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A novel method is proposed that allows accurate estimates of the local wall shear stress from near-wall mean velocity data in fully developed pipe and channel flows. DNS databases are used to demonstrate the accuracy of the method and to provide the reliability requirements on the experimental data.To demonstrate the applicability of the method, near-wall LDA measurements in turbulent pipe and channel flows were performed. The estimated wall shear stress is shown to be accurate to within 1%. Streamwise mean velocity and turbulence intensity profiles normalized with the wall friction velocity at several Reynolds numbers are presented.

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Abstract.A new method of interpreting the signals from triplesensor thermal anemometer probes has been developed based on fast solution for all the roots of the non-linear Jorgensen (1971) equations describing the directional response of each cylindrical sensor. The sensors can be oriented at arbitrary angles to each other, but always within a range of probe geometries that keep prong interference and thermal wake interference below acceptable levels. The properties of a class of non-orthogonal symmetric tetrahedral probe geometries are studied in relation to the range of flow vector angles that can be measured, the sensitivity of the probe with respect to changes in flow angle, and the sensitivity of the computed velocity components due to angular errors associated with the construction of the probe. The solutions of Jorgensen's equations are inherently multiple-valued, but if the velocity vector is restricted to be within a cone of angles, they are unique. It is shown that measurements with non-orthogonal triple sensor signals are sensitive to angular deviations of a few degrees of the sensor angles from the nominally orthogonal probe geometry, indicating the need of a non-orthogonal algorithm. The mean, rms, Reynolds stress, and power spectrum of the velocity in fully developed turbulent pipe flow were measured using a specially designed triple sensor probe and the proposed algorithm.

The use of single and multiple hot-wire/hot-film probes for subsonic flows is discussed with emphasis on the digital methods and procedures. An account is given of the major factors which influence the reliability of measurements and the application of many of the correction procedures required. The factors considered are: probe calibration, nonlinearity of the sensor-response equation, temperature effects, temporal and spatial resolution and probe design.Ioannis Lekakis was born and raised in Greece. He moved to USA to attend the George Washington University in Washington DC, where he earned a BSc in Mechanical Engineering. He continued his Mechanical Engineering studies at the University of Illinois in Urbana-Champaign, where he completed, in 1982, the MSc and, in 1988, the PhD degrees. After more than a year at the University of Iowa as an Assistant Professor, he worked at Imperial College, London and University of Erlangen, Germany, before his return to Greece in early 1996. He is presently employed at the University of Thessaly. His work is in the field of experimental turbulence, with special attention to the development of appropriate diagnostic techniques, HWA and LDA.