摘要:

The problem of turbulent diffusion is posed as determining the time evolution of the probability density of the concentration given those for the fluid velocity components, sources, and the initial concentration. At each time, all variables are elements of the Hilbert spaceLR2(R3), and a finite‐dimensional approximation based on expansions in orthonormal basis functions is developed. An expression for the joint probability density of all the Fourier coefficients is derived, the evaluation of which is shown to be particularly straightforward. Diffusion of material from a single source in an unbounded mildly turbulent fluid is considered as an application.

ISSN:0031-9171    

DOI:10.1063/1.863253

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Longitudinal and transverse structure functions of turbulent velocity fluctuations up to the fourth order have been measured in an adverse pressure gradient and compared with the original ideas of Kolmogoroff and with other experimental results. The results show the existence of an inertial subrange for a separation range of about one decade, and in turn the predictions of Kolmogoroff’s original theory have some validity even for a flow that is subjected to an adverse pressure gradient.

ISSN:0031-9171    

DOI:10.1063/1.863254

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Experimental results are presented describing the buckling type of instability in jets of very viscous liquids that flow from vertical slit‐type orifices. Under certain conditions, the jet may buckle in a manner similar to the buckling of a cantilever beam. The governing dimensionless parameters are identified and their critical values determined.

ISSN:0031-9171    

DOI:10.1063/1.863255

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The effect of a temperature‐dependent dielectric constants is considered in the stability analysis of a liquid layer subjected to an electric field, weak unipolar injection, and temperature gradient.

ISSN:0031-9171    

DOI:10.1063/1.863256

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Resonant shadowgraph and schlieren techniques provide instantaneous photographs of a laser‐produced barium plasma flowing across a transverse magnetic field. The field is seen to impose considerable structure upon the expanding plasma. The flow separates into several well‐defined regions, according to velocity. The slow plasma component displays internal striations. These are interpreted as shock waves excited by plasma flow across the field.

ISSN:0031-9171    

DOI:10.1063/1.863257

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Perturbative, spatially–periodic solutions of the Korteweg–deVries, the modified Korteweg–deVries, and the nonlinear Schro¨dinger equations are shown to be recurrent and nonstochastic, densely covering parts of the phase space bounded by level surfaces of the constants of motion. The connection of this result with the numerical phenomena of recurrences and the slow randomization of nonlinear systems is discussed.

ISSN:0031-9171    

DOI:10.1063/1.863258

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

It is shown that the magnetostatic mode can be excited by kinetic Alfve´n waves. The growth rates are calculated for various limits of interest in plasma diffusion studies as well as Alfve´n wave plasma heating experiments.

ISSN:0031-9171    

DOI:10.1063/1.863259

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

It is shown that convective cells can be excited by large‐amplitude propagating ion‐cyclotron waves in a plasma. Using the two‐fluid model, a coupled set of differential equations is obtained. A nonlinear dispersion relation for the modulational instability is then derived. This relation is investigated for different parameter regimes and expressions for the growth rates are obtained analytically. Relevance of the work to space physics as well as magnetically confined plasmas is discussed.

ISSN:0031-9171    

DOI:10.1063/1.863260

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The nonlinear evolution of the wave envelope of the fast ion beam mode has been studied experimentally. At large amplitudes above threshold, the beam mode is observed to be self‐amplitude‐modulated. When the self‐ modulation has fully evolved, the wave splits into a train of wave packets. Finally, it is shown that the self‐modulation observed can be explained by a theory for the modulational instability.

ISSN:0031-9171    

DOI:10.1063/1.863261

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

This numerical study considers the effect of a zero‐order density gradient on the development of Langmuir wave collapse in two dimensions. Two different situations are considered; (1) an initial soliton is pertubed in a direction transverse to the density gradient, and (2) the plasma is resonantly driven by an external pump electric field in the presence of transverse density fluctuations. The principal finding is that the density gradient can inhibit the development of Langmuir collapse for both the initial soliton and the externally driven cases. Over the limited parameter space surveyed it is found that collapse occurs for values of the scaled gradient parameterg≲2.5; whereg= (9/8)(M/m)3/2(&lgr;D/L),Mis the ion mass,mis the electron mass, &lgr;Dis the Debye length, andLis the gradient scale length. For larger values ofg, collapse is not observed.

ISSN:0031-9171    

DOI:10.1063/1.863252

出版商:AIP    

年代:1981

数据来源: AIP