摘要:

Pointed bubbles with a 120° angle at the apex rising in a two‐dimensional tube are considered. A solution is found at a unique Froude number of 0.36.

ISSN:0031-9171    

DOI:10.1063/1.865697

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Two‐stream supersonic turbulent mixing layers with a free‐stream Mach number of 2.3 on the high‐velocity side are experimentally investigated. A large‐scale vortical structure, which has been believed to dominate the development of incompressible mixing layers, is also observed in the present supersonic layers. The spreading rate of the layer is correlated with a velocity ratio of the free streams and a Mach number based on the velocity difference across the layer.

ISSN:0031-9171    

DOI:10.1063/1.865698

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A relativistic electron beam with a finite transverse dc momentum (&bgr;⊥=1/&ggr;0) passing through a region of combined uniform solenoidal and longitudinal wiggler magnetic fields is observed to convert 25% of its kinetic energy into coherent radiation at frequency &ohgr;=&ggr;20(kwV0+&OHgr;c/&ggr;0) if the phase velocity of the generated wave is slightly above the speed of light. In this situation, the bunchings of the slow electron‐cyclotron mode and free‐electron laser modes with combined solenoidal and longitudinal wiggler fields (lowbitron) are observed to compensate each other, which gives rise to a finite threshold for lowbitron operation. In order to attain high efficiency, the wiggler strength of a lowbitron must substantially exceed the threshold.

ISSN:0031-9171    

DOI:10.1063/1.865699

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

In the steady laminar flow past a sudden channel expansion at large Reynolds numberR, the longitudinal length scale of the separated eddy typically increases linearly withRwhereas the transverse length scale isO(1). If these trends were to continue indefinitely, the equations of motion in the bulk flow would reduce to the boundary layer equations asR→∞. Previous investigators obtained steady finite difference solutions to the latter for all values of the expansion ratio when the inlet velocity profile was parabolic; however, when the inflow was uniform, numerical instabilities were encountered below a certain value of the expansion ratio. In the present work these instabilities are avoided through the use of a global Newton method, and uniform inflows to several sudden expansion geometries are considered. In each case steady solutions are found only above a critical value of the expansion ratio where the pressure gradient becomes singular near the reattachment point. These results suggest that for uniform inflows and smaller values of the expansion ratio, the eddy length will no longer increase linearly withRwhen the latter is sufficiently large.

ISSN:0031-9171    

DOI:10.1063/1.865700

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Numerical results are presented for the streaming motion of an incompressible Newtonian fluid past axisymmetric ellipsoids of fixed shape, subject to the condition of zero tangential stress on the body surface. It is demonstrated that a substantial, closed‐streamline wake appears behind the ellipsoid at moderate Reynolds numbers,O(20–50), when the ellipsoid is sufficiently nonspherical, and that this wake initially increases in size with increase of Reynolds number but then eventually decreases in size and disappears when the Reynolds number is sufficiently large,O(200–250) in our cases. The existence of streaming flow past a smooth, slip surface without separation is consistent with predictions from laminar boundary‐layer theory. It is concluded that the recirculating wake that appears behind moderately deformed bubbles is strictly a finite Reynolds number phenomenon that cannot be predicted (or understood) via a boundary‐layer analysis.

ISSN:0031-9171    

DOI:10.1063/1.865701

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Stability of convective motion in a variable‐viscosity fluid contained in an infinite vertical slot generated by a lateral temperature difference was studied by means of linear stability analysis. The viscosity of the fluid was assumed to be an exponential function of the temperature. The undisturbed steady‐state motion was assumed to consist of purely vertical motion with a linear temperature distribution across the slot. The linear stability equations were solved by the Galerkin method. The results show that (i) the onset of instability is in the oscillatory, or traveling wave, mode in fluids whose mean Prandtl number is greater than 25, and (ii) the variable‐viscosity fluid is less stable than the constant‐viscosity fluid at the same mean Prandtl number when it exceeds 100.

ISSN:0031-9171    

DOI:10.1063/1.865702

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Consider a rectangular box filled with a fluid and heated from below. A two‐dimensional model is used to describe the effects of rigid sidewalls. For some aspect ratios the linear theory cannot predict which mode is preferred; thus a study of the bifurcating solutions of the nonlinear problem around these special ratios is made. The stability of the steady solutions is discussed when the competing modes are either the one and two roll modes or the three and four roll modes. In the first case as the Rayleigh number is increased, the flow prefers either the one roll mode for almost all Prandtl numbers or a mixed mode for a small range of low Prandtl numbers. In the second the flow always prefers the four roll mode.

ISSN:0031-9171    

DOI:10.1063/1.865703

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

An insulating isothermal spherical liquid layer subjected to the Coulomb force resulting from ionic injection by one electrode is considered. The electrohydrodynamic stability of this liquid layer is studied using a linear analysis. The critical conditions of stability are determined for a wide range of system parameters: from weak to strong injection, by either inner or outer electrode, and for a ratio of the radii varying over a large range of its permitted values. Injection by the outer sphere is the most unstable situation for weak injection; the converse is true for strong injection. This is explained using qualitative arguments.

ISSN:0031-9171    

DOI:10.1063/1.865704

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A fluid‐particle mixture occupying the interior of a long cylindrical centrifuge, initially in solid‐body rotation with the container, undergoes a transient process which leads to a final state consisting of separated fluid and particles in rigid rotation. The stability of this transient process is examined. It is found to be unstable to infinitesimal disturbances unless a diffusion term proportional to the gradient of volume fraction is added to the equations.

ISSN:0031-9171    

DOI:10.1063/1.865705

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

A new class of generalized self‐similar solutions for the problem of one‐dimensional unsteady outflow of a gas into a vacuum is found. It allows a unified and comprehensive description of plane, cylindrical, and spherical symmetric flows for arbitrary polytropic index. A key property is a moving inner boundary. Relative to this, subsonic and supersonic outflows are possible in certain parameter regions. Simple analytic expressions are found near the boundaries and an extensive parameter discussion is presented. The asymptotic solutions are of specific importance. As an application, it is shown that the isothermal corona of a laser‐generated plasma is in part described by one of these asymptotic solutions.

ISSN:0031-9171    

DOI:10.1063/1.865655

出版商:AIP    

年代:1986

数据来源: AIP