Control of Wind Turbine Flows
using Vortex Generators
Clara Velte
Martin O. L. Hansen
Dalibor Cavar
Knud Erik Meyer
MEK/Section of Fluid Mechanics
DTU
Vortex generators (VGs) are commonly used on wind turbine
blades and airplane wings
From: [1] Wind Turbine Technology, Spera D.A (editor) ASME Press 1994.
From: [2] Kermode A.C., Mechanics of flight, Pearson (11th edition) 2006.
Increase in aerodynamic efficiency
through passive techniques
Schematic figure of flow problem, 2D flow case.
Suction side of airfoil is represented by bump in wind tunnel.
Pressure distribution  unsteady, 3D separation bubble.
Separation might be controlled by vortex generators (VG:s).
VGs transfer momentum from the free stream flow into the BL and thereby delay
separation.
Schematic explanation of how a VG works:
By creating a longitudinal vortex, high momentum air is
transferred down into the bottom of the BL to increase
momentum and thus resistance to adverse pressure gradients
with the result of delaying stall.
Example of the potential of applying VGs on WT blades:
ELKRAFT 1000 kW Turbine at Avedøre
LM blade using NACA airfoils for outer part and thick FFA airfoils for
the inner part
~ 25 % power
increase @ 15 m/s
From: [3] S. Øye, The effect of Vortex Generators on the performance of the ELKRAFT 1000 kW
Turbine, 9th IEA Symp. On Aerodynamics of Wind Turbines, 1995.
To optimize the use of VGs, DSF has funded a project to study
the detailed flow behind the devices and thus better understand
the physics. A fundamental study.
• SPIV and LDA measurements in low speed wind tunnel at DTU
- to create a benchmark
• Simulations using CFD (LES & DES)
[5] Godard G., Stanislas M., Control of a decelerated boundary layer. Part 1: Optimization of
passive vortex generators, Aerospace Science and Technology Progress in Aerospace Sciences
10 (2006) 181-191
Optimization and characterization study of VGs.
A bump is designed to keep the flow on the verge of separation.
The wall shear stress is measured with a hot film
It is concluded that an optimum geometry is:
Counter rotating triangular VGs
h/ =0.37, L/h= 2.5, l/h=2, /h=6, =18o
Reproduced from [5]
SPIV was used for characterization of the flow
Baseline
VGs
x/h=22
x/h=38
x/h=57
Reproduced from [5]
The grey scale
indicates the
out-of-plane velocity
component and the
arrows the in-plane
velocities.
Non-dimensional streamwise velocity from HW
measurements at point of minimum skin friction in
plane of symmetry
(Reproduced from [5])
- The flowfield (velocity, turbulence) will be measured and
described very thoroughly (SPIV & LDA).
- CFD computations will be performed and compared with
measurements (LES & DES).
Status:
The wind tunnel is under severe reconstruction. The main
reason is for obtaining better optical access.
Methods are being tested to decrease effects from
reflections.
LES computations of the clean bump have been initiated.
Reconstruction of Test Section
Some sample results, stereoscopic PIV
x/h=4
x/h=6
x/h=10
The colour indicates
the out-of-plane
velocity component
and the arrows the inplane velocities.
Rhodamine 6G
Absorbs light at ~ 530 nm
Emits light at ~ 560 nm
Camera equipped with green
pass filter (~532 nm)
Low sensitivity to temperature
& pH-value
Application of R6G to model
Reflections from bump surface
Black paint
Black paint + R6G
x/h=0
x/h=1
x/h=2
x/h=4
x/h=6
Reflections from bump surface & VGs
Black paint
Black paint + R6G
x/h=0
x/h=1
x/h=2
Stay tuned… 