摘要:

A turbulent spot was created artificially in a transitional boundary layer by means of a point disturbance which was made as reproducible as possible. Single‐probe hot‐wire measurements of the streamwise velocity component were used to study the structure of the disturbance prior to its breakdown into a spot. The ’’incipient spot’’ in this experiment is found to be highly evolutionary, its length increasing by almost 100% during transit past a fixed location. The contour of the incipient spot, when projected on a plane parallel to the wall, is arrowheaded in shape with an apex which is considerably sharper than that of a turbulent spot. In the plane of symmetry, rapid fluctuations are observed far from the wall; closer to the wall, a velocity excess occurs and regions of streamwise momentum defect and excess develop alternately in the spanwise direction. Time history at a fixed streamwise location shows that the central excess region is crossed by a ’’defect bridge’’ connecting the downstream ends of the adjacent defect regions. The incipient spots in this experiment propagate considerably faster than the three‐dimensional wave packets, which have been investigated by others who applied the same, yet much weaker, disturbance source.

ISSN:0031-9171    

DOI:10.1063/1.863651

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Available measurements in turbulent boundary layers having zero, favorable, or adverse pressure gradients have been used here to show that a universal value of the nondimensional period between bursting does not exist.

ISSN:0031-9171    

DOI:10.1063/1.863652

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The transport equation for the probability density function of a scalar in turbulent shear flow is analyzed and the closure based on the gradient flux model for the turbulent flux and an integral model for the scalar dissipation term is put forward. The probability density function equation is complemented by a two‐equation turbulence model. Application to several shear flows proves the capability of the closure model to determine the probability density function of passive scalars.

ISSN:0031-9171    

DOI:10.1063/1.863653

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Growth of large fluctuations in the amplitude of high‐frequency waves or shocks propagating through turbulent, inhomogeneous media is investigated stochastically with geometrical acoustics. Previous efforts use linearization which is not uniformly valid at large distances and large variations in the ray tube area. The present analysis retains the nonlinear terms crucially important to focusing and growth of singular fluctuations in the amplitude. It is shown, in two dimensions, that the fluctuations of the ray‐tube area grow exponentially and caustics occur on every ray, repeatedly, on a distance scale ofa&sgr;−2/3, where the fluctuations in ray directions are still small,O(&sgr;2/3). Here, &sgr;≪1 is the standard deviation andais the correlation length of the wave velocity distribution in the medium. Probability densities for the occurrence of caustics are given. Also, the moments of the ray‐tube area distribution and amplitude related statistics are developed for distances well into the region of caustic formation. Finally, a method is presented for relating the theoretical predictions to measurements on an image plane.

ISSN:0031-9171    

DOI:10.1063/1.863654

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

A model is proposed for the time evolution of ion concentrations in a thin charge layer near an interface between an insulating solid and a semi‐insulating bipolar liquid, subjected to an applied electric field. Given the specific case of a traveling‐wave applied field of peak magnitudeE0and planar geometry, solutions for the charge density and electric field in the layer are used to calculate the time‐average stress moment and, hence, the pumping velocity of the liquid. Analytic solutions, valid in the regimes of small applied field magnitude (E0LD/Vt≪1, whereL0andVtare the Debye length and thermal voltage, respectively) or frequency (&ohgr;&egr;/&sgr;≪1, &ohgr; the angular frequency and &egr; and &sgr; the permittivity and equilibrium conductivity, respectively), predict charge layers with a characteristic dimension of the Debye length and fluid pumping in the direction of propagation of the traveling wave (forward). Numerical solutions, in the regime of large magnitude fields with &ohgr;&egr;/&sgr;∼1, predict either forward or backward pumping, as well as a charge layer with thickness on the order of a migration length (L= 2&pgr;bE0/&ohgr;, wherebis the ion mobility). Parameters such as ion mobility, thermal generation rate, and level of ionization in the liquid are important in determining the rate (and even direction) of the pumping.

ISSN:0031-9171    

DOI:10.1063/1.863655

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The influence of external magnetic fields on the viscosity of water at 295 K was investigated in a differential experiment. With fields up to 2.3 T either parallel to or perpendicular to the flow, an upper limit to relative changes was established as 3×10−5. This result is contradictory to earlier results by other authors, who reported relative viscosity increases up to 3×10−3at comparable field strengths.

ISSN:0031-9171    

DOI:10.1063/1.863656

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

Experiments in which the effective anode area of a multipole ion source is reduced, in order to enhance its ionization and gas efficiency, show that there is a minimum area below which efficient operation becomes impossible. This phenomena is analyzed in terms of a model based on particle balance equations and is shown to be closely related to the reversal in plasma potential with respect to anode potential. The theoretically predicted operation regime is shown to be in reasonable agreement with that found in a variety of multipole sources. Comparison is also made with the limitations set by cathode area.

ISSN:0031-9171    

DOI:10.1063/1.863657

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

This derivation of the gyrokinetic equations is based on the Hamiltonian formulation of the guiding center motion. The transformation to guiding center variables is accomplished by a sequence of canonical transformations. The kinetic equation is determined by Liouville’s equation. This equation is then ordered in accordance with the gyrokinetic ordering, and analyzed order by order to obtain the linear and nonlinear gyrokinetic equations.

ISSN:0031-9171    

DOI:10.1063/1.863658

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

The stabilization of the universal drift instability due to finite &bgr; effects (i.e., electromagnetic coupling and ∇Borbit modifications) is studied numerically and a marginal stability curve (as a function of &bgr; andk) is presented. It is found that the most difficult modes to stabilize have arbitrarily small wavenumbers and the critical value of &bgr; is &bgr;cr≃0.135 forTe=Ti.

ISSN:0031-9171    

DOI:10.1063/1.863659

出版商:AIP    

年代:1982

数据来源: AIP

摘要:

A detailed calculation is made of the nonlinear excitation of a low‐frequency whistler due to the beat of two high‐frequency electromagnetic waves. The whistler is excited near the reflection layers of the high‐frequency pumps where an effective antenna is generated by the beat ponderomotive force acting on electrons. Results relevant to ionospheric and laboratory experiments are obtained.

ISSN:0031-9171    

DOI:10.1063/1.863660

出版商:AIP    

年代:1982

数据来源: AIP