An integral boundary layer engineering model for vortex generators implemented in XFOIL

Abstract

To assess and optimize vortex generators (VGs) for flow separation control, the effect of these devices should be modelled in a cost and time efficient way. Therefore, it is of interest to extend integral boundary layer models to analyse the effect of VGs on airfoil performance. In this work, the turbulent boundary layer formulation is modified using a source term approach. An additional term is added to the shear-lag equation, to account for the increased dissipation due to streamwise vortex action in the boundary layer, forcing transition at the VG leading edge where applicable. The source term is calibrated and a semi-empirical relation is set up and implemented in XFOIL. The modified code is capable of addressing the effect of the VG height, length, inflow angle, and chordwise position on the airfoil's aerodynamic properties. The predicted polars for airfoils with VGs show a good agreement with reference data, and the code robustness is demonstrated by assessing different airfoil families at a wide range of Reynolds numbers. KEYWORDSintegral boundary layer, separation control, source term, vortex generator, Xfoil INTRODUCTIONFor the next generation of wind turbines, manufacturers aim to design multimegawatt rotors to improve the competitiveness of wind energy technology. To up-scale wind turbines, novel technologies are required and new design challenges will appear. One such aerodynamic challenge is the management of separated flow. Preventing or at least delaying separation over the blades can positively affect the annual energy production (AEP).On top of that, the magnitude and severe variations of the aerodynamic loads associated with separating flows can be reduced, mitigating structural fatigue issues. In the wind energy industry, separation control is often realized by using passive vortex generators (VGs).Vortex generators improve the resistance of a boundary layer against flow separation by re-energizing the flow close to the surface. The streamwise vortices shed from the free tips of the VGs enhance mixing between the high-energy flow in the outer part of the boundary layer with the low-energy regions near the walls (see Schubauer and Spangenberg. 1 ) The physics involved is nontrivial and poses a number of modelling challenges.To achieve a cost-effective scale up of current turbines, it will become necessary to evolve towards a multidisciplinary design process where VGs are already incorporated early in the design phase. To assess and optimize the use of VGs, there will be a need to effectively model these devices in a cost and time efficient manner. BackgroundNumerous VG modelling techniques have been explored in literature, most of which use computational fluid dynamics (CFD). The most direct and intuitive approach is to model the effect of VGs by including them as a local geometrical protrusion in the domain. This approach requires a fully . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...

Airfoil Selection Procedure, Wind Tunnel Experimentation and Implementation of 6DOF Modeling on a Flying Wing Micro Aerial Vehicle

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“…However, it has the limitation of the simplified nature of the shear term and the calibration for a single VG configuration. De Tavernier et al 32 extended this method for a broader application by proposing a semi-empirical relation between the source term, VG variables and boundary layer properties using a broad range of experimental and high-fidelity numerical data. The source-term integral (I ST ) was discovered to be subject to the most influnence of VG effects.…”

“…Daniele et al 31 suggested a sin-exponential increment instead of a step change in 30 , which was shown a good agreement with experimental results and increased numerical stability. De Tavernier 32 further extended Kerho and Kramer's approach 30 by introducing a smooth step function and decay rate to the source strength term to Xfoil 33 . However, all these work based on viscous-inviscid formulation only considers steady flow.…”

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“…Thus, the dimensions of the airfoil can affect the drag force and lift force that occurs. 14 An airfoil is a form of aerodynamics intended to produce a large lift with the smallest possible drag.…”