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Second Symposium on Turbulence and Shear Flow Phenomena
June, 27-29, 2001, KTH, Stockholm, Sweden

DOI: 10.1615/TSFP2

REFRACTIVE INDEX MATCHED LDA TECHNIQUE FOR INVESTIGATIONS OF LAMINAR TO TURBULENT BOUNDARY LAYER TRANSITION

pages 69-74
DOI: 10.1615/TSFP2.850
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RÉSUMÉ

Usage of laser-Doppler anemometry (LDA) requires optical access to the flow field of interest. This is not always easy, as in the case of complex geometries or very near-wall boundary layer measurements. One solution is to employ a solid material and fluid of the same index of refraction. In this case, there is no optical interference of the solid with the LDA. Although this technique is not new, previous studies have been limited to small flow apparatuses and relatively unpleasant fluids. Two new large scale flow facilities (LSTM and INEEL) have now been constructed which permit matched index of refraction (MIR) LDA measurements in difficult geometries at higher Reynolds numbers and with increased spatial resolution in the measurements. This paper describes the facilities and fluid, flow quality, and presents some new fundamental measurements for the transition process in flat plate boundary layers downstream of two-dimensional rectangular ribs. By use of LDA and a large MIR flow system, data for wall-normal fluctuations and Reynolds stresses were obtained in the near wall region to y+ < 0.1 in addition to the usual mean streamwise velocity component and its fluctuation. Consequently, results covered boundary layers which retained their laminar characteristics through those where a turbulent boundary layer was established shortly after reattachment downstream from the forcing rib. For "large" elements, evolution of turbulent statistics of the viscous layer for a turbulent boundary layer (y+ < ~30) was rapid even in flows where the mean velocity profile still showed laminar behaviour.

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