Back to the programme printer.gif Print




Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'How does the wind blow behind wind turbines and in wind farms?' taking place on Tuesday, 11 March 2014 at 16:30-18:00. The meet-the-authors will take place in the poster area.

Zeinab Ahmadi Zeleti Lappeenranta University of Technology, Finland
Co-authors:
Zeinab Ahmadi Zeleti (1) F P Heikki Haario (1) Jari Hämäläinen (1)
(1) Lappeenranta University of Technology, Lapeenranta, Finland

Printer friendly version: printer.gif Print

Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Zeinab Ahmadi Zeleti is working on porous media modeling of a complex forested terrain with lakes, buildings,etc. for wind park simulations. She is conducting her research at Lappeenranta University of Technology as doctoral student. She studied Master of Science in Bioenergy and Environmental Technology and involved as a research assistant in a scientific project "Transient heat transfer of nanofluids through porous media" at the Faculty of technology, Lappeenranta University of Technology.

Abstract

Experimental validation of a porous medium modeling for air flow through forest canopy

Introduction

Flow through forest is characterized by a large scale horizontal pressure gradient due to the resistance as a result of foliage, branches, and trunks. Accurately analyzing this pressure drop is very important specifically in predicting flow above and at the lee-side of the forest.
To take into account the existence of forest, most of numerical air flow models implemented the drag force approach and some used roughness length. However, this study utilizes the terrestrial laser scanning data to produce reliable information about a real forest essential for porous medium modeling of wind flow through and above forest.


Approach

The momentum balance equation written for incompressible wind flow through forested landscapes is
ερ DU/Dt=-ε∇p+ε∇.τ+S
where the source term representing the momentum absorbed within forest, S=-(μ/K U+C 1/2 ρ|U|U), is composed of two parts, viscous (Darcy) and inertial drag forces, respectively. Here, the porosity of the medium defined as the ratio of the volume occupied by air to the total volume is denoted as ε, the ability of the medium to permit flow is denoted as K, and the canopy inertial resistance coefficient as C.
This study follows that the viscous forces are not important for forest porous medium modeling. Thus, by neglecting this term and with help of drag-force approach [1, 2], the inertial resistance coefficient can be calculated as C=2 LAD C_(d )where C_(d )is the canopy drag coefficient. Also, Leaf area density, LAD, the key parameter to consider the vegetation of the canopy can be estimated based on methods available in literature, one of those is voxel based canopy model [3]. This method utilizes the laser scanning measurement points of the forest at several horizontal thickness layers to precisely estimate the LAD. Nevertheless, this method is capable to report the best estimate of LAD when the maximum horizontal layer thickness is used.
In this work, we first present the validation of the porous medium model with a real site before introducing the LAD into our RANS (Reynolds Averaged Navier-Stokes) simulation. Furthermore, we plan to collect couples of information on foliage distribution as a function of height or position aside from LAD, for example average height and diameter of tree trunks or porous vegetation canopy, porosity, etc. from terrestrial laser scanning measurement of the same field of interest with for instance image processing method. Then, we utilize these parameters as input data for CFD (Computational Fluid Dynamics) calculation to capture the effect of canopy on flow more precisely. This is an improved solution compared to the roughness or drag force approach to represent the complexity and the wake recirculation behind the forest, particularly for simulating a real site by large eddy simulation (LES).


Main body of abstract

The rectangular and hexagonal configuration of forest canopy, assumed in this study, as shown in is divided into two layers: the first layer is devoted to investigation of flow through solid stem of tree (a), and the second one deals with the porous vegetation canopy (b). In addition, the tree shapes are assumed to be cone-shaped and ball-shaped which provides a circular cross sectional area at any height for both layers. Therefore, a series of simulations was conducted in 2D for both layers where the size of the obstacles, defining the trunk and the vegetation canopy, varies. Note that, the diameter variation in a constant unit cell explains how dense the forest is at every height. Furthermore, it has been assumed that the specifications of the obstacles in the second layer are known, initially with porosity of 0.95, permeability of 2.28633 m^2 and inertial resistance factor of 20.411m^(-1) , since it is defined as a porous zone. Various inlet velocities ranging from 3 ms^(-1) to 12 ms^(-1) are given for every simulation case to solve the momentum equation based on the control-volume technique. The simple homogeneous porous medium momentum sink equation
∆p/L=μ/K_h U+(C_h ρ)/2 U^2
is used to approximate the homogenized permeability (K_h) and inertial resistance factor (C_h ) of the forest, with the pressure drop results of every case.
At any configuration with four different inlet velocities, the variation of pressure drop versus diameter is shown in for the first (a) and second (b) layer. As expected, the pressure drop decreases by minimizing the obstacle sizes. Similarly, the variation of homogenized forest permeability and resistance factor with diameter of hexagonally (red-asterisk-solid-line) and rectangularlly (black-point-solid-line) arranged trees at trunk (a) and porous vegetation (b) layers are presented in . It is obvious that the resistivity increases with maximizing the size of obstacles, resulting in lower forest permeability. Note that the permeability of solid stem layer with rectangular configuration remained unchanged for obstacles with diameter less than 0.3 m. Here, forest permeability and inertial resistance functions have been introduced as a function of diameter and height for both layers at different configurations.
Next, the effects of porosity and permeability on forest pressure loss are investigated. demonstrate the variation of homogenized permeability (a) and inertial resistance factor (b) with porosity of the first layer. Likewise, the red-asterisk-solid-line and the black-point-solid-line are representing the hexagonal and rectangular forest types, respectively. It is obvious that the permeability has increased whereas the resistivity has decreased by maximizing the void fraction. But, simulation results at the second layer were not sensitive when assigning reasonable porosity to the porous vegetation. Similarly, simulations reveal that the pressure drop of the media remains unchanged by varying the vegetation permeability. Moreover, the sensitivity of the resistance in the second layer on horizontal pressure gradient of forest is studied, as seen in .
After a successful agreement of 2D and 3D models with and without trees at each layer, shows the vertical velocity profiles along x-direction for forest with (red- dashdot-line) and without (black-solid-line) obstacles (homogenized forest) for hexagonal arrangement.
The aerial view of a real forest measurement located at Skinnarila, near Lappeenranta University of Technology, is shown in . The two lidar devices located at 6770864N, 559016E (before forest) and 6770848N, 558604E (after forest), shown as green spots, measure the wind velocity continuously at 11 different heights above ground, during neutral meteorological conditions from 24th of May till 6th of June, 2013. The area marked with dashdot-line displays the targeted area utilized for validation of our porous model. The directions of the wind which flows freely through the forest form Lake Saimaa are shown with arrows. Likewise, the lidar devices are marked in 3D laser scanning data over the field of interest which depicts the ground and objects on the ground (see ). The 10-min-average velocity samples with mean wind direction of 264° were selected at the inlet at 9 different heights. Similarly, the wind data corresponding to the same time step as the selected wind at the inlet were chosen for the outflow. clearly depicts the effect of the vegetation on the velocity profile after (outlet) and above the forest.


Conclusion

Porous medium modeling of wind flow through forested landscape is studied via CFD simulations. The effects of several parameters, namely the canopy tree diameter which may explain the density of the forest, the tree porosity, the permeability and resistance, as well as the forest tree configuration are examined on the prediction of the overall forest permeability, inertial resistance factor, and pressure gradient in the flow direction. In order to verify whether the 2D simple models were able to represent the forest correctly, the 2D models and 3D models with tree obstacles and together with homogenized forests (no canopy obstacles) were successfully compared. The simulations revealed that the permeability or more generally the viscous force is not a source of concern in calculating the momentum sink of the forest canopy. Similarly, simulations confirmed that high enough values of tree porosity which correspond to a real forest, for instance porosity above 0.85, have no effect on pressure gradient in the flow direction.
Furthermore, in order to validate the porous medium model and its applicability over any complex forested terrain, by the help of vegetation detailed information derived from terrestrial laser scanning data, the wind measurements are performed over a real site located next to the Lake Saimaa. On average, the wind was flowing through and over the forested land relatively from east (Lake Saimaa) to the west under a neutral meteorological condition. The analysis on the raw Skinnarila field data specified clearly the effect of vegetation obstacles in the forest over the wind behavior. Nevertheless, the CFD simulations over the real site are an ongoing work.



Learning objectives
To what extent the forest porous medium model can help in predicting the air flow behavior above and at the lee-side of the forest canopy?
To what extent the forest detailed information from LiDAR results can be used for CFD calculations?



References
[1] Ross A.N. & Baker T.P. (2013) Flow Over Partially Forested Ridges. Boundary-Layer Meteorology 146, 375-92.
[2] Schlegel F., Stiller J., Bienert A., Maas H.-G., Queck R. & Bernhofer C. (2012) Large-Eddy Simulation of Inhomogeneous Canopy Flows Using High Resolution Terrestrial Laser Scanning Data. Boundary-Layer Meteorology 142, 223-43.
[3] Hosoi, F. and K. Omasa (2006)., Voxel-Based 3-D Modeling of Individual Trees for Estimating Leaf Area Density Using High Resolution Portable Scanning Lidar. IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING 44(12): 3610-3618.