Description: Terrain		Version: 450		Updated: 01.06.05

	Terrain

	Getting started
	The first step in the set-up of flow field simulations, is the generation of a 3D model of the area of interest. This is done in the Terrain module. But first the basis for the 3D model must be available, which is a 2D dataset with elevation and roughness data in .gws format.
	Digital terrain conversion
	When starting a new project a grid.gws file is copied into the project. This file serves as a demo. The .gws format contains elevation and roughness data in a regular grid, the file can be viewed by using the Tools->View terrain model menu item.
	Conversion from other formats is performed by clicking the Tools->Convert terrain model menu item. The Convert terrain model is subject to continuous development, so please contact VECTOR if you need extensions in order to perform the conversion of your specific data, see examples of supported formats: third party formats. Alternatively, you can convert your terrain model directly to the .gws format by using your own tools, see: terrain field data.
	Properties
	1.		Terrain extension
	Coordinate system
	The coordinates in the grid.gws file can refer to any global orthogonal system. This coordinate system is called system 3 in the grid.gws file. After generation of the 3D model, a local coordinate system is introduced in the lower lefthand corner. Later in the Objects module the placements of objects can be referenced to by either the global or local coordinate systems.
	X-range and Y-range
	Extensions in the east-west direction and in the south-north direction. The specified coordinates may be increased to fit the nearest node in the grid.gws file, as interpolation of the underlying data is avoided. Default values are the minimum and maximum extensions in the grid.gws file (m).
	2.		Roughness
	Roughness height
	By default variable roughness heights are read from the grid.gws file. Alternatively a constant roughness height can be imposed in the model by specifying a non-zero value for the roughness height.
	The roughness height is defined in the log-law:
		U/Uτ = 1/κ ln(z/z 0)
	where:
			U = wind velocity
			Uτ = friction velocity (τ0/ρ)0.5
			τ0 = shear stress
			ρ = air density
			κ = 0.435, von Karmans constant
			z = coordinate in vertical direction
			z 0 = Roughness height
	3.		Numerical model
	Refinement type
	By default no refinement of the grid in the ground plane is performed. If a refinement area is specified, a denser distribution of nodes will be allocated within the refined area. The refined area is by default placed in the centre of the domain and the cell distribution is uniform within the refined area with an increasing cell size towards the borders. The data used for setting up the refinement area is stored in the current project under the folder dtm in the file simple_refinement.bws. Files with the extension .bws are blocking or refinement files. Finally .bws files can be loaded separately.
	Refinement area, X-range and Y-range
	Extensions in the east-west direction and in the south-north direction of the refinement area. By default the area is given in the centre with an extension equal to 1/3 of the total area extension.
	Refinement/blocking file
	Specification of a refinement or blocking file.
	Height above terrain
	The height above the terrain is defined as the vertical distance between the highest elevation point in the 3D model and the upper boundary. In order to set a proper value for this height, two seemingly contradictory requirements must be balanced. On the one hand, that the distribution of nodes in the vertical direction should be as dense as possible for obtaining accurate numerical solutions, in particular near the ground (see also Height distribution factor and Number of cells in Z direction). This requirement implies that the upper boundary should be placed as near the ground as possible. Yet, on the other hand, if the upper boundary is too close to the ground this would impose a blocking effect when the flow field passes over mountains.
	As a rule of thumb the fraction between the minimum and maximum open area between the ground and the upper boundary, calculated as the model is traversed in west-east and south-north direction, should be lager than 0.95, i.e. (Open area Minimum)/(Open area Maximum) > 0.95.
	By setting Height above terrain to Automatic a height will be calculated satisfying the relation (Open area Minimum)/(Open area Maximum) > 0.95. In some cases the automatic procedure might fail. The procedure will not capture the case with a ridge along the diagonal of the model, as the traverse in west-east and south-north direction will not detect significant changes in open area. But blocking might occur when the incoming flow is perpendicular to the ridge. Likewise for an isolated island or mountain the height of the upper boundary will be reduced as the total modelled area is increased, which could lead to blocking in extreme cases.
	Alternatively a specific height could be given.
	Height distribution factor
	The cell distribution in the vertical direction follows an arithmetic sequence. The height distribution factor gives the fraction between the cell at the ground and the cell at the upper boundary. The vertical cell distribution could be inspected under the 3D model section in the terrain report. Default value is 0.1 (-).
	Number of cells in Z direction
	Number of cells in the vertical direction. Default value is 20 (cells).
	Maximum number of cells
	The 3D model will consists of nx*ny*nz cells. The number of cells in Z direction is set in "Number of cells in Z direction". The number of cells in X and Y direction is set according to nx*ny*nz < "maximum number of cells", by skipping nodes in the grid.gws file. The obtained number of cells and corresponding grid resolution is found under the 3D model section in the terrain report. The computing time required is exponentially proportional to the number of cells. Consider splitting a large model into several smaller models using the nesting technique, described under the Wind Field module, or to use the refinement technique.
	Terrain smoothing limit
	In some cases a wind field simulation fails to give a physical solution. In these cases we do not get a converged solution, instead we get divergence. Smoothing of the terrain is a technique for battling divergence.
	Divergence could be caused by abrupt changes in the inclination between adjacent cells. Areas with abrupt changes in the inclination would be found in narrow valleys or at sharp mountain peaks. Typically, these areas would not be of interest for placing wind turbines, and as such a slight modification of the terrain could be accepted.
	In mathematical terms changes in the inclination are represented by the second order derivatives of the height. If the second order derivate surpasses a set limit of 0.005 then a warning is written during the execution of the module Terrain, with an advice of performing smoothing.
	Activation of smoothing will set a default smoothing limit of 0.004. All places where the second order derivatives are higher than the smoothing limit are smoothed, until the second order derivatives is less than the smoothing limit.
	Terrain smoothing should be used with care, as excessive use will significantly change the heights of the terrain. Both the second order derivatives and the changes between the original heights and the heights after smoothing are given in the report. Inspect these results, and make sure that areas of interest have not been significantly changed.
	The default setting is no smoothing.
	Orthogonalize 3-D grid
	Making the grid orthogonal is a technique for improving the convergence in cases with high inclination angles. By default the grid is not made orthogonal and then the grid extends in the vertical direction along straight vertical lines. Making the grid orthogonal means that the grid extends in the vertical direction along curved lines in an attempt of being perpendicular to the ground. In this way the cells will be less skewed in areas with high inclinations. When the inclination angles are higher than 50 ° it would be advisable to activate orthogonalization. The overall simulation time will go up when smoothing is activated.
	The default setting is no orthogonalization.
	4.		Forest
	Forest
	By default forest is disregarded. There is two ways of including a forest. The first method associates one roughness height present in the file grid.gws with given forest characteristics, see below. The second method reads all the forest characteristics directly from the grid.gws file, where these data has already been stored under specific keywords.
	Roughness height
	The roughness height used for specifying a forest. A forest is established at all locations with this given roughness height.
	Forest height
	The height of the forest. Default value is 30 (meters)
	Forest porosity
	The porosity of the forest. Allowed values between 0 and 1. A fully open area is obtained with porosity equal to 0, while porosity equal to 1 will give a fully blocked area.
	Forest cell count in Z direction
	The number of cells in the vertical direction used for the forest. The cell distribution is uniform. The remaining number of cells used above the forest, is Number of cells in Z direction - Forest cell count in Z direction. The vertical cell distribution could be inspected under the 3D model section in the terrain report.