Time-resolved multi-parameter flow diagnostics by filtered Rayleigh scattering: system design through multi-objective optimisation

Abstract

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 usually employ combinations of different methods relying on tracer particles or molecules. This typically requires usage of at least two laser systems and detection units as well as elaborate calibration of the luminous properties of the applied tracer species with regard to the specific thermodynamic conditions anticipated for the flow case at hand. 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) three 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 shown that the use of circularly polarised laser light offers no advantage over linear polarisation. By demonstrating a moderate sensitivity of the optimised observation arrangement to alignment errors, the presented FRS concept provides a practical solution for the simultaneous measurement of time-resolved 3C flow velocity and scalar fields.