Wind Energy

44
Numerical Simulation of Dynamic Stall
using Spectral/hp Method
B. Stoevesandt, J. Peinke, A. Shishkin and C. Wagner
Reduction of the weight of wind turbine blades is of large interest to manufacturers.
To do so without losing the necessary stability, blade loads caused by
dynamic stall effects have been a major task in aerodynamic research during
the last years [1].
The aim of the project is to calculate lift and drag caused by the effect of
dynamic stall as they arise in turbulent wind fields. This is done by means
of wind tunnel measurements and numerical flow simulations. For the flow
simulation a spectral/hp code has been chosen to achieve high accuracy [2].
In order to simulate the dynamic stall the boundary conditions have to be
flexible. Currently the main focus of the works lies on reliability tests of the
numerical code.
44.1 Introduction
Due to the increasing size of wind turbine airfoils their weight is becoming an
increasing problem. The aim of highest efficiency, best lift at a minimum of
cost and weight seems to be an unsolvable contradiction. One of the problems
in the design of airfoils is, that mechanical stability investigations are based
on estimations of the lift caused by dynamic stall.
Dynamic stall is induced by an unsteady inflow on the airfoil. For wind
turbines a main factor is gusty inflow on the profile causing rapid changes
of wind speed and direction. Thus at a tip with a tip speed of 80 m s −1 a
change of wind speed of 8 m s −1 would even cause an inflow deviance of 5.7 ◦ .
Figure 44.1 illustrates that these occurrences are of relevant order.
Simulating turbulence is difficult with traditional CFD approaches since
these methods are stabilized using numerical dissipation. In contrast the numerical
dissipation of spectral/hp-element methods is comparably low. The
aim of the project is to investigate the flow separation at the profile in 3D
and to analyse the extreme loads on the airfoil caused by turbulence.