Title: Optical flow sensor for cryogenic conditions using Filtered Rayleigh Scattering
(Optischer Strömungssensor für Kryobedingungen auf Basis von Filtered Rayleigh Scattering)

Leader: ILA R&D GmbH, Partners: DLR e.V., ETW GmbH

The main goal of the OFRS project was to develop a non-intrusive optical sensor to measure the following parameters: velocity (v), pressure (p), temperature (T) and density (ρ). This sensor should provide information on the flow inside of the ETW test section e.g. during a model test without influencing the flow, as seen when using a pressure probe or a hot-wire. In comparison to alternative measurement techniques, e.g. LDV measurements, the sensor does not require additional particles as it is based on Rayleigh scattering, which is the scattering of light off of the molecules of the air.

The sensor developed may measure the temporal mean values as well as fluctuations of v, p, T and ρ at one point in the flow with a resolution of some Hz. The required measurement system was developed for later application in ETW. The challenging boundary conditions like cryogenic temperatures, pressures of up to 4.5 bar, limited accessibility during the measurement campaign and the obstacles of long cable lengths or optical fibres, must be taken into account. An initial validation of the measurement principles was performed in the pilot ETW facility (PETW) at relevant flow conditions.

In perspective, the FRS (Filtered Rayleigh Scattering) method offers more opportunities to visualize and measure flow peculiarities (e.g. shock oscillation, separation) without disturbance or inducing additional tracer particles. Hence, an optical sensor based on FRS is a cutting edge innovation, as comparable sensors that cope without additional tracer particles either use complex hardware or do not measure the parameters with the required measurement accuracy. In addition, the application of a non-intrusive reference sensor in the test section of ETW will deliver additional information on the instationary flow quality and is of particular significance to tests with unsteady flow phenomena and as inputs for corresponding numerical simulations. For the comparison of numerical results and ETW test data, it is necessary to know the instationary free stream conditions upstream of the model.

This innovative measurement approach consists of a combination of cryogenic compliant, non-intrusive endoscopic hardware with fast frequency modulation to transfer the slow frequency scan for planar measurements (FSM - FRS), which was successfully developed at the DLR Institute of Propulsion Technology, Department of Engine Measurement Systems (AT OMT), into a relatively precise, unsteady point measurement method. The inclusion of the scattering angle influence on to the measurement signal by use of large apertures to increase the signal has so far been used for error estimation, but not yet for data evaluation.

Further information on the development of the laser-optical point sensor based on the Filtered Rayleigh scattering technology for use under cryogenic conditions can be found in German in the article "Entwicklung eines laseroptischen Punktsensors auf Basis der Filtered Rayleigh Scattering Technik zum Einsatz unter kryogenen Bedingungen", which was presented by A.-K. Hensch, P. Guntermann, M. Dues, and G. Stockhausen at the 30. Fachtagung "Experimentelle Strömungsmechanik" of the GALA e.V. in September 2023.

This work received funding from the German Federal Ministry for Economic Affairs and Energy LuFo V-3/2018-2022 under grant agreement no 20Q1729B.

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