Numerical Simulation of Turbulent Flow Through a Straight Square Duct

Turbulent duct flows are investigated using large eddy simulation at bulk Reynolds numbers, from 4410 to 250,000. Mean secondary flow is found to reveal the existence of two streamwise counter-rotating vortices in each corner of the duct. Turbulence-driven secondary motions that arise in duct flows act to transfer fluid momentum from the centre of the duct to its corners, thereby causing a bulging of the streamwise velocity contours towards the corners. As Reynolds number increases, the ratio of centreline streamwise velocity to the bulk velocity decreases and all turbulent components increase. In addition, the core of the secondary vortex in the lower corner-bisector tends to approach the wall and the corner with increasing Reynolds number. The turbulence intensity profiles for the low Reynolds number flows are quite different from those for the high Reynolds number flows. Typical turbulence structures in duct flows are found to be responsible for the interactions between ejections from wall and this interaction results in the bending of the ejection stems, which indicates that the existence of streaky wall structures is much like in a channel flow

Adaptive Detached Eddy Simulation and Passive Scalar Transport Modeling for Hybrid RANS/LES

The work described in this dissertation, follows the attempt made in Reddy et al. (2014a), to make Detached Eddy Simulation model more like traditional LES in eddy simulation region. Work done by Reddy et al. (2014a) proposed the l^2w DDES model that shares a similar formulation with Smagorinsky model in eddy simulation region. In the present research, an adaptive procedure was devised (Yin et al., 2015), to allow automatic adjustment of a model coefficient CDES to flow condition and grid resolution. The adaptive method is based on the Germano identity, and on a lower limiting value that is a function of the grid resolution and the Kolmogoroff length scale. The function, being a gauge for grid resolution, allows the model coefficient to be computed dynamically, wherever suitable. To extend adaptive DES to compressible flow and heat transfer, a passive scalar transport model is proposed for Hybrid RANS/LES (Yin and Durbin, 2016b). This too is an adaptive model. Adaptivity is based on computing test-filter fluxes. The formulation proves to be especially effective on coarse grids, as occur in DES. Under the principle that DES should converge to wall-resolved LES as the mesh becomes fine near the wall, a modification is made on the adaptive DES model to make this limit feasible (Yin and Durbin, 2016a). The modification is to the limiting function. It is found that the RANS region shrinks to y+ ∼ 5 on fine meshes, thus allowing the model to be almost equivalent to wall-resolved LES. One place where the wall-resolved asymptote can play a role is in laminar to turbulent transition. Both the original and modified formulation are tested in orderly, bypass, and separation induced transition. Three separated test cases are also included here: a series of 3-D diffusers, jet in cross flow, and rotating channel flows | for more elaborate testing of proposed model. The 3-D diffuser series, reveals that the adaptive method in Yin et al. (2015) has discrepancies with LES even on fine meshes, which partially motivates the revised model (Yin and Durbin, 2016a). The JICF test case validated both the passive scalar transport model in Yin and Durbin (2016b) and the revised model in Yin and Durbin (2016a). And rotating channel flow gives a more detailed assessment of the revised model. In summary, adaptive DES and passive scalar transport models are proposed. They adapt to flow and geometry. The passive scalar transport model is compatible with both wall-resolved LES and hybrid RANS/LES models

Investigation of the 3D flow characteristics in a rotating channel setup

Tableau d'honneur de la Faculté des études supérieures et postdoctorales, 2006-200

Institute for Computational Mechanics in Propulsion (ICOMP)

The Institute for Computational Mechanics in Propulsion (ICOMP) is a combined activity of Case Western Reserve University, Ohio Aerospace Institute (OAI) and NASA Lewis. The purpose of ICOMP is to develop techniques to improve problem solving capabilities in all aspects of computational mechanics related to propulsion. The activities at ICOMP during 1991 are described

Large Eddy Simulations of complex turbulent flows

In this dissertation a solution methodology for complex turbulent flows of industrial interests is developed using a combination of Large Eddy Simulation (LES) and Immersed Boundary Method (IBM) concepts. LES is an intermediate approach to turbulence simulation in which the onus of modeling of “universal” small scales is appropriately transferred to the resolution of “problem-dependent” large scales or eddies. IBM combines the efficiency inherent in using a fixed Cartesian grid to compute the fluid motion, along with the ease of tracking the immersed boundary at a set of moving Lagrangian points. Numerical code developed for this dissertation solves unsteady, filtered Navier-Stokes equations using high-order accurate (fourth order in space) finite difference schemes on a staggered grid with a fractional step approach. Pressure Poisson equation is solved using a direct solver based on a matrix diagonalization technique. Second order accurate Adams-Bashforth scheme is used for temporal integration of equations. Dynamic mixed model (DMM) is used to model subgrid scale (SGS) terms. It can represent large scale anisotropy and back-scatter of energy from small-to-large scale through scale-similar term and maintain the energy drain through eddy viscosity term whose coefficient is allowed to change with in the computational domain. This code is validated for several bench-mark problems and is demonstrated to solve complex moving geometry problem such as stator-rotor interaction. A number of parametric studies on jets-in-crossflow are performed to understand complex fluid dynamics issues pertaining to film-cooling. These studies included effects of variation of hole-aspect ratio, jet injection angle, free-stream turbulence intensity and free-stream turbulence length scales on the coherent structure dynamics for jets-in-crossflow. Fundamental flow physics and heat transfer issues are addressed by extracting coherent structures from time-dependent three dimensional flow fields of film-cooling by inclined jet and studying their influence on the film-cooled surface heat transfer. A direct method to perform heat transfer calculations in periodic geometries is proposed and applied to internal cooling in rotating ribbed duct. Immersed boundary method is used to render complex geometry of trapped vortex combustor on Cartesian grid and fluid mixing inside trapped vortex cavity is studied in detail