consistent with our numerical results and the experimental data.