Further Information

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• Is an interface model used in RWIND Simulation?

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FAQ 004391 EN

In RWIND Simulation, each model surface in the wind current is treated as a "smooth" wall. This definition results in a boundary layer in the areas around the flow, which has an influence on the velocity profile perpendicular to the wall depending on the air viscosity. This boundary layer is realized in RWIND Simulation according to the so-called "law of the wall". This law describes the velocity profile perpendicular to the wall and can be represented by the dimensionless variables u + and y + .

Dimensionless variable u + :
$\mathrm u^+=\frac{\mathrm U}{{\mathrm u}_{\mathrm\tau}}$
where
U ... Velocity on the wall
uτ ... Frictional velocity

Dimensionless variable y + :
$\mathrm y^+=\frac{{\mathrm u}_{\mathrm\tau}\cdot\mathrm y}{\mathrm\nu}$
where
y ... Wall distance
uτ ... Frictional velocity
ν ... Kinematic viscosity of air

Using the friction velocity uτ:
${\mathrm u}_{\mathrm\tau}=\sqrt{\frac{{\mathrm\tau}_{\mathrm w}}{\mathrm\rho}}$
where
τw ... Shear
ρ is air density

Describes boundary layer model in the viscous partial layer next to the wall,
$\mathrm u^+=\mathrm y^+$

and in the subsequent logarithmic layer
$\mathrm u^+=\frac1{\mathrm\kappa}\cdot\ln\;\mathrm y^++\mathrm C$

following velocity diagram.

where
κ ... Kármán constant (κ = 0.41 for the simulation of a smooth wall)
C ... Constant (C = 5 for the simulation of a smooth wall)

To ensure that the solution process is relatively fast and robust, the program specifies the corresponding boundary layer model directly in the first cell next to the model surface. The remaining part of the boundary layer results from the solution of the globally applied Navier-Stokes equations.

• What is the formula used to calculate the ASCE 7 wind velocity profile?

The wind velocity profile in RWIND Simulation according to the ASCE 7-16 standard [1] is calculated based on Eq. 26.10-1. The coefficients and basic wind speed in this equation below are incorporated in the wind pressure equation.

Velocity wind pressure (imperial): qz = 0.00256 Kz Kzt Kd Ke V2

We must reference this equation to calculate the inlet velocity relative to elevation for the RWIND Simulation CFD wind tunnel. To consider only velocity rather than pressure from this equation, the basic wind speed is multiplied by the square-root of each coefficient. Notice the velocity variable in Eq. 26.10-1 is squared which requires the square root of the coefficients to be considered.

$Inlet\;velocity\;=\;V\sqrt{K_e\;\cdot\;K_{d\;}\cdot\;K_z\;\cdot\;K_{zt}}$

Because the ASCE 7-16 standard does not address wind CFD analysis and magnitude of the required inlet velocity, it is difficult to draw comparisons. Therefore, this is the closest estimate for calculating the RWIND Simulation inlet wind velocity per the code.

• Is it possible to only transfer a specific selection of elements to the wind tunnel in RWIND Simulation?

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FAQ 004373 EN

Yes, it is; the interface application "Simulate and Generate Wind Loads" with the "Export only active objects" option under the "Settings" tab controls which objects should be transferred to the wind tunnel of RWIND Simulation.

Figure 01 - Simulate and Generate Wind Loads

• If the "Export only active objects" option is not selected, all objects available in the model are always placed in the wind tunnel.

• In the other case, the program only moves the objects in the currently displayed visibility of the model into the wind tunnel. The other hidden objects are not taken into account.

• When trying to perform a calculation, I get the message "The path to RWIND Simulation working directory cannot contain any non-ASCII characters." What can I do?

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FAQ 004338 EN

The working directory is a local path where the data of the currently opened structure is temporarily handled and saved. It consists of the first letters of the file. Only ASCII characters may be used.

The non-ASCII characters are, for example, "ä," "ö," and "ß."

To avoid this problem, replace the special characters in the file name by the ASCII characters, such as "ae" and "ss" instead of "ä" and "ß." When you open the file again, the message will no longer appear.
• Where in RWIND Simulation can I highlight the locations with the maximum and the minimum surface pressures on the model?

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FAQ 004334 EN

The program provides the "Min/Max Values" option under the "Scalar Fields" branch in the Display navigator. If you activate this option, the program displays a result bubble in the graphic with the smallest and the largest globally occurring result for the following result types:

• In RWIND Simulation, I would like to rotate the model, adjust the wind tunnel, and then rotate the model again. How can I prevent the rotation point of the model from being changed?

FAQ 004332 EN

In RWIND Simulation, you can manually adjust the model position and the model rotation. To do this, simply select Model → Edit in the navigator, and then enter the desired rotation angle in the Position tab. In this case, the rotation is carried out without changing the displacement.
• Are there any facade surfaces in RFEM for the transmission of wind loads to the main structure?

In RFEM, there are no special facade surfaces for distributing wind loads. However, you can create such a load distribution element without any influence on the resistance of the main structure using the standard tools of RFEM. To do this, proceed as follows:

1. Define the isotropic linear elastic "facade material" with the average stiffness of the adjacent elements of the main structure without the weight, thermal expansion property, and stiffness modification.

Figure 02 - Material of Facade Surfaces

2. In the facade areas, describe the surfaces made of the facade material with the "Orthotropic" stiffness type. To ensure that there is no opposed load at the surface corners transferred to the main structure, the thickness and torsional stiffness must be defined near to zero. We recommend to apply the average thickness of the adjacent elements of the main structure / 1,000 as the thickness, and to divide the related torsional stiffness k33 by the factor of 1,000 as well.

Figure 03 - Properties of Facade Surfaces

3. Define a line release between the facade surfaces and the main structure elements so that the forces perpendicular to the main structure elements can only be transferred. All other directions must be specified without any force.

Figure 04 - Possible Definition of Connection Between Facade Surfaces and Elements of Main Structure

4. To ensure that the surfaces with the line releases do not slip in the wall plane, it is necessary to apply the surface support to the facade surfaces in the plane degrees of freedom x and y.

Figure 05 - Support of Facade Surfaces

This option allows for the distribution of compression loads acting perpendicular to the facade surfaces as well as of wind pressures from the RWIND Simulation calculation to the main structure. In this case, it is necessary to use this modeling of the facade surface in connection with the linear method of analysis.

• From which RFEM or RSTAB version can models be exported in RWIND Simulation?

FAQ 004306 EN

The 'Simulate and Generate Wind Loads' function for exporting models to RWIND Simulation is available in RFEM or RSTAB from version x.21.01 of 22.10.2019.

If you do not have a service contract, you can download the program version you have purchased on the following page:

• Is the Gust-effect (G or Gf) from the ASCE 7-16 Sect. 26.11 considered in RWIND Simulation?

Yes, the Gust-factor G or Gf, can be adjusted in RWIND Simulation. This value can be changed within the "Wind load" tab under "Wind velocity."

In the ASCE 7-16, the conservative value for the Gust-factor, G, is 0.85 for rigid buildings. The engineer can calculate an alternative and more accurate value. The Gust-effect, Gf, for flexible buildings accounts for size and gust size similar to rigid buildings but also considers dynamic amplification including wind speed, natural frequency, and damping ratio.

• Will other standards be added to the wind profile generation options in RWIND Simulation?

FAQ 004299 EN

Additional standards and codes to generate the wind profile automatically in RWIND Simulation will be added in the future. We are always considering feedback from our current customers on which standards will be beneficial.

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If not, contact us via our free e-mail, chat, or forum support, or send us your question via the online form.

First Steps

We provide hints and tips to help you get started with the main programs RFEM and RSTAB.

Wind Simulation & Wind Load Generation

With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.

The generated wind loads acting on these objects can be imported to RFEM or RSTAB.

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