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• ### How can I display wind speeds at a certain height above the ground for a wind comfort analysis?

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The RWIND Simulation program displays the size of the wind speeds around the model bodies under the Velocity Field result. By displaying this result type, the program automatically switches to the "slicer" display. This display type shows the scalar result, which is spatially available at all positions between the model bodies, only in the area of a plane (slicer).

For a wind comfort analysis, the wind velocity result at a certain distance to the traffic area is usually required. For this, proceed as follows:

1. Activate the Velocity Field result.

2. Rotate the slicer planes into a plane parallel to the traffic area to be analyzed.

3. In the settings, define the desired distance of the slicer plane to the traffic area by entering a height.

• ### I have generated a load from area load on circular opening. If the radius is 1.0 m and the load is 10 kN/m², the resultant should be 31.416 kN. The recalculated load on the line should be 5.0 kN/m. However, the value of the converted load is 4,968 kN/m. Why?

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The inaccuracy is caused by the fact that RFEM calculates the area of the circle approximately by dividing the boundary, that is, as a polygon. In this case, the result is 3.1214447 kN instead of 31.416 kN. On the other hand, the length of the boundary curve is calculated exactly as a circle.
If you need a graphically accurate number (that is, an image for the structural analysis), this can be solved by reducing the number of decimal places displayed.
• ### After importing loads from another model, Warning No. 11003 appears. Some free loads are obviously perpendicular to the projection plane. What can I do?

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The projection plane and the load direction are different parameters and independent of each other.

While the load direction is the actual direction of the acting load, the projection plane together with the corresponding coordinates of the load position describes the plane from which the position of the load is projected onto the surfaces subjected to the load.

In the example file, a surface perpendicular to the load was selected incorrectly for the free load. Thus, the warning appears during the calculation.

In the case of free loads, it is therefore always necessary to ensure that all surfaces rest in the correct projection plane.
• ### How can I create a load arranged in a checkered way?

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It is recommended to create separate load cases for each span to be loaded. It is then necessary to generate the loads by span in the individual load cases. Free rectangular or polygonal loads allow you to enter surface loads on a freely definable area of the surface. As an alternative, you can divide the surface into partial surfaces and apply the surface load.

Furthermore, it is possible to use the RF-MOVE Surfaces add-on module to create load cases including loads.

You can then create the checkered arrangement of the loads in the load combinations or result combinations by combining the individual load cases.

The video shows the manual creation and generation of load cases and loads with RF‑MOVE Surfaces.

• ### Where can I define a wind velocity diagram according to a specific wind standard for the numerical wind tunnel analysis in RWIND Simulation?

In RWIND Simulation, the wind velocity is organized by means of the height of the wind tunnel inlet in a table, depending on the height coordinates and wind speeds. You can enter the data in this table manually in RWIND, or by using specific function generators.

If you do not create the model directly in RWIND Simulation, but transfer it from RFEM or RSTAB to RWIND Simulation, you can also define the wind velocity diagram in the interface window according to one of the available wind standards:
• EN 1991-1-4
• ASCE/SEI 7-16
• NBC 2015
The wind velocity diagram then results from the respective standard parameters (zone, terrain category, and so on).

• ### How can I automatically generate result combinations?

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In RFEM and RSTAB, you can automatically create combinations according to various standards. In the case of a purely linear calculation, the generation of result combinations can be defined in General Data of the model. The attached video shows the automatic generation.
• ### How can I access the RWIND Simulation program from RFEM, assuming that it is possible to transfer a 3D model created in RSTAB?

RWIND Simulation is exclusively compatible with RFEM due to the CFD modeling, which requires a finite element calculation. The model created in RSTAB can be opened in RFEM. The wind load parameters can be defined in the Simulate and Generate Wind Loads dialog box of RFEM.

At the bottom of the Wind load tab, it is possible to remain and automatically generate loads for the corresponding load cases the CFD calculation has been performed in the background.

If you would rather perform the wind simulation in RWIND, click the Open in RWIND Simulation button in the Load Cases tab.

To obtain the trial version of RWIND Simulation, proceed as described in this FAQ.

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

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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.

• ### In the case of member loads, the loads are entered in Table "3.2 Member Loads." I can view or change the values.In my model, I have applied generated loads. Should the member loads not be also visible in Table "3.15 Generated Loads"?

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The load values of generated loads are recorded in Table "3.15 Generated Loads." It is not possible to make any changes there.

If you want to change the loads, you have to disconnect the generated loads. You can do this quickly by using the shortcut menu of the load (see the figure).

• ### I cannot graphically display a variable polygon load on a surface correctly. It is not clear which load is actually applied. Is there any display option for this?

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There is an option for this in the Results navigator of RFEM 5.

After the calculation of a load case or a CO, you can switch to the Results navigator and select the "Load Distribution" result type.

Thus, you can display the load applied in the respective element. This can also be used very well for a graphical documentation of the load and printed as a graphic in the report.

1 - 10 of 147 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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