Vortex shedding - Karman Vibration

Vortex shedding is a special case of harmonic loading. One of the most important mechanisms for wind-induced oscillations is the formation of vortices (concentrations of rotating fluid particles) in the wake flow behind certain types of structures such as chimneys, towers, suspended pipelines,… At a certain (critical) wind velocity, the flow lines do not follow the contours of the body, but break away at some points, thus the vortices are formed. These vortices are shed alternately from opposite sides of the structure and give rise to a fluctuating load perpendicular to the wind direction.

In SCIA Engineer, the Vortex Shedding was implemented according the Czech loading standard. The effect is only taken into account if the critical wind velocity calculated is between a minimal and maximal value. These two extremes can be defined by the user. These values are taken as 5 m/s and 20 m/s. In addition, in SCIA Engineer it is possible to specify the length of the structure where the Von Karman effect can occur. For each geometric node of the structure, it is possible to relate a length of the cylinder to the node. This implies that, in order to obtain precise results, the structure should be modelled with sufficient geometric nodes. By default the effect can occur over the entire height of the structure however, when there are specific obstacles on the surface of a chimney for example, these obstacles will hamper the formation of the vortices and thus reduce the Von Karman effect. In practice, this is exactly the solution to suppress vortex-induced vibration: the fitting of special ribs on the surface of the cylinder.

The procedure for use of Karman Vibration

1) Activate the Dynamics functionality

2) Create a Mass Group

3) Input Masses OR Generate Masses from Static Load cases

4) Create a Mass Combination

5) Create a Karman Vibration Load case and specify its parameters

Load type = Dynamic; Specification = Karman vibration

Diameter of he tube is the diameter of the cylindrical part of the structure that will be analysed in the given load case.

The Wind direction is defined in the Global Coordinate System. A direction of 0,00 deg specifies the global X-axis. This implies that the Karman vibration will occur in a direction along the Y-axis (perpendicular to the wind direction).

Option ‘User number of excited eigenshape’ can be used to manually specify for which eigenmode the Vortex shedding needs to be calculated. When this option is not selected, the program determines the eigenmode automatically.

6) Input Karman Loads (lengths)

The program allows you to relate a length of the chimney to each geometric node. This load can be input using function Load > Point Force > Karman Load.

7) Refine the Finite-Element Mesh if required

Use Mesh Setup dialogue for mesh refinement.

8) Specify the number of Eigenmodes to be calculated

Use the Solver setup dialogue for this.

9) Perform a Linear Calculation

Once the calculation has been performed, additional information can be found in the Report about the calculation. The following information is listed:

• the number of the excited eigenmode,

• maximum horizontal translation,

• critical velocity,

• Reynolds number, etc.

Read also Dynamics Theoretical Background.