Multipoint velocity measurements in flows using a Fabry–Pérot interferometer

A concept for simultaneous multi-point velocity measurements in flows based on the evaluation of the Doppler frequency shift by means of a Fabry–Pérot interferometer (FPI) is presented. At Doppler global velocimetry, a molecular absorption cell is used to transform the Doppler frequency shift of scattered light into an intensity change. This, however, strongly restricts the choice of applicable lasers and optical saturation may cause systematic deviations. Using Fabry–Pérot interferometers instead, these limitations do not exist but simultaneous multi-point measurements with concurrent low uncertainty are difficult to achieve due to the angular dependence of the FPI transmission function. A theoretical consideration revealed that an FPI in the collimated configuration yields lower uncertainties than in the telecentric configuration. Simultaneous multi-point measurements can be achieved using a setup with two annular fibre arrays. The feasibility is demonstrated by means of flow velocity profile measurements with low uncertainty of 0.9 m/s at 10 ms temporal resolution and 50 μm spatial resolution.

► Fabry–Pérot interferometers can replace absorption cells in Doppler global velocimetry.
► The collimated setup exhibits lower measurement uncertainties than the telecentric setup.
► Multi-point velocity measurements can be performed using an annular fibre array.
► The feasibility was demonstrated by means of velocity measurements at a nozzle flow.