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.

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.

Direct numerical simulation of a turbulent flow in a rotating channel with a sudden expansion

2014 ◽  
Vol 745 ◽  
pp. 92-131 ◽  
Author(s):  
Eric Lamballais
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Rotation Rate ◽  
Large Scale ◽  
Expansion Ratio ◽  
Sudden Expansion ◽  
Free Shear Layer ◽  
Turbulent Structures ◽  
Recirculation Bubble ◽  
Spanwise Rotation

AbstractThe effects of spanwise rotation on the channel flow across a symmetric sudden expansion are investigated using direct numerical simulation. Four rotation regimes are considered with the same Reynolds number$\mathit{Re}=5000$and expansion ratio$\mathit{Er}=3/2$. Upstream from the expansion, inflow turbulent conditions are generated realistically for each rotation rate through a very simple and efficient technique of recycling without the need for any precursor calculation. As the rotation is increased, the flow becomes progressively asymmetric with stabilization (destabilization) effects on the cyclonic (anticyclonic) side, respectively. These rotation effects, already present in the upstream channel, lead further downstream to an increase (reduction) of the separation size behind the cyclonic (anticyclonic) step. In the cyclonic separation, the free-shear layer created behind the step corner leads to the formation of large-scale spanwise vortices that become increasingly two-dimensional as the rotation is increased. Conversely, in the anticyclonic region, the turbulent structures in the separated layer are more elongated in the streamwise direction and also more active in promoting reattachment. For the highest rotation rate, a secondary separation is observed further downstream in the anticyclonic region, leading to the establishment of an elongated recirculation bubble that deflects the main flow towards the cyclonic wall. The highest level of turbulent kinetic energy is obtained at high rotation near the cyclonic reattachment in a region where stabilization effects are expected. The phenomenological model of absolute vortex stretching is found to be useful in understanding how the rotation influences the dynamics in the various regions of the flow.

A Numerical Investigation of Thermal Convection in a Heat-Generating Fluid Layer

1980 ◽  
Vol 102 (3) ◽  
pp. 531-537 ◽  
Author(s):  
A. A. Emara ◽  
F. A. Kulacki
Keyword(s):  
Thermal Convection ◽  
Large Scale ◽  
Fluid Layer ◽  
Lower Surface ◽  
Rectangular Domain ◽  
Temperature Fields ◽  
Free Boundaries ◽  
Low Velocity ◽  
Nusselt Numbers ◽  
Rayleigh Numbers

Finite difference solutions of the equations governing thermal convection driven by uniform volumetric energy sources are presented for two-dimensional flows in a rectangular domain. The boundary conditions are a rigid, (i.e., zero slip), zero heat-flux lower surface, rigid adiabatic sides, and either a rigid or free (i.e., zero shear) isothermal upper surface. Computations are carried out for Prandtl numbers from 0.05 to 20 and Rayleigh numbers from 5 × 104 to 5 × 108. Nusselt numbers and average temperature profiles within the layer are in good agreement with experimental data for rigid-rigid boundaries. For rigid-free boundaries, Nusselt numbers are larger than in the former case. The structure of the flow and temperature fields in both cases is dominated by rolls, except at larger Rayleigh numbers where large-scale eddy transport occurs. Generally, low velocity upflows over broad regions of the layer are balanced by higher velocity downflows when the flow exhibits a cellular structure. The hydrodynamic constraint at the upper surface and the Prandtl number are found to influence only the detailed nature of flow and temperature fields. No truly steady velocity and temperature fields are found despite the fact that average Nusselt numbers reach steady values.

Large-scale motion of submerged offset jets issuing from sharp-edged rectangular nozzles

2020 ◽  
pp. 095440622093483
Author(s):  
B Nyantekyi-Kwakye
Keyword(s):  
Large Scale ◽  
Expansion Ratio ◽  
Quality Data ◽  
Recirculation Region ◽  
Acoustic Doppler Velocimeter ◽  
Mean Flow ◽  
Turbulent Structures ◽  
Mean Velocity ◽  
Lateral Direction ◽  
Scale Motion

The mean flow, turbulence characteristics, and dynamics of large-scale vortices are investigated for an offset jet issuing from different nozzle expansion ratios using a four-receiver acoustic Doppler velocimeter. The jet was discharged from sharp-edged rectangular nozzles with expansion ratios of 0.24, 0.49, 0.75, and 1.00 at Reynolds number of 5.3 × 104. The decay rate of the maximum mean velocity decreased with increasing expansion ratio due to suppressed lateral entrainment of ambient fluid. The acoustic Doppler velocimeter proved capable of providing high-quality data to investigate the energy spectrum and turbulent structures embedded in the flow. Large-scale vortices dominated the recirculation region compared to the reattachment and developing regions of the jet. Increasing the expansion ratio resulted in larger order of magnitude of the vortices within the recirculation region. The turbulent structures stretched in the lateral direction in regions where smaller-sized structures existed in the streamwise direction and vice versa.

scholarly journals Dynamics and evolution of turbulent Taylor rolls

2019 ◽  
Vol 870 ◽  
pp. 970-987 ◽  
Author(s):  
Francesco Sacco ◽  
Roberto Verzicco ◽  
Rodolfo Ostilla-Mónico
Keyword(s):  
Turbulent Flows ◽  
Large Scale ◽  
Fluid Motion ◽  
Domain Size ◽  
Turbulent Regime ◽  
Computational Domain ◽  
Mean Flow ◽  
Concentric Cylinders ◽  
Angular Velocities ◽  
High Reynolds

In many shear- and pressure-driven wall-bounded turbulent flows secondary motions spontaneously develop and their interaction with the main flow alters the overall large-scale features and transfer properties. Taylor–Couette flow, the fluid motion developing in the gap between two concentric cylinders rotating at different angular velocities, is not an exception, and toroidal Taylor rolls have been observed from the early development of the flow up to the fully turbulent regime. In this manuscript we show that under the generic name of ‘Taylor rolls’ there is a wide variety of structures that differ in the vorticity distribution within the cores, the way they are driven and their effects on the mean flow. We relate the rolls at high Reynolds numbers not to centrifugal instabilities, but to a combination of shear and anti-cyclonic rotation, showing that they are preserved in the limit of vanishing curvature and can be better understood as a pinned cycle which shows similar characteristics as the self-sustained process of shear flows. By analysing the effect of the computational domain size, we show that this pinning is not a product of numerics, and that the position of the rolls is governed by a random process with the space and time variations depending on domain size.

scholarly journals Direct numerical simulation of the scouring of a brittle streambed in a turbulent channel flow

2021 ◽  
Author(s):  
Federico Dalla Barba ◽  
Francesco Picano
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Channel Flow ◽  
Large Scale ◽  
Stokes Equations ◽  
Wall Stress ◽  
Turbulent Channel Flow ◽  
Numerical Approach ◽  
Immersed Boundary ◽  
Turbulent Structures

AbstractThe natural processes involved in the scouring of submerged sediments are crucially relevant in geomorphology along with environmental, fluvial, and oceanographic engineering. Despite their relevance, the phenomena involved are far from being completely understood, in particular for what concerns cohesive or stony substrates with brittle bulk mechanical properties. In this frame, we address the investigation of the mechanisms that govern the scouring and pattern formation on an initially flattened bed of homogenous and brittle material in a turbulent channel flow, employing direct numerical simulation. The problem is numerically tackled in the frame of peridynamic theory, which has intrinsic capabilities of reliably reproducing crack formation, coupled with the Navier–Stokes equations by the immersed boundary method. The numerical approach is reported in detail here and in the references, where extensive and fully coupled benchmarks are provided. The present paper focuses on the role of turbulence in promoting the brittle fragmentation of a solid, brittle streambed. A detailed characterization of the bedforms that originate on the brittle substrate is provided, alongside an analysis of the correlation between bed shape and the turbulent structures of the flow. We find that turbulent fluctuations locally increase the intensity of the wall-stresses producing localized damages. The accumulation of damage drives the scouring of the solid bed via a turbulence-driven fatigue mechanism. The formation, propagation, and coalescence of scouring structures are observed. In turn, these affect both the small- and large-scale structures of the turbulent flow, producing an enhancement of turbulence intensity and wall-stresses. At the small length scales, this phenomenology is put in relation to the formation of vortical cells that persist over the peaks of the channel bed. Similarly, large-scale irregularities are found to promote the formation of stationary turbulent stripes and large-scale vortices that enhance the widening and deepening of scour holes. As a result, we observe a quadratic increment of the volumetric erosion rate of the streambed, as well as a widening of the probability density of high-intensity wall stress on the channel bed.

Evolution laws of distributed vortex-induced pressures and energy of a flat-closed-box girder via numerical simulation

2020 ◽  
Vol 23 (13) ◽  
pp. 2776-2788
Author(s):  
Xingyu Chen ◽  
Yongle Li ◽  
Xinyu Xu ◽  
Haojun Tang ◽  
Bin Wang
Keyword(s):  
Numerical Simulation ◽  
Large Scale ◽  
Rapid Development ◽  
Wind Tunnel Test ◽  
Simulation Method ◽  
Lower Surface ◽  
Box Girder ◽  
Vortex Induced Vibration ◽  
Aerodynamic Damping ◽  
Evolution Laws

This article investigates the vertical vortex-induced vibration of a flat-closed-box girder using numerical simulation method. The accuracy of simulation results is verified at first by comparing the displacement responses and vortex-induced force of vertical vortex-induced vibration with those obtained in a previous wind tunnel test of large-scale sectional model. The precision of extracting the vortex-induced pressures from the surface pressures and decomposing the vortex-induced pressures via the mathematical model is validated later. Subsequently, the vortex-induced pressures and energy distribution, and the evolution laws of vortex-induced pressures and energy are discussed. The results show that the linear aerodynamic negative damping and nonlinear aerodynamic positive damping are key factors of the rapid development of vortex-induced vibration and the self-limiting phenomenon separately. The positive aerodynamic damping is mainly provided by the lower surface and the middle of the upper surface, and the negative aerodynamic damping is primarily provided by the middle and downstream of the upper surface.

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.

Large-scale Motion of Solar Filaments

2000 ◽  
Vol 179 ◽  
pp. 205-208
Author(s):  
Pavel Ambrož ◽  
Alfred Schroll
Keyword(s):  
Magnetic Flux ◽  
Proper Motion ◽  
Time Scale ◽  
Large Scale ◽  
Accuracy Of Measurements ◽  
Solar Filaments ◽  
General Movement ◽  
Scale Motion ◽  
Velocity Scatter

AbstractPrecise measurements of heliographic position of solar filaments were used for determination of the proper motion of solar filaments on the time-scale of days. The filaments have a tendency to make a shaking or waving of the external structure and to make a general movement of whole filament body, coinciding with the transport of the magnetic flux in the photosphere. The velocity scatter of individual measured points is about one order higher than the accuracy of measurements.

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