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Table of Contents

Transient Wind Options

Use this option to define sinusoidal wind speed and direction transients such as are defined in certain standards, such as, for example, IEC 61400-1. These transients are superimposed on any steady-state spatial variations defined by wind shear, tower shadow and an upturbine wake.

Click the Wind icon on the toolbar, select Time varying wind, and choose the Transients option.

First enter the following data:

  • Reference height at which the wind speed is defined, unless Refer wind speed to hub height has been selected. Please see reference height for more details. If wind shear is defined, the wind speed at any other height will be different.

  • Flow inclination: for non-horizontal wind flows (e.g. on the side of a hill). A positive value indicates a rising wind.

Separate transients may be defined for each of the following variables:

  • Wind speed: defined at the reference height.

  • Wind direction: measured clockwise from North - see wind direction.

  • Horizontal wind shear: defined as the difference in wind speed between the two sides of the rotor plane. The wind speed varies linearly with horizontal distance across the rotor plane.

  • Vertical wind shear: defined as the difference in wind speed between the top and bottom of the rotor plane. The wind speed varies linearly with height.

  • Vertical direction shear: defined as the difference in wind direction between the top and bottom of the rotor plane. The wind direction varies linearly with height.

All defined transients will be used, but if any is not required, specify an appropriate start value and a zero amplitude. For each transient, enter the following information for the variable in question:

  • Start value: the value of the variable before the start of the transient.

  • Amplitude of change: the largest range from maximum to minimum during the transient.

  • Time to start cycle: the time into the simulation at which the transient starts.

  • Time period of cycle: the duration of the transient.

  • Type of cycle: Three types of transient are provided, as specified by various standards. A half wave transient begins with the start value and increases sinusoidally by the amount defined by the Amplitude. A full wave transient begins with the start value and changes sinusoidally by the amount defined by the Amplitude in the first half of the time period, changing back to the start value by the end of the time period. The IEC-2 transient models a gust. The equations used for each cycle type are described in transient wind

Note that all three types of cycle are available for all combinations of the wind speed and shears, but the IEC-2 model is not recommended to be used for defining the vertical and horizontal shears because it does not conform with the IEC standard. Instead, the simplified cosine function shall be used when one intends to define certain transient effects by setting appropriate amplitudes, either using full or half wave transients.

Last updated 28-08-2024