Experiments, Objectives and Results

In order to successfully translate experimental results to practical airfoil applications, flow conditions need to be properly scaled. The Reynolds number and the boundary layer pressure gradient parameter are of particular importance for such wall-bounded flows.

The boundary layer wind tunnel (BLT) of the TU Delft allows prescription of arbitrary pressure gradients by adjusting the back wall of the test section (Figure 2). The length of the test section (~5m) also means that much thicker boundary layers can be obtained as compared to what is more commonly obtained on actual airfoils, facilitating boundary layer flow measurements. A stereo-PIV setup was built around the tunnel on a traversing system, enabling accurate control of the measurement planes up/downstream of the VG array.

An array of sub-boundary layer VGs

with the following parameters were

investigated:

• Rectangular/Triangular vanes

• Counter-rotating configuration

• Device height ~0.5δ

Experimental vortex

generator study for Wind Energy applications

Background

Aerodynamic lifting surfaces are designed to operate in predefined flow conditions. The separation of flow from said surfaces may be encountered during some stage of operation and is generally undesired. Active and/or passive flow control offers the possibility to eliminate or manipulate this flow phenomenon. Boundary layer suction, passive vortex generators (VGs) and plasma actuators are just some examples found in aeronautics and engineering applications.

Wind turbines are no exception. Early evidence of the use of simple passive VGs on a wind turbine rotor emerged in the 1990s with field tests suggesting an increase in 25% of the power output of a stall regulated 1MW machine1.

Progress and Outlook

Following a review of literature on the subject, the first experimental campaign was conducted on characterising the flow induced by a fixed vane-type vortex generator. Post-processing of these first results will shed insight on the downstream evolution of the embedded streamwise structures, subject to different pressure gradients.

An additional experimental campaign before the end of the first year of research will shed further insights on

• the three-dimensionality of the vortex generation, evolution and breakdown processes, under pressure gradients closer to those experienced on thick airfoil sections found near the blade root region

References

[1] 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. [2] AdVanced Aerodynamic Tools for lArge Rotors (AVATAR), Grant No. FP7-ENERGY-2013-1/no. 608396; http://www.eera-avatar.eu/home/ [3] Smart-Blade GmbH; http://smart-blade.com/products-services/vortex-generators.html

Linking experiments with theory

Numerous computational techniques (CFD) are aimed at correctly modelling the effect of vortex generators. However, the ability to rapidly assess the performance of airfoils mounted with such devices cannot be overstated. Integral Boundary layer (IBL) codes such as XFOIL and RFOIL are examples of such dedicated tools which are of particular significance.

Such codes couple the inviscid outer flow with the viscous boundary layer equations, enabling an elegant and rather accurate solution for the flow around airfoils. There is therefore great value in investigating the possibilities of modelling VGs in the context of an IBL approach. Past researchers have done so from the perspective of “added” turbulence. Advances in optical measurement techniques however have allowed for detailed characterisation of the induced flow from VGs.

With this new knowledge, in addition to the contributions from the present work, it is desired to formulate new boundary layer closure relations which are based on a more physical interpretation of the VG induced flow. Doing so would allow for successful integration of improved airfoil design and optimisation in wind turbine design cycles.

PhD Candidate: Daniel Baldacchino Department: AWEP Section: Wind Energy (DUWIND) Supervisor: dr. ir. C.J. Simão Ferreira Promoter: prof. dr. G.J.W. van Bussel Start date: September 2013 Funding: AVATAR (FP7) Type: Engineering Scientific

Aero

space

Engin

eering

Fig. 4 Successive regions of down/upwash induced within and between VG pairs. Contours depict in-plane

(vertical) velocity.

Injected momentum

Fig. 2 Boundary layer wind tunnel modified to produce an adverse pressure gradient

Test-Wall

Adjustable back-wall

Reducing the extent of separated flow, typically experienced near the blade root sections, boosts blade aerodynamic performance. A recent report suggested, based on field monitoring, that an annual energy production (AEP) increase of 2.2% for wind turbines mounted with VGs is possible2. Apart from the AEP, VG-actuated flow may also serve to mitigate other unsteady effects viz. fatigue loading and aerodynamic noise.

All this is accentuated when considering next generation 10-20MW wind turbine rotors exceeding 200m diameter. Tackling the challenges posed by such rapid growth is the AVATAR project, within which this research is conducted3.

Fig. 1 VG vanes visible near wind turbine blade root1.

VG cascade

Fig. 3 Vortex rollup & diffusion in the region behind a rectangular vane-type vortex generator. Contours represent out-of-plane, streamwise velocity component (plane spacing not shown to scale). 𝑼𝒆

With these setup parameters, the following is studied:

• Influence of the pressure gradient on integral vortex properties (e.g. circulation, core size and convection velocity)

• Influence of crossflow at the VG (apparent yaw) representing radial flow near the blade root

These new insights will allow for further validation of semi-empirical vortex generator flow models, which could also offer routes towards the desired IBL modelling approach.

Preliminary experimental results show the roll-up process of the streamwise vortices emanating from the VGs, shown in Figure 3. Also evident is the growth of the vortex core in the streamwise direction. These first results show that the current setup is able to deliver detailed flow fields of the boundary layer as well as to resolve the coherent streamwise vortices. Figure 4 shows the vertical (in-plane) velocity, with downwash evident behind the VG. Upwash regions are observed on either side, due to the presence of adjacent VGs from the rest of the array.

• The influence of the state of the boundary layer (laminar, turbulent) on the near wake region of the VG viz. vortex formation

The new data and insights will be the starting point for further analytical work to improve the viscous-inviscid design codes, and extend their usability to incorporate these passive control devices.