# BEM algorithm

The Blade Element Momentum (BEM) theory is used to compute the aerodynamic loads on the rotor. For that, each blade is divided into a number of Blade aerodynamical station, and the force exerted by the air on each station is computed.

The BEM theory uses the concept of induced velocity to calculate the aerodynamic loads on the blade elements: as a result of extracting kinetic energy from the flow, the velocity of the air is modified. This variation is represented by the so-called induced velocity. Estimating the induced velocity is key in determining the aerodynamic loads on the blade element.

The Induced Velocity is a mathematical representation of the action of the blade on the air: when air particles hit the blade, they will produce an aerodynamic load on the element that will move the blade. As a consequence of Newton's third law, the element produces an equal and opposite force that modifies the velocity of the air particles. The difference between the air particle velocity upstream and downstream the blade is represented by the induced velocity

Two different BEM algorithms have been implemented in Ashes.
• The Steady BEM can be used to compute loads in steady conditions, and is used as the initial condition for the Unsteady BEM. The steady BEM will calculate the aerodynamic loads on the rotor assuming quasi-static conditions, therefore it is not recommended to use it in situations where the incoming wind velocity on a blade element varies (i.e. any realistic situation)
• The Unsteady BEM accounts for the time variations within the simulation and calculates the aerodynamic loads on the rotor at a time step based on the characteristics of the system at that time step and the loads at the previous time step. The Unsteady BEM uses the loads calculated by the Steady BEM for the first time step of the simulation.
It is possible to swicth between these two implementations in the Advanced section of the Aerodynamics tab in the Analysis parameters dialog.