Date

2022-1-03

Author

Abdulrahim, Anas

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This thesis study presents the results of an experimental investigation focusing on the effects of inflow boundary layer on the wake characteristics of a porous disc (disc diameter-to-boundary layer height is 0.2) with radially non-uniform porosity in terms of mean flow, turbulence, wake scaling, and proper orthogonal decomposition. Two-dimensional two-component particle image velocimetry measurements within the wake are performed up to 7.5 diameters downstream as the disc is lowered deeper in to a boundary layer that is representative of a neutral atmospheric boundary layer over a flat terrain (corresponds to 435 scale factor compared to field length scale). Results show that otherwise symmetrical wake velocity profiles that exist outside the boundary layer get skewed and sheared around the disc centerline in the boundary layer due to the inflow wind shear. The turbulent kinetic energy, its production and Reynolds shear stress levels in the wake get asymmetrical around the centerline of the disc such that the production of turbulent kinetic energy is observed to be higher above centerline. Due to the inflow shear, the wake centerline gets shifted downwards (i.e., towards the wind tunnel wall), which is in contrast to the observations on real wind turbine wakes in the literature where the wake actually lifts up. The asymmetrical and skewed velocity profiles both in the streamwise and cross-stream directions can be collapsed onto a single function by using proper wake scaling parameters based on the ratio of local strain to average strain within the velocity profile calculated separately for either side of the wake. Proper orthogonal decomposition reveals that the most energetic modes are the first and second modes. The differences in the modes are significant when the disc is at different positions within the boundary layer. Finally, the wake velocity profiles of the porous disc are compared against model predictions from the Bastankhah and Porté-Agel wake model and the Ishihara and Qian wake model under different inflow conditions. A method is proposed as an extension to the wake models to generate asymmetric wake velocity profiles. The comparison illustrates the deviation due to the wake growth rate and initial wake width estimation. The wake growth rate of the porous disc can be significantly higher than field turbines as well as those estimated through empirical relations suggested in the literature in the context of engineering wake models, particularly at low ambient turbulence conditions. This suggests caution in the use of porous discs to simulate wind turbine wakes and wind farms in wind tunnel studies. Results show that the when using the estimated wake growth rate and initial wake width of the porous disc, the wake models show better predictions overall for all the test cases considered.

Subject Keywords

Atmospheric Boundary Layer, Porous Disc, Wind Turbine, Wake Model, Wake Turbulence, Wake Growth Rate, Proper Orthogonal Decomposition

URI

https://hdl.handle.net/11511/95947

Collections

Graduate School of Natural and Applied Sciences, Thesis