Employing a high-speed video system and hydrogen bubble-wire flow visualization, the characteristics of the low-speed streaks which occur in the near-wall region of turbulent boundary layers have been examined for a Reynolds-number range of 740 [les ] Reθ < 5830. The results indicate that the statistics of non-dimensional spanwise streak spacing are essentially invariant with Reynolds number, exhibiting consistent values of $\overline{\lambda^{+}} \approx 100$ and remarkably similar probability distributions conforming to lognormal behaviour. Further studies show that streak spacing increases with distance from the wall owing to a merging and intermittency process which occurs for y+ [simg ] 5. An additional observation is that, although low-speed streaks are not fixed in time and space, they demonstrate a tremendous persistence, often maintaining their integrity and reinforcing themselves for time periods up to an order of magnitude longer than the observed bursting times associated with wall region turbulence production. A mechanism for the formation of low-speed streaks is suggested which may explain both the observed merging behaviour and the streak persistence.

The flow induced by a vortex ring approaching a plane wall on a trajectory normal to the wall is investigated for an incompressible fluid which is otherwise stagnant. The detailed characteristics of the interaction of the ring with the flow near the surface have been observed experimentally for a wide variety of laminar rings, using dye in water to visualize the flow in the ring as well as near the plane surface. Numerical solutions are obtained for the trajectory of the ring as well as for the unsteady boundary-layer flow that develops on the wall. The experimental and theoretical results show that an unsteady separation develops in the boundary-layer flow, in the form of a secondary ring attached to the wall. A period of explosive boundary-layer growth then ensues and a strong viscous-inviscid interaction occurs in the form of the ejection of the secondary vortex ring from the boundary layer. The primary ring then interacts with the secondary ring and in some cases was observed to induce the formation of a third, tertiary, ring near the wall. The details of this process are investigated over a wide Reynolds number range. The results clearly show how one vortex ring can produce another, through an unsteady interaction with a viscous flow near the wall.

Secondary vortices have been observed in the near-wake of circular cylinders in the Reynolds number range of 1,200 to 11,000. Using both the hydrogen bubble flow visualization technique in conjunction with an INSTAR high speed video system and hot-wire anemometry measurements, vortex shedding frequency data were collected and correlated. It was concluded that "transition waves", reported by Bloor [1964], and secondary vortices are identical phenomena. It was established that the nondimensional shedding frequency of the secondary vortices demonstrates a 0.9 power-law relationship relative to Reynolds number, contrary to the 0.5 power-law reported by Bloor. The results suggest that the vortices result from a near-wake free-shear instability which causes the separated cylinder boundary layer to roll-up into the secondary vortices. Visual observations indicate a strong three-dimensional distortion of the vortices, immediately following formation, which may provide the mechanism for the transition from vortex streets to turbulence.

When measuring blood pressure indirectly, oscillations in the cuff pressure are observed. The cuff pressure for which these oscillations reach a maximum and its relationship to the true mean arterial pressure was investigated using a simple one-dimensional theoretical model of the cuff-arm-artery system. Results from this model indicate that the cuff pressure for maximal oscillation is strongly dependent on compression chamber air volume, pulse pressure, and arterial elasticity. Parallel experimental studies indicate general agreement with the theoretical model. The cuff pressure for maximal oscillations appears to provide a reasonable estimation of the true mean arterial pressure provided compression chamber air volume is kept small.

No abstract