University of Twente Student Theses


Improving the prediction of pressure gradient field in wind farm numerical simulation method, WAKEFARM

Sankara Subramanian, G.K. (2018) Improving the prediction of pressure gradient field in wind farm numerical simulation method, WAKEFARM.

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Abstract:The wind turbines in a wind farm interact aerodynamically through their wakes. The wakes are characterized by reduced wind speeds and increased turbulence. The numerical modelling of wind farms is vital, as it helps us to understand the wind turbine wake interactions and to predict the total power output of the wind farm better. The current wake model at the Energy research Centre of the Netherlands (ECN) is implemented into a Fortran code named WAKEFARM. It simulates the wake properties of a single turbine or a row of turbines. WAKEFARM solves RANS equations in perturbation form. The RANS equations used in WAKEFARM are parabolized in the streamwise direction. The two momentum equations in the transverse directions are elliptic. The axial pressure gradient in the axial momentum equation is prescribed along with the body force. It is calculated using inviscid vortex models. The induced velocity vector field calculated from the vortex model is given as initial guess to the perturbation variables in the three momentum equations. In this thesis work, two vortex methods are developed with circulation varying in spanwise direction: a model of a wind turbine rotor with more than three blades having constant axial induction and a model of a three-bladed wind turbine with spanwise varying axial induction. Both the models trail a helical wake. The root vortex is included in both the models. The axial pressure gradient is calculated from inviscid, incompressible Navier Stokes equations. The trend in the calculated axial pressure gradient is in good agreement with the momentum theory. The two new vortex models are implemented in WAKEFARM. The horizontal velocity profiles in the cross-flow direction at hub height are validated with field measurements in ECN’s wind turbine test site in Wieringermeer. The constant axial induction model correlates well with the experiments compared to the existing vortex models. The new method shows a flattened velocity profile near the centre of the wake due to the influence of root vortex. The centerline velocity profiles are validated with wind tunnel measurements in Marchwood laboratories. The centerline velocity profile at 5 rotor diameters downstream, predicted by the constant axial induction model has the best correlation with the Marchwood experiments.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Sustainable Energy Technology MSc (60443)
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