摘要:

Movie sequences of vortex patterns visualized by smoke in a two‐dimensional starting flow around an airfoil indicate the process of vortex splitting predicted by Moore and Saffman and by Christiansen and Zabusky for some vortical flows.

ISSN:0031-9171    

DOI:10.1063/1.864753

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Measurements of the dispersion coefficient in oscillating pipe flows for a wide range of Womersley numbers, tidal displacements, and tube radii are presented. The results are shown to be in good agreement with those of a recent laminar dispersion theory for oscillatory flows.

ISSN:0031-9171    

DOI:10.1063/1.864752

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

From an examination of all data to date on the dissipation of turbulent energy in grid turbulence, it is concluded that, for square‐mesh configuration, the ratio of the time scale characteristic of dissipation rate to that characteristic of energy‐containing eddies is a constant independent of Reynolds number, for microscale Reynolds numbers in excess of about 50. Insufficient data available for other grid configurations suggest a possibility that the ratio could assume different numerical values for different configurations. This persistent effect of initial conditions on the time scale ratio is further illustrated by reference to the jet‐grid data of Gad‐el‐Hak and Corrsin.

ISSN:0031-9171    

DOI:10.1063/1.864731

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Parametric excitation of ion Bernstein waves is observed with an ICRF fast wave induction loop antenna in the ion cyclotron frequency range (&ohgr;0∼2&OHgr;i−4&OHgr;i). Important features of the decay process are investigated and discussed.

ISSN:0031-9171    

DOI:10.1063/1.864732

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

It is demonstrated by particle simulations that a relativistic electron beam with 1.25 MeV energy, 1.5 kA current, and 0.5% momentum spread can convert 25% of its energy into electromagnetic radiation based on the Doppler‐shift dominated cyclotron maser mechanism. However, the increase in beam thermal spread reduces the available free energy &Dgr;&ggr;=&ggr;0−&ggr;Rby increasing the beam resonant energy &ggr;Rwhere &ggr;0is the initial beam energy and substantially decreases the conversion efficiency. Further increase in thermal spread eventually changes the saturation mechanism from phase trapping to energy depletion.

ISSN:0031-9171    

DOI:10.1063/1.864733

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

For field‐reversed configurations formed from a 20 mTorr static gas filling in the FRX‐C device, the density in the end regions is determined by side‐on interferometry. This allows us to understand the radial profiles obtained from end‐on holograms, and provides a more accurate estimate of the FRC particle confinement time than previously obtained.

ISSN:0031-9171    

DOI:10.1063/1.864734

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The spatial development of the inlet velocity profile in a uniformly porous channel and tube is investigated. For tubes which are very long compared with their radius, it is shown that above a critical Reynolds number of 2.3 the inlet velocity profile does not necessarily decay into the fully developed, similarity profile for an infinite tube. Rather, the structure of the flow throughout the entire tube is influenced by the inlet profile. This loss of validity of the similarity solution is due to the fact that the tube is of finite length and the inlet profile is not of the similarity form. The actual length of the tube is, however, unimportant. Analogous results hold for the flow in a porous channel, with a critical Reynolds number of approximately 6.

ISSN:0031-9171    

DOI:10.1063/1.864735

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The spatial stability of a class of exact similarity solutions of the Navier–Stokes equations whose longitudinal velocity is of the formxf′(y), wherexis the streamwise coordinate andf′(y) is a function of the transverse, cross‐streamwise, coordinateyonly, is determined. These similarity solutions correspond to the flow in an infinitely long channel or tube whose surface is either uniformly porous or moves with a velocity linear inx. Small perturbations to the streamwise velocity of the formx&lgr;g′(y) are assumed, resulting in an eigenvalue problem for &lgr; which is solved numerically. For the porous wall problem, it is shown that similarity solutions in whichf′(y) is a monotonic function ofyare spatially stable, while those that are not monotonic are spatially unstable. For the accelerating‐wall problem, the interpretation of the stability results is not unambiguous and two interpretations are offered. In one interpretation the conclusions are the same as for the porous problem—monotonic solutions are stable; the second interpretation is more restrictive in that some of the monotonic as well as the nonmonotonic solutions are unstable.

ISSN:0031-9171    

DOI:10.1063/1.864736

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The time‐dependent boundary‐layer flow past an impulsively started translating and rotating circular cylinder is investigated. Two different solution methods are used to calculate the initial flow development; these are an expansion of the solution in a power series in time after the impulsive start and a fully numerical integration of the governing equations. Both solution methods are in excellent agreement during the initial stages of the motion. Calculations are carried out for various rotational speeds of the cylinder and in the boundary‐layer limit of infinitely large Reynolds number. The results show a wide variety of unusual and complex boundary layer separation flow patterns; it is demonstrated how separation is affected by increasing rotation rates and further how it is ultimately completely suppressed by the rotation. This problem involves both an upstream and downstream moving wall for the boundary layer flow and consequently is of interest in connection with the Moore–Rott–Sears criterion for unsteady separation; the present results suggest the criterion for both the upstream moving wall and the downstream moving wall will be achieved in these flows.

ISSN:0031-9171    

DOI:10.1063/1.864908

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

The shape of a two‐dimensional bubble rising at a constant velocityUin a tube of widthhis computed. The flow is assumed to be inviscid and incompressible. The problem is solved numerically by collocation. The results confirm Garabedian’s findings. There exists a unique solution for each value of the Froude numberF=U/( gh)1/2smaller than a critical valueFc. Heregdenotes the acceleration of gravity. It is found thatFc=0.36. In addition the problem of a jet emerging from a vertical nozzle is considered. It is shown that the slope of the free surface at the separation points is horizontal forF<Fcand vertical forF>Fc. Graphs and tables of the results are included.

ISSN:0031-9171    

DOI:10.1063/1.864754

出版商:AIP    

年代:1984

数据来源: AIP