Aerodrag (for tower and nacelle drag)

Main command aerodrag

With this module, it is possible to apply aerodynamic drag forces at a given number of structures.

Subcommand aerodrag_element

Command block that can be repeated as many times as needed. In this command block aerodynamic drag calculation points are set up for a given main body.

Obl.	Command name	Explanation
*	mbdy_name	1. Main_body name to which the aerodynamic calculation points are linked.
	(old command body_name still usable)
*	aerodrag_sections	1. Distribution method: (“uniform” only possibility)
		2. Number of calculation points (min. 2).
	nsec	This command must be present before the sec commands.
		1. Number of sections given below.
	sec	This command must be repeated nsec times
		1. Distance in [m] along the main_body c2_def line. Positive directed from node 1 to node “last”.
		2. \(C_{d}\) drag coefficient (default=1.0)
		3. Width of structure (diameter)
	update_states	Logical parameter that determines whether the movement of the structure is included or not.
		1. parameter (1=states are updated (default), 0=not updated)

By choosing the uniform distribution, HAWC2 places \(n\) equidistant calculation points on the main body, from the first until the last node. The distributed aerodynamic drag is computed for each calculation point as

\[\begin{split}\begin{align*} f_x &= \frac{1}{2} \rho V_x^2 c C_d \mathrm{sgn}(V_x) \\ f_y &= \frac{1}{2} \rho V_y^2 c C_d \mathrm{sgn}(V_y) \\ f_z &= 0 \\ m_x &= 0 \\ m_y &= 0 \\ m_z &= 0 \end{align*}\end{split}\]

with \(\rho\) the air density, \(c\) the interpolated width at the calculation point and \(C_d\) the interpolated drag coefficient at the calculation point. \(V_x\) and \(V_y\) are the relative wind speed in the aerodynamic coordinate system at the calculation point, optionally including the structural one. The wind speed is evaluated only at the first iteration of each time step, while the structural one is always updated. The aerodynamic drag is not applied in the first 5 seconds of the simulation. Between 5 and 10 seconds it smoothly goes from 0 to 100%, where it remains until the end of the simulation.