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Fluctuating skin friction is measured in two- and three-dimensional turbulent boundary layers using a MEMS sensor and a wall-wire as reference. Skewness, flatness and spectra of the turbulent skin friction are presented to demonstrate the potential and limitations of the MEMS sensor. The measured turbulence intensities of the order of 0.4 are in general agreement with a number of experimental and DNS studies. However, the fluctuating quantities measured with this MEMS sensor, operated at an over-heat ratio of 1.3, are shown to depend on the Reynolds number or mean skin friction. Therefore, such a high over-heat ratio, which was proven to dramatically increase the accuracy of mean skin friction measurements in a previous study by the authors, may not be appropriate for the measurement of fluctuating wall-shear with MEMS sensors, particularly at low mean shear values.

Recent improvements in three techniques for measuring skin friction in two- and three-dimensional turbulent wall-bounded shear flows are presented. The techniques are: oil-film interferometry, hot wires mounted near the wall, and surface hot-film sensors based on MEMS technology. First, we demonstrate that the oil-film interferometry technique can be used to measure the skin-friction magnitude and its direction in two- and three-dimensional wall-bounded shear flows. Second, a simple method is outlined to measure the skin friction with a wall wire located outside of the viscous sublayer. Finally, a systematic study of the parameters influencing wall-friction measurements with MEMS sensors is presented. The results demonstrate that accurate measurements of the mean skin friction with MEMS sensors are possible in two- and three-dimensional wall flows. Measurements by the three techniques are compared to each other and to past measurements in the same facility.

The understanding of high Reynolds number turbulence remains a great challenge nowadays. Although recent laboratory xperiments, measurements in planetary boundary layer and direct numerical simulations provide a huge amount of information, none of these data sets provide high Reynolds number, high spatial resolution and well converged statistics at the same time. As a response to this problem, an international effort started some years ago to develop a new research facility allowing high spatial resolution even at high Reynolds number. This facility is currently under design and will be built at the Center for International Cooperation in Long Pipe Experiments (CICLoPE), at the University of Bologna in 2008. The Reynolds number trends of various wall-bounded flows and the challenges in achieving good spatial resolution for measurements is presented and the need for a new facility for collaborative investigations of wall-bounded flows is then discussed together with its basic specifications.

High Reynolds number turbulence is ubiquitous in a number of flow of practical interest and crucial to draw conclusions regarding the physics of turbulence. Although recent laboratory experiments, measurements in planetary boundary layer and direct numerical simulations provide a huge amount of information, none of these data sets provide high Reynolds number, high spatial resolution and well converged statistics at the same time. As a response to this problem, an international collaboration between a group of universities and research centers started some years ago to build large scale infrastructures for high Reynolds number experiments. The Center for International Cooperation in Long Pipe Experiments, CICLOPE (www.ciclope.unibo.it) at the University of Bologna, was created for this purpose and will be open to international scientists through different collaboration programs. The laboratory is currently under construction and the first facility, which will be installed there is a large pipe flow experiment that will allow fully resolved turbulence measurements even at high Reynolds number.