摘要:

Mixed convection flow in a heated pipe with a heated inner core is examined. Analytic solutions are derived in power series of the Dean number and the product of Reynolds and Rayleigh numbers. The results are compared with flows in the absence of an axial core. The core modifies the flow and the fluid temperature distribution.

ISSN:0899-8213    

DOI:10.1063/1.857803

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this paper, an experimental investigation of variations of the global lift coefficient as a function of the Reynolds number is discussed. The experimental unit was capable of measuring the lift coefficient at Reynolds numbers ranging from 3000 to 30 000. Since the intensity of the unsteady lift forces was of the order of few millinewtons, a sensor and signal‐conditioning circuit was specially designed for measurement. The velocity field and the flow turbulence rate were measured upstream from the cylinder using laser‐Doppler anemometry. Tube aspect ratio, blockage, and ends effects are discussed. The evolution of the Strouhal number was observed as the Reynolds number passed from 3000 to 30 000. When measured as a function of the Reynolds number, the global lift coefficient displayed considerable variation for Reynolds numbers between 3000 and 10 000.

ISSN:0899-8213    

DOI:10.1063/1.857804

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An experimental study on the development of turbulent spots in plane Poiseuille flow at a Reynolds number of 1600 has been carried out with the aim of achieving a better understanding of the transition to and maintenance of turbulence at low Reynolds numbers. Spots were triggered by a loudspeaker‐induced jet of high velocity. The initial disturbance was found to undergo a first stage of rapid expansion, in which sharp internal shear layers form at locations away from the symmetry plane and precede the transition to turbulence. After this initial stage, a nearly self‐similar structure develops with the typical features of a turbulent spot. The general features, turbulent properties, and spanwise spreading of the spot were investigated and compared both to previous experimental data and to numerical simulations. High‐frequency fluctuations are absent at the front of the spot, whereas the turbulence is apparently self‐sustained at the rear and displays features similar to fully turbulent Poiseuille flow at much higher Reynolds numbers. The waves accompanying the wing tips of the spot extend well within the spot, and reach amplitudes far in excess of those previously found outside the spot. The high level of random fluctuations in this part of the spot indicates that the breakdown of the waves is important for the spanwise propagation of the turbulent region.

ISSN:0899-8213    

DOI:10.1063/1.857805

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A finite‐difference method for solving the discrete Boltzmann equations, which are the governing equations for a model gas in which molecules have many discrete velocities, is developed. The method is applied to three fundamental problems in rarefied gas flow to study the features of discrete‐velocity gases: normal shock wave structure, heat transfer between two parallel plates, and two‐dimensional vapor deposition. Two different discrete‐velocity gas models are used.

ISSN:0899-8213    

DOI:10.1063/1.857825

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The nonlinear Boltzmann equation is solved numerically to examine the Maxwellization of spatially homogeneous gases, using the multigroup method. By applying the Krook–Wu scattering model, an exact solution of the Boltzmann equation (BKW mode) is reproduced with high accuracy. The numerical code is also used for hard‐sphere molecules. Initial distributions are a Maxwellian with tail cutoff and distributions composed of two &dgr; peaks. For the latter class, a strong transient overpopulation of the distribution function is observed, which may amount to several orders of magnitude.

ISSN:0899-8213    

DOI:10.1063/1.857806

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

This paper concerns an analytical/numerical study of molecular mixing and infinitely fast chemical reactions in the field of a two‐dimensional vortex pair. Calculations of scalar concentration fields and its mixing statistics are presented for a range of nondimensional vortex strengths, &Ggr;/2&pgr;&ngr;=10–50, Schmidt numbers, &ngr;/D=1–100, and a time parameter. A measure of molecular mixing ‘‘mixedness,’’ defined as the variance of the spatial concentration field from its well‐mixed value, shows that the augmentation of mixedness resulting from the presence of vortex pair is linearly proportional to the vortex pair strength and time elapse since the inception of the vortex pair.

ISSN:0899-8213    

DOI:10.1063/1.857807

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A direct simulation of turbulent flow in a channel is analyzed by the method of empirical eigenfunctions (Karhunen–Loe`ve procedure, proper orthogonal decomposition). This analysis reveals the presence of propagating plane waves in the turbulent flow. The velocity of propagation is determined by the flow velocity at the location of maximal Reynolds stress. The analysis further suggests that the interaction of these waves appears to be essential to the local production of turbulence viaburstingorsweepingevents in the turbulent boundary layer, with the additional suggestion that thefastactingplane waves act astriggers.

ISSN:0899-8213    

DOI:10.1063/1.857808

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Exact similarity solutions are presented for the spatial‐dependent McKean–Boltzmann equation, when creation and removal processes are allowed. The effects of such processes on the spatial dynamics of the gas are analyzed through these solutions.

ISSN:0899-8213    

DOI:10.1063/1.857809

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The use of strong magnetic fields for the control of particle settling in metallic systems is investigated in the limit of small inertial and magnetic Reynolds numbers. Finite element calculations of flow around axisymmetric bodies show that the drag increases proportional to the intensity of the magnetic fieldBor the Hartmann number Ha. The flow field forms boundary layers, which thin with increasing Ha, along surfaces parallel to the flow. For axisymmetric bodies, the boundary layer separates as the poles of the surface are approached and encloses regions of almost stagnant fluid. These regions spread upstream and downstream along the body with increasing Ha, thereby trapping the particle in a column of stagnant fluid. The pressure difference between the leading and trailing fluid columns is responsible for the increased particle drag. Asymptotic analysis with Ha≫1 confirms the scalings from the computations and clarifies the flow structure near the body.

ISSN:0899-8213    

DOI:10.1063/1.857810

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this paper the stability characteristics of an infinitely long, uniformly elongating metal jet are investigated. The application to a metallic jet formed from an explosive charge, or shaped‐charge jet, is of particular interest. The effect of an axial electric current on the stability of the jet is determined. The jet is assumed to be perfectly plastic, perfectly conducting, nonswirling, and isothermal. The governing equations are solved to determine the idealized motion of the jet, which is then perturbed by an arbitrary three‐dimensional disturbance. The resulting first‐order equations are solved numerically for the time evolution of this perturbation. For a given initial condition, the growth rate of the disturbance depends on the relative importance of the inertial, electrical, and plastic forces. The details of growth rate characteristics are explained in terms of the appropriate physical principles.

ISSN:0899-8213    

DOI:10.1063/1.857811

出版商:AIP    

年代:1990

数据来源: AIP