The Field of Flow Structures Generated by a Wave of Viscous Fluid Around Vertical Circular Cylinder Piercing the Free Surface

AbstractThe diffraction of water waves induced by a large-diameter, surface-piercing, vertical circular cylinder is studied numerically. The Navier-Stokes equations in primitive variables are considered for the simulation of a given wave case, and the technique is followed of the Direct Numerical Simulation (DNS). The criterion of the imaginary part of the complex-eigenvalue pair of the velocity-gradient tensor for the extraction of the flow vortical structures is applied to the computed fields, so unveiling a complex configuration of structures at the free surface, and at the cylinder external walls. The most energetic modes of the flow are further extracted from the DNS-simulated fields by using the Karhunen-Loève decomposition (KL). A “reduced” velocity field is reconstructed using the first three most energetic eigenfunctions of the decomposition, and its evolution is followed through a sequence of time steps

Master-modes in 3D turbulent channel flow

Turbulent flow fields can be expanded into a series in a set of basic functions. The terms of such series are often called modes. A master- (or determining) mode set is a subset of these modes, the time history of which uniquely determines the time history of the entire turbulent flow provided that this flow is developed. In the present work the existence of the master-mode-set is demonstrated numerically for turbulent channel flow. The minimal size of a master-mode set and the rate of the process of the recovery of the entire flow from the master-mode set history are estimated. The velocity field corresponding to the minimal master-mode set is found to be a good approximation for mean velocity in the entire flow field. Mean characteristics involving velocity derivatives deviate in a very close vicinity to the wall, while master-mode two-point correlations exhibit unrealistic oscillations. This can be improved by using a larger than minimal master-mode set. The near-wall streaks are found to be contained in the velocity field corresponding to the minimal master-mode set, and the same is true at least for the large-scale part of the longitudinal vorticity structure. A database containing the time history of a master-mode set is demonstrated to be an efficient tool for investigating rare events in turbulent flows. In particular, a travelling-wave-like object was identified on the basis of the analysis of the database. Two master-mode-set databases of the time history of a turbulent channel flow are made available online at http://www.dnsdata.afm.ses.soton.ac.uk/. The services provided include the facility for the code uploaded by a user to be run on the server with an access to the data

Simulação de grandes escalas de escoamentos turbulentos incompressíveis e compressíveis na presença de paredes

Orientadores: William Roberto Wolf, João Luiz Filgueiras de AzevedoTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia MecânicaResumo: O desenvolvimento de escoamentos turbulentos na presença de paredes ocorre tanto em aplicações industriais quanto na natureza. Essa classe de escoamentos turbulentos possui grandes diferenças comparando-se com turbulência livre e os avanços em técnicas experimentais e poder computacional estão permitindo entender mais profundamente sobre as estruturas turbulentas encontradas em tais escoamentos. Neste trabalho, estudamos escoamentos turbulentos incompressíveis e compressíveis em canais utilizando a técnica de simulação de grandes escalas. Escoamentos incompressíveis em canais com variações de forma são estudados com o propósito de se entender os efeitos de variação de gradiente de pressão e curvatura na formação de instabilidades de streaks. Além disso, os efeitos de variação de forma na anisotropia do escoamento também são investigados através de uma análise dos invariantes to tensor de Reynolds e do balanço espectral de energia cinética turbulenta. Escoamentos supersônicos em canais lisos também são analisados com o intuito de se compreender os efeitos de compressibilidade e número de Reynolds na turbulência e a importância dos modos hidrodinâmicos, térmicos e acústicos nas estruturas turbulentas coerentes. Para isso, uma análise dos efeitos de velocidade de advecção de estruturas de velocidade e propriedades termodinâmicas é apresentada. Finalmente, técnicas de análise modal são aplicadas para investigar a física das estruturas coerentes em escoamentos supersônicos na presença de paredes para diferentes números de Mach e ReynoldsAbstract: The development of turbulent flows in the presence of walls occurs in both industrial and nature applications. This class of turbulent flows has great differences when compared to free turbulence and the advances in experimental techniques and computational power allow a deeper understanding of the turbulent structures found in such flows. In this work, we study incompressible and compressible turbulent flows in channels using the large eddy simulation technique. Incompressible flows in channels with shape modifications are studied in order to understand the effects of pressure gradient variations and curvature in the formation of streak instabilities. In addition, the effects of shape variation on flow anisotropy are also investigated through an analysis of the anisotropic Reynolds stress tensor invariants and the turbulent kinetic energy spectral balance. Supersonic flows over smooth channels are also analyzed in order to understand the effects of compressibility and Reynolds number on turbulence and the importance of the hydrodynamic, thermal and acoustic modes in the coherent turbulent structures. This analysis is performed through an assessment of the advection velocities of the velocity components and thermodynamic properties in different regions of the boundary layer. Finally, flow modal decomposition techniques are applied to investigate the physics of coherent structures in supersonic wall-bounded flows for different Mach and Reynolds numbersDoutoradoTermica e FluidosDoutor em Engenharia Mecânica2014/07623-6; 2016/03985-6FAPES

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

An investigation of road vehicle rear surface contamination and its simulation

Vehicle surface contamination is an important design consideration as it affects aesthetic appearance, driver s vision and the performance of the onboard camera and sensor systems. This work investigates the soiling process and the requirements for credible numerical simulation of the rear surface contamination for two quarter-scale generic car-like bluff bodies, which represent a vehicle type particularly susceptible to this type of contamination. It was shown that the accurate prediction of the mean flow field is a prerequisite for credible soiling estimation. However, modelling the full unsteady behaviour of particle motion with concurrent particle tracking is crucial in order to accurately capture the details of soiling, which is highly unsteady in nature. It was shown that the spray generally entrains into the wake behind the core of the bottom wake vortex and the details of deposition are controlled by the wake structure and the size and orientation of the ring vortex. It was shown that the modelling of secondary spray processes, although computationally expensive, may be important as they tend to change the properties of spray and affect its dynamics. Consideration of realistic boundary conditions, such as the rotating tyres and ground motion was also shown to affect the spray. Wheel rotation increases deposition and the vertical distribution of spray on the base. The ground motion, on the other hand, leads to a wider contamination pattern. An investigation into the tyre spray modelling methods showed that the spray generated behind the contact patch is primarily responsible for the rear surface contamination. Therefore, this study suggested that the tyre spray model currently in use by industrial companies and researchers could potentially be simplified, which would result in a reduced computational effort and would speed up computations of vehicle surface contamination

Turbulent Boundary Layer Features via Lagrangian Coherent Structures, Proper Orthogonal Decomposition and Dynamic Mode Decomposition

High-speed stereo PIV-measurements have been performed in a turbulent boundary layer at Reθ of 9800 in order to elucidate the coherent structures. Snapshot proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) are used to visualize the flow structure depending on the turbulent kinetic energy and frequency content. The first six POD and DMD modes show the largest and the lowest amount of energy and frequency, respectively. Lagrangian coherent structure (LCS) based on the algorithm developed using the variational theory is also applied to track the flow via attracting and repelling trajectories. The shapes and the length of the trajectories show variation with increasing advection time. LCS trajectories are overlayed with the individual POD and DMD modes. Repelling and attracting lines cover the structure of these modes. Reconstructed flow fields from individual POD modes are also used to generate new LCS trajectories. The energy and frequency content have a direct impact on the length of the trajectories, where the longest reconstructed trajectories associate with the higher energy and lower frequency modes, and vise verse. The multiple intersection points between the repelling and attracting lines marked the low momentum regions