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Proper scaling of a fluid flow permits convenient, dimensionless representation of experimental data, prediction of one flow based on a similar one, and extrapolation of low-Reynolds-number, laboratory-scale experiments to field conditions. This is a particularly powerful technique for turbulent flows where analytical solutions derived from first principles are not possible. We review in the present paper the topical development in scaling the canonical turbulent boundary layer and pipe and channel flows. Additional to utilizing some of the most comprehensive and high-quality databases available to date, the article focuses on contemporary advances in analytical and asymptotic approaches to determine the meanvelocity profile as well as to scale higher-order statistics. The current debate concerning the mean-velocity profile of turbulent wall-bounded flows has ruled out neither a logarithmic nor power law behavior. Furthermore, a Reynolds number dependence of the mean-velocity profile has not been excluded either. Clearly, a more complex functional form is needed to describe the profile. The present results can be utilized to extrapolate the available low-Reynolds-number physical and numerical data to the more practically important high-Reynolds-number field conditions. Knowledge of the proper scaling of the canonical cases can also be useful to non-canonical wall-bounded flows as well as to calibrate turbulence models and flow sensors in the vicinity of walls. Keywords: Turbulent wall-bounded flows; Turbulent boundary layers: Pipe flows: Channel flows; Similarity laws: Self-similarity: Scaling; Asymptotic methods

Compliant coating research is one of those areas which experienced its fair share of triumphs and debacles. For over forty years, the subject has fascinated, frustrated and occasionally gratified scientists and engineers searching for methods to delay laminar-to-turbulence transition, to reduce skin-friction drag in turbulent wall-bounded flows, to quell vibrations, and to suppress flow-induced noise. In its simplest form, the technique is passive, relatively easy to apply to an existing vehicle or device, and perhaps not too expensive. Through the years, however, claims for substantial drag and noise reductions were made, only to be later refuted when the results were more critically examined. There are several important issues with regard to the reliability of available analytical, numerical and experimental results. In this article, some of these issues, particularly as they relate to the search for drag-reducing compliant coatings, will be addressed with the objective of elucidating the potential pitfalls to newcomers to the field. Problem formulation with proper boundary conditions, impossibility of obtaining first-principles analytical solutions when the wall-bounded flow is turbulent, and limitations of existing numerical simulations will be elaborated. The effects of background turbulence in a wind or water tunnel, accurate drag measurements, compliant wall motion, and the geometry and properties of the coating used will be among the outstanding experimental issues discussed. Attempts will be made to explain some of the seemingly contradictory results available in the open literature.

This article reviews recent developments in the field of flow visualization, revisits the many possible pitfalls in interpreting the results and illustrates the splendor of fluids in motion. Most of the new techniques introduced during the past 10-20 years became possible as a result of the introduction and accessibility of three artifacts: the computer, the laser and the video camera. Laser-induced fluorescence, particle-image velocimetry, focused schlieren, optical tracking and holography are among the rapidly improving flow visualization techniques that increasingly provide quantitative field information. Digital image processing algorithms are used to process and display the colossal data included in physical and numerical flow visualizations.