摘要:

The small scale structure of free convection at high Rayleigh number was investigated by a flow visualization technique and temperature measurements. The dominant feature of the flow field was many sheetlike plumes consisting of several thermals, which transport heat from the conduction layer to the interior region. It was found that the boundary heat flux and the fluid’s properties determine the average characteristics of the thermals, namely, (a) the distance from the boundary where the thermals are generated, (b) the temperature excess transported by thermals, (c) the upward velocity of the fluid in a thermal, and (d) the horizontal dimension of the thermal. Velocity fluctuations and the heat transport rate can be explained through these characteristic values.

ISSN:0899-8213    

DOI:10.1063/1.857383

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

An underexpanded supersonic circular jet was studied experimentally in a pressure ratio range of 2–15, yielding an equivalent Mach number range of 1.1–2.4 downstream of the nozzle. Spark Schlieren photography and near‐field pressure measurements showed large changes in the jet structure for different Mach numbers. For nearly sonic exit velocity, the jet had a dominant symmetric structure and spreading rate similar to subsonic conditions. For an exit velocity equivalent toM>1.2, the jet switched mode to a helical structure with an accompanying substantial increase in the spreading rate. A transitional region was determined for 1.12<M<1.18, in which the jet switched modes intermittently from axisymmetric to helical. For Mach numbers higher than 1.6, another structural change was observed, related to the emergence of a normal shock in the first shock cell. The structural changes affected the near‐field pressure fluctuations of the jet. The jet with the helical structure had a strong pressure field component in the upstream direction that could be related to the higher spreading rate of the jet.

ISSN:0899-8213    

DOI:10.1063/1.857384

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The induction zone characteristics of a planar subsonic high‐speed reactive flow downstream of a specific origin are investigated theoretically for the global irreversible reactionF+Ox→&ngr;P. The equation of state for the reacting gas mixture is more general than that for a constant molecular weight gas. Perturbation methods based on the limit of high activation energy are used to construct the general parameter‐dependent analytical solutions. The dependence of the ignition delay distance on the kinetic, stoichiometric, and flow parameters is discussed in detail. Significantly, it is shown that the reaction with a mole decrement (&ngr;=1) yields the minimum, and a mole increment (&ngr;=3) yields the maximum ignition delay distance when the chemical heat addition and the origin values of parameters are fixed. The physics and length scales found from the perturbation analysis are used as a guide in generating supporting numerical solutions.

ISSN:0899-8213    

DOI:10.1063/1.857385

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Very little attention has been given, so far, to the problem of the propagation of axisymmetric second‐sound shock waves, although they may occur in practical applications. In this paper, research on the propagation of moderate cylindrical heat pulses in superfluid helium is reported. A theoretical model is built based on the experimental observation of the evolution of the structure of rectangular heat pulses. The agreement between the experimental measurements and the theoretical calculations of the evolution of the main parameters is very satisfactory.

ISSN:0899-8213    

DOI:10.1063/1.857386

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The dynamics of the cooling of a strongly heated spherically symmetric cavity produced by an explosion in the upper atmosphere is investigated, allowing for the gas motion during the heat conduction. Conditions for a self‐similar cooling process are found, and the corresponding self‐similar dynamics equations are solved. The characteristic cooling time of the heated cavity is determined as a function of the energy release and parameters of the neutral atmosphere at the height of explosion.

ISSN:0899-8213    

DOI:10.1063/1.857387

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Measurements have been made of the size of the region of secondary flow in linearly stratified fluids (Brunt–Va¨isa¨la¨ frequencies in the range 1.07–2.69 rad/sec that has been caused by torsionally oscillating disks, with radii of 3.0 cm≤r≤4.5 cm. Amplitudes of oscillation were held constant at approximately 1 rad and the frequency of oscillation varied from 0.69 rad/sec≤&ohgr;≤4.78 rad/sec. The fluid was bounded in a cylindrical container with a radius of 7.05 cm and the kinematic viscosity in the vicinity of the disk &ngr;=1.08 cS, was approximately constant. Experiments were performed to extend the work of Folseetal. [Phys. Fluids29, 667 (1986)] to include variations in the Ekman number,Ek=&ngr;/&ohgr;r2. The size of the secondary flow region was measured from shadowgraphs and the flow field was mapped using a two‐component laser‐Doppler velocimeter. An empirical correlation of the data with the results of Folseetal. is presented.

ISSN:0899-8213    

DOI:10.1063/1.857388

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The Rayleigh–Taylor instability in a finite thickness layer of viscous fluid has been studied in the creeping flow limit. The wavelength of fastest growth was found to be controlled by the larger of the layer thickness or a parameter that was a function of the ratio of the surface tension to the pressure gradient.

ISSN:0899-8213    

DOI:10.1063/1.857389

出版商:AIP    

年代:1989

数据来源: AIP

ISSN:0899-8213    

DOI:10.1063/1.857390

出版商:AIP    

年代:1989

数据来源: AIP

ISSN:0899-8213    

DOI:10.1063/1.857559

出版商:AIP    

年代:1989

数据来源: AIP