Multimode Laser Doppler Anemometer for Turbulence Measurements With High Spatial Resolution

The advantageous employment of multimode fibres for beam delivery in laser Doppler anemometers (LDA) is presented. Turbulent boundary layers can be investigated with high precision and high spatial resolution. For measurements of turbulence data usually hot wire anemometers are employed which, however, do not fulfil the requirement of non-intrusiveness. Therefore laser Doppler anemometers are employed which are limited by the size of their measurement volume of usually about 50 μm × 100 mm. The spatial resolution can be improved by a stronger focusing or by using a side receiver, which restricts the detection area. Furthermore, the measurement of turbulence data is limited by the varying fringe spacing, which pretends a non-existing degree of turbulence (“virtual turbulence”) and is caused by the wave-front curvature of the employed Gaussian laser mode. In this contribution it is demonstrated, that the employment of multimode-light with beam quality factors M² >> 1 the length of the measurement volume is reduced to a few percent compared to the intersection volume length of the two laser beams because of the low spatial coherence of the multimode light. The uniformity of the fringe spacing is significantly improved. The variation of fringe spacing (“virtual turbulence”) is less than 0.05%. The multimode-fibre LDA (MMF-LDA) combines the advantages of both a short measurement volume guaranteeing a high spatial resolution as well as low virtual turbulence in one device. It is therefore well suited for high accurate determination of velocity gradients in laminar or turbulent boundary layers. A MMF-LDA with about 100 fringes and 5·10⁻⁴ fringe spacing variation within a measurement volume of length 80 μm was used to perform fluid measurements in a wind tunnel. The remaining turbulence intensity of the free wind tunnel stream was determined to 0.3%. Boundary layer measurements on a well-known laminar velocity profile, the Blasius boundary layer, were performed and the wall shear stress was determined. All results are in excellent agreement with the theory. Measurements of turbulent boundary layers are presented. Multimode fibres allow the transfer of significantly higher power into the LDA measurement volume and need lower alignment effort compared to the usually employed single-mode fibres. Powerful laser diodes can now be applied for LDA set-ups, enabling sensitive velocity measurements of fluid flows.