@phdthesis{Jozefik2016, author = {Jozefik, Zoltan}, title = {Application of ODT to turbulent combustion problems in incompressible and compressible regimes}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-38653}, school = {BTU Cottbus - Senftenberg}, year = {2016}, abstract = {The one-dimensional turbulence (ODT) model is applied to a reactant - to - product counterflow configuration as well as to a shock tube configuration in non-reactive flow and in deflagration and detonation regimes. The model employed herein solves conservation equations for momentum, energy, and species on a one dimensional (1D) domain corresponding to the line spanning the domain between nozzle orifice centers in the counterflow configuration and corresponding to the tube length in the shock tube configuration. The effects of turbulent mixing are modeled via a stochastic process, while the Kolmogorov and reactive length and time scales are explicitly resolved. In the counterflow configuration, comparisons between model and DNS results for spatial mean and root-mean-square (RMS) velocity, temperature, and major and minor species profiles are shown. The ODT approach shows qualitatively and quantitatively reasonable agreement with the DNS data. Scatter plots and statistics conditioned on temperature are also compared for heat release rate and all species. ODT is able to capture the range of results depicted by DNS. However, conditional statistics show signs of underignition. To carry out the shock tube simulations, the ODT methodology is extended to include an efficient compressible implementation and a model for capturing shock-induced turbulence is presented. The necessary algorithmic changes to include compressibility effects are highlighted and the model for capturing shock-turbulence interaction is presented. To validate the compressible solver, results for Sod's shock tube problem are compared against a finite volume Riemann solver. To validate the model for shock-turbulence interaction, comparisons for a non-reactive and a reactive case are presented. First, results of a shock traveling from light (air) to heavy (SF6) with reshock have been simulated to match mixing width growth data of experiments and turbulent kinetic energy results from LES. Then, for one-step chemistry calibrated to represent an acetylene/air mixture, the interaction of a shock wave with an expanding flame front is simulated, and results with 2D simulation (2D-sim) data for flame brush formation and ensuing deflagration-to-detonation transitions (DDT) are compared. Results for the Sod shock tube comparison show that the shock speed and profile are captured accurately. Results for the nonreactive shock-reshock problem show that interface growth at all simulated Mach numbers is captured accurately and that the turbulent kinetic energy agrees in order of magnitude with LES data. The reactive shock tube results show that the flame brush thickness compares well to 2D-sim data and that the approximate location and timing of the DDT can be captured. The known sensitivity of DDT characteristics to details of individual Wow realizations, seen also in ODT, implies that model agreement can be quantified only by comparing Wow ensembles, which are presently unavailable other than in an ODT run-to-run sensitivity study that is reported herein.}, subject = {Turbulence; Combustion modeling; One dimensional turbulence (ODT); Counterflow; Shock tube; Turbulenz; Verbrennungsmodellierung; Gegenstrom; Stoßrohr; Turbulente Str{\"o}mung; Gegenstr{\"o}mung; Stoßwellenrohr; Simulation}, language = {en} } @phdthesis{Kadasch2016, author = {Kadasch, Eckhard}, title = {Controlling entrainment in large-eddy simulation of stratocumulus clouds}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-40393}, school = {BTU Cottbus - Senftenberg}, year = {2016}, abstract = {A front-tracking algorithm for large-eddy simulation (LES) is developed to untangle the numerical and physical contributions to entrainment in stratocumulus-topped boundary layers. The front-tracking algorithm is based on the level set method. Instead of resolving the cloud-top inversion, it is represented as a discontinuous interface separating the boundary layer from the free atmosphere. The location of the interface is represented as an isosurface of an evolving marker function the evolution of which is governed by an additional transport equation. The algorithm has been implemented in an existing LES code based on the anelastic approximation of the Navier-Stokes equations. The original LES algorithm is verified against direct-numerical simulation (DNS) data of an idealized two-dimensional cloud-top mixing layer. For this, the subgrid-scale model of the LES code was replaced by a constant molecular viscosity in order to focus on numerical errors only. A grid convergence study confirmed the anticipated global second-order rate of convergence and the convergence to the DNS solution. The slower convergence of the LES code as compared to the higher-order DNS yielded leading-order errors in the mixing layer growth at the coarsest resolutions, which were finer still than typical LES resolutions. The front-tracking algorithm is verified by LESs of two different convective atmospheric boundary layers: the smoke cloud, a solely radiatively driven boundary layer, and a stratocumulus-topped boundary layer based on data from the DYCOMS II field study. Specifying zero entrainment, it was shown that entrainment in LES can be controlled effectively by the front-tracking algorithm. The algorithm drastically reduces entrainment errors and reduces dependencies of the solution to numerical parameters such as the choice of flux-limiters and grid resolution.}, subject = {Entrainment; Large-eddy simulation; Level set method; Stratocumulus; Atmospheric boundary layer; Atmosph{\"a}rische Grenzschicht; Entrainment; Large Eddy Simulation; Level Set Methode; Stratocumulus; Kumulus; Atmosph{\"a}rische Grenzschicht; LES }, language = {en} }