基于SPH方法的湿润性固壁边界条件模拟研究

固体壁面由于表面特殊结构和材料属性，时常表现出对交界面上水体的吸附作用，而这一特征对微小水体作用尤为明显。本文提出了一种湿润性固壁边界条件的计算方法，即假设壁面粒子的亲水性以及毛细吸附作用统一表现为对支持域内流体粒子的吸附力。基于光滑粒子流体动力学（SPH）方法，模拟了静态液滴在不同湿润性壁面上的变形至稳定过程。模拟了液滴撞击疏水壁面的过程，将液滴的运动过程分为碰撞、铺展、回缩和回弹四个阶段，分析各阶段壁面受力分布情况。研究表明：根据模拟液滴静态接触角的变化特点，本文湿润性固壁边界条件可以较好的反映出壁面湿润性；液滴撞击输水表面的模拟数据与试验结果趋势上吻合良好；壁面压力波伴随着液滴的铺展和回缩传播并衰减；只有在回弹后期液滴即将脱离壁面时壁面拉力起主导作用，其余各时刻壁面均以压力为主。

Simulation of Boundary Conditions at a Wettable Solid Wall Based on the SPH Method

Due to the special structure of surface and material properties, solid wall always shows adsorption of water at the interface, and this feature is particularly evident to small water droplets. Surface wettability is used to reflect the magnitude of adsorption force. A Lagrange wettable solid surface boundary condition is presented. It is assumed that the hydrophilia and capillary action of solid wall particles are unified as an adsorption force acting on the liquid particles of supported domains. The adsorption force is deemed relevant to fluid pressure, saturation and surface hydrophilia. To investigate the adsorption phenomenon, several interactions between single droplet and wettable solid surface are simulated by a stress-correction Smoothed Particle Hydrodynamics (SPH) model. First, deformation processes of static droplets on different wettable solid surfaces are simulated to rate the relationship between the droplet’s static contact angle and the wettability coefficient. Then, droplet impact on wettable solid surface is studied to investigate the influence of wettability on the deformation of droplet. Finally, the propagation of stress wave on solid surface during the deformation is analyzed. Droplet’s impacting movement is divided into four stages: collision, spreading, retraction and rebound. Research shows that: according to the changing characteristics of droplet’s static contact angle, the mentioned boundary condition can clearly reflect the wettability of solid surface. Simulation of droplet impact on wettable solid surface shows good agreement with the experimental result. During the rebound stage, a sufficiently large wettability will cause droplet to deform into liquid column. The pressure wave of solid surface propagates and decays along with the spreading and retraction of droplet. Only at the end of the rebound stage, when the droplet is about to detach from the solid surface, tension plays a leading role. For the rest of time, the solid surface is under pressure. The study has improved the theory of interaction between free surface fluid and solid surface, which provides a reference for further study on the effects of free surface fluid and granular materials.