LDA MEASUREMENTS IN THE NEAR-WALL REGION OF A TURBULENT PIPE-FLOW

This paper presents laser-Doppler measurements of the mean velocity an d statistical moments of turbulent velocity fluctuations in the near-w all region of a fully developed pipe flow at low Reynolds numbers. A r efractive-index-matched fluid was used in a Duran-glass test section t o permit access to the near-wall region without distortion of the lase r beams. All measurements were corrected for the influence of the fini te size of measuring control volume. Measurements of long-time statist ical averages of all three fluctuating velocity components in the near -wall region are presented. It is shown that the turbulence intensitie s in the wall region do not scale with inner variables. However, the l imiting behaviour of the intensity components very close to the wall s how only small variations with the Reynolds number. Measurements of hi gher-order statistical moments, the skewness and flatness factors, of axial and tangential velocity components confirm the limiting behaviou r of these quantities obtained from direct numerical simulations of tu rbulent channel flow. The comparison of measured data with those obtai ned from direct numerical simulations reveals that noticeable discrepa ncies exist between them only with regard to the flatness factor of th e radial velocity component near the wall. The measured v' flatness fa ctor does not show the steep rise close to the wall indicated by numer ical simulations. Analysis of the measured data in the near-wall regio n reveals significant discrepancies between the present LDA measuremen ts and experimental results obtained using the hot-wire anemometry.