Parrot Sphinx can simulate the effects of wind on the drone. You can configure its speed magnitude but also its direction. The speed can not only be static but also dynamic according to the provided ExprTk expressions and other available options.

## How to configure#

Wind parameters are accessible by the means of sphinx-cli, or any other JSON-RPC tool. For example, to set the mean wind to 5 m/s directed toward East, enter the following:

$sphinx-cli param -m world wind/wind magnitude_mean 5.0$ sphinx-cli param -m world wind/wind direction_mean 0.
$sphinx-cli param -m world wind/wind elevation_mean 0.  If you want to add to the mean wind a horizontal 2 m/s wind with an orientation that turns 360° every 60 seconds: $ sphinx-cli param -m anafi wind/wind magnitude_expr "2."
$sphinx-cli param -m anafi wind/wind direction_expr "360 * time / 60"$ sphinx-cli param -m anafi wind/wind elevation_expr 0.


The wind is modeled as the sum of two independent parts controlled by two different wind components:

• the mean part. Speed vector that is constant or slowly varying according to drone altitude. This part is controlled by the wind world component.

• the dynamic part. Speed vector computed from ExprTk expressions. It can typically contain speed variation representing gusts. This part is controlled by the wind drone component.

These two parts are optional and can be set to zero.

They all are defined by their respective characteristics:

• Full magnitude in m/s

• Horizontal direction in degrees

• Angle in degrees formed by the horizontal plane and the wind direction

Each part of the simulated wind can be smoothed by low-pass filters on magnitude, direction and elevation. The aim is to prevent any brutal discontinuity produced by either by parameter change at run-time, or by the behavior of any ExprTk expression.

Let’s delve into each aspect of the model.

## Low-pass filtering#

To prevent the wind from changing too fast in a non-realistic way, wind values can optionally be low-pass filtered by setting their time_for_rise_xxxxx to a value greater than 1 ms.

The time_for_rise_mean parameter is found in the wind world component, while the time_for_rise_dynamic parameter is found in the wind drone component.

The equation of the filter is:

$v(t) = (1 - 0.001 / time\_for\_rise) * v(t-1) + 0.001 / time\_for\_rise * target$

Therefore when time_for_rise_mean or time_for_rise_dynamic is set to 0.001 second (or lower), the filter is deactivated and the means change instantly creating potential discontinuities.

## Height-dependent mean wind#

The mean part of the wind can optionally depend on:

• The height $$z$$ above the ground

• The terrain roughness $$c_r$$ defined by the terrain_category parameter of the wind world component

By default, the height above the ground and terrain roughness are ignored. To activate their effects, the enable_terrain_effect parameter of the wind world component must be set to true.

Important

When the enable_terrain_effect parameter is set to true, the magnitude_mean parameter defines the mean wind velocity at 10 m above ground level.

When activated, the mean wind velocity at height $$z$$ is determined using the expression found in section 4.3 of [EN1991-1-4]:

$v_m(z) = c_r(z) * c_0(z) * v_b$

… where $$v_b$$ is the characteristic mean wind velocity, at 10 m above ground level in open country terrain with low vegetation. The orography factor $$c_0$$ accounts for the increase of mean wind speed over isolated hills and escarpments. In the simulator, for ease of use, the orography factor $$c_0$$ is always set to 1, which means that the average slope of the terrain exposed to the wind is always considered to be less than 3°.

Available terrain_category values are:

Value

Description

0

Sea, or coastal area, exposed to sea winds; lakes and bodies of water traversed by wind over a distance of at least 5 km.

1

Open countryside, with or without a few isolated obstacles (trees, buildings, etc.) separated from each other by more than 40 times their height.

2

Countryside with hedges; vineyards; bocage; scattered habitat.

3

Urbanized or industrial areas; dense bocage; orchards.

4

Urban areas, of which at least 15% of the surface is covered by buildings whose average height is greater than 15 m.

Example of use:

$sphinx-cli param -m world wind/wind enable_terrain_effect true$ sphinx-cli param -m world wind/wind terrain_category 2


European Committee for Standardization, EN 1991-1-4:2005+A1:2010 - Eurocode 1. Actions on structures. General actions. Wind actions, CEN, 2005/2010.

## Gusts#

### Parametric profile#

You can define periodic wind gusts using the ExprTk function gust_magnitude(duration, transition, pause).

… Where:

• duration (in s): time of the gust

• transition (in s): transition time between no gust and gust (before and after the duration time)

• pause (in s): delay between gusts

This function is expected to be used only in the field magnitude_expr of the wind drone component. It generates a gust of magnitude 1 m/s for the defined period. You can adjust to the desired speed by simply multiplying the function by a scalar.

A pause value of 0.0 means a single gust cycle.

The function gust_presence() returns:

• a value of 1.0 only during the duration phase.

• a value between 0.0 and 1.0 for each transition phase. One at the start of the cycle, another one at the end.

• a value of 0.0 the rest of the time or if there is no gust.

The total time for a gust cycle is:

$duration + 2 * transition + pause$

You can optionally add noise to the magnitude (e.g. markov_noise()) for more realistic results.

$sphinx-cli param -m anafi wind/wind magnitude_expr "10.0 * gust_magnitude(18, 5, 2) + markov_noise(0.5, 0.05)"  To simulate changes in gust elevation and direction, use markov_noise() combined to gust_presence(). $ sphinx-cli param -m anafi wind/wind elevation_expr "gust_presence() * markov_noise(0.999, 1.0)"
$sphinx-cli param -m anafi wind/wind direction_expr "gust_presence() * markov_noise(0.999, 0.5)"  ### Predefined profile# You can also run prerecorded gusts data by setting the predefined_gust parameter of the wind drone component. • medium_intensity: a medium strength gust of wind between 3 m/s and 7 m/s • high_intensity: a strong gust of wind between 10 m/s and 18 m/s $ sphinx-cli param -m anafi wind/wind predefined_gust "medium_intensity"


To deactivate the use of prerecorded data, set predefined_gust to an empty string. By default, predefined_gust is set to an empty string.