Simulation and Optimisation of unsteady aerodynamic performance of a 2D VAWT rotor

Simulation and Optimisation of unsteady aerodynamic performance of a 2D VAWT rotor: With emphasis on airfoil design

Boohalli Shivamallegowda, Greeshma (TU Delft Aerospace Engineering)

Ferreira, Carlos (mentor)

Delft University of Technology

Aerospace Engineering

2018-04-23

Airfoil design plays an important role in improving the performance of Vertical Axis Wind Turbines(VAWTs). The aim of this thesis is to arrive at optimal airfoil geometries to enhance the performance of VAWTs and analyse the performance of these optimal airfoils under steady and unsteady operation conditions of the rotor. The airfoil design is aimed at rotors operating in the Reynolds number range of 10ዀ. The inverse airfoil design approach is used to arrive at the airfoil geometries. The design process involves formulating a multiobjective optimisation problem with the aerodynamic and structural performance of the airfoil as objectives.
After a comprehensive literature study, the objective derived by Ferreira and Geurts [25] was chosen for its suitability to the current work. The aerodynamic performance of the airfoil is gauged by the ratio of lift slope and drag coefficient. The structural objective is limited to the flapwise bending stiffness. To account for the weather impact, the aerodynamic objective also includes the effect of surface roughness.
An effective optimisation framework is built with the Non-dominated Sorting Genetic Algorithm(NSGA-II) to arrive at the Pareto front with best suitable airfoils for VAWTs. This framework is coupled with XFOIL which evaluates the geometries and another python based framework, Distributed Evolutionary Algorithms in Python(DEAP) which helps in setting up the genetic algorithm. Once the convergence is reached, few airfoils from the final Pareto front are chosen for further analysis.
Based on the literature study, the Actuator Cylinder Model(ACM) was chosen to simulate the performance of the airfoils in a VAWT rotor. The performance of the selected airfoils were analysed under steady flow conditions with the ACM. To account for the effects of unsteady flow over the airfoil, a Beddoes-Leishman type dynamic stall model formulated by the Risø national laboratories was integrated into the AC Model loop along with a correction to the effective angle of attack. The inviscid part of this model was validated with another AC Model
code from Li [42] and the inviscid panel method 푈2퐷푖푉퐴 [65].
The performance of the optimised airfoils is an improvement over the performance of the NACA airfoils of similar thicknesses which proves that the optimisation process has been successful. The performance of the airfoils in steady flow indicates that the power performance of the airfoil suffers more from the surface roughness as the thickness of the airfoil increases. The thicker airfoils perform better at high solidity applications at low tip speed ratios. The range of operational tip speed ratios decreases as the airfoils become thicker.
The performance of these airfoils under the effects of unsteady flow under the inviscid flow assumption provides an overview of the differences in the performance of the airfoils under unsteady flow conditions. The range of tip speed ratios where the airfoils show their best performance moves towards higher values indicating that under unsteady flow effects, the rotor needs to operate at a higher rotational velocity to achieve its best performance. However, to obtain a better insight on the performances of the airfoil under unsteady flow effects, further studies with the complete validated Risø model have to be carried out.

Airfoil optimisation
vertical axis wind turbine
unsteady flow effects
Actuator cylinder model
performance simulation of VAWTs

http://resolver.tudelft.nl/uuid:f1f934a6-6c10-4d59-a3b1-5b8cc6407572

51.9900, 4.3754

Student theses

master thesis

© 2018 Greeshma Boohalli Shivamallegowda