This study presents a numerical optimization of a ducted wind turbine (DWT) to maximize power output. The cross section of the duct was an Eppler 423 airfoil, which is a cambered airfoil with a high lift coefficient (CL). The rotor was modeled as an actuator disk, and the Reynolds-averaged Navier–Stokes (RANS) k–ε model was used to simulate the flow. The optimization determined the optimal placement and angle for the duct relative to the rotor disk, as well as the optimal coefficient of thrust for the rotor. It was determined that the optimal coefficient of thrust is similar to an open rotor in spite of the fact that the local flow velocity is modified by the duct. The optimal angle of attack of the duct was much larger than the separation angle of attack of the airfoil in a freestream. Large angles of attack did not induce separation on the duct because the expansion caused by the rotor disk helped keep the flow attached. For the same rotor area, the power output of the largest DWT was 66% greater than an open rotor. For the same total cross-sectional area of the entire device, the DWT also outperformed an open rotor, exceeding Betz's limit by a small margin.

An experimental and computational comparison of the turbulent flow field for a sharp 90 deg elbow and plugged tee junction is presented. These are commonly used industrial geometries with the tee often retrofitted by plugging the straight exit to create an elbow. Mean and fluctuating velocities along the midplane were measured via two-dimensional (2D) particle image velocimetry (PIV), and the results were compared with the predictions of Reynolds-averaged Navier–Stokes (RANS) simulations for Reynolds numbers of 11,500 and 115,000. Major flow features of the elbow and plugged tee were compared using the mean velocity contours. Geometry effects and Reynolds number effects were studied by examining the mean and root-mean-square (RMS) fluctuating velocity profiles at six positions. Finally, the asymmetry of the flow as measured by the position of the centroid of the volumetric flux and pressure loss data were examined to quantify the streamwise evolution of the flow in the respective geometries. It was found that in both geometries there was a large recirculation zone in the downstream leg but the RANS simulations predicted an overly long recirculation which led to significantly different mean and fluctuating velocities in that region when compared to the experiments. Comparison of velocity profiles showed that both experiments and numerics agree in the fact that the turbulence intensities were greater at higher Re downstream of the vertical leg. Finally, it was shown that the plugged tee recovered its symmetry more rapidly and created less pressure loss than the elbow.

No abstract

Ducted wind turbines (DWTs), when compared to an open rotor of the same rotor diameter, have the potential to extract more energy from the wind. Positioning the duct at the optimal orientation has a significant impact on the achievable performance. This work focuses on a numerical optimization study to determine this orientation. The Reynoldsaveraged-Navier-Stokes (RANS) k− model was used to simulate the flow. An axisymmetric geometry was utilized to model the DWT. The cross-section of the duct was an Eppler 423 airfoil, which is a cambered airfoil with a high lift coefficient (CL). The rotor was modeled as an actuator disc. The results showed the optimal orientation of the duct as well as the ideal coefficient of thrust for the rotor, this coefficient of thrust is analogous to an open rotor despite the local flow velocity being altered by the duct. The optimal angle of attack for the duct is substantially higher than what one would expect based on the maximum lift angle of an airfoil in a freestream. The higher angle of attack can be attributed to the expansion due to the rotor wake, which keeps the flow attached to the duct. The DWT has a higher power output than an open rotor based on both the area of the rotor and the total cross-sectional area of the DWT, generating a performance increase of 66% and 4.7% respectively. However the addition of the duct is not without cost. To produce the equivalent power per unit cross-sectional area requires a stronger tower than needed for an open rotor.

No abstract