scholarly journals Turbulent Flow Fields Over a 3D Hill Covered by Vegetation Canopy Through Large Eddy Simulations

Energies ◽  
2019 ◽  
Vol 12 (19) ◽  
pp. 3624 ◽  
Author(s):  
Zhenqing Liu ◽  
Yiran Hu ◽  
Yichen Fan ◽  
Wei Wang ◽  
Qingsong Zhou
Keyword(s):  
Three Dimensional ◽  
Flow Fields ◽  
Large Eddy Simulations ◽  
Wind Tunnel Experiments ◽  
Turbulence Structures ◽  
Large Eddy ◽  
Turbulence Kinetic Energy Budget ◽  
The Mean ◽  
Spanwise Rotation ◽  
The Hill

The flow fields over a simplified 3D hill covered by vegetation have been examined by many researchers. However, there is scarce research giving the three-dimensional characteristics of the flow fields over a rough 3D hill. In this study, large eddy simulations were performed to examine the coherent turbulence structures of the flow fields over a vegetation-covered 3D hill. The numerical simulations were validated by the comparison with the wind-tunnel experiments. Besides, the flow fields were systematically investigated, including the examinations of the mean velocities and root means square of the fluctuating velocities. The distributions of the parameters are shown in a three-dimensional way, i.e., plotting the parameters on a series of spanwise slices. Some noteworthy three-dimensional features were found, and the mechanisms were further revealed by assessing the turbulence kinetic energy budget and the spectrum energy. Subsequently, the instantaneous flow fields were illustrated, from which the coherent turbulence structures were clearly identified. Ejection-sweep motion was intensified just behind the hill crest, leading to a spanwise rotation. A group of vertical rotations were generated by the shedding of the vortex from the lateral sides of the hill.

scholarly journals Large Eddy Simulations of the Flow Fields over Simplified Hills with Different Roughness Conditions, Slopes, and Hill Shapes: A Systematical Study

Energies ◽  
2019 ◽  
Vol 12 (18) ◽  
pp. 3413 ◽  
Author(s):  
Liu ◽  
Hu ◽  
Wang
Keyword(s):  
Turbulent Flow ◽  
Flow Fields ◽  
Large Eddy Simulations ◽  
International Electrotechnical Commission ◽  
Roughness Effects ◽  
Turbulence Structures ◽  
Hill Slope ◽  
Large Eddy ◽  
Speed Up ◽  
The Hill

Turbulent flow fields over topographies are important in the area of wind energy. The roughness, slope, and shape of a hill are important parameters affecting the flow fields over topographies. However, these effects are always examined separately. The systematic investigations of these effects are limited, the coupling between these effects is still unrevealed, and the turbulence structures as a function of these effects are still unclear. Therefore, in the present study, the flow fields over twelve simplified isolated hills with different roughness conditions, slopes, and hill shapes are examined using large eddy simulations. The mean velocities, velocity fluctuations, fractional speed-up ratios, and visualizations of the turbulent flow fields are presented. It is found that as the hill slope increases, the roughness effects become weaker, and the roughness effects will further weaken as the hill changes from 3D to 2D. In addition, the fractional speed-up ratio at the summit of rough hills can even reach to three times as large as that over the corresponding smooth hills. Furthermore, the underestimation of the ratios of spanwise fluctuation to the streamwise fluctuation by International Electrotechnical Commission (IEC) 61400-1 is quite obvious when the hill shape is 3D. Finally, coherent turbulence structures can be identified for smooth hills, and as the hill slope increases, the coherent turbulence structures will experience clear evolutions. After introducing the ground roughness, the coherent turbulence structures break into small eddies.

scholarly journals Coherent structure of the convective boundary layer derived from large-eddy simulations

1989 ◽  
Vol 200 ◽  
pp. 511-562 ◽  
Author(s):  
Helmut Schmidt ◽  
Ulrich Schumann
Keyword(s):  
Boundary Layer ◽  
Boundary Condition ◽  
Large Scale ◽  
Three Dimensional ◽  
Large Eddy Simulations ◽  
Stable Layer ◽  
Convective Boundary ◽  
Velocity Fluctuations ◽  
Large Eddy ◽  
The Mean

Turbulence in the convective boundary layer (CBL) uniformly heated from below and topped by a layer of uniformly stratified fluid is investigated for zero mean horizontal flow using large-eddy simulations (LES). The Rayleigh number is effectively infinite, the Froude number of the stable layer is 0.09 and the surface roughness height relative to the height of the convective layer is varied between 10−6 and 10−2. The LES uses a finite-difference method to integrate the three-dimensional grid-volume-averaged Navier–Stokes equations for a Boussinesq fluid. Subgrid-scale (SGS) fluxes are determined from algebraically approximated second-order closure (SOC) transport equations for which all essential coefficients are determined from the inertial-range theory. The surface boundary condition uses the Monin–Obukhov relationships. A radiation boundary condition at the top of the computational domain prevents spurious reflections of gravity waves. The simulation uses 160 × 160 × 48 grid cells. In the asymptotic state, the results in terms of vertical mean profiles of turbulence statistics generally agree very well with results available from laboratory and atmospheric field experiments. We found less agreement with respect to horizontal velocity fluctuations, pressure fluctuations and dissipation rates, which previous investigations tend to overestimate. Horizontal spectra exhibit an inertial subrange. The entrainment heat flux at the top of the CBL is carried by cold updraughts and warm downdraughts in the form of wisps at scales comparable with the height of the boundary layer. Plots of instantaneous flow fields show a spoke pattern in the lower quarter of the CBL which feeds large-scale updraughts penetrating into the stable layer aloft. The spoke pattern has also been found in a few previous investigations. Small-scale plumes near the surface and remote from strong updraughts do not merge together but decay while rising through large-scale downdraughts. The structure of updraughts and downdraughts is identified by three-dimensional correlation functions and conditionally averaged fields. The mean circulation extends vertically over the whole boundary layer. We find that updraughts are composed of quasi-steady large-scale plumes together with transient rising thermals which grow in size by lateral entrainment. The skewness of the vertical velocity fluctuations is generally positive but becomes negative in the lowest mesh cells when the dissipation rate exceeds the production rate due to buoyancy near the surface, as is the case for very rough surfaces. The LES results are used to determine the root-mean-square value of the surface friction velocity and the mean temperature difference between the surface and the mixed layer as a function of the roughness height. The results corroborate a simple model of the heat transfer in the surface layer.

scholarly journals An Effective Three-Dimensional Layout of Actuation Body Force for Separation Control

2012 ◽  
Vol 2012 ◽  
pp. 1-12 ◽  
Author(s):  
Ittetsu Kaneda ◽  
Satoshi Sekimoto ◽  
Taku Nonomura ◽  
Kengo Asada ◽  
Akira Oyama ◽  
...  
Keyword(s):  
Body Force ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Separated Flow ◽  
Flow Fields ◽  
The Body ◽  
Large Eddy Simulations ◽  
Separation Control ◽  
Large Eddy

We conducted large eddy simulations of the control of separated flow over an airfoil using body forces and discuss the role of a three-dimensional vortex structure in separation control. Two types of cases are examined: (1) the body force is distributed in a spanwise uniform layout and (2) the body force is distributed in a spanwise intermittent layout, with three-dimensional vortices being expected to be generated in the latter cases. The flow fields in the latter cases have a shorter separation bubble than those in the former cases although the total momentum of the body force in the latter cases is the same as or half of the former cases. In the flow fields of the latter type, the three-dimensional vortices, which are not observed in the former cases, are generated by the body force downstream of the body force distributed. Thus, three-dimensional vortices are considered to be effective in controlling the separated flow.

scholarly journals Large-Eddy Simulations of a Drizzling, Stratocumulus-Topped Marine Boundary Layer

2009 ◽  
Vol 137 (3) ◽  
pp. 1083-1110 ◽  
Author(s):  
Andrew S. Ackerman ◽  
Margreet C. vanZanten ◽  
Bjorn Stevens ◽  
Verica Savic-Jovcic ◽  
Christopher S. Bretherton ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Cloud Water ◽  
Large Eddy Simulations ◽  
Cloud Base ◽  
Entrainment Rate ◽  
Liquid Water Path ◽  
Average Maximum ◽  
Large Eddy ◽  
The Mean

Abstract Cloud water sedimentation and drizzle in a stratocumulus-topped boundary layer are the focus of an intercomparison of large-eddy simulations. The context is an idealized case study of nocturnal stratocumulus under a dry inversion, with embedded pockets of heavily drizzling open cellular convection. Results from 11 groups are used. Two models resolve the size distributions of cloud particles, and the others parameterize cloud water sedimentation and drizzle. For the ensemble of simulations with drizzle and cloud water sedimentation, the mean liquid water path (LWP) is remarkably steady and consistent with the measurements, the mean entrainment rate is at the low end of the measured range, and the ensemble-average maximum vertical wind variance is roughly half that measured. On average, precipitation at the surface and at cloud base is smaller, and the rate of precipitation evaporation greater, than measured. Including drizzle in the simulations reduces convective intensity, increases boundary layer stratification, and decreases LWP for nearly all models. Including cloud water sedimentation substantially decreases entrainment, decreases convective intensity, and increases LWP for most models. In nearly all cases, LWP responds more strongly to cloud water sedimentation than to drizzle. The omission of cloud water sedimentation in simulations is strongly discouraged, regardless of whether or not precipitation is present below cloud base.

scholarly journals Waving plants and turbulent eddies

2010 ◽  
Vol 652 ◽  
pp. 1-4 ◽  
Author(s):  
J. J. FINNIGAN
Keyword(s):  
Three Dimensional ◽  
Large Eddy Simulations ◽  
Plant Canopy ◽  
Turbulent Eddies ◽  
Plant Canopies ◽  
Large Eddy ◽  
Flexible Plant ◽  
Paradoxical Results

New large-eddy simulations of flow over a flexible plant canopy by Dupont et al. (J. Fluid Mech., 2010, this issue, vol. 652, pp. 5–44) have produced apparently paradoxical results. Work over the last three decades had suggested that turbulent eddies could ‘lock onto’ to the waving frequency of uniform cereal canopies. Their new simulations contradict this view, although a resolution may lie in the essentially three-dimensional nature of the instability process that generates the dominant eddies above plant canopies.

Large Eddy Simulations of Film-Cooling Flows With a Micro-Ramp Vortex Generator

2011 ◽  
Author(s):  
Aaron F. Shinn ◽  
S. Pratap Vanka
Keyword(s):  
Film Cooling ◽  
Cross Flow ◽  
Vortex Generator ◽  
Vortex Pair ◽  
Large Eddy Simulations ◽  
Wind Tunnel Experiments ◽  
Cooling Effectiveness ◽  
Film Cooling Effectiveness ◽  
Cooling Flows ◽  
Large Eddy

Large Eddy Simulations were performed to study the effect of a micro-ramp on an inclined turbulent jet interacting with a cross-flow in a film-cooling configuration. The micro-ramp vortex generator is placed downstream of the film-cooling jet. Changes in vortex structure and film-cooling effectiveness are evaluated and the genesis of the counter-rotating vortex pair in the jet is discussed. Results are reported with the jet modeled using a plenum/pipe configuration. This configuration was designed based on previous wind tunnel experiments at NASA Glenn Research Center, and the present results are meant to supplement those experiments. It is found that the micro-ramp improves film-cooling effectiveness by generating near-wall counter-rotating vortices which help entrain coolant from the jet and transport it to the surface. The pair of vortices generated by the micro-ramp are of opposite sense to the vortex pair embedded in the jet.

THE PLUNGING OF HYPERPYCNAL PLUMES ON TILTED BED BY THREE-DIMENSIONAL LARGE-EDDY SIMULATIONS

2020 ◽  
Author(s):  
Felipe Nornberg Schuch ◽  
Jorge Silvestrini ◽  
Eckart Meiburg ◽  
Sylvain Laizet
Keyword(s):  
Three Dimensional ◽  
Large Eddy Simulations ◽  
Large Eddy

On the Influence of Swell Propagation Angle on Surface Drag

2019 ◽  
Vol 58 (5) ◽  
pp. 1039-1059 ◽  
Author(s):  
Edward G. Patton ◽  
Peter P. Sullivan ◽  
Branko Kosović ◽  
Jimy Dudhia ◽  
Larry Mahrt ◽  
...  
Keyword(s):  
Surface Roughness ◽  
Three Dimensional ◽  
Surface Drag ◽  
Wave State ◽  
Propagation Angle ◽  
Predictive Skill ◽  
Large Eddy ◽  
Wave Effects ◽  
The Mean ◽  
Wave Models

AbstractA combination of turbulence-resolving large-eddy simulations and observations are used to examine the influence of swell amplitude and swell propagation angle on surface drag. Based on the analysis a new surface roughness parameterization with nonequilibrium wave effects is proposed. The surface roughness accounts for swell amplitude and wavelength and its relative motion with respect to the mean wind direction. The proposed parameterization is tested in uncoupled three-dimensional Weather and Research Forecasting (WRF) simulations at grid sizes near 1 km where we explore potential implications of our modifications for two-way coupled atmosphere–wave models. Wind–wave misalignment likely explains the large scatter in observed nondimensional surface roughness under swell-dominated conditions. Andreas et al.’s relationship between friction velocity and the 10-m wind speed under predicts the increased drag produced by misaligned winds and waves. Incorporating wave-state (speed and direction) influences in parameterizations improves predictive skill. In a broad sense, these results suggest that one needs information on winds and wave state to upscale buoy measurements.

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