Hassan M. Nagib

Can We Ever Rely on Results from Wall-Bounded Turbulent Flows without Direct Measurements of Wall Shear Stress?

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. The results also demonstrate that accurate measurements of the mean skin friction with MEMS sensors are possible. Second, fluctuating skin friction is measured in two- and three-dimensional turbulent boundary layers using a MEMS sensor and a wall-wire as reference. Statistics like skewness, flatness and spectra of the turbulent skin friction are presented to demonstrate the potential and limitations of the MEMS sensor. Finally, the skin friction is measured using the oil film technique with an accuracy of about 1.5%, over the range of Reynolds numbers 10,000 < Reθ < 70,000, in a zero pressure-gradient boundary layer. The results are very well represented by the log-law with κ = 0.38, C = 4.1.

CICLoPE – A Large Pipe Facility for Detailed Turbulence Measurements at High Reynolds Number

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.

Scaling of High Reynolds Number Turbulent Boundary Layers Revisited (invited)

Flat plate turbulent boundary layers at high Reynolds number are studied based on experiments at the National Diagnostic Facility (NDF) and other measurements. The range of Reynolds number Re based on momentum thickness for NDF is between 12,000 and 62,000 for zero pressure gradient. Experimental results for the mean flow are analyzed to reveal appropriate scale relations. It is found that the ratio of mean time scale and turbulent time scale can be used as a significant flow parameter and not just for order of magnitude estimations. Then, the Reynolds number dependence of the wake parameter in the outer region is reevaluated and its strong dependence on the constants of the logarithmic law is assessed. Next, the behavior of the shape factor H for zero pressure gradient boundary layers is discussed in the limit of very large Reynolds numbers, where H approaches unity very slowly. Turning to the mean velocity profiles, the conventional outer scaling of the mean velocity defect with u is compared with other recently proposed velocity scales and it is concluded that u is the proper velocity scale in the overlap and wake regions. Finally, a new outer length scale , analogous to the outer scale used in pipes and channels, is proposed to clarify issues related to the limit of infinite Reynolds number.