Master of Engineering Science
Mechanical and Materials Engineering
A numerical validation study of under-expanded impinging jet is conducted using OpenFOAM, an open-source computational fluid dynamics (CFD) library. RhoCentralFoam, a density based, compressible flow solver with a two-equation shear stress transport (SST) turbulence model is used on an axisymmetric model to reduce the computation cost. Major features of the flow were compared to an experimental study by Henderson et al., with a nozzle pressure ratio (NPR) of 4.0 and nozzle to plate spacing between 1.65-4.16. Of the features measured, the Mach diamond spacing, super-sonic core, and shear layer are all accurately predicted, while the recirculation bubble in the impingement region and acoustic phenomenon are suppressed. The model is then applied pneumatic nebulizer medical device, which generates a low-pressure vortex by confining the impingement region. Several geometric features are varied to determine their influence on the rotating vortex, of which the nozzle to plate spacing was most influential.
Summary for Lay Audience
Many engineering problems require an understanding of how their project interacts with air, water, or any other fluid involved. In recent decades, new computer techniques have been developed that enable the behaviour of fluids to be better predicted, providing valuable insight into many problems. This work will check the accuracy of a computer simulation software called OpenFOAM, as it models a super-sonic jet of air impacting a wall. The settings in the program were selected to be compatible with super-sonic flows, with separate settings to check the impact of turbulence on the flow. To make the simulation faster, only a small slice of the flow region was simulated, reducing the number of calculations required by the computer. The results of the computer simulation were compared to a well-documented experiment to ensure they were correct. Many aspects of the computer simulation match the experimental results well, although there were some errors found where the flow impacted the wall. Some parts of the jet will move around as time passes, these features of the jet were also suppressed in the computer simulation. Once the accuracy of the computer simulation was known, it was applied to a medical device that operates under similar conditions, to predict the flow for that specific application. In that device, after the air flow hit the wall it would begin to rotate rapidly in a vortex, creating a suction between the vortex and the jet flow. The amount of suction in the device was experimentally measured and compared to the computer simulations, finding good agreement between the two results. Finally, the impact that changing some of the dimensions in the device had on the suction pressure was explored. The most important geometric feature was the distance between the start of the jet flow, and the wall it impacted on.
Nielsen, Peter, "Validation of rhoCentralFoam for Engineering Applications of Under-Expanded Impinging Free Jets" (2019). Electronic Thesis and Dissertation Repository. 6789.