PhD Defence Yuedi Bai | Flow field analysis of a single vortex generator in turbulent boundary layers

Flow field analysis of a single vortex generator in turbulent boundary layers

Yeudi Bai is a PhD student in the department Engineering Fluid Dynamics.(Co)Promotors are prof.dr.ir. C.H. Venner and dr. H. Ozdemir from the Faculty of Engineering Technology (ET), University of Twente.

With a global installed capacity exceeding 1,000 Terawatts, wind energy represents a dominant share of renewably generated power. This share is projected to increase further in response to the ongoing imperative to mitigate climate change. Beyond the growth in total installed capacity, individual wind turbines are becoming increasingly massive; the current generation features power ratings of up to 20 MW and rotor diameters exceeding 250 m.

 The efficiency of a wind turbine is governed by its aerodynamics, specifically the airflow around the airfoil profile. Achieving optimal lift and minimized drag—and thus maximized power output—requires the flow to remain attached to the blade profile across its entire span and under all operating conditions. Flow separation (or even stall) results in significant energy losses. This separation is typically initiated within the boundary layer between the blade surface and the freestream, triggered by high angles of attack, low flow velocities, or adverse pressure gradients. It can also be induced by local surface deformations caused by contamination (e.g., insects) or erosion (e.g., rain droplets), as well as highly fluctuating inflow velocities (inflow turbulence). The susceptibility to separation is particularly high near the blade root, where the profiles are thick due to structural requirements and local flow velocities are relatively low. One method for preventing boundary layer separation is the implementation of vortex generators (VGs). These are small devices placed on the blade surface that induce vortices in the flow, primarily near the boundary layer edge. These vortices facilitate the mixing of high-momentum air from the outer flow into the low-momentum boundary layer, thereby increasing the flow velocity near the surface and delaying or preventing separation. VGs are typically applied in pairs, using either co-rotating or counter-rotating configurations. While they increase lift and reduce base drag, they also introduce parasitic drag themselves, which can negatively impact power generation. Consequently, the optimal design of vortex generators is critical to turbine efficiency.

To improve the effectiveness of VGs, various facets of their aerodynamics have been investigated, though usually in pairwise configurations. However, little is known about the three-dimensional influence of a single VG on the flow and its interaction with the boundary layer - particularly regarding its impact on displacement thickness and momentum thickness, which are parameters widely used in integral boundary layer methods. Improving this understanding would make these methods highly suitable for the preliminary design phase.

At the University of Twente wind tunnel measurements were conducted on the flow surrounding a single vortex generator within a boundary layer. The study varied both the geometric parameters of the VG (height and angle of incidence) and the characteristics of the boundary layer (natural transition versus turbulent flow with tripped transitions of varying thicknesses). Using hot-wire anemometry, the velocity profiles within the boundary layer were determined at various positions upstream and downstream of the VG for each boundary layer type. These measured profiles were then compared with theoretical models.

Furthermore, Particle Tracking Velocimetry (PTV) was employed to characterize the three-dimensional velocity field. Analyses using the $\varOmega$ criterion demonstrated that a secondary vortex is generated alongside the primary vortex. The primary vortex can be characterized as a stable, rotating cylindrical structure, while the secondary vortex is spiral-shaped and dissipates as it propagates along the periphery of the primary vortex. The incidence angle of the vortex generator significantly influences the secondary vortex, while the size of the primary vortex is dictated by both the VG height and its angle of incidence relative to the boundary layer.

The results indicate that the secondary vortex negatively affects the development of the primary vortex. Convective momentum transfer within the vortex was analyzed using various velocity components; the findings show that the geometry of the vortex created by a specific VG is independent of the thickness of the boundary layer in which it is immersed. However, a thicker boundary layer does result in less effective momentum transfer from the vortex to the boundary layer. Finally, the PTV data were analyzed using Proper Orthogonal Decomposition (POD) to identify which structures within the flow field are most energy-containing at specific frequencies.

Finally, the effects of VG height and incidence angle on the boundary layer displacement thickness and momentum thickness are analyzed. From the result several conclusions were drawn. For the single VG case, the streamwise flow direction is dominant, while crossflow components are more visible than when pairwise VGs are considered. Moreover, the primary vortex is not pushed down by a neighboring VG and thus persists longer and stays effective in streamwise direction. In addition, a secondary vortex originating from the tip of the VG is more visible, but weaker than the primary vortex. The boundary layer parameters like displacement and momentum thickness show distinct behavior for the single VG case. Detailed analysis of the parameters in this study help understanding and facilitate modeling the effects of VGs for engineering applications.