(667e) CFD Simulation of the Wind Turbine Performance Under Rainy Conditions

Wind turbines operate in open environments and their performance is affected by the atmospheric conditions, including rain. Rain affects the performance of a wind turbine by reducing the momentum of the boundary layer as a result of rain droplets forming a water layer adhering to the turbine blade surface and consequently changing  the condition of its properties.

To obtain a better understanding of the effect of rain, which provides valuable information in deciding the site for a new wind turbine, a multiphase computational fluid dynamics (CFD) model to simulate wind turbine performance was developed.  In our model, the Lagrangian Discrete Phase Model (DPM) capable of simulating rain droplets was coupled with the Eulerian Volume of Fluid (VOF) models capable of simulating the water layer formation and flow over the wind turbine blades. The coupling between DPM and VOF models was implemented by adding two source terms to the VOF model’s continuity and momentum equations. As each rain droplet hits the blade’s surface or enters the water layer around it, the droplet is removed from the computational domain and its effect is accounted for by adding an instantaneous source term to each of the VOF’s mass and momentum equations.

Initially, we applied our model to simulate air and rain flow patterns using wind turbine blade airfoil NREL S809 and studied the effect of rain at different angles of attack, surface property of the airfoil, and rainfall rate. Our simulation showed that, the wetting surface has higher lift and lower drag coefficients, both favorable, in comparison with a non-wetting surface. This behavior is mainly due to the added roughness caused by water on a non-wettable surface, which leads to increase in drag force and decrease in lift force. Due to the dominating effect of surface tension compared to droplet momentum, increasing the rainfall rate will alter the performance of the airfoil only up to a certain rainfall rate at which a water layer is formed all over the airfoil.  Once the rainfall rate is high enough to immerse the entire upper surface of the airfoil under water, a further increase in the rainfall rate will not have a tangible effect on the performance of the airfoil. Regarding the effect of angle of attack, our calculated results showed that, due to the upward momentum that the droplets add to the airfoil at higher angles of attack, in rainy conditions the lift coefficient continues to increase even past the single-phase stall angle. However, this increase in lift is accompanied by a significant increase in drag coefficient caused by flow separation.

In addition, numerical simulations were performed to study the performance of three dimensional wind turbines in both no-rain and rainy conditions.