Direct Numerical Simulation of Breakup Phenomena in Liquid Jets and of Colliding Raindrops

Summary Since powerful computational ressources are available, numerical simulation is one of the most attractive tools to bridge the gap between the experimental and analytical description of fluid flow phenomena. One of these tools is the direct numerical simulation (DNS) technique relying on a very high spatial and temporal resolution of fluid systems. Thus, all length scales of a fluid flow, only limited by the grid size, are captured by DNS. Since investigations of many fluid systems require a very fine resolution, considerable progress can only be made by both applying sophisticated numerical methods and using high performance computers. With the inhouse 3D DNS program FS3D (Free Surface 3D) based on the Volume-of-Fluid method it is possible to simulate two phase flows of fundamental interest in the automotive and aerospace industry, in meteorology and agriculture but also in the oil industry or medicine. One part of this study focuses on the numerical simulation of the physical phenomena leading to disintegration of liquid jets. Although many researchers focused in the past on primary breakup phenomena and many experimental, analytical and also numerical results are available, these processes are not well understood. Therefore it was decided to simulate this breakup process by DNS. The results presented here demonstrate the capability of DNS on modern supercomputers. The first numerical simulation results on jet breakup agree qualitatively well with present experimental results. Differences are mainly due to the inflow condition which is a crucial problem for this phenomena. The other part of this study is dedicated to a cloud microphysical process called ‘collision-induced breakup’ limiting the maximum size of raindrops. This process comprises a binary collision of raindrops and a subsequent disintegration of the coalesced body resulting in creation of a number of smaller fragment drops. The only very few experimental investigations of this process dating from the very past are reexamined by using the advanced fluid-mechanic program FS3D. The advantage of this numerical method is its high accuracy in simulating drop collisions. First results of fragment size distributions due to collisional breakup of raindrops are presented showing a good global agreement with those from laboratory experiments.