scholarly journals On regime C flow around an oscillating circular cylinder

2018 ◽  
Vol 849 ◽  
pp. 968-1008 ◽  
Author(s):  
Chengwang Xiong ◽  
Liang Cheng ◽  
Feifei Tong ◽  
Hongwei An
Keyword(s):  
Circular Cylinder ◽  
Large Scale ◽  
Three Dimensional ◽  
Domain Size ◽  
Wave Pattern ◽  
Travelling Wave ◽  
Marginal Stability ◽  
Computational Domain ◽  
Mode Iii ◽  
Vortex Pattern

This paper focuses on the characteristics of the regime C flow (Tatsuno & Bearman, J. Fluid Mech., vol. 211, 1990, pp. 157–182) around an oscillating circular cylinder in still water. The regime C flow is characterised by the formation of large-scale vortex cores arranged as opposed von Kármán vortex streets, resulting from a regular switching of vortex shedding directions with respect to the axis of oscillation. Both Floquet analysis and direct numerical simulations (DNS) are performed to investigate the two- (2-D) and three-dimensional (3-D) instabilities. The present study reveals that the low-wavenumber 3-D instability can emerge slightly before the 2-D instability in regime C. In total, five spanwise vortex modes were identified: (i) standing-wave pattern, S-mode; (ii) travelling-wave pattern, T-mode; (iii) mixed ST-mode; (iv) X-type vortex pattern, X-mode; and (v) U-type vortex pattern, U-mode. The modal analysis conducted in this study demonstrates that the vortex patterns and the corresponding spatial and temporal modulations of the dynamic loads of the S-, T- and mixed ST-modes are mainly induced by the 3-D instability of a single wavenumber. The characteristics of the X-mode are due to the superposition of the 3-D instabilities of multiple wavenumbers. The U-mode is dominated by a 2-D instability and its interaction with 3-D instabilities. The domain size dependence study demonstrates that the regime C flow is very sensitive to the spanwise length of the computational domain. The subcritical nature of the regime C flow is responsible for the discrepancy in the marginal stability curves obtained by independent Floquet stability analysis, DNS and physical experiments.

Multi-Objective Optimization of a Microturbine Compact Recuperator

2008 ◽  
Vol 130 (3) ◽  
Author(s):  
Diego Micheli ◽  
Valentino Pediroda ◽  
Stefano Pieri
Keyword(s):  
Heat Transfer ◽  
Three Dimensional ◽  
Numerical Procedure ◽  
Design Procedure ◽  
Domain Size ◽  
Multidisciplinary Optimization ◽  
Computational Domain ◽  
Surface Geometry ◽  
Multi Objective ◽  
Flux Boundary Conditions

An automatic approach for the multi-objective shape optimization of microgas turbine heat exchangers is presented. According to the concept of multidisciplinary optimization, the methodology integrates a CAD parametric model of the heat transfer surfaces, a three-dimensional meshing tool, and a CFD solver, all managed by a design optimization platform. The repetitive pattern of the surface geometry has been exploited to reduce the computational domain size, and the constant flux boundary conditions have been imposed to better suit the real operative conditions. A new approach that couples cold and warm fluids in a periodic unitary cell is introduced. The effectiveness of the numerical procedure was verified comparing the numerical results with available literature data. The optimization objectives are maximizing the heat transfer rate and minimizing both friction factor and heat transfer surface. The paper presents the results of the optimization of a 50kWMGT recuperator. The design procedure can be effectively extended and applied to any industrial heat exchanger application.

Study of Wake Profiles of a Simplified Model of High Speed Train Using RANS and LES Turbulent Models

2014 ◽  
Vol 629 ◽  
pp. 426-430
Author(s):  
Sufiah Mohd Salleh ◽  
Mohamed Sukri Mat Ali ◽  
Sheikh Ahmad Zaki Shaikh Salim ◽  
Sallehuddin Muhamad ◽  
Muhammad Iyas Mahzan
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Sudden Change ◽  
Domain Size ◽  
Recirculation Region ◽  
Bluff Bodies ◽  
Computational Domain ◽  
High Speed Train ◽  
Free Shear Layer ◽  
Turbulent Models

Flow structure over bluff bodies is more complex in wake. The wake is characterized by the unsteady behavior of the flow, large scale turbulent structure and strong recirculation region. For the case of high speed train, wake can be observed at the gap between the coaches and also on the rear coach. Wakes formation of high speed train are generated by free shear layer that is originated from the flow separation due to the sudden change in geometry. RANS and LES turbulent models are used in this paper to stimulate the formation of wakes and behavior of the flow over a simplified high speed train model. This model consists of two coaches with the gap between them is 0.5D. A total of four simulations have been made to study the effect of computational domain size and grid resolution on wake profiles of a simplified high speed train. The result shows that the computational domain can be reduced by decreasing the ground distance to 1.5D without affecting the magnitude of the wake profile. Both RANS and LES can capture the formation of the wake, but LES requires further grid refinement as the results between the two grid resolutions are grid dependent.

scholarly journals Double-Diffusive Recipes. Part I: Large-Scale Dynamics of Thermohaline Staircases

2014 ◽  
Vol 44 (5) ◽  
pp. 1269-1284 ◽  
Author(s):  
T. Radko ◽  
A. Bulters ◽  
J. D. Flanagan ◽  
J.-M. Campin
Keyword(s):  
Numerical Simulations ◽  
Large Scale ◽  
Density Ratio ◽  
Three Dimensional ◽  
Flux Ratio ◽  
Double Diffusion ◽  
Spontaneous Generation ◽  
Computational Domain ◽  
The North ◽  
Basin Scale

Abstract Three-dimensional dynamics of thermohaline staircases are investigated using a series of basin-scale staircase-resolving numerical simulations. The computational domain and forcing fields are chosen to reflect the size and structure of the North Atlantic subtropical thermocline. Salt-finger transport is parameterized using the flux-gradient formulation based on a suite of recent direct numerical simulations. Analysis of the spontaneous generation of thermohaline staircases suggests that thermohaline layering is a product of the gamma instability, associated with the variation of the flux ratio with the density ratio . After their formation, numerical staircases undergo a series of merging events, which systematically increase the size of layers. Ultimately, the system evolves into a steady equilibrium state with pronounced layers 20–50 m thick. The size of the region occupied by thermohaline staircases is controlled by the competition between turbulent mixing and double diffusion. Assuming, in accordance with observations, that staircases form when the density ratio is less than the critical value of , the authors arrive at an indirect estimate of the characteristic turbulent diffusivity in the subtropical thermocline.

Coherent drift-wave structures in toroidal plasmas

1996 ◽  
Vol 56 (3) ◽  
pp. 605-613 ◽  
Author(s):  
W. Horton ◽  
T. Tajima ◽  
J.-Y. Kim ◽  
Y. Kishimoto ◽  
M. Ottaviani
Keyword(s):  
Large Scale ◽  
Three Dimensional ◽  
Marginal Stability ◽  
Ion Temperature ◽  
Ion Temperature Gradient ◽  
Toroidal Plasmas ◽  
Convective Structures ◽  
Drift Wave ◽  
Particle Simulations ◽  
Two Phases

Using the ion-temperature-gradient-driven drift waves as a paradigm for drift-wave anomalous transport, we explore the structure of the linear and nonlinear modes. Two phases of transport are shown to exist: (i) Bohm-like transport for parameters close to marginal stability; (ii) gyro-Bohm transport for turbulent convection cells in systems driven away from marginal stability. Nonlinear relaxation to large-scale coherent convective structures is observed in three-dimensional toroidal particle simulations.

scholarly journals Numerical simulation of turbulent convection over wavy terrain

1992 ◽  
Vol 237 ◽  
pp. 261-299 ◽  
Author(s):  
Kilian Krettenauer ◽  
Ulrich Schumann
Keyword(s):  
Numerical Simulation ◽  
Large Scale ◽  
Fluid Layer ◽  
Domain Size ◽  
Lower Surface ◽  
Constant Heat Flux ◽  
Computational Domain ◽  
Turbulent Structures ◽  
Convective Boundary ◽  
Scale Motion

Thermal convection of a Boussinesq fluid in a layer confined between two infinite horizontal walls is investigated by direct numerical simulation (DNS) and by large-eddy simulation (LES) for zero horizontal mean motion. The lower-surface height varies sinusoidally in one horizontal direction while remaining constant in the other. Several cases are considered with amplitude δ up to 0.15H and wavelength λ of H to 8H (inclination up to 43°), where H is the mean fluid-layer height. Constant heat flux is prescribed at the lower surface of the initially at rest and isothermal fluid layer. In the LES, the surface is treated as rough surface (z0 = 10−4H) using the Monin-Oboukhov relationships. At the flat top an adiabatic frictionless boundary condition is applied which approximates a strong capping inversion of an atmospheric convective boundary layer. In both horizontal directions, the model domain extends over the same length (either 4H or 8H) with periodic lateral boundary conditions.We compare DNS of moderate turbulence (Reynolds number based on H and on the convective velocity is 100, Prandtl number is 0.7) with LES of the fully developed turbulent state in terms of turbulence statistics and Characteristic large-scale-motion structures. The LES results for a flat surface generally agree well with the measurements of Adrian et al. (1986). The gross features of the flow statistics, such as profiles of turbulence variances and fluxes, are found to be not very sensitive to the variations of wavelength, amplitude, domain size and resolution and even the model type (DNS or LES), whereas details of the flow structure are changed considerably. The LES shows more turbulent structures and larger horizontal scales than the DNS. To a weak degree, the orography enforces rolls with axes both perpendicular and parallel to the wave crests and with horizontal wavelengths of about 2H to 4H. The orography has the largest effect for λ = 4H in the LES and for λ = 2H in the DNS. The results change little when the size of the computational domain is doubled in both horizontal directions. Most of the motion energy is contained in the large-scale structures and these structures are persistent in time over periods of several convective time units. The motion structure persists considerably longer over wavy terrain than over flat surfaces.

scholarly journals Three-Dimensional Hydrostatic Curved Channel Flow Simulations Using Non-Staggered Triangular Grids

Water ◽  
2022 ◽  
Vol 14 (2) ◽  
pp. 174
Author(s):  
Wei Zhang ◽  
Miguel Uh Uh Zapata ◽  
Damien Pham Van Pham Van Bang ◽  
Kim Dan Nguyen
Keyword(s):  
Finite Volume Method ◽  
Finite Volume ◽  
Channel Flow ◽  
Large Scale ◽  
Three Dimensional ◽  
Ocean Modeling ◽  
Curved Channel ◽  
Triangular Grids ◽  
Vortex Pattern ◽  
Volume Method

Non-staggered triangular grids have many advantages in performing river or ocean modeling with the finite-volume method. However, horizontal divergence errors may occur, especially in large-scale hydrostatic calculations with centrifugal acceleration. This paper proposes an unstructured finite-volume method with a filtered scheme to mitigate the divergence noise and avoid further influencing the velocities and water elevation. In hydrostatic pressure calculations, we apply the proposed method to three-dimensional curved channel flows. Approximations reduce the numerical errors after filtering the horizontal divergence operator, and the approximation is second-order accurate. Numerical results for the channel flow accurately calculate the velocity profile and surface elevation at different Froude numbers. Moreover, secondary flow features such as the vortex pattern and its movement along the channel sections are also well captured.

Multi-Objective Optimization of a Microturbine Compact Recuperator

2007 ◽  
Author(s):  
Diego Micheli ◽  
Valentino Pediroda ◽  
Stefano Pieri
Keyword(s):  
Heat Transfer ◽  
Three Dimensional ◽  
Numerical Procedure ◽  
Design Procedure ◽  
Domain Size ◽  
Computational Domain ◽  
Surface Geometry ◽  
Micro Gas Turbine ◽  
Multi Objective ◽  
Flux Boundary Conditions

An automatic approach for the multi-objective shape optimization of micro gas turbine heat exchangers is presented. According to the concept of Multi Disciplinary Optimization (MDO), the methodology integrates a CAD parametric model of the heat transfer surfaces, a three dimensional meshing tool and a CFD solver, all managed by a design optimization platform. The repetitive pattern of the surface geometry has been exploited to reduce the computational domain size, and constant flux boundary conditions have been imposed to better suit the real operative conditions. A new approach that couples cold and warm fluids in a periodic unitary cell is introduced. The effectiveness of the numerical procedure was verified comparing the numerical results with available literature data. The optimization objectives are maximizing the heat transfer rate and minimizing both friction factor and heat transfer surface. The paper presents the results of the optimization of a 50 kW MGT recuperator. The design procedure can be effectively extended and applied to any industrial heat exchanger application.

scholarly journals Three dimensional flows beneath a thin layer of 2D turbulence induced by Faraday waves

2020 ◽  
Vol 62 (1) ◽  
Author(s):  
Raffaele Colombi ◽  
Michael Schlüter ◽  
Alexandra von Kameke
Keyword(s):  
Large Scale ◽  
Spatial Scales ◽  
Fluid Motion ◽  
Three Dimensional ◽  
Wave Pattern ◽  
Velocity Fields ◽  
Surface Flow ◽  
Two Dimensional ◽  
Faraday Waves ◽  
2D Turbulence

Abstract Faraday waves occur on a fluid being subject to vertical shaking. Although it is well known that form and shape of the wave pattern depend on driving amplitude and frequency, only recent studies discovered the existence of a horizontal velocity field at the surface, called Faraday flow. This flow exhibits attributes of two-dimensional turbulence and is replicated in this study. Despite the increasing attention towards the inverse energy flux in the Faraday flow and other not strictly two-dimensional (2D) systems, little is known about the velocity fields developing beneath the fluid surface. In this study, planar velocity fields are measured by means of particle image velocimetry with high spatio-temporal resolution on the water surface and at different depths below it. A sudden drop in velocity and turbulent kinetic energy is observed at half a Faraday wavelength below the surface revealing that the surface flow is the main source of turbulent fluid motion. The flow structures below the surface comprise much larger spatial scales than those on the surface leading to very long-tailed temporal and spatial velocity (auto-) correlation functions. The three-dimensionality of the flow is estimated by the compressibility, which increases strongly with depth while the divergence changes its appearance from intermittent and single events to a large scale pattern resembling 2D cut-planes of convection rolls. Our findings demonstrate that the overall fluid flow beneath the surface is highly three-dimensional and that an inverse cascade and aspects of a confined 2D turbulence can coexist with a three-dimensional flow. Graphic abstract

scholarly journals Fast Particle Acceleration in Three-dimensional Relativistic Reconnection

2021 ◽  
Vol 922 (2) ◽  
pp. 261
Author(s):  
Hao Zhang ◽  
Lorenzo Sironi ◽  
Dimitrios Giannios
Keyword(s):  
Large Scale ◽  
Magnetic Energy ◽  
Three Dimensional ◽  
Domain Size ◽  
Lorentz Factor ◽  
High Energy ◽  
Upstream Region ◽  
Cross Sectional ◽  
High Energy Cosmic Rays ◽  
Free Particles

Abstract Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ large-scale 3D particle-in-cell simulations of reconnection in magnetically dominated (σ = 10) pair plasmas to study the energization physics of high-energy particles. We identify an acceleration mechanism that only operates in 3D. For weak guide fields, 3D plasmoids/flux ropes extend along the z-direction of the electric current for a length comparable to their cross-sectional radius. Unlike in 2D simulations, where particles are buried in plasmoids, in 3D we find that a fraction of particles with γ ≳ 3σ can escape from plasmoids by moving along z, and so they can experience the large-scale fields in the upstream region. These “free” particles preferentially move in z along Speiser-like orbits sampling both sides of the layer and are accelerated linearly in time—their Lorentz factor scales as γ ∝ t, in contrast to γ ∝ t in 2D. The energy gain rate approaches ∼eE rec c, where E rec ≃ 0.1B 0 is the reconnection electric field and B 0 the upstream magnetic field. The spectrum of free particles is hard, dN free / d γ ∝ γ − 1.5 , contains ∼20% of the dissipated magnetic energy independently of domain size, and extends up to a cutoff energy scaling linearly with box size. Our results demonstrate that relativistic reconnection in GRB and AGN jets may be a promising mechanism for generating ultra-high-energy cosmic rays.

scholarly journals Exploiting self-organized criticality in strongly stratified turbulence

2021 ◽  
Vol 933 ◽  
Author(s):  
Gregory P. Chini ◽  
Guillaume Michel ◽  
Keith Julien ◽  
Cesar B. Rocha ◽  
Colm-cille P. Caulfield
Keyword(s):  
Large Scale ◽  
Ad Hoc ◽  
Three Dimensional ◽  
Boussinesq Equations ◽  
Marginal Stability ◽  
Small Scale ◽  
Stratified Turbulence ◽  
Self Organized ◽  
Reduced Equations ◽  
Self Organized Criticality

A multiscale reduced description of turbulent free shear flows in the presence of strong stabilizing density stratification is derived via asymptotic analysis of the Boussinesq equations in the simultaneous limits of small Froude and large Reynolds numbers. The analysis explicitly recognizes the occurrence of dynamics on disparate spatiotemporal scales, yielding simplified partial differential equations governing the coupled evolution of slow large-scale hydrostatic flows and fast small-scale isotropic instabilities and internal waves. The dynamics captured by the coupled reduced equations is illustrated in the context of two-dimensional strongly stratified Kolmogorov flow. A noteworthy feature of the reduced model is that the fluctuations are constrained to satisfy quasilinear (QL) dynamics about the comparably slowly varying large-scale fields. Crucially, this QL reduction is not invoked as an ad hoc closure approximation, but rather is derived in a physically relevant and mathematically consistent distinguished limit. Further analysis of the resulting slow–fast QL system shows how the amplitude of the fast stratified-shear instabilities is slaved to the slowly evolving mean fields to ensure the marginal stability of the latter. Physically, this marginal stability condition appears to be compatible with recent evidence of self-organized criticality in both observations and simulations of stratified turbulence. Algorithmically, the slaving of the fluctuation fields enables numerical simulations to be time-evolved strictly on the slow time scale of the hydrostatic flow. The reduced equations thus provide a solid mathematical foundation for future studies of three-dimensional strongly stratified turbulence in extreme parameter regimes of geophysical relevance and suggest avenues for new sub-grid-scale parametrizations.

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