Profilometry using structured illumination
This seminar presents a profilometer to resolve liquid-gas interfaces in 3D space based on ray tracing and structured illumination. A camera, in the gas phase, observes the refraction of a regular fluorescent grid through a deformed liquid surface. A robust ray-tracing algorithm is implemented to reconstruct the 3D topography in front of the fluorescent grid. The latter is generated by Talbot-effect structured illumination. The vertical orientation of the structured illumination plane provides boundary conditions for the surface reconstruction and minimizes optical occlusions. The experimental technique and the profilometry algorithm will be presented in detail and illustrated with samples of reconstructed surfaces from a droplet impact on a deep pool.
Practical aspects of designing background-oriented schlieren (BOS) experiments for vortex measurements
Setup-related aspects of background-oriented schlieren (BOS) experiments are discussed focusing on a sensitivity parameter S, which represents the relation between light deflection and resulting BOS signal, and the geometric blur. An analytic expression for the geometric blur by means of the circle of confusion (CoC) was derived which shows a proportional relation to the sensitivity factor S. The theoretical findings were validated in a reference experiment using generic distortions in glass plates. It was found that the filtering effect of the blur decreases the maximum background shift and its influence can be expressed with a blur loss factor B, which depends on the size of the CoC in relation to the investigated object. Multiplying the setup sensitivity S with the blur loss B results in the effective sensitivity S_eff that determines the maximum achievable BOS signal of a schlieren object. For the investigated reference objects, the maximum effective sensitivity S_eff was found to occur at CoC sizes in the object domain from 2.5 to 3.8 times the extent of the investigated objects. A step-by-step method is proposed for designing BOS experiments to obtain a maximum signal strength. The design parameters are further discussed specifically in regard to rotor tip vortex visualization, for which a variety of previously reported experiments are compared. A simple prediction method for the BOS signal of blade tip vortices is proposed and validated with experimental data from a rotor test stand. The application of the method to rotor systems of different size shows the requirement for increasingly higher sensitivity values for visualizing vortices of small-scale rotors.
