摘要:

The instability of flows around hump and dip imperfections is investigated. The mean flow is calculated using interacting boundary layers, thereby accounting for viscous/inviscid interaction and separation bubbles. Then, the two‐dimensional linear stability of this flow is analyzed, and the amplification factors are computed. Results are obtained for several height/width ratios and locations. The theoretical results have been used to correlate the experimental results of Walker and Greening (British Aeronautical Research Council 5950, 1942). The observed transition locations are found to correspond to amplification factors varying between 7.4 and 10.0, consistent with previous results for flat plates. The method accounts for both viscous and shear‐layer instabilities. Separation is found to increase significantly the amplification factor.

ISSN:0031-9171    

DOI:10.1063/1.866815

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The aim of this work is an experimental study of the development of perturbations of a gaseous interface impulsively accelerated by a plane shock wave. The experiments are performed in a double diaphragm shock tube, where the second diaphragm is a very thin Mylar film which can be initially bulged because of a pressure difference between the two gases. The shape of the leading front of the contact zone is measured at three locations along the tube using a transversal array of heat transfer gauges. After the shock passage, the evolution of the interface is sensitive to vorticity production and boundary layer effects so that the impulsive Rayleigh–Taylor theory is inadequate for the description of this evolution. In particular, the predicted perturbation reversal when the shock wave passes from the heavy gas to the light one may not occur because of the boundary layer effect.

ISSN:0031-9171    

DOI:10.1063/1.866816

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The structure and growth rate of the radiative instability in a sheared magnetic fieldBhave been calculated analytically using the Braginskii fluid equations. In a shear layer, temperature and density perturbations are linked by the propagation of sound waves parallel to the local magnetic field. As a consequence, density clumping or condensation plays an important role in driving the instability. Parallel thermal conduction localizes the mode to a narrow layer wherek∥=k⋅B/‖B‖ is small and stabilizes short wavelengthsk>kc, wherekcdepends on the local radiation and conduction rates. Thermal coupling to ions also limits the width of the unstable spectrum. It is shown that a broad spectrum of modes is typically unstable in tokamak edge plasmas and it is argued that this instability is sufficiently robust to drive the large‐amplitude density fluctuations often measured there.

ISSN:0031-9171    

DOI:10.1063/1.866817

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A kinetic theory in the form of an integral equation is provided to study the electrostatic oscillations in a collisionless plasma immersed in a uniform magnetic field and a nonuniform transverse electric field. In the low temperature limit (ky&rgr;i≪1, wherekyis the wave vector in theydirection and &rgr;iis the ion gyroradius) the dispersion differential equation is recovered for the transverse Kelvin–Helmholtz modes for arbitrary values ofk∥, wherek∥is the component of the wave vector in the direction of the external magnetic field assumed in thezdirection. For higher temperatures (ky&rgr;i>1) the ion‐cyclotron‐like modes described earlier in the literature by Ganguli, Lee, and Palmadesso [Phys. Fluids28, 761 (1985)] are recovered. In this article the integral equation is reduced to a second‐order differential equation and a study is made of the kinetic Kelvin–Helmholtz and the ion‐cyclotron‐like modes that constitute the two branches of oscillation in a magnetized plasma including a transverse inhomogeneous dc electric field.

ISSN:0031-9171    

DOI:10.1063/1.866818

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The presence of magnetically reflected particles is shown to allow the existence of large amplitude magnetosonic solitary waves in relativistic electron–positron plasmas. If the flow is assumed to contain a single loop of gyrating particles, self‐consistent structures are found with peak field amplitudes (B/B∞)max<(11)1/2, whereB∞is the magnitude of the upstream magnetic field. In contrast, without reflected particles, the amplitude of a relativistic magnetosonic soliton is restricted to (B/B∞) −1<2/&ggr;∞, where &ggr;∞is the upstream Lorentz factor. Therefore, if &ggr;∞≫1, reflected particles greatly increase the allowable amplitudes of these nonlinear waves. It is also shown that when &ggr;∞≫1, the wave properties are independent of &ggr;∞, and are completely parametrized by the ratio of the Poynting flux to the kinetic energy flux in the upstream flow. Some new features of solitary waves without reflected particles are also derived, and a heuristic model is presented which gives a simple physical interpretation of many of these results.

ISSN:0031-9171    

DOI:10.1063/1.866765

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

In an underdense plasma a large‐amplitude plasma oscillation may be produced by the beating of two electromagnetic waves with a frequency difference approximately equal to the plasma frequency. In the spatially one‐dimensional, cold, and collisionless plasma the large‐amplitude plasma oscillation is limited by the nonlinearity caused by relativistic effects. In this paper a simple nonlinear equation, resembling the original equation of Rosenbluth and Liu [Phys. Rev. Lett.29, 701 (1972)], is derived in the weak beat power limit from the fully relativistic fluid model proposed by Sprangle, Sudan, and Tang [Appl. Phys. Lett.45, 375 (1984); Phys. Fluids28, 1974 (1985)]. This equation also contains the effects of the relativistic transverse motion. Its analytical solution, describing the plasma oscillation dynamics, is given in a closed form by using Jacobian elliptic functions. The analytical computations are compared with numerical computations. Finally the fully relativistic equation, describing free plasma oscillations, is studied analytically.

ISSN:0031-9171    

DOI:10.1063/1.866766

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An electrostatic linear analysis is performed for a finite‐width electron or ion beam streaming along a strong magnetic field in the presence of a homogeneous background plasma. The linear fluid equations have been solved as a boundary value problem for planar and cylindrical beam shapes, and the dispersion which results as an eigenvalue problem is solved numerically without approximation as a function of the electron beam width. The solution gives unstable modes for any beam width and the dispersion relation shows different branches. There is a branch in both configurations that represents the most unstable mode, and the wavelength of this unstable mode for the cylindrical beam is larger than the corresponding wavelength for the most unstable mode of the slab‐shaped beam.

ISSN:0031-9171    

DOI:10.1063/1.866767

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A two‐dimensional electrostatic particle‐in‐cell code is used to simulate a finite‐width plasma streaming across a uniform magnetic field. The simulations show that the plasma polarizes, and non‐neutral charge layers develop along its edges. In an electron–ion plasma, the charge layers are asymmetric and the electron layer is unstable to the diocotron mode. The simulations show that this instability has smaller growth rate for plasma streams that are relatively less dense and wider. For a positive/negative ion plasma with equal mass ions the charge layers are symmetric and the plasma is stable to the diocotron mode. The results show that the diocotron instability leads to vortex structure when the plasma width is greater than the ion gyroradius, but this instability disrupts the entire plasma when the plasma width is of the order of or smaller than the ion gyroradius.

ISSN:0031-9171    

DOI:10.1063/1.866768

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The recent conclusion of Terry, Diamond, and Hahm (TDH) [Phys. Rev. Lett.57, 1899 (1986)] that in a turbulent, collisionless plasma ‘‘magnetic transport including quasilinear magnetic flutter transport  . . .  does not contribute to the relaxation of ⟨f⟩, and thus is not responsible for electron energy or momentum transport’’ is argued to be irrelevant or incorrect for a variety of situations of physical interest, including saturation by quasilinear plateau formation, induced scattering, and, most important, conventional mode coupling. The well‐established theory of the mean infinitesimal response function and the spectral balance equation provides a unifying framework for understanding the work of TDH. In particular, the cancellations which lead to the TDH conclusion are special cases of well‐known relationships between the response functionparticle propagator, and dielectric function. A more general, concise, and manifestly gauge‐invariant algebraic derivation of the cancellations is given. Although the cancellations occur in a certain limit, the conclusions of TDH do not follow in general: The TDH picture of steady‐state turbulence as consisting of small‐scale ‘‘incoherent’’ ballistic ‘‘clumps’’ shielded by long‐wavelength ‘‘coherent’’ dielectric response is misleading physically and incomplete mathematically since it does not describe correctly the often dominant process of renormalizedn‐wave coupling, particularly important for the ions. Although ion ballistic response is negligible, ions are important nevertheless: Their nonlinear contribution to the saturated potential can drive parallel electron currents, hence magnetic fluctuations, through linear mechanisms. Thus, when ion nonlinearities are considered, formulas for the magnetic contribution to transport emerge which are quite similar to the quasilinear formula.

ISSN:0031-9171    

DOI:10.1063/1.866769

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Exact solutions of the nonlinear magnetohydrodynamic equations for a highly conducting fluid within an axisymmetric container are obtained. Attention is focused on fluids in which the unperturbed velocity and magnetic field are axially symmetric and purely zonal. It is shown that there are exact solutions with large amplitude but restricted form, indicating that an arbitrary disturbing force produces other motions as well as Alfve´n waves propagating along an azimuthal magnetic field whose strength varies with radius.

ISSN:0031-9171    

DOI:10.1063/1.866770

出版商:AIP    

年代:1988

数据来源: AIP