摘要:

The popular view that juncture flows are dominated by an array of steady horseshoe vortices is not universally correct. Visualizations are presented that show that vortices do form but that they undergo a periodic generation and shedding motion. The temporal scale of this cycle does not depend strongly on the characteristics of the incoming boundary layer. Also, the vortices exhibit an unusual pinching‐off process in the wake of the cylinder leading to new horseshoe‐vortex filaments existing behind the cylinder, which are then convected downstream.

ISSN:0031-9171    

DOI:10.1063/1.866374

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Using the compressible resistive magnetohydrodynamic (MHD) equations in a finite aspect ratio cylinder, it is found that them=1 mode (the sawtooth oscillation) can saturate when the pressure inside the magnetic island is higher than that of the original core plasma. The saturation condition is of the form &Dgr;&bgr;p≳8&egr;−1q=1(1−q0)2. This saturation effect can be used to actively stabilize sawteeth by heating or raising the density in the island. This mechanism, together with a stabilizing toroidal effect, may also explain recent lower‐hybrid wave‐driven tokamak experiments where the saturation of sawteeth has been observed.

ISSN:0031-9171    

DOI:10.1063/1.866375

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

This study is concerned with the effects of surface tension on the steady potential flow of a liquid issuing from a two‐dimensional slot with inclined walls. The mathematical problem, which is characterized by a nonlinear boundary condition along the free streamline, is solved using a novel numerical technique based on the fast direct Poisson solver. Numerical solutions were obtained for various Weber numbers and slot angles, and the results are compared with the free streamline shapes and contraction coefficients of the corresponding classical potential solutions without surface tension, and with some available theoretical results. When the numerical grid size is reduced or when the Weber number is sufficiently small, undesirable standing waves appear on the free surface that violate a causality condition. The cause of these waves was examined. A criterion was found for the occurrence of the waves.

ISSN:0031-9171    

DOI:10.1063/1.866376

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The effect of slow rotation on the propagation of solitary internal waves in shallow fluids is studied. The Kadomtsev–Petviashvili (KP) equation, appropriately modified to account for Coriolis effects, is solved numerically for two‐layer flow in a rotating channel of finite width. In the presence of rotation, an initially straight‐crested soliton evolves to a three‐dimensional disturbance, which, to a reasonable approximation, has a sech2profile along the channel but decays slowly as it propagates. Furthermore, the wave amplitude varies exponentially across the channel and the wave crest is curved backward. These trends are in agreement with recent laboratory experiments.

ISSN:0031-9171    

DOI:10.1063/1.866377

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Time‐dependent complex eigenfrequencies for traveling inertial waves in a rotating cylinder of fluid have been measured while the fluid was spun up from rest. The waves were excited by precessing the lid of the cylindrical container at a frequency near a predicted eigenfrequency for a given rotation speed. Time sequences of disturbance pressure differences measured during subsequent free ringdown of the excited wave were inverted to recover inertial eigenfrequency and decay rate using a generalized least‐squares procedure.

ISSN:0031-9171    

DOI:10.1063/1.866378

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The convection of cold water in the vicinity of its density maximum in a rectangular enclosure is studied in the limit of small Rayleigh number. Results for the stream function (to lowest order) and the first corrections (for convective effects) to the temperature field and heat transfer are obtained. Results are discussed in the context of a single parameter, which fixes the orientation of the wall temperatures with respect to the extremum temperature. The effects of changing the enclosure aspect ratio are also discussed.

ISSN:0031-9171    

DOI:10.1063/1.866379

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Steady bimodal convection of an infinite Prandtl‐number Boussinesq fluid in a cylinder is considered. An asymptotic analysis similar to the one used by Buell and Catton [Phys. Fluids29, 23 (1986)] for axisymmetric convection yields a solvability condition that determines the radial wavenumber. The analysis is valid for convection far away from the origin, the lateral boundary, and any pattern dislocations. The azimuthal wavenumber is treated as a parameter, although in real systems it is dependent on the initial and boundary conditions. Results are presented for Rayleigh numbers between 14 000 and 60 000, and for azimuthal wavenumbers between 5 and 7. It is shown that for increasing Rayleigh numbers, the selected radial wavenumber and the heat transfer tend to become independent of the azimuthal wavenumber. No quantitative experimental data are available, but one qualitative comparison is good.

ISSN:0031-9171    

DOI:10.1063/1.866380

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The onset of double‐diffusive convection is studied for an infinite fluid layer possessing a density maximum in its interior. Linear stability analyses are performed on a basic state with constant temperature and concentration gradients in which the density‐temperature relationship is quadratic. Regions of stability and instability to both steady and oscillatory modes are delineated in the positive quarterplane of the Rayleigh and solutal Rayleigh numbers plane. It is found that decreasing the position of maximum density in the vertical leads to an increased stability range and also increased regions of oscillatory instability in that quadrant. It is also found that the extent of penetration of the convective motion into the stable region is diminished with increased solute concentration. The effects of both the rigid–rigid and the free–free boundary conditions on the instabilities are investigated. Both conditions lead to dynamically similar motions with significant quantitative differences.

ISSN:0031-9171    

DOI:10.1063/1.866381

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

Experimental results of the nonlinear growth of perturbations on a shocked interface between liquids having a density ratio of 10 are reported. After the shock passes from the higher‐density liquid into the lower‐density liquid, spikes of the higher‐density liquid are observed to grow into the other liquid without producing severe turbulence. The measured growth is compared with analytical estimates. The time‐resolved shadowgraphs provide excellent data for testing hydrodynamics models.

ISSN:0031-9171    

DOI:10.1063/1.866382

出版商:AIP    

年代:1987

数据来源: AIP

摘要:

The anisotropy of scalar fluctuations induced by scalar flux and the return to isotropy of the anisotropic scalar field are investigated by using the modified zero‐fourth‐cumulant (MZFC) expansion. The spectral equations of axisymmetric scalar flux and variance in decaying isotropic turbulence are formulated by applying the MZFC expansion. Numerical calculation is made under the initial conditions for two different degrees of anisotropy: one is an isotropic state with a flux, and another is a two‐dimensional state that is strongly anisotropic in the three dimensions. Results show that the scalar flux produces an anisotropy of the scalar distribution so that scalar concentration increases in the direction parallel to the symmetric axis and decreases in the direction transverse to it. The return to isotropy is rapid, except for large scale fluctuations, even in the strongly anisotropic case.

ISSN:0031-9171    

DOI:10.1063/1.866383

出版商:AIP    

年代:1987

数据来源: AIP