Spray formation in non-Newtonian uids

虽然对牛顿喷雾进行了一些实验和数值研究，但对非牛顿喷雾的探索相对较少受到关注。对控制这种类型的水喷雾形成的物理现象的基本理解仍然是一个挑战。本项目旨在为非牛顿雾化和喷雾系统的数值研究奠定基础。为了实现这一点，采用了直接数值模拟（DNS）方法，其中所有的时间和空间尺度都被完全解析。本文首先模拟了Oldroyd-B粘弹性二维/轴对称框架的细丝减薄过程，利用流体体积技术跟踪界面，利用对数构象变换求解粘弹性本构方程。这允许快速探索参数空间，捕捉弹性、粘性和惯性力（即Deborah和Ohnesorge数）的影响，它们表征了粘弹性所呈现的内部松弛和宏观时间尺度。这为首次开发二维Oldroyd-B脉冲数值模拟提供了出发点，并以等速射流释放到停滞气相，以探索粘弹性对喷射液滴尺寸的影响。这些模拟为粘弹性射流雾化过程的三维模拟奠定了基础。非牛顿流体的喷雾形成的数值模拟仍然存在很大的挑战，但它反映了工业应用（例如喷雾干燥），并且可以优化喷雾过程，包含非常复杂行为的流体。

While both several experimental and numerical studies for Newtonian sprays have been conducted, the exploration of the non-Newtonian ows has received comparatively little attention. Achieving fundamental understanding of the physical phenomena governing spray formation of this type of ow remains a challenge. The present project aims to set the basis for the numerical examination of non-Newtonian atomisation and spray systems. To achieve this, a Direct Numerical Simulations (DNS) approach is followed where all the temporal and spatial scales are completely resolved. We begin with the simulation of the filament thinning of an Oldroyd-B viscoelastic two-dimensional/axisymmetric framework, using the volume-of-fluid technique to track the interface and the log-conformation transformation for the solution of the viscoelastic constitutive equation. This permits the rapid exploration of parameter space, capturing the effect of the elastic, viscous and inertia forces (i.e. Deborah and Ohnesorge numbers), which characterise the internal relaxation and macroscopic time scales that viscoelasticity presents. This serves as a departure point for the development for the very first time of two-dimensional numerical simulations of an Oldroyd-B impulsive, as well as released with constant velocity jets into a stagnant gas phase for exploring the effect of viscoelasticity on the ejected droplet size. These simulations constitute the basis for further work involving three-dimensional simulations of atomisation processes of viscoelastic jets. The numerical simulations of the spray formation of a non-Newtonian fluid still offers substantial challenges, but it is reflective of industrial applications (e.g. spray-drying) and can lead to the optimisation of spray processes, containing fluids of a very complex behaviour.