4,669 research outputs found
Energy and velocity of a forming vortex ring
It is known that vortex rings formed by large stroke ratios (in a piston/cylinder arrangement) pinch off from their generating jets at a fairly constant universal time scale. In this paper we show that the hypothesis that at the pinch off the translational velocity of the ring equals the jet flow velocity near the ring is equivalent to the recently proposed idea based on a variational principle by Kelvin and Benjamin that the pinch off occurs when the apparatus is no longer able to deliver energy at a rate required for steady vortex ring existence. A formula for the propagation velocity of a thick vortex ring is also proposed and compared with available experimental data and empirical relations
Defocusing digital particle image velocimetry and the three-dimensional characterization of two-phase flows
Defocusing digital particle image velocimetry (DDPIV) is the natural extension of planar PIV techniques to the third spatial dimension. In this paper we give details of the defocusing optical concept by which scalar and vector information can be retrieved within large volumes. The optical model and computational procedures are presented with the specific purpose of mapping the number density, the size distribution, the associated local void fraction and the velocity of bubbles or particles in two-phase flows. Every particle or bubble is characterized in terms of size and of spatial coordinates, used to compute a true three-component velocity field by spatial three-dimensional cross-correlation. The spatial resolution and uncertainty limits are established through numerical simulations. The performance of the DDPIV technique is established in terms of number density and void fraction. Finally, the velocity evaluation methodology, using the spatial cross-correlation technique, is described and discussed in terms of velocity accuracy
Jamaan at the pass of Bi’rein. An Iron Age IIB-C Ammonite stronghold in central Jordan
In years 2015-2016 the Zarqa Directorate of the Department of Antiquities of the Hashemite Kingdom of Jordan carried out a rescue excavation at the site of Jamaan, an Iron Age IIB-C Ammonite stronghold 16 Km north of ‘Amman. The site survey and limited soundings allow to plot a plan of the structure, comprising an outer enclosure with a casemate wall, two cisterns, and a square podium tower, and to collect ceramic material dating from Iron Age IIB-C (c. 840-580 BC), as well as the head of a soft limestone statue, possibly depicting a local chief or an official. The latter adds to the relatively conspicuous number of statues from the Kingdom of Ammon, possibly illustrating the production of a non-royal commission
Structures in stratified plane mixing layers and the effects of cross-shear
A two-dimensional temporal mixing layer is generated in a stratified tilting tank similar to that used by Thorpe (1968). Extensive flow dynamics visualization is carried out using, for the top and bottom layers, fluids of different densities but of the same index of refraction. The two-dimensional density field is measured with the laser-induced fluorescence technique (LIF). The study examines further the classical problem of the two-dimensional mixing layer and explores the effects of cross-shear on a nominally two-dimensional mixing layer, a situation widespread in complex industrial and natural flows. Cross-shear is another component of shear, in plane with but perpendicular to the main shear of the base flow, generated by tilting the tank around a second axis
Flexibility effects on vortex formation of translating plates
Vortex structures made by impulsively translating low aspect-ratio plates are studied
experimentally using defocusing digital particle image velocimetry. The investigation
of translating plates with a 90° angle of attack is important since it is a fundamental
model for a better understanding of drag-based propulsion systems. Rectangular flat rigid,
flexible and curved-rigid thin plates with the same aspect ratio are studied in
order to develop qualitative and quantitative understanding of their vortex structures
and hydrodynamic forces. We find that the vortex formation processes of all three
cases are drastically different from each other. The interaction of leading-edge vortices
and tip flow near the tip region is an important mechanism to distinguish vortex
patterns among these three cases. The drag trends of three cases are correlated
closely with vortex structure and circulation. The initial peak of hydrodynamic force
in the flexible plate case is not as large as the initial peak of the flat and curved
rigid plate cases during the acceleration phase. However, after the initial peak, the
flexible plate generates a large force comparable to that of the flat-rigid plate case in
spite of its deformed shape, which results from the slow development of the vortex
structure
A model for vortex ring formation in a starting buoyant plume
Vortex ring formation in a starting axisymmetric buoyant plume is considered. A model describing the process is proposed and a physical explanation based on the Kelvin–Benjamin variational principle for steady vortex rings is provided. It is shown that Lundgren et al.'s (1992) time scale, the ratio of the velocity of a buoyant plume after it has travelled one diameter to its diameter, is equivalent to the time scale (formation time) proposed by Gharib et al. (1998) for uniform-density vortex rings generated with a piston/cylinder arrangement. It is also shown that, similarly to piston-generated vortex rings (Gharib et al. 1998), the buoyant vortex ring pinches off from the plume when the latter can no longer provide the energy required for steady vortex ring existence. The dimensionless time of the pinch-off (the formation number) can be reasonably well predicted by assuming that at pinch-of the vortex ring propagation velocity exceeds the plume velocity. The predictions of the model are compared with available experimental results
Vector bundles on toric varieties
CORRECTION. One of the main results in this paper contains a fatal error. We
cannot conclude the existence of nontrivial vector bundles on X from the
nontriviality of its K-group. The K-group that is computed here is the
Grothendieck group of perfect complexes and not vector bundles. Since the
varieties are not quasi-projective, existence of nontrivial perfect complexes
says nothing about the existence of nontrivial vector bundles. We thank Sam
Payne for drawing our attention to the error and Christian Haesemeyer for
explanations about the K-theory.
Abstract: Following Sam Payne's work, we study the existence problem of
nontrivial vector bundles on toric varieties. The first result we prove is that
every complete fan admits a nontrivial conewise linear multivalued function.
Such functions could potentially be the Chern classes of toric vector bundles.
Then we use the results of Corti\~nas, Haesemeyer, Walker and Weibel to show
that the (non-equivariant) Grothendieck group of the toric 3-fold studied by
Payne is large, so the variety has a nontrivial vector bundle. Using the same
computation, we show that every toric 3-fold X either has a nontrivial line
bundle, or there is a finite surjective toric morphism from Y to X, such that Y
has a large Grothendieck group.Comment: There is an error in one of the main conclusions of the paper. Please
see the abstract for more detail
The effect of flow oscillations on cavity drag
An experimental investigation of flow over an axisymmetric cavity shows that self-sustained, periodic oscillations of the cavity shear layer are associated with low cavity drag. In this low-drag mode the flow regulates itself to fix the mean-shear-layer stagnation point at the downstream corner. Above a critical value of the cavity width-to-depth ratio there is an abrupt and large increase of drag due to the onset of the ‘wake mode’ of instability. It is also shown by measurement of the momentum balance how the drag of the cavity is related to the state of the shear layer, as defined by the mean momentum transport and the Reynolds stress , and how these are related to the amplifying oscillations in the shear layer. The cavity shear layer is found to be different, in several respects, from a free shear layer
3D vortex formation of rigid and flexible plates in impulsively starting motion
This fluid dynamics video shows three-dimensional vortex formation process
for plates in impulsive motion which is investigated experimentally by using
defocusing digital particle image velocimetry (DDPIV). Rigid and flexible plate
cases are compared in order to study the effect of flexibility on 3D vortex
formation and associated hydrodynamic forces. This study was motivated by the
general question of how the flexibility of flapping propulsors in flying and
swimming animals affects vortex formation and propulsive force. For translating
and rotating motion, the flexible plate generates a vortex morphology which is
drastically different from that of the rigid plate. We identified the
deflection of the tip region as the source of this difference. The flexible
plate does not produce a large peak for the hydrodynamic force at the impulsive
start and stop. This force trend is correlated with smooth vortex formation and
shedding processes
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