Optical Diagnostics with Background Oriented Schlieren: A practical perspective on reactive and non-reactive flow scenarios

The oldest and without a doubt one of the most versatile non-intrusive
measurement approaches is the schlieren imaging technique. Although they
have been used over a century for visualization of flow fields, they are associated with certain drawbacks. In this regard, background oriented schlieren (BOS) provides a unique opportunity to quantitatively characterize density varying flows which has a major superiority of adaptability and scalability over a wide range of applications. However, traditionally BOS is referred to lack necessary sensitivity and resolution specifications to compete with classical schlieren. Beyond the many hardware developments that mitigate this weakness, it is shown the application of optical flow (OF) image processing technique can address these issues without altering the complexity level of the measurement setup. Accordingly, the enhanced capabilities of BOS using optical flow is characterized through experimental and numerical assessments. Although optical flow is shown to prevail over the conventional block-matching algorithms, the background patterns employed in BOS were still designed to suit the needs of the prior. Therefore, a dedicated parametric study is performed to unlock the full potential of optical flow, the resolution and sensitivity response of optical flow to different background configurations, in which guideline pattern generation parameters with respect to the flow field of interest are established.

Focusing on exploring the capabilities of BOS in challenging flow visualization applications, qualitative visualizations and quantitative characterizations of density varying features are performed via BOS. In this regard, thermodynamics of a heated swirling subsonic jet, passage and trailing edge flow topology in a turbine cascades, aeroacoustics of dual-stream co-axial supersonic jets, aerodynamics of supersonic objects, dynamics of shock wave transitional and turbulent boundary layer interactions at supersonic and hypersonic speeds are investigated by means of constructing custom BOS experimental setups that are compatible to the specifications and constraints of each facility. Transitioning to reactive flow applications, BOS is demonstrated as an alternative approach for flame visualization in a Bunsen burner and to perform laminar burning velocity measurements of methane/air mixtures. The capability of BOS in terms of providing relevant flow field information in combustion environments is extended to the temperature and heat release characterization of swirl stabilized premixed flames in a model gas turbine combustor.