Comparison of turbulent models for predicting the flow behavior in a mould during continuous casting of steel

V delu je opisana zgradba numeričnega modela in rezultati simulacije vodnega modela, ki posnema tok jekla pri procesu kontinuirnega ulivanja jekla. Simulacije napovedujejo tokovne razmere v kokili, ki vplivajo na kakovost in učinkovitost tega procesa v industriji. Razvita sta geometrijski model vodnega modela kokile in 3D blok strukturirana prostorska diskretizacija. Z uporabo RANS pristopa za modeliranje turbulence ter z uporabo VOF modela za obravnavanje proste površine je izdelan tri-razsežni model. Skupaj s pravilno izbranimi računskimi parametri omogoča natančno, ponovljivo, od računske mreže neodvisno in numerično stabilno simulacijo hitrostnih in tlačnih razmer v vodnem modelu. Za modeliranje turbulence sta uporabljena modela realizacijski k-epsilon in SST k-omega. Izvedena je simulacija in analiza hitrostnih profilov in hitrostnih polj na izbranih lokacijah. Rezultati simulacij izbranih turbulentnih modelov so primerjani med seboj ter z rezultati eksperimentalnih podatkov, pridobljenih na podlagi meritev z metodo sledenja delcev. Primerjava rezultatov simulacij posameznih modelov z eksperimentom pokaže, da je k-omega model nekoliko primernejši za napovedovanje. Primerjava prav tako pokaže, da lahko z uporabo obeh turbulentnih modelov kvalitativno napovemo primarni tok v zgornjem in srednjem delu kokile. V spodnjem delu kokile pa se rezultati primarnega toka pri obeh modelih ne ujemajo povsem z eksperimentom. Čeprav k-epsilon in k-omega zadovoljivo opišeta primarni tok pa njuna uporaba ni primerna za dobro napovedovanje sekundarnega toka.In the thesis is the structure of the numerical model and the simulation results of a water model which imitates the flow of steel during continuous casting of steel process. The simulations predict the flow behaviour in the mould, which affects the quality and the efficiency of this process in the industry. A geometric model of mould\u27s water model and a 3D block structured spatial discretization are developed. Using the RANS approach for modeling turbulence and the VOF model to address the free surface, a three-dimensional model is developed. Together with accurately selected computational parameters, the model enables precise, robust, independent and numerically stable simulation of velocity and pressure conditions of the water model. The realizable k-epsilon and SST k-omega models are used to model the turbulence. A simulation and analysis of velocity profiles and velocity fields at selected locations is performed. The simulation results of both turbulent models are compared with each other and with the results of the particle image velocity based experimental data. The comparison of the simulation results of the individual models with the experiment shows that the k-omega model is slightly more suitable for prediction. The comparison also shows that both turbulent models can qualitatively predict the primary flow in the upper and middle section of the mould. However, in the lower section of the mould, the results of both models do not exactly match the experiment in the primary flow. Although k-epsilon and k-omega models adequately describe primary flow, their use is unsuitable for proper predicting of the secondary flow

RANS versus scale resolved approach for modeling turbulent flow in continuous casting of steel

Numerical modeling is the approach used most often for studying and optimizing the molten steel flow in a continuous casting mold. The selection of the physical model might very much influence such studies. Hence, it is paramount to choose a proper model. In this work, the numerical results of four turbulence models are compared to the experimental results of the water model of continuous casting of steel billets using a single SEN port in a downward vertical orientation. Experimental results were obtained with a 2D PIV (Particle Image Velocimetry) system with measurements taken at various cut planes. Only hydrodynamic effects without solidification are considered. The turbulence is modeled using the RANS (Realizable k-(epsilon), SST k-(omega)), hybrid RANS/Scale Resolved (SAS), and Scale Resolved approach (LES). The models are numerically solved by the finite volume method, with volume of fluid treatment at the free interface. The geometry, boundary conditions, and material properties were entirely consistent with those of the water model experimental study. Thus, the study allowed a detailed comparison and validation of the turbulence models used. The numerical predictions are compared to experimental data using contours of velocity and velocity plots. The agreement is assessed by comparing the lateral dispersion of the liquid jet in a streamwise direction for the core flow and the secondary flow behavior where recirculation zones form. The comparison of the simulations shows that while all four models capture general flow features (e.g., mean velocities in the temporal and spatial domain), only the LES model predicts finer turbulent structures and captures temporal flow fluctuations to the extent observed in the experiment, while SAS bridges the gap between RANS and LES

Time-resolved crystallography captures light-driven DNA repair

International audiencePhotolyase is an enzyme that uses light to catalyze DNA repair. To capture the reaction intermediates involved in the enzyme’s catalytic cycle, we conducted a time-resolved crystallography experiment. We found that photolyase traps the excited state of the active cofactor, flavin adenine dinucleotide (FAD), in a highly bent geometry. This excited state performs electron transfer to damaged DNA, inducing repair. We show that the repair reaction, which involves the lysis of two covalent bonds, occurs through a single-bond intermediate. The transformation of the substrate into product crowds the active site and disrupts hydrogen bonds with the enzyme, resulting in stepwise product release, with the 3′ thymine ejected first, followed by the 5′ base