@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{Meiselbach2015, author = {Meiselbach, Falko T.}, title = {Application of ODT to turbulent flow problems}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-34952}, school = {BTU Cottbus - Senftenberg}, year = {2015}, abstract = {The continual optimization process for more efficiency of industrial flows has raised the need for providing deeper understanding of turbulence. These details can be provided by direct numerical simulation (DNS), which is impossible for most flows with current computers. Therefore, progress in optimizing Reynolds averaged Navier-Stokes (RANS) and large eddy simulation (LES) modeling strategies will need to continue. Another ansatz is the reduction to 2D or 1D models to reduce the numerical cost. One dimensional turbulence (ODT) as presented by A. R. Kerstein is a new modeling strategy that reduces the 3D simulation to a 1D line of sight through the flow region. Due to the higher resolution afforded by the 1D model, it is possible to simulate even the smallest scales and to provide insight into turbulence statistics. To assess the advantages and disadvantages of the model, ODT has to be validated against several flows. Within this thesis, ODT is validated against the channel flow, the passive scalar transport and the channel flow with a fluctuating pressure gradient. These flows are simplified test cases for the phenomena present in single-phase industrial flows. ODT produces meaningful results for friction Reynolds numbers up to Reτ = 6·10⁵ and for Prandtl numbers from Pr = 0.025 to 50. Statistics of the wall shear stress are presented and the influence of pressure fluctuations is discussed. Based on these channel results, the non-breaking and breaking jet are simulated. While the former is a simplified case of a free-surface flow, the latter is of primary interest for spray formation and fuel injection. Detailed statistics of the TKE budgets and the breakup are presented. As the last case, the cloud top of a stratocumulus-topped boundary layer (STBL) was simulated. The case combines the interaction of an active and a passive scalar. It further combines the simulation of a stable and an unstable stratified region that suppresses and enhances turbulence respectively. The simulations reproduce the entrainment velocity and generate comparable mean and flux profiles compared to DNSs.}, subject = {One-Dimensional Turbulence; ODT; Channel flow; Heat transfer; Smoke cloud; One-Dimensional Turbulence; ODT; Kanalstr{\"o}mung; W{\"a}rme{\"u}bertragung; Rauchwolke; Turbulente Str{\"o}mung; Numerische Str{\"o}mungssimulation}, language = {en} } @phdthesis{MedinaMendez2020, author = {Medina Mendez, Juan Ali}, title = {Application of the One-Dimensional Turbulence model to electrohydrodynamically enhanced internally forced convective flows}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-53388}, school = {BTU Cottbus - Senftenberg}, year = {2020}, abstract = {This thesis is an attempt to assess some of the effects that electroquasistatic body forces exert on turbulent internally forced convective flows. In order to do that, a stochastic turbulence model is employed, namely, the One-Dimensional Turbulence (ODT) model. The reduced dimensionality of ODT demands a reduction of the Navier-Stokes equations (and in this case, also the Maxwell equations), into a 1-D system. This is done by performing an asymptotic analysis in terms of the nondimensional numbers of the flow. Also, a validation step due to the relative novel character of the cylindrical ODT formulation is done for an incompressible and constant properties flow regime and a variable density flow regime. The validation is presented for both the temporal (T-ODT) and a novel spatial (S-ODT) formulation in both planar and cylindrical geometries. Results in the constant property case show that wall normal (and radial) profiles, in both the T-ODT and S-ODT formulations, show good agreement with each other and to the data of Direct Numerical Simulations (DNSs). For the evaluated variable density heated pipe flow, gradients at the wall can be better reproduced with S-ODT. After validating the model, ODT is applied first into a planar configuration which emulates the flow in a wire-plate Electrostatic Precipitator (ESP). For this flow, the additional input energy due to the electroquasistatic body force has an effect on the modification of the bulk velocity, and subsequently, the skin friction coefficient. Some qualitative DNS trends are confirmed with ODT, such as the localized increase of the Reynolds stress, as a consequence of increased eddy activity close to the discharge electrodes. Next, the results of ODT simulations in a cylindrical wire-tube ESP are presented. Here, ODT results are compared to experimental results. ODT results for global integral quantities such as the streamwise pressure gradient and the Nusselt number enhancement ratio are able to match in a reasonable way the experimental results. The competing relevance between the EHD contribution to turbulence by momentum, and by affecting the temperature and density due to the Joule heating effect is also analyzed, showing the leading order relevance of the former one. Specifically for the Nusselt number results, the sensitivity of the EHD flow to transition effects is shown to be very significant. This thesis may open the door to a vast new field of phenomena which can not only serve for the further validation of the ODT model against DNSs or experiments, but also for the real use of ODT in applications which are so far inaccessible for traditional DNSs.}, subject = {EHD; ODT; Forced convection; Turbulente Str{\"o}mung; Numerische Str{\"o}mungssimulation; Elektrohydrodynamik; Abscheider; Turbulenzmodell}, 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} } @phdthesis{Rakhi2023, author = {Rakhi,}, title = {Stochastic modeling of the turbulent boundary layer}, doi = {10.26127/BTUOpen-6416}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-64162}, school = {BTU Cottbus - Senftenberg}, year = {2023}, abstract = {The incompressible temporally developing turbulent boundary layer (TBL) and spatially developing turbulent boundary layer (SBL) with and without blowing is analyzed using the map-based stochastic one-dimensional turbulence (ODT) model. An understanding of these idealized flows is of fundamental relevance for boundary layer-type problems, which are frequently encountered in several applications, from atmospheric sciences to engineering. In the ODT model, the flow variables are resolved on all scales along a wall-normal, 1-D domain. These variables are evolved by a deterministic process representing molecular diffusion and by a stochastic process modeling the effect of turbulent advection and pressure fluctuations. Due to the reduction in dimensionality, the model is particularly appropriate for high Reynolds number flow. It is shown that the ODT model is able to capture salient features of the turbulent boundary layer-type flows by comparing the results with various available reference direct numerical simulation (DNS), large eddy simulation (LES) and experimental results. The comparison is presented for the mean velocity profiles, turbulent velocity fluctuation profiles (up to fourth order), the skin friction coefficient and shape factor for different bulk (Reb) and momentum Reynolds numbers (Reθ) using fixed model parameters. The influence of the model parameters is also discussed for various momentum Reynolds numbers for each investigated flow configuration. The results discussed in this thesis suggest that the ODT model is an economical and reasonably accurate approach for the simulation of turbulent boundary layer flows.}, subject = {Turbulent flow; Turbulent boundary layer; Stochastic modeling; One-dimensional turbulence; Turbulente Str{\"o}mung; Turbulente Grenzschicht; Stochastische Modellierung; Eindimensionale Turbulenz; Turbulente Str{\"o}mung; Turbulente Grenzschicht; Statistisches Modell}, language = {en} } @phdthesis{Sharma2019, author = {Sharma, Sparsh}, title = {Stochastic modelling of leading-edge noise in time-domain using vortex particles}, doi = {10.26127/BTUOpen-5085}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-50858}, school = {BTU Cottbus - Senftenberg}, year = {2019}, abstract = {The conceptual designing of rotating machines such as fans, wind turbines, contra-rotating open rotors and helicopter blades require low-cost, easy-to-run tools which allow quick noise assessments and optimization analyses underlying this phenomenon. The state-of-the-art numerical and experimental methods are far more expensive to conduct an optimisation study, whereas inexpensive methods like the analytical ones can have significant errors in realistic geometries at high-frequency ranges, higher angles of attack. The response to large coherent disturbances and the statistical modeling of turbulence is required because turbulence, by far its nature, is stochastic. Determining the accurate unsteady response of airfoil is crucial for noise prediction. The primary goal of the project is to develop a new low-cost and easy-to-use numerical technique for aero-acoustic designs, focused primarily on airfoil-turbulence interaction. The development of the statistical method is divided into three sections; namely - 1) calculating the background flow, 2) modeling of statistically optimized inflow disturbance, 3) constructing a vortex database to predict the noise in multiple flow fields characterized by different values of turbulent intensities and length scales. In the framework of this work a new approach to model inflow turbulence, a significant noise-generating element, is suggested, which does not depend on heavy computations requiring supercomputers. Through this approach, the influence of turbulence parameters on the noise generated in turbomachinery can be quantified. The approach also considers the geometrical parameters of the airfoil in the noise prediction. The background flow is numerically simulated via solving the vorticity transport equations in the Lagrangian form (vortex methods). The acoustic influence of a finite number of vortices, characterized by all the possible combinations of size, circulation and injection position/time defined using the ranges of probability distribution functions, released from injection points upstream of the airfoil are precomputed and stored in a matrix. The method is computationally inexpensive compared to classical vortex methods since the effect due to particles are precomputed, stored in a/an matrix/array. The matrix can be called as a library while predicting the noise from a specific airfoil.}, subject = {Leading-edge noise; Aeroacoustics; Ffowcs-Williams Hawkings; Broadband noise; Airfoil-turbulence interaction; Tragfl{\"u}gelvorderkantenschalls; Breitbandger{\"a}usch; Tragfl{\"a}chenger{\"a}usch; Aeroakustik; Zeitbereich; Tragfl{\"u}gel; Vorderkante; Tragfl{\"u}gelumstr{\"o}mung; Str{\"o}mungsakustik}, language = {en} }