Effects of diffusion in hypersonic flow

Gür, Hilmi Berk
Hypersonic flows become popular due to its use in space explorations and military applications. It occurs normaly, when a space craft re-enter to the atmosphere. Space crafts are exposed to high temperatures and high pressures because of Earth’s atmposhere and gravity. In addition in high temperatures, gases tend to react with each other. These reactions also have effects on space craft’s surface. In order to calculate these effects, all physical phenomena must be calculated. One of these phenomena is diffusion. Diffusioncanbeidentifiedasmovementofthespecies(O2,N2,ie.). There are various models for diffusion. Fick’s Law of Diffusion is simplest one. In Fick’s Law, concentration differences of the species are taken as a driving force. However,therearemoreeffectsratherthanconcentrationdifferences. Therefore,themost frequently applied model is the Stefan-Maxwell Diffusion Equation. In this model, diffusion fluxes are calculated not only with concentration differences but also, with diffusion fluxes of other species. In diffusion calculation, diffusion coefficient or diffusivity plays an important role as conductivity constant. One method to calculate diffusivityisConstantLewisNumbermethod. Itisassumedthat,Lewisnumberdoes not change with the temperature gradient. Bertin [4] used more complex and accurate model. He calculated diffusivity with the effect of molecular properties. Lastly, Stefan-Maxwell calculated diffusivity with the effect of different species and their collision. These calculations are examined with each cases. In this thesis, Apollo AS-202 Command Module is selected as a re-entry space craft. Structural grid is applied around the geometry. Newton-GMRES method is used to solve flow field. Different CFD considerations are used in order to calculate the physical effects accurately in terms of numerical efficiency. The solution must be independent of grid size. Therefore four different grids are used; course, medium and fine grid, to analyze grid independence. Van Leer flux splitting method is selected as Flux splitting method. In this study diffusion effects in hypersonic flow are studied and compare it with experimental results and calculation with different models.  
Citation Formats
H. B. Gür, “Effects of diffusion in hypersonic flow,” M.S. - Master of Science, Middle East Technical University, 2016.