摘要:

A new phenomenon of successive branchings of a liquid jet injected from a nozzle into the atmosphere is reported. It is observed that an intact jet can be made to bifurcate successively into a two‐, three‐, and multiple‐pronged jet by oscillating the nozzle along its axis at a successively higher critical frequency. While the two‐ and three‐pronged jets are found to be structurally stable in certain frequency ranges, the different branches in a multiple‐pronged jet appear to merge and split among themselves chaotically without any discernible periodicity.

ISSN:0899-8213    

DOI:10.1063/1.858130

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effect of a free surface on the structure of a submerged turbulent jet is investigated experimentally. Three‐component LDV measurements beneath a clean free surface show that the mean flow spreads laterally outward in a shallow surface layer much wider than the mean flow well below the surface. As the free surface is approached, velocity fluctuations normal to the surface are diminished while those parallel to the surface are enhanced. Laser‐induced fluorescence is used to show that the surface layer contains fluid ejected from the jet. With the addition of surface‐active agents, the surface layer is suppressed.

ISSN:0899-8213    

DOI:10.1063/1.858131

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The use of axisymmetric distributions of singularities to describe the translation of an annular disk in the presence of fixed boundaries or other disks is complicated by the appearance of a singularity at the inner rim. This makes the numerical solution of the integral equations more difficult and less accurate than for the corresponding motions of a unit disk discussed in an earlier paper [Phys. Fluids A2, 301 (1990)]. However it is observed here that the asymptotic methods developed by a previous author [D. A. Spence, Proc. Cambridge Philos. Soc.68, 529 (1970)], for the limit in which the ratio of radii approaches unity, can, to leading order, be used to obtain more simply estimates of the influence of fixed walls or other disks on the drag force and fluid flux through the hole. Interpolation then could provide estimates of these physical quantities for radii ratios that are close to neither unity nor zero.

ISSN:0899-8213    

DOI:10.1063/1.858132

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The motion of a bubble rising at a constant velocityUin a vertical tube of circular cross section is considered. The problem is formulated by using the Stokes’ streamfunction as one of the independent variables. The resulting equations are solved by finite differences. It is found that there is a flow for each value of the Froude numberF=U/&sqrt;gD. HereDis the diameter of the tube andgis the acceleration of gravity. ForF≳Fd∼0.49, there is a cusp at the apex of the bubble. ForF<Fdthe slope of the free surface profile is continuous at the apex. ForF=Fd, the solution is a pointed bubble with a 130° angle at the apex. The results forF<Fdagree with the solutions previously computed by Levine and Yang. In addition, the problem of an axisymmetric jet falling from a vertical nozzle is considered. It is shown that there is a flow for each value of the Froude number.

ISSN:0899-8213    

DOI:10.1063/1.858133

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effect of surface tension on the cavitating flow past a circular cylinder is considered. The problem is solved numerically for various values of the Weber number &agr; and of the angular position &ggr; of the separation points. It is shown that the angle &bgr; between the free surface and the surface of the cylinder is determined uniquely by specifying the values of &agr; and &ggr;.

ISSN:0899-8213    

DOI:10.1063/1.858134

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Thermocapillary convection (TC) in cylindrical liquid bridges (floating zones) of liquids with Prandtl numbers Pr=1, 7, and 49 is investigated experimentally. The zones have been heated from above or from below to study the influence of buoyant forces. Fourier analyses of temperature signals from zones covering systematically wide ranges of aspect ratiosAand Marangoni numbers Ma have shown the existence of various forms of periodic and nonperiodic TC. This paper reports on periodic TC existing under certain conditions between the onset of time‐dependent TC at the critical Marangoni number Macand 7×Mac. From the measurements of the onset of periodic TC the dependence is reported for the threshold Macand the period near the threshold &tgr;con the aspect ratio. The development of periodic TC when further increasing Ma is shown by typical examples from measurements of the frequency and the amplitude of the oscillations. By correlation analyses from three temperature signals, different structures of periodic TC in the investigatedA–Ma–Pr range were identified. Both the running waves with an azimuthal component (m≥1) and the axially running waves (m=0) were found and the findings of these and various other spatiotemporal structures of periodic TC were displayed inA–Ma/Macstate maps. These maps indicate as well the influence of the Prandtl number and of the buoyant forces on the preferred spatial structure of periodic TC, as discussed in the light of an already existing theory.

ISSN:0899-8213    

DOI:10.1063/1.858135

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Steady thermocapillary flows of low‐Prandtl‐number fluids in shallow rectangular enclosures under an imposed‐heat‐flux configuration are studied in the absence of gravitational forces. The Navier–Stokes equations are solved numerically for Reynolds numbers up to 104. The pressure correction method is used to treat the pressure‐velocity coupling, in particular, the SIMPLEC approximation is considered. In the numerical simulation, the free surface is assumed flat. This hypothesis is justifiedaposteriori; free surface deformations are computed by domain perturbation for small capillary numbers. The numerical results show the existence of a boundary layer along the free surface and the existence of a strong vortex close to the vertical wall. The characteristic velocity in the boundary layer responds to Ostrach’s scaling of thermocapillary boundary layers, and the wall vortex responds to Batchelor’s model for steady laminar flows with closed streamlines at large Reynolds numbers. The calculated surface deformations have the following feature: When inertia effects are important, the wall vortex induces a depression of the free surface in the region above it, which results in an elevation in the region below the heater; this behavior is opposite to that for the case of small Reynolds numbers.

ISSN:0899-8213    

DOI:10.1063/1.858136

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Linear stability analysis of the Be´nard–Marangoni problem in a layer of fluid with a deformable free surface is considered. The analysis is restricted to fixed values of the Prandtl and the Biot numbers in order to determine the role of the Crispation number on convection. For a deformable upper interface both stationary and oscillatory instabilities are obtained. These two kinds of instabilities have been studied separately and the corresponding critical wave numberskcand critical Rayleigh numbersRchave been obtained numerically. The conditions under which two stationary states, an overstable mode and stationary mode, or two overstable modes can coexist simultaneously are determined. In the last case the possibility to obtain a strong resonance between two overstable modes is also discussed.

ISSN:0899-8213    

DOI:10.1063/1.858137

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The Eckhaus, cross‐roll, and zigzag instabilities of convective patterns in the form of rolls are examined in the case that the minimum in the marginal stability curve is degenerate (flat). The effects of asymmetry in the growth rate curves are also considered. Examples are given for the quartic degeneracy.

ISSN:0899-8213    

DOI:10.1063/1.858138

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Average intact lengths of round liquid jets generated by airblast coaxial atomizers were measured from over 1500 photographs. The intact lengths were studied over a jet Reynolds number range of 18 000 and Weber number range of 260. Results are presented for two different nozzle geometries. The intact lengths were found to be strongly dependent on Re and We numbers. An empirical equation was derived as a function of these parameters. A comparison of the intact lengths for round jets and flat sheets shows that round jets generate shorter intact lengths.

ISSN:0899-8213    

DOI:10.1063/1.858139

出版商:AIP    

年代:1991

数据来源: AIP