摘要:

Using digital image processing of particle streak photography, the streamwise and perpendicular components of the velocity field were investigated, in the mid‐span plane of a two‐dimensional mixing layer, with a 6:1 velocity ratio. The Reynolds number of the flow, based on the local vorticity thickness and the velocity difference across the layer, ranged from 1360 to 2520, in the plane of observation. The significant result of this experiment was that the region of vorticity bearing fluid is confined to a small fraction of the flow. A second finding, consistent with the small regions of concentrated vorticity, was the observation of instantaneous streamwise velocity reversal, in the laboratory frame, in small regions of the flow.

ISSN:0031-9171    

DOI:10.1063/1.863481

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The temporal evolution of small three‐dimensional disturbances with a large streamwise scale in viscous, parallel, semi‐bounded flows is studied. In the limit of the initial disturbance being independent of the streamwise coordinate, the vertical velocity component consists solely of a continuous spectrum part. Tollmien–Schlichting waves do not appear in this special case. The streamwise perturbation velocity is obtained by solving a forced initial value problem. While the vertical velocity remains constant for small times, the streamwise perturbation velocity exhibits a linear growth due to the forcing. Eventually, viscous dissipation becomes dominant and the disturbance decays. Asymptotic solutions valid for small and large times are presented. The relation of these results to the longitudinal streaky structure found in many shear flows is discussed.

ISSN:0031-9171    

DOI:10.1063/1.863490

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Vorticity contours are used to show that two‐dimensional separation falls into three stagnation region classes for steady flows; one type on fixed walls and two others for moving walls. The existing data on laminar unsteady flows indicate that they fill the same three classes.

ISSN:0031-9171    

DOI:10.1063/1.863491

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The flow arising from an initial pressure discontinuity across a perturbed interface of two ideal gases is studied using analytical and numerical methods. In particular, the stability of the shock wave, the interface, and the rarefaction wave in the resulting flow are investigated. The equations of motion and the initial and boundary conditions are linearized for small perturbations, and a Fourier analysis is made in the lateral direction. The equations are then solved by the method of characteristics. The results show that the interface is unstable and its perturbations asymptotically acquire a constant rate of growth. The shock wave is stable and has rapidly damped oscillations, which appear to be unaffected by the instability of the interface.

ISSN:0031-9171    

DOI:10.1063/1.863492

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An investigation of diverging supersonic nozzles with throat diameters in the range of 0.025 to 0.25 mm has been carried out using gasdynamic measurements for a variety of gases and mixtures with sulfur hexafluoride. Due to the small nozzle size, boundary layers, although thin, may constitute a major fraction of the flow or, indeed at low pressures, may be fully developed. Nevertheless these nozzles have been shown to be very efficient for production of clusters condensed from the expanding gas. They produce orders of magnitude increases in molecular beam intensities, relative to the conventional, ’’isentropic’’ free jet sources. The effects of gas properties and nozzle design on performance have been analyzed and compared to theoretical solutions of the governing equations of motion including cluster nucleation and growth.

ISSN:0031-9171    

DOI:10.1063/1.863493

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An experimental study of the effects of thermal and velocity nonuniformities is performed in an equilibrium plasma for a range of Hall coefficients. By introducing equally spaced cold blades in the radial flow of an electrodeless magnetohydrodynamic disk deivce, it is possible to create well‐defined two‐dimensional wake nonuniformities with strong variations of the plasma properties in the direction normal to the magnetic field and the flow. This type of nonuniformity and orientation theoretically provides the strongest reduction of Hall coefficient and effective conductivity for high values of the Hall coefficient. This degradation which reached more than 50% in some cases, is controlled by both the level of nonuniformities and the value of the ideal Hall coefficient. The former is dependent upon the number of blades (root mean square deviation of the conductivity), and the latter is dependent upon the values of the magnetic field intensities. The results provide basic quantitative information about the effects of conductivity and velocity nonuniformities on the performance of equilibrium magnetohydrodynamic generators over a wide range of Hall coefficients. The theoretical predictions are derived from a detailed two‐dimensional electrodynamic analysis and a simplified engineering model based on a generalization of Rosa’s layer model. These experiments validate the analytical studies and support the use of the theoretical layer nonuniform models in describing the effect of boundary layers on the performance of linear magnetohydrodynamic devices.

ISSN:0031-9171    

DOI:10.1063/1.863494

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Under certain operating conditions, a high power, multifilament ion source can effectively function as a spontaneously excited ion‐acoustic resonator. The fluctuations, initially excited by a current‐driven instability at the thermionic source cathode, exhibit a Lorentzian spectral profile where the resonant frequency is solely determined by the spatial cathode configuration, and the resonance half‐width is set by the mean plasma ion lifetime. The ion wave‐cathode interaction, responsible for the resonant behavior, entails a feedback mechanism dependent on the electrostatic wave scattering properties of thermionic filaments operated under space‐charge‐limited conditions. Fluctuation amplitude levels are insufficient to significantly perturb the ions and consequently, the instability does not affect the performance of a multifilament source neutral beam system.

ISSN:0031-9171    

DOI:10.1063/1.863495

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

By solving the Korteweg–deVries equation in a wide range of the ratio between the nonlinearity and the dispersion, the recurrence of the initial state of the ion wave is examined. The recurrence is assured of taking place only when the dispersion of the initial ion wave predominates over the nonlinearity. If the initial wave has strong nonlinearity compared with the dispersion, the recurrence is indistinct, and the initial monochromatic wave evolves to a turbulent state.

ISSN:0031-9171    

DOI:10.1063/1.863496

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Cylindrical arrays of parallel wires can be imploded by the magnetic forces generated by currents through the wires to form hot, denseZ‐pinch plasmas. Analytic growth rates of displacements and deformations of wires in imploding wire arrays are calculated. Arrays of six or more wires are reasonably stable against asymmetric displacements. The growth rate of the kink instability on a single wire of linear mass density &mgr;, radiusa, and carrying currentIpeaks at &lgr;max= 0.7I/&mgr;1/2caat a kink wavelength of about 4a. The most unstable kink modes ofn‐wire arrays are the symmetric (l= 0) radial modes and antisymmetric (l=n/2) tangential modes. The effect of a center wire is to tend to destabilize radial kink modes and stabilize tangential modes. In the experimental parameter range of the largest current generators, the number of kink growth times before collision is insensitive to values of maximum current and current pulse width. In general, the kink instability will grow nonlinearly if the initial array radius is more than a few plasma wire radii. In the limit of long wavelengths, the kink instability is shown to be equivalent to the displacement instability.

ISSN:0031-9171    

DOI:10.1063/1.863497

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

A decay type instability is observed in a linearly stable ion beam‐plasma system. The spatial development of the instability agrees with the solution of the mode coupling equations describing a three‐wave interaction.

ISSN:0031-9171    

DOI:10.1063/1.863483

出版商:AIP    

年代:1981

数据来源: AIP