Helge Madsen Aagaard

Simulation of low frequency noise from a downwind wind turbine rotor

One of the major drawbacks of a wind turbine with a downwind rotor is the generation of considerable low frequency noise (so-called thumping noise) which can cause annoyance of people at a considerable distance. This was experienced on a number of full-scale turbines in e.g. US and Sweden in the period from around 1980 to 1990. One of the common characteristics of this low frequency noise, emerging from analysis of the phenomenon, was that the sound pressure level is strongly varying in time. We have investigated this phenomenon using a model package by which the low frequency noise of a downwind rotor can be simulated. In order to investigate the importance of wake unsteadiness, time true CFD computations of the flow past a 4 m diameter cylinder were performed at 8 m/s, and the wake characteristics were subsequently read into the aeroelastic code HAWC, which finally gives output to the aero acoustic model. The results for a 5 MW two-bladed turbine with a downwind rotor showed an increase in the sound pressure level of 5-20 dB due to the unsteadiness in the wake caused mainly by vortex shedding. However, in some periods the sound pressure level can increase additionally 0-10 dB when the blades directly pass through the discrete shed vortices behind the tower. The present numerical results strongly confirm the experiences with full scale turbines showing big variations of sound pressure level in time due to the wake unsteadiness, as well as a considerable increase in sound pressure level if the blade passing frequency is close to the Strouhal number controlling the vortex shedding from the tower.

On the Dynamics of a Teeter Hinge with Delta3 Angle on a Two Bladed Multi MW Wind Turbine

This study analyses the effect of delta-3 on a 5 MW turbine using a simple analytical model of the teeter system in combination with time simulations on the same turbine with the aeroelastic code HAWC2. The analysis shows that the delta 3 angle changes both the eigen-frequency and the damping of the teeter system. The effect is strongly dependent on the lock number which further seems to change with the up-scaling of the turbines. For a modern turbine the lock number is typically above 20 and the teeter motion is strongly damped and the influence of the delta-3 is small. However, a reduction of the out-of plane movement of the blade tip can be obtained using a positive delta-3 angle. Also the influence on flapwise moment and the yaw moment is discussed. The most used wind turbine concept of today is the three blade upwind turbine, but the two teetering bladed downwind turbine could be a rival for the off-shore market. The major problems with two bladed turbines are the increased noise 1 from the blade passing the tower and the visually impact of the two blade concept, but for off-shore this will not be a problem. The blade cost and the tower top mass of a two bladed turbine will be reduced compare to the traditionally three bladed design, and the teeter mechanism of the two bladed turbine reduce the blade loads. The teeter motion dynamics can be changed by including a �3 angle, which couples the teeter motion to a pitching of the blade. However, as described in a recent paper by Malcolm 2 the role of the delta-3 angle has been shrouded in some mystery and there seems not to be clear guide-lines in the literature about how to choose the optimal delta-3 for a specific turbine configuration and what the main effects are on the teeter dynamics. In the present paper a simple analytical model, suggested by Anderson 1, is used to study and characterize the dynamics of a two-bladed teetering rotor with a delta 3 angle. The results of the analytical model are compared with numerical results from simulations with the aeroelastic model HAWC2, developed at Risoe National Laboratory.

A Stochastic Static Stall Model Applied to Wing Turbine Blade

The aim of this work is to improve aeroelastic simulation codes by accounting for the unsteady aerodynamic forces that a blade experiences in static stall. A model based on the spectral representation of the aerodynamic forces is defined. Some three-dimensional features of the actual flow are taken into account in the model. The input data for the model can be collected either from wind tunnel measurements or numerical results from Computational Fluid Dynamics calculations of airfoil sections at constant angles of attack. An analysis of these data is provided which helps to understand the characteristics of stall. The model is applied to the case of a wind turbine rotor and results are compared to experimental data.