18.6: Moment-of-Momentum Equation

The moment-of-momentum equation is a critical tool for analyzing the torque produced by the rotating blades of a wind turbine. This equation is derived by applying Newton's second law to a fluid particle, which states that the rate of change of linear momentum is equal to the external force acting on the particle.

In the case of rotating systems like wind turbines, the moment of this force, or torque, is determined by considering the position of the fluid particle relative to the axis of rotation.

To calculate torque, the position vector of the fluid particle is combined with the external force acting on it. This combination yields the torque experienced by the particle.

The next step involves expanding the time derivative of the moment of momentum. This expansion is divided into two parts: one representing the rate of change of angular momentum due to the particle's velocity, and the other accounting for the fluid's movement through space.

Since the change in position over time is equal to the particle's velocity, the mathematical expansion simplifies, leading to a clearer understanding of how forces act within the rotating system.

The final form of the moment-of-momentum equation is obtained by integrating the relevant terms over the control volume, which encompasses the region of fluid interacting with the turbine.

This equation provides a relationship between the torque exerted on the system and the angular momentum entering and exiting the control volume, as well as the external forces acting on the system. In the context of wind turbines, this allows for a detailed analysis of how the blades generate rotational energy from the wind, improving the efficiency and performance of the turbine.