Experimentally measured steady and unsteady forces induced by a cross flow over a smooth circular cylinder are reported in this paper. The measurements were made in the 0·305 m research water tunnel of the Pennsylvania State University. The parameters examined include the Reynolds number, which was varied over a range of 2 × 104 to 2 × 106, free-stream turbulent intensity, integral length scale-to-diameter ratio and active span-to-diameter ratio; however, this paper includes only some of these results.Among the results reported are the complete mean drag data on all the cylinders tested and some fluctuating force data chosen to illustrate the effects of Reynolds number and active span-to-diameter ratio on the measured forces in uniform and turbulent cross flows. From these results, it can be concluded that the unsteady forces bear a functional relation to Reynolds number in the range tested which is very similar to the well-documented behaviour of the mean drag. Thus the effect of free-stream turbulence on both the steady drag forces and the unsteady forces is to shift the transitional region to a lower Reynolds number. Decreasing the active span-to-diameter ratio increases the buffeting lift coefficient at the same Reynolds number. Finally, it is observed that the Strouhal lift signal at transitional Reynolds numbers is no longer quasi-periodic but rather resembles a narrow-band random signal.

No abstract

No abstract

The influence of a 60-Hz ac electric field on a thermally stratified flow of a dielectric between two parallel plates is considered. The coupled hydrodynamic problem is first cast as a stability problem and solved for a Poiseuille base flow in the limit of low Reynolds number. Two modes of instability were examined, transverse roll cells and longitudinal roll cells. Of the two, the latter appears to be the more probable mode and the critical dielectrophoretic Rayleigh number seems to be independent of Reynolds number. The transverse modes exhibit an overstability. A crude verification of the theory is to be had from data obtained on a parallel plate test apparatus. The experimental parametric results clearly demonstrate the role of the dielectrophoretic augmentation of heat transfer. It is found that the heat transfer coefficient is a direct function of the heat flux rate and the applied potential difference, and an inverse function of the flow rate.

No abstract