摘要:

The destabilizing effect of the over‐reflection mechanism in stratified fluids in horizontal sheared motion is considered. It is shown that the behavior of unstable modes, induced by the contemporary presence of jumps in both the mean horizontal velocity and the Brunt–Va¨isa¨la¨ frequency located at different heights, can be interpreted in terms of successive reflections and over‐reflections at the Bru¨nt–Va¨isa¨la¨ frequency jump and at the vortex sheet, respectively. A very good accord is obtained with the predictions of the linear theory of unstable modes with small associated growth rates.

ISSN:0031-9171    

DOI:10.1063/1.862641

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Measurements were made of the terminal velocity of a slender circular cylinder moving transverse to its axis at low Reynolds number in a fluid bounded by a rectangular container. The measurements are compared to theoretical results in four limiting cases: (1) a cylinder of infinite length in an unbounded fluid, (2) a cylinder of finite length (but large length to diameter ratio) in an unbounded fluid, a cylinder of finite length moving midway between infinite plane parallel boundaries when (3) the boundary influence is strong, and (4) when the boundary influence is weak. The effects of other boundaries are shown to be small. Empirical correction terms are presented and the practical limitations of the theoretical solutions are discussed. The most important result is that when the boundary influence is strong, there is an unexpectedly large correction term due to the finite length of the cylinder. In these experiments the Reynolds number based on cylinder length ranges from 8.1 to 0.065 and the ratio of sidewall boundary separation to cylinder length ranges from 18 to 0.022.

ISSN:0031-9171    

DOI:10.1063/1.862642

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Some characteristic features of the large scale in a fully developed, two‐dimensional, turbulent duct flow have been investigated with the use of a wall shear stress probe and a single hot wire, traversed across the duct. From space‐time correlation measurements of wall shear stress and velocity fluctuations, the large scale structure is observed to be inclined at an angle of about 13° fory/d ≳ 0.15, wheredis the duct half‐width. The convection speedUcof wall shear stress fluctuations is equal to 0.62U0forRd ≳ 3.5×104, whereU0is the center‐line velocity andRdis the Reynolds number based on center‐line velocity and duct half‐width. The convection speed of velocity fluctuations is approximately constant over a wide range of Reynolds numbers. The integral length scale of wall shear stress fluctuations is nearly 0.38d. Joint probability density functions of velocity and wall shear stress fluctuations are presented and discussed.

ISSN:0031-9171    

DOI:10.1063/1.862643

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The turbulent properties of a thermal wake are investigated in the particular case where the initial conditions producing the kinematic wake and the temperature field are different from one another. In such a case it is observed that the correlation coefficientRv&Vthgr;is not zero at the point where the mean temperature gradient is zero. Accordingly, there is a region where the ’’production terms’’ of the temperature variance are negative. Comparisons are made between kinematic and thermal structures determined from a spectral analysis. The thermal field is found to have some degree of independence from the kinematic field. The large and small structures of each field are different, but they can be related by translating the axisuv¯=0 of the thermal wake until it coincides with the axisv&thgr;¯=0 of the kinematic wake.

ISSN:0031-9171    

DOI:10.1063/1.862644

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The temperature field of axisymmetric laminar plumes generated in water from a small heat source located in a large horizontal plane was measured interferometrically for heat injection rates of 1.5 and 0.72 cal/sec at heights of up to 19 cm above the source. For a height of 19 cm and heat injection rate of 1.5 cal/sec, steady state was reached within 5 sec of the passage of the starting cap; an axial temperature of 3.1° C above ambient was measured, and the diameter at half that temperature was 4 mm. The experimental results were compared with previous analytic solutions for plumes generated from ideal isolated point sources. Good agreement was obtained if the concept of an apparent source is utilized.

ISSN:0031-9171    

DOI:10.1063/1.862645

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Liquid metal in a closed container made of dissimilar metal under a magnetic field is, in general, set into motion by thermoelectric effects if the interfacial temperature is nonuniform, a situation likely to occur in fusion reactor blankets owing to the high thermoelectric power of lithium. The paper sets out the general problem for the case where the field is uniform and so strong that inertial and viscous effects are negligible and where the container wall thickness is small and uniform. Extended cylindrical portions of interface, parallel to the magnetic field, are excluded. Particular attention is paid to containers which are axisymmetric about the field direction and several solutions are found for the case of a spherical container. Many of these imply drastic changes in the total heat transport as compared with pure conduction in the static liquid.

ISSN:0031-9171    

DOI:10.1063/1.862646

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The response of a heavy ion seed component to the passage of a plane shock in the neutral background gas has been analyzed. The translational equilibrium of the ion trace occurs in a region of effectively frozen ionization level and electron temperature where the ion behavior is controlled by a momentum balance of electrical, inertial, and collisional forces. Three distinct types of solutions are found to occur: quasi‐ neutral downstream relaxation at low electron temperatures; quasi‐neutral upstream accommodation at high electron temperatures; and non‐quasi‐ neutral damped electrostatic oscillations at intermediate electron temperatures.

ISSN:0031-9171    

DOI:10.1063/1.862647

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A radioactivity‐related increase in the rate of drift of xenon tracer isotopes in xenon gas was observed while measuring the coefficient of self‐diffusion. The effect occurred at pressures near one atmosphere and room temperature. Some auxiliary experiments are described.

ISSN:0031-9171    

DOI:10.1063/1.862635

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Numerical hydrodynamic‐heat flow simulations of spherical ablation have been done with sufficient spatial resolution to show the details of the ablation front structure. These simulations show a continuous qualitative change in the velocity spectrum of expanding ions with increasing laser pulse length, ranging from approximately isothermal behavior from short pulse to ablative behavior from longer pulses. The ablative behavior is characterized by an energetic peak in the ion spectrum, a characteristic of ablation that is experimentally observable. The longer pulse cases are found to be in good agreement with the stationary flow model of ablation.

ISSN:0031-9171    

DOI:10.1063/1.862636

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Electron collisionless motion in an infinite plasma column is studied under the conditions of relaxed Bennet equilibrium in order to understand the macroscopic structure of the equilibrium, and serve as a preliminary step in an expanded study aimed at determining the relative importance of drift reversed currents. The model assumes, in the framework of the Vlasov theory, a Bennett equilibrium released of the constraint of a constant transverse temperature, a constant macroscopic speed of the charged particles with a constant ratio of their densities; the particles, however, are not required to be monoenergetic, which leads to a diffuse plasma edge. The results, in graphical form, define the values of the parameters for which drift reversals occur, and the conditions under which variations of the parameter values do not affect drift reversals, thus suggesting simplifications in the parametric study of particle trajectories.

ISSN:0031-9171    

DOI:10.1063/1.862637

出版商:AIP    

年代:1979

数据来源: AIP