Multimode fibre laser Doppler anemometer with

The advantageous employment of multimode fibres for beam delivery in laser Doppler anemometers (LDA) is 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 singlemode fibres. Powerful laser diodes can now be applied for LDA set-ups, enabling sensitive velocity measurements of fluid flows. However, due to the obeserved signal degeneration caused by the speckle-effect, this technique could never become established. In this work the speckle pattern of the multimode beam is sufficiently suppressed by choosing highaperture fibres with high intermodal dispersion and by the use of laser diode arrays with low coherence lengths, see figure (1). It is demonstrated, that 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 long axis of the measurement volume is in this case aligned in the flow direction, whereas in conventional LDA ́s employing single mode radiation it is perpendicular. The uniformity of the fringe spacing is significantly improved. The variation of fringe spacing, pretending a nonexisting degree of turbulence (“virtual turbulence”), is less than 0.1%. 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 80 μm measurement volume length, about 100 fringes and 5⋅10 fringe spacing variation was used to perform fluid measurements in a wind tunnel. The remaining degree of turbulence of the wind tunnel 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. 1 ,4 1 ,5 1 ,6 1 ,7 1 ,8 1 ,9 2 ,0 2 ,1 2 ,2 2 ,3 2 ,4 0 1 2 3 4 -4 0 -3 0 -2 0 -1 0 0 1 0 2 0 -0 ,0 1 0 ,0 0 0 ,0 1 0 ,0 2 0 ,0 3 0 ,0 4 0 ,0 5 0 ,0 6 0 ,0 7 0 ,0 8 Am pl itu de / a. u.