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The effects of a porous fence with a nonuniform porosity on flow fields are investigated numerically. First, an experiment with a non-uniform porous fence located in a wind tunnel is performed to obtain a reference data set. Then, a numerical model that utilizes the finite volume scheme with a weakly compressible-flow method to solve the continuity and momentum equations is developed. The numerical simulation is compared to experimental measurements for validation purposes. As a result, the numerical predictions show good agreements with the experimental data. Finally, the numerical investigations of the flow fields around porous fences with various combinations of upper and lower fence porosity are also presented. When the upper porosity is greater than the lower porosity, the Protection Index PI_{0.1}, PI_{0.3} and PI_{0.5}, representing the adverse sheltering effect, decreases compared to that of the uniform porous fence. When the upper porosity is less than the lower porosity, the PI_{0.5} increases and the variations of the PI_{0.1} and PI_{0.3}, depend on the upper porosity, compared to that of the uniform porous fence. The results show that the porous fence with the upper fence porosity

Various kinds of fences have been used as windbreaks to reduce the wind erosion effectively. A fence blocks the oncoming flow and reduces the mean velocity of the flow behind the fence. Flows around a fence are of complex characteristics. Flow separation from fences results in strong shear layers, along which turbulence intensities are large.

The characteristics of turbulent flows around porous fences have been reported in several studies [

The above experimental results demonstrated that the characteristics of turbulent flow downstream of the porous fence significantly depended on the porosities. Hence, the numerical model will meet some considerable both in modeling the fluid dynamics of the recirculation flow and the porous effects. Usually, practical engineers used a drag law to represent the porous effects due to low computational cost [

How to handle the porous fence. (a) Reproduced as a drag law. (b) Directly reproduced configuration.

The above literature demonstrates the detailed investigation of the flow structures around porous fences, and the role played by the porosity has been emphasized. However, data for the cases of a porous fence with a nonuniform porosity are limited. An elevated fence constructed in a road becomes nonuniform when support structures create a gap between the bottom of the fence and the ground. Also, the solid fences act as nonuniform porous fences when it receives a strong wind during its construction. Cho [

The main objective of this study is to numerically investigate the effects of porous fences with nonuniform porosities on flow structures, by varying the porosity in the upper and lower halves of a fence. We recognized in literature survey that no experimental data was available for examining the performance of a model predicting flow through a nonuniform porous fence. Therefore, experimental results with a nonuniform porous fence are presented to provide a reference data set for validating the numerical model. The validated numerical model is used to study the sheltering effect of a nonuniform porous fence. The Protection Index described by Van et al. [

The experiments were conducted in an open-suction-type wind tunnel with a test section of 0.6 W × 0.6 H × 8.0 L (m^{3}). Spires and roughness elements were installed in front of the test section to create a thermally neutral atmospheric boundary layer. A porous fence with a nonuniform porosity was tested. The porosity of the lower half of the porous fence (

Schematic diagram of the porous fence and coordinate system.

The flow characteristics depicted in Figure

Grid system used in this study (green grids showing downstream of the fence; red grids showing upstream of the fence).

Computational grid local to the porous fence.

Figure

Comparison between calculated results and measured data of mean streamwise velocity profiles around the nonuniform porous fence (

The numerical model in this study was validated by comparing the computed results with the experimental data. Future applications of the numerical model were to be the numerical analysis of the manipulated flow cases. The computational conditions, including the boundary conditions and model parameters, of the experiment in the previous section were used as the basis for the following numerical analysis. Various combinations of upper and lower fence porosity were numerically studied. Five porous fences and the combinations of upper and lower fence porosity used in this study are shown in Figure

Areas under contours

Protection Index ( |
Case | ||||
---|---|---|---|---|---|

A | B | C | D | E | |

PI_{0.1} |
9.44 | 7.44 | 8.56 | 7.78 | 14.49 |

PI_{0.3} |
16.67 | 13.05 | 13.63 | 14.26 | 21.33 |

PI_{0.5} |
22.97 | 21.92 | 20.96 | 24.06 | 28.37 |

Porous fences: (a)

Comparison between calculated streamline patterns around various porous fences.

Variations of calculated mean streamwise velocity profiles around various porous fences.

Variations of calculated mean vertical velocity profiles around various porous fences.

Contour plots of the calculated mean streamwise velocity for various porous fences.

Flows around nonuniform porous fences are numerically investigated. The numerical model developed in this work is based on the finite volume scheme with a weakly-compressible-flow method. Additionally, the experimental data of a nonuniform porous fence are presented mainly for the validation of the numerical model. As a result, the numerical model is shown to be useful and appropriate for predicting the flows around a nonuniform porous fence. The computation results are consistent with the experimental data. The effect of nonuniform porous fence on flow fields are simulated by varying the combinations of upper and lower fence porosity. The bleed flow passing through a nonuniform porous fence has a velocity gradient in the vertical direction. This manipulation of the bleed flow of the porous fence has a significant effect on the sheltering effect evaluated by the Protection Index. In the porous fences with the upper porosity being greater than the lower porosity, the Protection Index decreases compared to that of the uniform porous fence. Additionally, the porous fences with the upper porosity being less than the lower porosity effectively enhance the sheltering effect. The porous fence with the porosity of the upper half of fence is 0% and the lower half of fence is 30% demonstrates best performance in sheltering effect among the porous fences in this study.

The authors gratefully acknowledge Ching-Hua Yang, former master student of the National Chung Hsing University (Taiwan), who carried out the wind tunnel measurements. The J.- T. Lee would also like to thank the National Science Council (Taiwan) for providing financial support (NSC 100-2626-M-005-004-MY3) during preparing this paper.