Abstract
A hypothesis was tested and validated for predicting the vortex strength induced by a vortex generator in wall-bounded flow by combining the knowledge of the Vortex Generator (VG) geometry and the approaching boundary layer velocity distribution. In this paper, the spanwise distribution of bound circulation on a vortex generator was computed by integrating the pressure force along the VG height, calculated using Computational Fluid Dynamics (CFD). It was then assumed that all this bound circulation was shed into a wake to fulfill Helmholtz’s theorem which then curls up into one primary tip vortex. To validate this, the trailed circulation estimated from the distribution of the bound circulation was compared to the one in the wake behind the vortex generator, determined directly from the wake velocities at some downstream distance. In practical situations, the pressure distribution on a vane is unknown and consequently other estimates of the spanwise force distribution on a VG must instead be applied, such as using 2D airfoil data corresponding to the VG geometry at each wall-normal distance. Such models have previously been proposed and used as an engineering tool to aid preliminary VG design. Therefore, it is not the purpose of this paper to refine such engineering models, but rather to validate their assumptions, such as applying a lifting line model on a VG that has a very low aspect ratio and is placed in a wall boundary layer. Herein, high Reynolds number boundary layer measurements of VG-induced flow were used to validate the Reynolds-Averaged Navier–Stokes (RANS) model circulation results, which were used for further illustration and validation of the hypothesis.
Highlights
Vortex generators (VGs), as first described by Taylor in the late 1940s [1,2,3], were originally used to suppress flow separation in diffusors but are today commonly used aerodynamic devices in wind turbine blades, mainly to delay separation at high angles of attack
Particle Image Velocimetry (PIV) measurements and Reynolds-Averaged Navier–Stokes (RANS) simulations were used to verify the the hypothesis that the strength of the streamwise vortexand behind a VG, which is responsible forverify mixing
VG, which which is responsible for mixing the boundary layer, could be found from the distribution of bound circulation, which in an the boundary layer, could be found from the distribution of bound circulation, which in an engineering model could be estimated from 2-D airfoil data, assuming local 2-D flow at different engineering model could be estimated from 2-D airfoil data, assuming local 2-D flow at different heights from the root of the VG
Summary
Vortex generators (VGs), as first described by Taylor in the late 1940s [1,2,3], were originally used to suppress flow separation in diffusors but are today commonly used aerodynamic devices in wind turbine blades, mainly to delay separation at high angles of attack. This increases the maximum lift coefficient and, reduces the necessary chord for the same lift, resulting in more slender and cost-effective blades [4]. VGs in principle consist of small vanes, typically with a triangular or rectangular geometry They are mounted in the boundary layer with an angle of incidence to the incoming flow that creates. The boundary layer becomes more resistant to flow reversal in adverse pressure gradients
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