#### Further Information

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• ### Which formula is used in the RF‑/TOWER Loading add-on module to calculate the first natural frequency for the determination of the structure coefficient?

The first natural frequency is required to determine the structure coefficient. It is not determined by using a generalized formula, but the integrated eigenvalue solver RF‑/DYNAM, taking into account the real mass distribution and displaying the results in Column A of Table 2.3.

• ### How can I reactivate an expired trial license of RWIND Simulation on a single-user computer?

The RWIND Simulation program is protected against unauthorized use. Before you buy the program, you can use a limited trial license to test all program features before the purchase. After this trial period, the program switches to a limited demo mode and deactivates the function to activate a new trial license. If you indicate a justified reason, the program can be authorized again with a temporary trial license via the e-mail activation.

For this, proceed as follows:

1. Run RWIND Simulation "As Administrator." To do this, right-click the RWIND Simulation icon and select "Run As Administrator."

2. Click the "Activate Now" button to confirm the authorization check.

Then, the "RWIND Simulation 1.xx Demo Authorization Status" dialog box appears.

3. In the "RWIND Simulation 1.xx Demo Authorization Status" dialog box, select the "Software Key" option in the Authorization Method area and click the "Activation via E-Mail" function in the lower part.

This opens the "Activation by E-Mail" dialog box.

Furthermore, enter a user-defined description of the workstation to be activated under "Description of Workspace" and the RWIND Simulation level with an expiry date of the activation to be carried out under "Required Authorization."
Then, click the "Generate Request Code" button to confirm your entries.

The "RWIND Simulation 1.xx Demo" dialog box appears.

5. Check the data entered in the "RWIND Simulation 1.xx Demo" dialog box. If they comply, click "OK" to confirm the dialog box.

This opens the "Activation by E-Mail" dialog box.

6. The "Activation by E-Mail" dialog box consists of three parts. In the "Step 2" tab, the required request code is displayed on the left. Copy this text block to the Windows clipboard by clicking the "Copy Request Code" button and then paste the code into a blank e‑mail with [Ctrl]+[V] or the Windows paste function. Send this e-mail to support@dlubal.com with the justified reason for a new trial period for further processing.

7. You will receive an activation code from us. In the dialog box, go to the "Step 3" tab and enter this code in the "Activation Code" text box.
Then, click the "Activate Now" button to run the activation process.

After the successful authorization, you receive a confirmation message and can test the program for the specified trial period without any restrictions. After the expiration of the trial period, the program automatically switches to the limited demo mode.

• ### How are the properties of the elasticity and shear modulus of a membrane fabric with the usual force/length syntax transformed into the general force/surface syntax to be entered in RFEM?

The thickness of membranes is usually very thin compared to the planar extension. Due to these extreme geometric conditions, the stiffness of membrane fabrics is usually related directly to a strip width, that is the line (compare with a line spring), without considering the thickness.

In contrast, the general FEA software RFEM processes the material definitions (E, G, ν, and so on) and surface properties (shell, membrane, and so on) independently of each other. Thus, the pure definition of the material still does not clarify whether there is a rigid plate structure or a flexible membrane structure subjected to a tensile load. The final element specification is not clear until the surface properties are considered additionally for the simulation. Therefore, RFEM always requires the description of stiffness in the general unit syntax of force/surface, regardless of the geometric conditions of the structural component to be simulated.

Thus, the line-related membrane stiffness in the force/length syntax can be transferred to the force/surface syntax in RFEM by considering the reference thickness d:

$\frac{\mathrm F}{\mathrm A}=\frac{\left({\displaystyle\frac{\mathrm F}{\mathrm L}}\right)}{\mathrm d}$

where
F is the force,
L is the length,
d is the reference thickness,
A is the surface.

The stiffness transformed into the force/surface format in this way is thus related to the reference thickness and can convert the initially specified membrane stiffness in the force/length format in RFEM by specifying the reference thickness d as the membrane surface thickness.

• ### Does RWIND Simulation apply a boundary layer model?

In RWIND Simulation, each model surface in the wind flow is treated as a "smooth" wall. This definition results in a boundary layer in the areas around the flow close to the walls, which has an influence on the velocity profile perpendicular to the wall depending on the air viscosity. This boundary layer is created in RWIND Simulation according to the so-called "wall law." 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 is the velocity on the wall,
uτ is the frictional velocity.

Dimensionless variable y+:
$\mathrm y^+=\frac{{\mathrm u}_{\mathrm\tau}\cdot\mathrm y}{\mathrm\nu}$
where
y is the wall distance,
uτ is the frictional velocity,
ν is the kinematic viscosity of the air.

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

By describing the boundary layer model in the viscous partial layer directly next to the wall
$\mathrm u^+=\mathrm y^+$

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

you obtain the following velocity distribution,

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

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

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.

• ### Why are there the intermediate results for some members exactly at the member end node in the result table of member internal forces?

The result display of intermediate values is generally linked to the member division property. Also, member loads defined in sections in RFEM generate further intermediate results. For example, if a member load defined in sections is distributed from the member start to the member end, the intermediate results are obtained at the member end nodes.

• ### Where in RWIND Simulation can I highlight the locations with the maximum and the minimum surface pressures on the model?

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:

• ### 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?

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:

• ### How can I extend an active license od the RWIND Simulation based on a software key online before the activation period expires?

The RWIND Simulation program is protected against unauthorized use. If the software key is authorized, the program is only authorized for a specific activation period, which must be defined by the user. After this period, the program automatically switches to the demo mode; the previously used license is freely availablea gain for a new activation. To avoid the reactivation on the same single‑user computer, it is possible to renew the license online (a computer is connected to the Internet) by specifying a new activation period before the activation period expires. Proceed as follows:

1. Run RWIND Simulation "As Administrator." To do this, right-click the RWIND Simulation icon and select "Run As Administrator."

2. Select the "RWIND Simulation - License and Authorization" function in the "Help" menu.

This opens the "RWIND Simulation 1.xx Authorization Status" dialog box.

3. Check in the "RWIND Simulation 1.xx Authorization Status" dialog box if an active software key authorization is available. This is the case if the "Software Key" option is selected in the "Authorization Method" section, and an active authorization is indicated by a green dot in the "Authorization Status" section.

If the check is negative, and a red dot is displayed, no RWIND Simulation license based on a software key is activated on the single-user computer. As a result, the activation period for a license cannot be extended.

If the requirements mentioned above are met, click the "Extend Activation Online" button. The "Extend Activation Online" dialog box appears.

4. The "Extend Activation Online" dialog box shows the license specification to be extended.

Enter your online activation key based on your contract documents and define the new activation period in the date field.

Then, click the "Extend Activation Now" function to run the online extension.

This opens the "RWIND Simulation 1.xx" dialog box.

5. In the "RWIND Simulation 1.xx" dialog with the confirmation prompt about the planned modification, click "OK" to finally change the activation period.

After successfully changing the activation period, you receive a confirmation message and can use the program without restrictions.
Thus, your purchased license is used in our system for the newly defined activation period. During this time, it is not possible to activate the RWIND Simulation program with the activation key on any other computer. The assignment of the license in our system ends with a proper deactivation or after the defined activation period has expired.

To avoid unnecessary license assignments in our system due to unexpected computer problems on your part (defect, loss, hardware replacement, employee change, and so on), we recommend you to keep the activation period as short as possible.

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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.

#### Your support is by far the best

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