Investigation of Vortex-Generator Induced Flow Structures on a Flat Plate using IR Thermography, Particle Image Velocimetry and Oil Flow Visualization

The use of passive flow control, e.g. by means of vortex generators attached to the blade or endwall surface, is one approach to reduce boundary layer separation induced aerodynamic losses in axial fans by separation point delay. An efficient use of this approach requires a sufficient analysis and understanding of the three-dimensional flow structures. Even though computational fluid dynamics (CFD) methods based on the Reynolds-Averaged-Navier-Stokes equation approach (RANS) occupy a significant position in the development phase of aforementioned machines, experimental validation of novel, but also established numerical models is still indispensable and essential. Especially concerning flow separation, CFD models inhabit an inevitable sensitive dependency of separation point prediction on mesh quality and mesh characteristics. In this work we present a detailed and comprehensive study of the flow structures on a flat plate using several established vortex generator geometries, i.e. spheres, cones, ramps and wheeler-type wedges. The experiments are conducted in a plane wind tunnel with Reynolds numbers about Re ≈ 1e05 at a fixed turbulence intensity of 1.5% and under steady-state flow conditions. The surface flow structure is investigated using oil flow visualization and IR thermography utilizing the dependency of wall heat transfer on local flow characteristics. For this purpose, the flat plate is heated internally through a temperature-controlled adhesive thermo polyester heating foil. The plate surface was coated with a thin, low heat conducting polymer (PTFE) limiting the heat flux and allowing local flow phenomena to be resolved sufficiently. For a comprehensive insight into the three-dimensional flow field, Particle Image Velocimetry (PIV) measurements were conducted on several planes parallel and perpendicular to the main flow direction, supplementing the above-mentioned surface flow analysis. This investigation provides a broad description of local three-dimensional flow phenomena induced by common vortex generator designs, applicable to boundary layer separation control in axial fans. Within the scope of this study, it is shown that IR thermography is a suitable approach for contact-free measurements of surface flow structures.