Showing papers on "Vortex shedding published in 1968"

Unsteady lift forces generated by vortex shedding about a large, stationary, and oscillating cylinder at high Reynolds numbers.

Abstract: Wind tunnel study of unsteady lift forces generated by vortex shedding about large, stationary, and oscillating cylinder at high Reynolds numbers

Energy dissipated by atmospheric eddies in the wake of islands

Abstract: The dissipation rates for kinetic energy in the large eddies observed in the wake of three islands are calculated to be 22, 25, and 34 erg g−1 sec−1 some three hours after the formation of the eddies. The tangential velocity at the visible edge of the eddy at that time is estimated to be about 14 knots. A periodic fluctuation in pressure should exist on the steep sides of the islands exposed tangentially to the free stream velocity vector. These pressure fluctuations, caused by flow separation during vortex shedding, would have a periodicity of 8 to 9 hours and a magnitude possibly as great as 1 mb. Assumption is made that the eddies are an atmospheric analog of the Karman trails observed in the laboratory and that the two-dimensional theory built on these observations will suffice to describe the most important features of the atmospheric vortex trails.

Vortex streets in stably stratified fluids

Abstract: : Periodic vortices (vortex street) formed behind a circular cylinder in a flow of a stably stratified fluid with constant upstream velocity and density gradient are investigated experimentally. The vortices are elliptic in shape, and the vertical spacings between them decrease rapidly along the wake. The vortex street collapses into internal waves far downstream. The on-set of the vortex street is determined in terms of a stability parameter sigma ( = Ri to the 1/2 power/Log10(Re/40), a function of Reynolds number Re and Richardson number Ri. It is found that for sigma > 1, the flow is extremely stable and there are no periodic vortices formed in the wake. As sigma decreases to less than unity (sigma < 1), periodic vortices begin to appear in the wake. (Author)

Vortex Flow in Superconductors

Abstract: Electrical resistance observed1 in superconductors in the mixed state is interpreted as a measure of the motion of Abrikosov vortices in a direction transverse to the imposed net current. Additional evidence of flow of vortices has been provided by dc transformer action2,3 and by heat transport4 in the direction of vortex flow. The connection between the resistive voltage drop and the flow of vortices is understood5 in terms of the superconducting order parameter, which is a complex number varying in space and time. A vortex, which is formed at one edge of the sample, moves across it, and is destroyed at the other edge, has a ``zipper'' effect on the phase of the order parameter. On one side of the path of the vortex, the phase is raised by π (for a single quantum vortex); on the other side it is lowered by the same amount. This process makes no net change in the physical state of the sample; yet it requires a pulse of voltage difference between the ends of the sample, because the time derivative of phase of the order parameter is proportional6 to electrostatic potential (more generally, to the chemical potential for electrons). A state of steady flow of vortices thus involves a steady difference of potential between the ends of the sample. A voltmeter registers this difference. There is no net induced emf to be registered. The dissipation associated with the electrical resistance of a sample in which there is vortex flow occurs in the form of Joule heating produced by normal (i.e., nonsuper) currents.7 Most of this dissipation is in the cores of the vortices, where the material is at least approximately normal and where the electric field is strongest. The electric field in a moving core is partly induced magnetically but is mostly the gradient of electrostatic potential which is associated with the rapid changes of order parameter on opposite sides of the core. A moving vortex not only produces heat but also carries heat along with it, transversely to the electric current and to the magnetic field. A plausible model for the mechanism of this heat transport is based on the available excited states of the superconducting system of electrons as described by BCS. Each available level has a thermal probability of being occupied. The spectrum of levels available varies from place to place in the material according to the local value of the energy gap, which practically vanishes in the core of each vortex, but is significantly large between cores. A particular excitation can migrate only in regions where the energy gap is less than the excitation energy. Each low‐energy excitation is therefore trapped, rattling about within a definite core. When a core moves, the trapped excitations are carried along. When a vortex is eventually destroyed at the edge of the sample, its trapped excitations are stranded at the last position of the core. As the gap there goes up, so does the energy of each excitation. The excitation probability which corresponded to thermal equilibrium at the orignal energy is excessive at higher energies. Until the energy becomes so great that the excitation is no longer trapped, the excitation probability can readjust only by a net probability of conversion of energy from the electronic excitation into lattice heat. Similarly, when a vortex is formed, its core absorbs heat from the lattice. The net result is transportation of heat from the location of formation to the location of destruction. The detailed mechanisms by which forces are applied to vortices remain obscure. But by thermodynamic arguments8 we find a force in the direction of j×B due to net electrical current and a thermal force in the direction of − ▿T. In a superconductor in which the pinning of vortices is slight, we should be able (at least as laboratory curiosities) to use vortex flow as the basis of an electrically driven low‐temperature refrigerator and of a thermally driven source of electrical energy.

Experimental Measurements of Lift and Drag on an Oscillating, Smooth Circular Cylinder in Crossflow.

Abstract: : In certain Reynolds number regimes vortices are shed periodically from a smooth circular cylinder which moves normal to its axis through a viscous fluid. This alternate shedding of vortices induces a fluctuating lift force, in a direction perpendicular to its motion. A small fluctuating drag force is also induced in the direction of motion with a frequency of twice the vortex shedding frequency. For an elastic cylinder or an elastically mounted cylinder, if the frequency at which the vortices are shed is approximately equal to the free frequency of vibration of the cylinder, resonance can occur and the amplitude of vibration of the cylinder may become quite large. This phenomenon is responsible for the serious vibration problems of towed cables, submarine perioscopes, and other strut appendages subject to these unsteady hydrodynamic forces. The present work was undertaken to repeat and extend some of the measurements made in former investigations, to collect the data on magnetic tape, and to analyze the results of a high-speed computer.