Extension of the familiar concept of boundary-layer separation to flow along moving walls and unsteady flows is a subject that attracted some interest in the 1950's and has been investigated further in the past few years. The well-known criterion of vanishing wall-shear does not apply in such flows, and therefore the definition of the phenomenon becomes more difficult than in the simpler types of flow considered by Prandtl. The practical importance of extending the concept is discussed and arguments in favor of its definition in terms of Goldstein's singularity are reviewed. The model proposed by the present authors in 1971 is described, together with available numerical and experimental evidence that supports it. Numerical studies of steady and unsteady separating boundary-layer flows are reported, in which singularities of Goldstein's type are detected and comparisons can be made between the position-vs.-time curves of the singularity and of the point of vanishing wall-shear. One of these studies involves the classic case of the circular cylinder started impulsively, which was treated by Blasius and by Goldstein and Rosenhead and for which detailed numerical results are available. The distinction between vanishing wall-shear and separation is dramatic. This review closes with some remarks about the mathematical state of this subject.

An experimental study has been made of a circular cylinder in steady and oscillatory flow with non-zero mean velocity up to a Reynolds number of 40000. The results for the stationary cylinder are in close agreement with previously published data. Skin-friction measurements revealed the amplitude of fluctuation of the boundary layer for different angular locations. It has been universally accepted that bluff bodies shed vortices at their natural frequency of shedding (Strouhal frequency), or, when synchronized with an external unsteadiness, at the frequency of the disturbance or half of it, depending of the direction of the unsteadiness. Our findings, instead, indicate that the shedding frequency may vary smoothly with the driving frequency before locking on its subharmonic. Moreover, the present results indicate that, at the lowest frequency limit of lock-on, vortices are shed simultaneously on both sides of the model. A more traditional alternate pattern of vortex shedding is then recovered at higher driving frequencies.

Shen, and W. R. Sears. However, I feel I should single out Drs. U. B. Mehta and L. van Dommelen who have done a very thorough job reviewing the first few chapters and the last chapter, respectively. Finally, Miss H. L. Reed has scrutinized the galleys, catching an embarrassing number of errors. I thank them all deeply.

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The performance of the heart after a mitral valve replacement operation greatly depends on the flow character downstream of the valve. The design and implanting orientation of valves may considerably affect the flow development. A study of the hemodynamics of two orientations, anatomical and anti-anatomical, of the St. Jude Medical (SJM) bileaflet valve are presented and compared with those of the SJM Biocor porcine valve, which served also to represent the natural valve. We document the velocity field in a flexible, transparent (LV) using time-resolved digital particle image velocimetry (TRDPIV). Vortex formation and vortex interaction are two important physical phenomena that dominate the filling and emptying of the ventricle. For the three configurations, the following effects were examined: mitral valve inlet jet asymmetry, survival of vortical structures upstream of the aortic valve, vortex-induced velocities and redirection of theflow in abidance of the Biot-Savart law, domain segmentation, resonant times of vortical structures, and regions of stagnantflow. The presence of three distinct flow patterns, for the three configurations, was identified by the location of vortical structures and level of coherence corresponding to a significant variation in the turbulence level distribution inside the LV. The adverse effect of these observations could potentially compromise the efficiency of the LV and result in flow patterns that deviate from those in the natural heart.