Characterization of engine inlet flows by optical measurements of velocity profiles with high measurement rate

For characterizing inlet flows of aircraft engines, grids of 40 pressure probes are applied as state-of-the-art for measuring the pressure fluctuations in the inlet with a typical measurement rate of 4 kHz. In order to get a more flexible and non-intrusive measurement setup, frequency modulated Doppler global velocimetry (FM-DGV) is applied in an inlet flow experiment for the first time measuring the flow velocity fluctuations. The novel approach includes using an imaging fiber bundle and an endoscope for light detection from inside the inlet without disturbing the flow significantly. The key challenge was to achieve a sufficiently high measurement rate, which is limited by the signal to noise ratio. In the experiment, measurement rates of up to 8 kHz were achieved. This proves the principle applicability of FM-DGV for inlet flow characterization. The single component setup consisted of 16 measurement points linearly aligned at a radial line (compared to 10 points with a probe rake). The measured mean velocity profiles were validated by the air mass flow rate. Measurements with 0.5 kg/s and 1 kg/s were accomplished. The resolved velocity spectra indicate significant flow oscillations up to about 1 kHz. As expected, the spectra show stronger oscillations when inserting a distortion plate. Hence, FM-DGV is proven to be a promising technique for the non-intrusive characterization of inlet flows at high measurement rates