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.

Seeding-free inlet flow distortion measurement by filtered Rayleigh scattering: diagnostic approach and verification

“…Measurement data are obtained at two neighboring absorption lines, leading to a two-branch structure with a frequency gap in between. The resulting intensity spectra for each perspective strongly differ in level and shape, which is mainly related to the variation in scattering geometry and its inŕuence on the Rayleigh lineshape [33]. The Rayleigh lineshape model used in this work is a combination of a calibrated analytical model [35] and a recently introduced machine learning approximation [36].…”

“…To extend the FRS technique to the planar measurement of multiple time-resolved ŕow quantities, one idea is to split the scattering signal obtained from a single laser pulse between several detection channels, each equipped with a molecular őlter of different vapour density [28ś32]. A second approach involves using a single molecular őlter in combination with at least őve directions of observation to derive the őve unknown ŕow variables from a single-pulse measurement (pressure, temperature and three velocity components) [16].The latter approach leads to a complex detection scheme that has recently been optimised to maximise sensitivities of an instantaneous multi-parameter FRS measurement with regard to the planned application at the CCITF based on a six-branch image őbre bundle [33]. It is the aim of this work to experimentally verify this diagnostic approach by performing FRS measurements with an optimised multiple-view detection scheme on a simpliőed ŕow conőguration.…”

“…In our recently published work [33], it is argued that in a single-frequency multi-view FRS arrangement, it is not sufficient to place the different camera perspectives randomly in the space around the ŕow experiment. Instead, multi-objective optimisation is used to identify a suitable optical conőguration to reach the lowest measurement uncertainties.…”

“…The choice is motivated by the location of the wavenumber close to the end of the blocking region of the molecular őlter, where the slope of the transmission curve increases rapidly and changes in the Rayleigh scattering's spectral shape due to temperature or pressure or Doppler shifts from ŕow velocity lead to strong dynamics of the FRS intensiy. For detailed information about the multi-variate dependency of the FRS signal on the wavenumber, the scattering angle as well as the optimised detection conőguration that follows can be found in our previous work [33].…”

“…Then, the respective branch is őxed at the speciőed radius 𝑅 from this reference point. With this procedure, any error in angular alignment is assumed to be within ±5°, which was shown to have an acceptable effect on measurement accuracy [33].…”