Unsteady Multi-Parameter Flow Diagnostics By Filtered Rayleigh Scattering: System Design By Multi-Objective Optimisation

The measurement of the time-resolved three-component (3C) velocity field together with scalar flow quantities such as temperature or pressure by laser-optical diagnostics is a challenging task. Current approaches typically employ combinations of different methods relying on tracer particles or molecules, which requires elaborate calibration procedures of the tracer's photo-physical properties and extensive instrumentation. In contrast to this, the tracer-free filtered Rayleigh scattering (FRS) technique has been proven to obtain combined time-averaged velocity and scalar fields and might offer a viable alternative for unsteady flow diagnostics. By applying multiple perspective views, two detection system variants are presented, combining 1) six observation branches with one camera/molecular filter and 2) four camera views with two cameras and molecular filters of differing vapour densities. Both configurations in principle allow for the simultaneous measurement of instantaneous 3C velocity, temperature and pressure fields. Multi-objective optimisation is used to enhance the detection setups for different sets of experimental configurations. It is shown that a higher number of observation positions and the associated dynamics of the FRS signal prove to be advantageous compared to the use of less views in combination with two acquisition channels equipped with different molecular filters. It is also demonstrated that the use of linearly polarised laser light is preferred over circular polarisation. Future work will focus on the realisation of the multiple-view FRS concept for the combined measurement of 3C velocity and scalar fields.