Development of a flow visualization model using the transport tube method for application in vertical axis wind turbine analysis

As wind farms continue to grow in size and number of turbines, the reduced-speed region behind a turbine (its wake) is becoming a significant factor that can affect other turbines in a farm. In order to understand how air speed (thus momentum and kinetic energy) is recovered by a turbine’s wake before reaching another downstream turbine, the characteristics of wake flow needed to be examined. The structure of wake flow was studied using the transport tube method, an analysis model that visualizes the transport of mass, momentum, and kinetic energy as three-dimensional tube structures. Numerical integration was used to create transport tubes in velocity, momentum flux, and kinetic energy flux vector fields. Multiple sample flow fields of varying conditions were used to gauge the method’s accuracy before it was further applied to idealized axisymmetric wake flow. Our results indicated that the transport tube method was effective at identifying flow features (e.g. eddies) and transport patterns of mass, momentum, and kinetic energy in a variety of flows both 2D and 3D, laminar and turbulent. Additionally, the analysis of idealized wake flow demonstrated the mechanism by which kinetic energy was recovered in downstream flow. The viscous effect caused a component of kinetic energy transport to direct towards the center of the wake, resulting in flow of kinetic energy from surrounding high-speed freestream flow to the low-speed wake.