  The current study uses numerical approaches to investigate the effect of
droplet deformation and internal circulation on droplet dynamics. Although
droplet drag is a classical area of study, there are still theoretical gaps in
understanding the motion of large droplets. In applications like spray
combustion, droplets of various sizes are generated and move with the flow.
Large droplets tend to deform in the flow, and have complex interactions with
the flow because of this deformation. To better model spray, the physical
understanding of droplets need to be improved. Under spray conditions, droplets
are subjected to a high temperature and pressure environment, and the coupling
between liquid and gas is enhanced. Therefore, the deformation and internal
circulation will affect droplet drag coefficient more significantly than in
atmospheric conditions. To study the mechanism on how droplet shape and
internal circulation influence droplet dynamics, we will use direct numerical
simulation (DNS) to simulate a droplet falling at its terminal velocity in high
pressure air. An in-house code developed for interface-capturing DNS of
multiphase flows will be employed for the simulation. The drag coefficient is
calculated, and the results are consistent with existing literature for
slightly deformed droplets. The results show that the drag coefficient is
directly related to the droplet deformation and droplet internal circulation.
The paper also develops a theory to account the effect of Weber number and
liquid/gas properties in droplet deformation.
