scholarly journals Transient flows past arrays of yawed finite prisms

2017 ◽  
Vol 95 (12) ◽  
pp. 1285-1298 ◽  
Author(s):  
I-Han Liu ◽  
Bashar Attiya ◽  
Alparslan Oztekin
Keyword(s):  
Drag Coefficient ◽  
Three Dimensional ◽  
Simulation Method ◽  
Offshore Structures ◽  
Wake Flow ◽  
Transient Flows ◽  
Eddy Simulation ◽  
Marine Current ◽  
Yaw Angle ◽  
Energy Harvesting Devices

Transient flows past arrays of yawed finite-length prisms in tandem and staggered arrangements are simulated. Large eddy simulation method is employed to characterize spatial and temporal characteristics of flows in the vicinity of prisms. Drag coefficients of prisms are determined for different spacing and yaw angle of prisms. Three-dimensional effects are investigated for different spacings of prisms in a tandem array. Drag coefficient of downstream prism is significantly lower compared to that of upstream prism when prisms are tightly spaced. Drag coefficient of downstream prism predicted by simulations in three-dimensional geometry is recovered fully as spacing approaches 10 times the height of the prism while drag coefficient predicted by simulations in two-dimensional geometry is only about 30% of the upstream prism even when two prisms are separated by 20 times the height of the prism. The wake flow patterns do not influence the drag coefficient strongly for the staggered arrays of yawed prisms. Prisms can be placed in a tighter arrangement without a significant penalty of drag reduction. Mathematical models and numerical methods employed are validated by comparing simulation results against experimental results. Drag exerted on these prisms can directly be related to power generation by devices containing translating blades of marine-current applications. This study demonstrates that arrangement, spacing, and yaw angle can be used to design and optimize energy harvesting devices and other offshore structures.

scholarly journals On the aerodynamic flow around a cyclist model at the hoods position

2019 ◽  
Vol 23 (1) ◽  
pp. 35-47 ◽  
Author(s):  
Jiun-Jih Miau ◽  
Shang-Ru Li ◽  
Zong-Xiu Tsai ◽  
Mai Van Phung ◽  
San-Yi Lin
Keyword(s):  
Wind Tunnel ◽  
Water Channel ◽  
Three Dimensional ◽  
Simulation Method ◽  
Reynolds Numbers ◽  
Numerical Results ◽  
Wake Flow ◽  
Model Surface ◽  
Eddy Simulation ◽  
Separation Pattern

Abstract Aerodynamic flow around an 1/5 scale cyclist model was studied experimentally and numerically. First, measurements of drag force were performed for the model in a low-speed wind tunnel at Reynolds numbers from $$5.5 \times 10^{4}$$5.5×104 to $$1.8 \times 10^{5}$$1.8×105. Meanwhile, numerical computation using a large eddy simulation method was performed at three Reynolds numbers of $$1.1 \times 10^{4}$$1.1×104, $$6.5 \times 10^{4}$$6.5×104 and $$1.5 \times 10^{5}$$1.5×105 to obtain the drag coefficients for comparison. Second, flow visualization was made in a water channel and the wind tunnel mentioned to examine the three-dimensional flow separation pattern on the model surface, which could also be realized from the numerical results. Finally, a wake flow survey based on the hot-wire measurements in the wind tunnel showed that in the near-wake region, the flow was featured with the formation of multiple streamwise vortices. The numerical results further indicated that these vortices were evolved from the separated flows occurred on the model surface. Graphic Abstract

Numerical simulation of the effects of obstacle deflectors on the aerodynamic performance of stationary high-speed trains at two yaw angles

2017 ◽  
Vol 232 (3) ◽  
pp. 913-927 ◽  
Author(s):  
Ji-qiang Niu ◽  
Dan Zhou ◽  
Xi-feng Liang
Keyword(s):  
High Speed ◽  
Simulation Method ◽  
Aerodynamic Performance ◽  
Wake Flow ◽  
Detached Eddy Simulation ◽  
Aerodynamic Forces ◽  
Eddy Simulation ◽  
Delayed Detached Eddy Simulation ◽  
High Speed Trains ◽  
Yaw Angle

In this study, based on the shear-stress transport κ–ω turbulent model, the improved delayed detached eddy simulation method has been used to simulate the unsteady aerodynamic performance of trains with different obstacle deflectors at two yaw angles (0° and 15°). The numerical algorithm is used and some of the numerical results are verified through wind tunnel tests. By comparing and analysing the obtained results, the effects of the obstacle deflectors on the force of the trains as well as the pressure and flow structure around the trains are elucidated. The results show that the obstacle deflectors primarily affect the flow field at the bottom of the head car as well as the wake flow, and that the internal oblique-type obstacle deflector (IOOD) markedly improves the aerodynamic performance of the trains, by decreasing most of the aerodynamic forces of the train cars and minimising their fluctuations. Further, a nonzero yaw angle weakens or even changes the effect of the IOOD on the aerodynamic forces of the train cars. However, the effect of the IOOD is more on the tail car.

Three-Dimensional Numerical Simulation of the Flow around Two Side-by-Side Cylinders of Different Diameters

2014 ◽  
Vol 721 ◽  
pp. 199-202
Author(s):  
Zhen Xiao Bi ◽  
Zhi Han Zhu
Keyword(s):  
Numerical Simulation ◽  
Flow Direction ◽  
Three Dimensional ◽  
Cross Flow ◽  
Simulation Method ◽  
Scale Model ◽  
Hydrodynamic Characteristics ◽  
Eddy Simulation ◽  
Drag And Lift ◽  
Different Diameters

This paper presents the calculation of hydrodynamic characteristics of two side-by-side cylinders of different diameters in three dimensional incompressible uniform cross flow by using Large-eddy simulation method based on dynamical Smagorinsky-Lilly sub-grid scale model. Solution of the three dimensional N-S equations were obtained by the finite volume method. The numerical simulation focused on investigating the characteristic of the pressure distribution (drag and lift force), vorticity field and turbulence Re=. Results shows that, the asymmetry of the time –averaged velocity distribution in the flow direction behind the two cylinders is very obvious; the frequency of eddy shedding of the small cylinder is about twice of the large one. The turbulence of cylinders is more obvious.

The Effect of Boundary Proximity Upon the Wake Structure of Horizontal Axis Marine Current Turbines

2008 ◽  
Author(s):  
L. E. Myers ◽  
A. S. Bahaj ◽  
R. I. Rawlinson-Smith ◽  
M. Thomson
Keyword(s):  
Flow Field ◽  
Three Dimensional ◽  
Tidal Energy ◽  
Wake Flow ◽  
Small Scale ◽  
Energy Yield ◽  
Novel Approach ◽  
Marine Current ◽  
Downstream Flow ◽  
Hydraulics Laboratory

An experimental and theoretical investigation of the flow field around small-scale mesh disk rotor simulators is presented. The downstream wake flow field of the rotor simulators has been observed and measured in the 21m tilting flume at the Chilworth hydraulics laboratory, University of Southampton. The focus of this work is the proximity of flow boundaries (seabed and surface) to the rotor disks and the constrained nature of the flow. A three-dimensional Eddy-viscosity numerical model based on an established wind turbine wake model has been modified to account for the change in fluid and the presence of a bounding free surface. This work has shown that previous axi-symmetric modelling approaches may not hold for marine current energy technology and a novel approach is required for simulation of the downstream flow field. Such modelling solutions are discussed and resultant simulation results are given. This work has been conducted as part of a BERR-funded project to develop a numerical modelling tool which can predict the flow onto a marine current turbine within an array. The work presented in this paper feeds into this project and will eventually assist the layout design of arrays which are optimally spaced and arranged to achieve the maximum possible energy yield at a given tidal energy site.

NUMERICAL ANALYSIS OF BULK DRAG COEFFICIENT IN DENSE VEGETATION BY IMMERSED BOUNDARY METHOD

2011 ◽  
Vol 1 (32) ◽  
pp. 48 ◽  
Author(s):  
Tomohiro Suzuki ◽  
Taro Arikawa
Keyword(s):  
Drag Coefficient ◽  
Immersed Boundary Method ◽  
Three Dimensional ◽  
Vertical Cylinder ◽  
Immersed Boundary ◽  
Dense Vegetation ◽  
Eddy Simulation ◽  
Boundary Method ◽  
Large Eddy ◽  
Good Agreement

In this paper, bulk drag coefficient in rigid dense vegetation is investigated mainly by using a three dimensional numerical simulation model CADMAS-SURF/3D by incorporating Immersed Boundary Method to calculate flow around the vertical cylinder in the Cartesian grid. Large Eddy Simulation is also incorporated as a turbulence model. Firstly, validation of the developed model is conducted with a single cylinder in the flow field based on literature. All the results obtained here (Re=300, 3,900 and 8,000) show good agreement with the reference data in literature. After the validation, multiple cylinders are allotted in three different densities (S/D=2.8, 2.0, 1.4) in a numerical wave tank and numerical simulations are conducted to investigate bulk drag coefficient. The result shows that the ratio of bulk drag coefficient to drag coefficient, which represents a reduction, is not just a function of density but a function of parameter 2a/S, in which 2a is stroke of the motion and S is cylinder distance. 2a is less than S, the effect of the density is neglected because the wake does not reach the other cylinders even when the density is high. On the contrary, it might affect the ratio of bulk drag coefficient to drag coefficient when the stroke of the motion is larger than the cylinder distance even when the density is low. In general, the ratio of bulk drag coefficient to drag coefficient decreases when 2a/S increases.

WAVELET MULTI-RESOLUTION ANALYSIS ON LARGE-EDDY SIMULATION OF TURBURLENT FLOW BEHIND A VEHICLE EXERNAL MIRROR

2013 ◽  
Vol 11 (04) ◽  
pp. 1360007 ◽  
Author(s):  
A. RINOSHIKA ◽  
Y. ZHENG ◽  
E. SHISHIDO
Keyword(s):  
Large Eddy Simulation ◽  
Large Scale ◽  
Three Dimensional ◽  
Wake Flow ◽  
Small Scale ◽  
Three Dimensional Flow ◽  
Eddy Simulation ◽  
Multi Resolution Analysis ◽  
Large Eddy ◽  
Resolution Analysis

The three-dimensional orthogonal wavelet multi-resolution technique was applied to analyze flow structures of various scales around an externally mounted vehicle mirror. Firstly, the three-dimensional flow of mirror wake was numerically analyzed at a Reynolds number of 105 by using the large-eddy simulation (LES). Then the instantaneous velocity and vorticity were decomposed into the large-, intermediate- and relatively small-scale components by the wavelet multi-resolution technique. It was found that a three-dimensional large-scale vertical vortex dominates the mirror wake flow and makes a main contribution to vorticity concentration. Some intermediate- and relatively small-scale vortices were extracted from the LES and were clearly identifiable.

scholarly journals Numerical Simulation of Fluctuating Wind Effects on an Offshore Deck Structure

2017 ◽  
Vol 2017 ◽  
pp. 1-17 ◽  
Author(s):  
Jin Ma ◽  
Dai Zhou ◽  
Zhaolong Han ◽  
Kai Zhang ◽  
Jennifer Nguyen ◽  
...  
Keyword(s):  
Oil And Gas ◽  
Parametric Design ◽  
Structural Response ◽  
Simulation Method ◽  
Offshore Structures ◽  
Vital Role ◽  
Structural Dynamic ◽  
Eddy Simulation ◽  
Wind Pressures ◽  
Fluctuating Wind

The offshore structures that play a vital role in oil and gas extraction are always under complicated environmental conditions such as the random wind loads. The structural dynamic response under the harsh wind is still an important issue for the safety and reliable design of offshore structures. This study conducts an investigation to analyze the wind-induced structural response of a typical offshore deck structure. An accurate and efficient mixture simulation method is developed to simulate the fluctuating wind speed, which is then introduced as the boundary condition into numerical wind tunnel tests. Large eddy simulation (LES) is utilized to obtain the time series of wind pressures on the structural surfaces and to determine the worst working condition. Finally, the wind-induced structural responses are calculated by ANSYS Parametric Design Language (APDL). The numerically predicted wind pressures are found to be consistent with the existing experimental data, demonstrating the feasibility of the proposed methods. The wind-induced displacements have the certain periodicity and change steadily. The stresses at the top of the derrick and connections between deck and derrick are relatively larger. These methods as well as the numerical examples are expected to provide references for the wind-resistant design of the offshore structures.

Effect of Base Slant on Flow in the Near Wake of an Axisymmetric Cylinder

1980 ◽  
Vol 31 (2) ◽  
pp. 132-147 ◽  
Author(s):  
Thomas Morel
Keyword(s):  
Drag Coefficient ◽  
Three Dimensional ◽  
Local Maximum ◽  
The Body ◽  
Wake Flow ◽  
Force Measurements ◽  
Near Wake ◽  
Slant Angle ◽  
Cylinder Length ◽  
Axisymmetric Bodies

SummaryThe effects of slanting the base of a slender axisymmetric cylinder (length/diameter ratio of 9), aligned with the flow, was studied experimentally. The body was equipped with interchangeable rear ends covering a range of slant angles between 0° (vertical) and 70°. It was found that the base slant has a very dramatic effect on body drag, particularly in a relatively narrow range of slant angles where the drag coefficient exhibits a large local maximum (over-shoot). Detailed study of the flow showed that the drag overshoot is related to the existence of two very different Separation patterns on the slanted base. One pattern is similar to that found behind axisymmetric bodies with no base slant, and its main feature is the presence of a closed Separation region adjacent to the base. The other pattern is highly three-dimensional with two streamwise vortices forming along the sides of the slanted base. This pattern sets in very abruptly at a “critical” slant angle α ∼ 47°. Drag force measurements showed that, at first, the drag coefficient slowly increases with the slant angle, but then jumps suddenly upwards to more than double its baseline value (from CD = 0.24 to CD = 0.625) at the critical angle. At angles higher than that CD decreases again, and at 70° it is about equal to the baseline value. Further effects of the slant angle are the generation of a large side force and a significant increase in near-wake flow periodicity.

scholarly journals Large-eddy simulation of contrail evolution in the vortex phase and its interaction with atmospheric turbulence

2015 ◽  
Vol 15 (13) ◽  
pp. 7369-7389 ◽  
Author(s):  
J. Picot ◽  
R. Paoli ◽  
O. Thouron ◽  
D. Cariolle
Keyword(s):  
Large Eddy Simulation ◽  
Turbulent Flows ◽  
Atmospheric Turbulence ◽  
Three Dimensional ◽  
Vertical Direction ◽  
Ambient Air ◽  
Wake Flow ◽  
Initial Flow ◽  
Eddy Simulation ◽  
Large Eddy

Abstract. In this work, the evolution of contrails in the vortex and dissipation regimes is studied by means of fully three-dimensional large-eddy simulation (LES) coupled to a Lagrangian particle tracking method to treat the ice phase. In this paper, fine-scale atmospheric turbulence is generated and sustained by means of a stochastic forcing that mimics the properties of stably stratified turbulent flows as those occurring in the upper troposphere and lower stratosphere. The initial flow field is composed of the turbulent background flow and a wake flow obtained from separate LES of the jet regime. Atmospheric turbulence is the main driver of the wake instability and the structure of the resulting wake is sensitive to the intensity of the perturbations, primarily in the vertical direction. A stronger turbulence accelerates the onset of the instability, which results in shorter contrail descent and more effective mixing in the interior of the plume. However, the self-induced turbulence that is produced in the wake after the vortex breakup dominates over background turbulence until the end of the vortex regime and controls the mixing with ambient air. This results in mean microphysical characteristics such as ice mass and optical depth that are slightly affected by the intensity of atmospheric turbulence. However, the background humidity and temperature have a first-order effect on the survival of ice crystals and particle size distribution, which is in line with recent studies.

The Effect of Boundary Proximity Upon the Wake Structure of Horizontal Axis Marine Current Turbines

2011 ◽  
Vol 134 (2) ◽  
Author(s):  
A. S. Bahaj ◽  
L. E. Myers ◽  
R. I. Rawlinson-Smith ◽  
M. Thomson
Keyword(s):  
Flow Field ◽  
Three Dimensional ◽  
Tidal Energy ◽  
Wake Flow ◽  
Small Scale ◽  
Energy Yield ◽  
Modeling Tools ◽  
Sea Bed ◽  
Marine Current ◽  
Hydraulics Laboratory

An experimental and theoretical investigation of the flow field around small-scale mesh disk rotor simulators is presented. The downstream wake flow field of the rotor simulators has been observed and measured in the 21m tilting flume at the Chilworth hydraulics laboratory, University of Southampton. The focus of this work is the proximity of flow boundaries (sea bed and surface) to the rotor disks and the constrained nature of the flow. A three-dimensional Eddy-viscosity numerical model based on an established wind turbine wake model has been modified to account for the change in fluid and the presence of a bounding free surface. This work has shown that previous axi-symmetric modeling approaches may not hold for marine current energy technology and a novel approach is required for simulation of the downstream flow field. Such modeling solutions are discussed and resultant simulation results are given. In addition, the presented work has been conducted as part of a UK Government funded project to develop validated numerical modeling tools which can predict the flow onto a marine current turbine within an array. The work feeds into the marine energy program at Southampton to assist developers with layout designs of arrays which are optimally spaced and arranged to achieve the maximum possible energy yield at a given tidal energy site.

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