Numerical Investigation of the Boundary Layer Control and Wind Turbine Power Production Enhancement Using Solid Vortex Generators (SVGs) Yannis G. Perivolaris InFlow Wind Energy Consultant yiannis@inflow.gr Spyros G. Voutsinas National Technical University of Athens spyros@fluid.mech.ntua.gr Abstract The aerodynamic behavior of blades equipped with solid vortex generators (SVGs) is analyzed by means of 3D RANS CFD simulations considering non-rotating and rotating cases. The presence of the SVGs is modeled by introducing the corresponding loading as extra source term in the momentum equations. Furthermore, taking into account that SVGs are applied in series of co- and counter-rotating layouts of small periodic span, one such periodic strip is modeled with either symmetry or cyclic side conditions. The model is first validated against existing wind tunnel measurements on non- rotating (fixed wing) configurations. Then, the rotating case is considered in which case the preservation of the total pressure in the spanwise direction is added. Keywords: solid vortex generators, stall delay, boundary layer control, CFD, 3D effects 1 Operational characteristics of SVGs An SVG is a small thin plate fitted normal to the surface of the blade at a fixed angle α with respect to the chordwise direction. Operating as a lifting surface, the SVG will generate a pronounced tip vortex which will exchange retarded fluid from the near wall region with high-energy fluid from outside of the boundary layer. Momentum is transported within the boundary layer which will increase the skin friction and result separation delay. SGVs are applied on fixed wings or blades in series covering part or the entire span. There exist two lay-out possibilities: the co-rotating and the counter-rotating (Figure 1). Theoretically, the counter-rotating layout achieves better mixing, thus more appropriate for industrial applications involving primarily heat exchange (Shabaka et al. [1], Mehta & Bradshaw [2].). Typically, SVGs are placed over the suction side of the inboard part of wind turbine blades and at a distance from the leading edge ranging between 10% and 30% of the chord length. They will increase the maximum lift coefficient by delaying separation, but there will be also a penalty in drag. It is worth noticing that over the inboard part of the blade, 3D rotational effects are pronounced, especially within the separated region (Zhaohui Du & Selig [3], Chaviaropoulos et al. [4]). The radial pressure gradient will induce a radial flow pumping which is responsible for the increased lift coefficient as compared to 2D values at the same incidence. This is a separate mechanism, so when SVGs are applied, their action will interact with the pressure pumping effect. The prediction of this combined effect on the performance of the blade is a challenge for CFD also because the involved length scales are small and usual computer resources do not allow an adequate resolution of the complete blade. 2 CFD modelling options SVGs are known to be effective on suppressing flow separation and widely used in various thermo-fluid applications. An in depth review on the boundary layer flow separation control using low-profile vortex generators, is given by Lin [5]. Key results are presented in several aspects. It is concluded that SVGs are best suited for applications in which flow-separation locations are relatively fixed and the generators can be placed reasonably close to the separation line. As regards numerical simulations, a lot of effort has been put in analyzing the dependence on the choice of the turbulence model [6, 7, 8, 9]. It is concluded that the frequently used Boussinesq hypothesis provides acceptable approximations. Another important aspect is the way the SVGs interact with the flow. In [10] a single SVG was fully meshed using 6.5 10 6 cells. The simulations indicated that the intensity of the vortices was under predicted suggesting that a finer grid is needed.