3.4.1 Wind Load

Wind Load

In this tab, you can define the common parameters that are relevant to determine the wind load.

This tab section controls the wind directions which will affect both the wind exposure of the model and the number of generated load cases. The angle φ to set the wind directions refers to the positive global X-axis of the model. The rotation is defined clockwise about the positive Z-axis.

You can apply a Uniform division of the wind rose by equidistant steps Δφ. The finer the division steps are, the more load cases will be created. Define the start angle φ_A (0° is aligned to X-axis) and the end angle φ_B of the distribution.

Alternatively, you can apply Manually defined directions: enter the values of the angles and separate them by semicolons. To save user-defined directions, click the button and enter a description. Click the button to import a user-defined list of wind directions.

In this section, you can define the parameters of the wind velocity and turbulence. There are two options for the wind velocity.

The wind load is determined over the height of the building, depending on standard-specific parameters. The standards EN 1991-1-4 [1], ASCE/SEI 7-16 , and NBC 2015 [3] are available for selection. The Image above illustrates the parameters associated with EN 1991-1-4. When a different standard is set, the content of this section is adjusted to the regulations of the code.

For EN 1991-1-4, select the National Annex. If applicable, you can set the relevant Wind zone. Use the button to open a map showing the wind zone of your country.

When you double-click a location on the map, the corresponding wind zone will be set in the list.

Select the terrain Category in the list. Depending on your National Annex, different categories are available.

For some National Annexes, the wind velocity is also dependent on the Altitude H_S of the terrain. The value can be entered manually.

As the result of all parameters, the Fundamental wind velocity v_b,0 is displayed. The wind profile is calculated from this value, taking into account the overall height of the model. According to EN 1991-1-4 [1], 4.2, this value is the characteristic 10 minutes mean wind velocity, irrespective of wind direction and time of year, at 10 m above ground level in open country terrain with low vegetation such as grass and isolated obstacles with separations of at least 20 obstacle heights.

You can review the wind velocities of every level in the left table below and left chart to the right.

Instead of the fundamental wind velocity, you can Consider the mean velocity. According to EN 1991-1-4 [1], 4.3, the mean wind velocity v_m(z) at a height z above the terrain depends on the terrain roughness and orography. It is determined according to EN 1991-1-4 [1], Expression (4.3). This accounts for the fact that effects of peak velocity pressure appearing on the model are considered by a specific wind tunnel simulation, which allows for the application of lower mean velocities. When this option is activated, the table and chart are updated.

The profiles of the mean velocities v_m are specified in the National Annex for Germany, Table NA.B.2. for the four terrain categories, for example.

The Density of the air is dependent on the altitude, temperature, atmospheric pressure, and humidity. It has an effect on the dynamic behaviour of the fluid.

You can define any wind profile not represented by the available standards in a table sheet. Enter the Level z and allocate the corresponding Wind Velocity. [Enter] creates a new table line for the next level.

The [Import from MS Excel] button enables you to import a specific pressure curve from an active Excel sheet.

The velocity diagram is displayed dynamically in the left chart.

The buttons below the table have the following functions.

Table 3.2 Buttons for user-defined diagrams

Button	Function
	Opens a dialog box where you can select a user-defined wind profile
	Opens a dialog box where you enter a name for the current wind profile to save it
	Exports the current wind profile to MS Excel
	Imports a wind profile from MS Excel
	Restores the default settings
	Opens the Windows calculator where you can compute values

The check box and table are accessible when the Use uniform turbulence option to consider turbulence is disabled (see Section 'Turbulence Properties' below). Here you can define any turbulence profile in a table sheet. Enter the Level z and allocate the corresponding Turbulence Intensity. [Enter] creates a new table line for the next level.

The [Import from MS Excel] button enables you to import a specific turbulence curve from the active Excel sheet.

The other buttons below the table are described in the Table above.

The turbulence intensity diagram is displayed dynamically in the right chart.

For the calculation with RWIND Simulation, the effects of turbulence are essential. If you want to Consider turbulence, activate this option. The effects of turbulent flow are characterised by chaotic changes of pressure and flow velocity, contrasting laminar flow. For further explanation, see the Chapter 'Turbulence'.

The Model of turbulence can be based on the correlations between k and epsilon or k and omega.

The first option is set as default. Depending on the selection, you can specify either the turbulent dissipation rate ε or the specific dissipation rate ω. In some cases, the simulation using k_ω is more stable than the one using k_ε.

The fluctuating component of the wind is represented by the Turbulence intensity defined in EN 1991-1-4 [1], 4.4 or ASCE/SEI 7-16 , 26.11.4, for example. It is the standard deviation of the turbulence divided by the mean wind velocity. The recommended rules are given in EN 1991-1-4 [1], Expression (4.7) or ASCE/SEI 7-16 , Eq. (26.11-7).

When the Use uniform turbulence at the inlet option is set, you can define the intensity I as a global percentage, i.e. the ratio of root-mean-square of the turbulent velocity fluctuations and the averaged velocity at the same location over some time period. An idealised flow of air with absolutely no fluctuations in air speed or direction would have a turbulence intensity value of 0%. For high-turbulence cases, the turbulence intensity is typically between 5% and 20% (see CFD Online). The turbulence intensity is set to 1% by default to cover most medium- and low-turbulence cases. When you deactivate the Use uniform turbulence at the inlet option, you can define the turbulence intensity at specific height levels in the User-defined Turbulence table (see Section 'Wind Profile' above).

The Terrain level option controls which part of the building is exposed to the wind. The default value is zero, which means that the wind flows around the lowest point of the model. If the model contains a basement storey or foundations, for example, the terrain level should be adapted accordingly. The value refers to the orientation of the global Z-axis: if it points upwards, a positive value moves the model downwards. If the Z-axis points downwards, however, you have to enter a negative value. The terrain level represents a global value, i.e. it is not possible to define an inclined terrain.