# Environment: Wind data

OrcaFlex includes the effects of wind on:

You may choose whether or not wind loads are included for vessels, lines, 6D buoys and 6D buoy wings.

### Air density

The air density is assumed to be constant and the same everywhere.

### Air kinematic viscosity

Used to calculate Reynolds number. The value here is fixed and cannot be edited.

### Air speed of sound

Used to calculate the wind turbine unsteady aerodynamics.

### Vertical wind variation

Wind speed is assumed to be the same at all heights, unless a vertical wind speed profile is specified. To specify a vertical wind speed profile, you may define the wind speed variation with height above the mean water level (MWL) as a dimensionless multiplicative factor. To do so, you define a vertical variation factor variable data source. Negative factors may be used, allowing you to model reversing wind profiles.

A value of '~' means that there is no vertical variation.

If you are using the OCIMF model for wind load on vessels, the speed is expected to be that at an elevation of 10m (32.8 ft) above the mean water level (MWL). If you have the wind speed $v(h)$ at some other height h (in metres), then the wind speed $V(10)$ at 10m can be estimated using the formula $v(10) = v(h)\,(10/h)^{1/7}$.

 Note: The vertical wind variation profile data is not available, and is not applied, when full field wind is modelled. For full field wind the vertical variation in the wind velocity is specified directly in the external full field wind data.

## Wind type

Wind can defined a number of different various ways, by setting the wind type to one of the following.

### Constant

The wind is defined by specifying its speed and direction, which remain constant over time.

### NPD spectrum, API spectrum, ESDU spectrum

The wind speed varies randomly over time, using a choice of either the NPD spectrum, API spectrum or the ESDU spectrum.

In these cases:

• The wind direction remains constant over time.
• The spectrum is defined by the ref. mean speed (the 1-hour mean speed at an elevation of 10m (32.8 ft) above MWL) and the elevation above MWL at which the wind speed is to be calculated. From these, OrcaFlex calculates the mean speed at the given elevation and parameterises the spectrum which determines the statistical variation about that mean. If a value of '~' is specified for the elevation, it is taken to be that of the reference mean speed, i.e. 10m (32.8 ft) above MWL. The view spectrum button plots a graph of the resulting spectrum.
• The min and max frequencies bound the range considered by the spectral discretisation algorithm: only spectral energy between these frequencies will contribute to the wind velocity. The default values correspond to the range of frequencies for which the NPD spectrum is defined, as documented in ISO 19901-1:2005. To include all the energy in the spectrum, use values of 0.0 and infinity. If the max frequency is infinity, we use an approximation to integrate the tail of the spectrum out to infinity which requires that the min frequency must be set to 0.0.
• The wind speed is modelled by the superposition of sinusoidal functions of time, their number given by number of components, whose amplitudes and frequencies are chosen by OrcaFlex to match the spectral shape. OrcaFlex uses the same equal-energy approach to choosing the amplitudes and frequencies as for wave spectra discretisation. You should choose a number of components large enough to give a reasonable representation of the spectrum.
• The wind speed is ramped from the mean speed to the dynamic speed during the build-up period.
• The phases of the components are chosen using a pseudo-random number generator that generates phases which are uniformly distributed. The phases generated are repeatable – i.e. if you re-run a case with the same data then the same phases will be used – but you can choose to use different random phases by altering the seed used by the random number generator.
• The view wind components button gives a report of the components that OrcaFlex has chosen. This will tell you the width of the frequency intervals, which can help you to judge whether the number of components is sufficient.
• If the ESDU spectrum is selected, the absolute value of the site latitude must be specified.
 Note: When frequency domain dynamic analysis is enabled, the mean wind speed is used during static analysis, and the wind spectrum specifies the dynamic wind behaviour.

### User defined spectrum

A user-defined spectrum is given by a table of pairs of values of frequency $f$ and S, the spectral energy $S(f)$.

The given values of $f$ do not need to be equally-spaced. For intermediate values of $f$, OrcaFlex will obtain $S(f)$ by linear interpolation. S(f) is taken as zero for values of $f$ outside the range of the table. Your table should therefore include enough points to adequately define the shape of $S(f)$ (particularly where $S(f)$ has a wide range or high curvature) and should cover the full frequency range over which the spectrum has significant energy.

The above description of wind speed calculation for NPD, API and ESDU spectra applies equally to user defined spectra, with the following exceptions:

• The mean speed is entered directly, rather than being calculated from the ref. mean speed and elevation.
• The min and max frequencies are determined by the range of frequencies in the table.
 Note: When frequency domain dynamic analysis is enabled, the mean wind speed is used during static analysis, and the wind spectrum specifies the dynamic wind behaviour.

### User specified components

The wind is defined as the sum of a number of given sinusoidal components. For each component you give:

• Frequency or period: you may specify either one of these – the other is automatically updated using the relationship period = 1 / frequency.
• Amplitude: the single amplitude of the component – that is, half the peak to trough height.
• Phase lag: the phase lag relative to the wind time origin.

The randomise phases button will generate a random phase value for each component, replacing all the existing data.

### Time history (speed)

The wind speed variation with time is specified explicitly by time history. Linear interpolation is used to obtain the wind speed at intermediate times.

You must also provide mean speed and mean direction to apply in the statics calculation. The wind direction remains constant over time.

The wind speed is ramped from the mean speed to the dynamic speed during the build-up period.

### Time history (speed & direction)

Both the wind speed and direction variation with time are specified explicitly by time history. Linear interpolation is used to obtain the wind speed and direction at intermediate times.

You must also provide mean speed and mean direction to apply in the statics calculation.

The wind speed and direction are ramped from the mean values to the dynamic values during the build-up period.

### Full field

Full field wind allows for variation of wind velocity in both space and time, with data specified in an external file. At the moment the only supported format is the binary TurbSim .bts full field file. The coordinate system used in the .bts files is a right-handed system with $x$ horizontal in the direction of propagation, $y$ horizontal normal to $x$, and $z$ vertically upwards.

The .bts files contain time series of 3D wind velocity, $V_g(y,z,t)$, at points on an evenly spaced grid in the vertical $yz$ plane. Optionally, the file may also contain time series of 3D wind velocity, $V_t(z,t)$, at tower points in a single line below the grid.

For full field wind you must define the following:

• The name of the .bts file. You can give either its full path or a relative path. Clicking file header allows you to view the information contained in the .bts file header.
• The wind direction and origin which determine how the .bts file's coordinate system is mapped on to the OrcaFlex coordinate system. A value of '~' for the $Z$ wind origin can be used to specify that the vertical origin is at the mean water level.
• The wind time origin, specified relative to the global time origin.
• Ramped, which allows you to control how the wind is applied during the build-up period. When checked, the wind velocity is ramped from zero to the full value during the build-up period, otherwise the full value is applied at all times. When wind is ramped, no wind is applied during static analysis, otherwise the wind velocity at the simulation start time is applied during static analysis.

OrcaFlex uses Taylor's frozen turbulence hypothesis, and the mean wind speed recorded in the .bts file, to map between $V_g(y,z,t)$ and $V_g(x,y,z,t)$, and between $V_t(z,t)$ and $V_t(x,z,t)$.

To interpolate in the grid, OrcaFlex uses barycentric interpolation in $y,z,t$ space. Over the tower region, if defined, OrcaFlex uses barycentric interpolation in $z,t$ space.

For points outside the grid, OrcaFlex clips to the edge of the grid, along each primary axis. For example, consider a .bts file with no tower points, and with a grid defined at $y_1, y_2, \ldots, y_{n_y}$ and $z_1, z_2, \ldots, z_{n_z}$. For values of $y < y_1$ or $y > y_{n_y}$, OrcaFlex clips $y$ to $y_1$ or $y_{n_y}$ respectively. Similarly, for values of $z < z_1$ or $z > z_{n_z}$, OrcaFlex clips $z$ to $z_1$ or $z_{n_z}$ respectively.

The .bts file format supports periodic time histories. If the file is periodic, as recorded in the file header, OrcaFlex will interpret the data accordingly. For non-periodic files, if extrapolation in time is required it is performed by clipping to the defined range.