Qualitative visualization of butane and helium tracers in air using a digital background oriented schlieren method

Fluid flow around turbine blades, inside burners or on aerofoil is responsible for operation and overall design of the fluid energy machinery or airplane. Stall effects and turbulence behind blades are responsible for losses in efficiency. Because of this, exact knowledge of separation behaviour and vortex development of fluid flow around profile geometries or inside burners is mandatory. For this reason, aerodynamic research focuses on in-vestigations and qualitative visualization of turbulence. The aim of this work is to develop a digital camera filter to visualize turbulence and stall effects directly around the object. One possibility is to use a digitally generated background pattern together with a filtered video image to visualize differences in diffraction coefficient of the gas. This so-called Background Oriented Schlieren (BOS) imaging can be used to visualize strains for comparison against Computational Fluid Dynamics (CFD) [1]. The BOS is based on the index of refraction of different gas phase species and temperatures. A qualitative evaluation of the flow field was also carried out, as an adjustment experiment for further turbulence models. It allows an evaluation of the flow resulting from different Reynolds numbers [2]. The 2-dimensional, dynamic image of the flow field allows to develop further approaches for mathematical models which are needed for validation. Different background patterns are generated and tested for their suitability for BOS im-aging. Also, the experimental setup is explained, and different real time software filter methods are tested to get better quality of the visual results. Several images of different blade geometries are presented, and streams of hot air, butane and helium have been visualized. A filter from a Particle Image Velocity (PIV) method (here: PIVlab) was used to calculate velocities from the flow fields [3,4]. This enables further evaluation and visual repre-sentation i.e., of Reynolds numbers or a vector field [2].