Multiphase turbulence mechanisms identification from consistent analysis of direct numerical simulation data

Direct Numerical Simulation (DNS) serves as an irreplaceable tool to probe the complexities of multiphase flow and identify turbulent mechanisms that elude conventional experimental measurement techniques. The insights unlocked via its careful analysis can be used to guide the formulation and development of turbulence models used in multiphase computational fluid dynamics simulations of nuclear reactor applications. Here, we perform statistical analyses of DNS bubbly flow data generated by Bolotnov (Reτ= 400) and Lu–Tryggvason (Reτ= 150), examining single-point statistics of mean and turbulent liquid properties, turbulent kinetic energy budgets, and two-point correlations in space and time. Deformability of the bubble interface is shown to have a dramatic impact on the liquid turbulent stresses and energy budgets. A reduction in temporal and spatial correlations for the streamwise turbulent stress (uu) is also observed at wall-normal distances of y+= 15, y/δ = 0.5, and y/δ = 1.0. These observations motivate the need for adaptation of length and time scales for bubble-induced turbulence models and serve as guidelines for future analyses of DNS bubbly flow data. Keywords: Budget Equations, Bubble-Induced Turbulence, DNS, M&C2017, Multiphase CFDUnited States. Department of Energy. Naval Reactors Division (Rickover Fellowship Program in Nuclear Engineering

Extending bubble-induced turbulence modeling applicability in CFD through incorporation of DNS understanding

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 147-153).Precise knowledge and understanding of the multiphase flow distribution is essential for light water reactor design. Multiphase Computational Fluid Dynamics (M-CFD) modeling and simulation techniques provide three-dimensional resolution of complex flow structures, which can be used to improve operation and safety in current systems, while driving optimization and performance enhancement in next generation designs. Introducing bubbles into liquid flow dramatically modifies the turbulent kinetic energy profile. Examination of experimental and Direct Numerical Simulation (DNS) research reveals a complicated, incomplete, and conflicting picture of bubble-induced turbulence (BIT). Incorporating these physical mechanisms into a BIT model compatible within the Eulerian-Eulerian framework remains a formidable challenge. Two-equation BIT models share a general formulation, manifesting as additional source terms in traditional turbulence models to account for production and dissipation of bubble-induced turbulence. Existing formulations struggle with reliably predicting the turbulent kinetic energy profile, routinely yielding non-physical results that subsequently worsen mean flow predictions. The present work encompasses two research objectives that include (1) advancing the understanding of the complex effects bubbles pose on liquid turbulence, and (2) proposing an approach to incorporate these physical phenomena into a BIT closure relation. Greater understanding of two-phase turbulent mechanisms is advanced through statistical analysis of upward bubbly channel flow DNS data generated by Bolotnov and Lu/Tryggvason. The impact of bubble deformability on the resulting turbulent distributions, energy budgets, and scales are quantified and examined. A methodology that incorporates these fundamental mechanisms into a new BIT model is proposed. The closure comprises five components that include new turbulent viscosity and time-scale formulations in addition to optimized values for the modulation parameter, dissipation coefficient, and newly proposed turbulent viscosity multiplier. Model performance and improvement is confirmed through simulation of the entire Liu (1989) experimental database and comparison with existing closures. The model is incorporated into the Bubbly And Moderate void Fraction (BAMF) formulation (Sugrue et al., 2017) in order to deliver best practices and guidelines for application of momentum closures with BIT modeling. This is accomplished through redefinition of the Wobble number, calibration of expressions for the turbulent dispersion coefficient and lift inversion function, and assessment via simulation of experimental databases.by Benjamin Lawrence Magolan.Ph. D

Implementation of a non-linear eddy viscosity turbulence model into Hydra-TH for fuel related applications

Thesis: S.M., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 91-93).A quadratic [kappa]-[epsilon]non-linear eddy viscosity model (NLEVM) has been implemented into Hydra-TH, a Computational Fluid Dynamics (CFD) software being developed to support the nuclear reactor thermal-hydraulics modeling and simulation needs of the CASL program. This model adopts a non-linear extension of the stress-strain relationship that allows it to capture the anisotropy of flow conditions. Modeling this behavior is essential for the accurate simulation and prediction of the flow profile in fuel rod arrays, where secondary flow vortices arise and act to modify the flow profile. The quadratic model formulation in the greater context of Reynolds-Averaged Navier- Stokes (RANS) turbulence modeling is first presented. This is followed by a discussion of the key aspects of the standard and quadratic [kappa]-[epsilon] model implementations, which have been incorporated into the Hydra-TH source code to supplement the already fully-functioning RNG [kappa]-[epsilon] model. The three [kappa]-[epsilon] model variants are then applied to the 'classic' engineering test cases of flow in a square duct and a U-channel bend in order to highlight the relative merits and deficiencies of the quadratic model. Next, the quadratic model is validated on triangular and square rod fuel array test cases that are representative of the flow profile that develops in nuclear reactor subchannels. This thesis concludes with a rigorous sensitivity study of the triangular fuel rod array simulations, whereby guidelines and best practices for the quadratic model's use for nuclear fuel related applications are derived.by Benjamin Lawrence Magolan.S.M