Direct Numerical Simulation of Incompressible Flows in Domains of Close Packed Spheres

This study aimed to investigate and quantify turbulent flow effects for incompressible, isothermal fluid flows in computational domains consisting of regularly packed spheres using high-fidelity computational fluid dynamics. The flow domains treated in this study are analogous in an idealized sense to those encountered in pebble bed based high temperature nuclear reactors. The quantification of turbulent flow effects serves two purposes. Firstly, it assists in the development of lower-fidelity engineering tools such as Reynolds averaged Navier-Stokes based methodologies. Secondly, the quantification of turbulent flow effects adds to our fundamental understanding of the physics of incompressible flows in complex geometries. The study was conducted using an open-source spectral element computational fluid dynamics code, Nek5000, which was used to perform a series of direct numerical simulations in several flow domains representing both theoretical geometries and idealized sections of a practical reactor core at low to moderate Reynolds numbers. Selected results include the development of a high-fidelity database of numerical data for an expanded unit-cell geometry, the identification of possible very low frequency temporal dynamics in domains featuring several close packed spheres, and the calculation of turbulence statistics in a domain approximating the near-wall region of a reactor core

Large-scale streaks in a turbulent bluff body wake

A turbulent circular disk wake database (Chongsiripinyo \& Sarkar, \textit{J. Fluid Mech.}, vol. 885, 2020) at Reynolds number $\textit{Re} = U_\infty D/\nu = 5 \times 10^{4}$ is interrogated to identify the presence of large-scale streaks - coherent elongated regions of streamwise velocity. The unprecedented streamwise length - until $x/D \approx 120$ - of the simulation enables investigation of the near and far wake. The near wake is dominated by the vortex shedding (VS) mode residing at azimuthal wavenumber $m=1$ and Strouhal number $\textit{St} = 0.135$. After filtering out the VS structure, conclusive evidence of large-scale streaks with frequency $\textit{St} \rightarrow 0$, equivalently streamwise wavenumber $k_x \rightarrow 0$ in the wake, becomes apparent in visualizations and spectra. These streaky structures are found throughout the simulation domain beyond $x/D \approx 10$. Conditionally averaged streamwise vorticity fields reveal that the lift-up mechanism is active in the near as well as the far wake, and that ejections contribute more than sweep to events of intense $-u'_xu'_r$. Spectral proper orthogonal decomposition (SPOD) is employed to extract the energy and the spatiotemporal features of the large-scale streaks. The streak energy is concentrated in the $m=2$ azimuthal mode over the entire domain. Finally, bispectral mode decomposition (BMD) is conducted to reveal strong interaction between $m=1$ and $\textit{St} = \pm 0.135$ modes to give the $m=2, \textit{St} = 0$ streak mode. Our results indicate that the self-interaction of the VS mode generates the $m=2, \textit{St} = 0$ streamwise vortices, which leads to streak formation through the lift-up process. To the authors' knowledge, this is the first study that reports and characterizes large-scale low-frequency streaks and the associated lift-up mechanism in a turbulent wake

Unsteady aerodynamics of single and tandem wheels

The major unsteady aerodynamic forces and major physics of a generic single wheel and tandem wheels are studied for the first time using wind tunnel tests. The wind-tunnel tests are performed in the 2.1 m × 1.5 m wind tunnel at the University of Southampton. The tandem-wheel configuration consists of two in-line wheels that can be tested at different inter-axis distances and various installation angles. A vibration test is performed in situ on the model assembly to validate the unsteady-load measurements. Mean and unsteady aerodynamic loads and on-surface pressures are measured. Particle Image Velocimetry is used to acquire the velocity fields in the wake downstream of the model and surface oil-flow technique is used to identify the flow features on the surface of the wheels. Proper Orthogonal Decomposition is also used to characterise the wake in terms of unsteady fluctuations. The results of the experiments on the tandem wheels show that higher values of inter-axis distance correspond to slightly higher total mean drag coefficients and remarkably lower drag coefficient RMS values. Higher installation angles are associated with higher mean drag coefficients but generally lower fluctuations of the force coefficients. Non-zero mean lift coefficients are found for low inter-axis distance configurations at zero installation angle. The flow on the single wheel and on the front wheel of the tandem wheels is affected by laminar-turbulent transitional features. The vortical structures past the tandem wheels consist of four vortices that detach from the tyre shoulders of the front wheel and interact with the rear wheel. The study and obtained databases contribute to the general understanding of the complex flow and help to improve engineering predication of the gear aerodynamic loads

Reduced Order Models for Hydrodynamic Analysis of Pipelines based on Modal Analysis and Machine Learning

Bluff bodies have extensive implementation in engineering. For instance, marine risers, jumpers, umbilicals, and bundle flowlines are the examples of the circular bluff bodies which are used in the field of offshore engineering. They can be designed both to be fixed or flexible supported and to be a single or tandem configurations. The subsea structures are subjected to the external hydrodynamic loads such as cyclic loads, vibrations, high pressure, etc. For example, one of the commonly observed damaging flow features associated with hydrodynamics of the flexible supported bluff bodies is vortex shedding. Therefore, investigation of the instantaneous flow structures and the hydrodynamic forces acting on the bluff bodies at the operational conditions need to be performed to prevent the degradation mechanisms and increase service life of the subsea structures. Nowadays, the modern techniques are used in order to analyse the flow field data with high efficiency. For example, neural networks (NNs) are trained with massive experimental or numerical simulation data to predict the spatial-temporal evolution of the dominant coherent structures of the flow field and structural behaviour can be considered as an alternative to the conventional computational fluid dynamics (CFD) simulations. In the present thesis, numerical investigations of the flow around cylindrical bluff bodies in the upper transition Reynolds number regime ( = 3.6 ∙ 10^6 ) are performed. Two pipeline operational conditions are considered. First one is tandem configuration of the two stationary pipelines subjected to steady flow. Second one is a pipeline undergoing the vortex-induced-vibrations (VIV) subjected to a steady current. Two-dimensional (2D) Unsteady Reynolds-Averaged-Navier-Stokes (URANS) equations combined with the standard k−w SST turbulence model are solved.  The open source CFD toolbox OpenFOAM v2012 is employed to perform the simulations. The Reduced Order Models (ROMs) which can provide a low-dimensional representation of the simulation data with reduced computational time and cost are designed. Dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) techniques are implemented for the first and second cases, respectively. In addition, further development of the ROMs for the VIV cylinder case is done by implementing the long short-term neural network (LSTM-NN). The neural network based model allows to make the predictions of the dominant hydrodynamic characteristics of the flow around the cylindrical bluff bodies subjected to a high Reynolds number flow at a future time instances with a reduced computational cost

On the wake dynamics of an oscillating cylinder via proper orthogonal decomposition

The coherent structures and wake dynamics of a two-degree-of-freedom vibrating cylinder with a low mass ratio at Re=5300 are investigated by means of proper orthogonal decomposition (POD) of a numerical database generated using large-eddy simulations. Two different reduced velocities of U*=3.0 and U*=5.5, which correspond with the initial and super-upper branches, are considered. This is the first time that this kind of analysis is performed in this kind of system in order to understand the role of large coherent motions on the amplification of the forces. In both branches of response, almost 1000 non-correlated in-time velocity fields have been decomposed using the snapshot method. It is seen that a large number of modes is required to represent 95% of the turbulent kinetic energy of the flow, but the first two modes contain a large percentage of the energy as they represent the wake large-scale vortex tubes. The energy dispersion of the high-order modes is attributed to the cylinder movement in the inline and cross-stream directions. Substantially different POD modes have been found in the two branches. While the first six modes resemble those observed in the static cylinder or in the initial branch of a one-degree of freedom cylinder in the initial branch, the modes not only contain information about the wake vortexes in the super-upper branch but also about the formation of the 2T vortex pattern and the Taylor–Görtler structures. It is shown that the 2T vortex pattern is formed by the interplay between the Taylor–Görtler stream-wise vortical structures and the cylinder movement and is responsible for the increase in the lift force and larger elongation in the super-upper branch.This work has been partially financially supported by the Ministerio de Economía, Industria y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación, Spain (Ref. PID2020-116937RB-C21, PID2020-116937RB-C22), and by the European High-Performance Computing Joint Undertaking (JU) under grant agreement No 956104. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Spain, France, Germany. O. Lehmkuhl work is financed by a Ramón y Cajal postdoctoral contract by the Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación, Spain (RYC2018-025949-I).Peer ReviewedPostprint (published version

Support interference of wind tunnel models: A selective annotated bibliography

This bibliography, with abstracts, consists of 143 citations arranged in chronological order by dates of publication. Selection of the citations was made for their relevance to the problems involved in understanding or avoiding support interference in wind tunnel testing throughout the Mach number range. An author index is included