Session: 04-01 Numerical and Data Based Methods for Multiphase Flows

Paper Number: 87895

87895 - A Numerical Method and Study of Viscoelastic Droplet Breakup 

Droplet breakup has been well studied and characterized over the years, especially for water. In these conditions it is well understood that the Weber and Ohnesorge numbers delineate regimes of the physical drivers of breakup physics. In the context of the recent COVID-19 pandemic, there is a strong need to understand droplet breakup physics with respect to saliva. Although saliva is primarily composed of water, it exhibits a viscoelastic property due to the proteins dispersed within the fluid often described as a mucus. These micro-scale forces of these protein chains are difficult to model relevant to continuum-scale salivary droplet released in a cough or sneeze, so it is desired to quantify this new behavior. Specifically, there is a need to understand droplet breakup with a droplet of an underlying viscoelastic behavior relevant to various human respiratory function (i.e., slip velocities from speech, cough, and sneeze) to better understand the subsequent ambient dispersion for public health and safety purposes.

This study presents a comprehensive study of viscoelastic, droplet-breakup physics using multiphase computational fluids dynamics (CFD) based on the volume of fluid (VOF) method. The specific challenge and novelty is the overall outcome and methods used to exploration of viscoelastic breakup physics. In the context of VOF, the method approximating both viscous and elastic characteristics of the saliva with a function based on the Carreau-Yasuda (CY) model. The CY model is traditionally used for modeling blood flow and here is extended to approximate saliva. The foundation of the model couples the shear-rate to drive both a variable viscosity and relaxation time.

The proposed paper will present both an approach and results assessing a CY-based VOF method to study viscoelastic droplet breakup relevant to human respiratory function. The model wis formulated into a single, spherical, viscoelastic droplet exposed to variable thermal conditions driven by a macroscale model of respiratory function. Overall, the methodology is developed by adapting using user code with the commercial CFD tool, Star-CCM+, to consider both a viscoelastic multiphase capability and coupling to the human respiratory results. The droplet-scale model focuses on a multiphase domain initialized for a salivary droplet. The fluid-gas interaction is calculated using the VOF method to track the fluid volume fraction on the domain. The velocity inlet implements a time-dependent boundary based on the relative velocity a droplet experiences from a sneeze measured in full scale models of our previous work.

The framework is an expansion of previously benchmarked, liquid water droplet breakup. Hence, the focus demands establishing the CY-model implementation and its model constants where we use experimental results for calibration and validation. The CY model constants are based on measurements of saliva from previous work. The relaxation time values of the study are taken from elongational viscometer data. While the numerical model formulation is benchmarked using an elongational capillary study of saliva samples (essentially saliva ligaments forming between two expanding plates). We specifically use the diameter of a saliva capillary pulled apart by two plates is measured as a function of time as validation of the CY-model implementation.

On acceptance, the final paper will document the methods, processes, and results. In addition, the paper will present and document results that include benchmarks with experiments and mesh sensitivity studies. Lastly, the studied explored in this effort will include parametric studies outlining the effect of viscoelastic effects on droplet breakup and explore the drivers in the changes. Finally, the results and conclusions will be discussed and documented.

Presenting Author: Michael Kinzel University of Central Florida