Control of Wind Turbine Flows using Vortex

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					Control of Wind Turbine Flows
  using Vortex Generators

          Clara Velte
      Martin O. L. Hansen
        Dalibor Cavar
       Knud Erik Meyer

 MEK/Section of Fluid Mechanics
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
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

                                           The grey scale
                     x/h=22               indicates the
                                           out-of-plane velocity
                                           component and the
                                           arrows the in-plane


               Reproduced from [5]
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).

The wind tunnel is under severe reconstruction. The main
reason is for obtaining better optical access.

Methods are being tested to decrease effects from

LES computations of the clean bump have been initiated.
Reconstruction of Test Section
Some sample results, stereoscopic PIV


                                    The colour indicates
                           x/h=6    the out-of-plane
                                    velocity component
                                    and the arrows the in-
                                    plane 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





Reflections from bump surface & VGs
Black paint              Black paint + R6G



Stay tuned… 

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