摘要:

The onset of three‐dimensionality in the von Ka´rma´n vortex street behind a circular cylinder is investigated by carrying out the first global, nonparallel, three‐dimensional stability analysis of the periodic flow. This flow is found to become unstable at a Reynolds number of 170 by a critical, three‐dimensional Floquet mode with a spanwise wavelength of 1.8 diam. The spatial structure of this mode indicates that the onset of three‐dimensionality is due to a near‐wake instability and not caused by a stagnation‐line or a boundary‐layer instability.

ISSN:0899-8213    

DOI:10.1063/1.858616

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A general methodology is developed for simulating complicated reacting flow problems. This method combines the large‐eddy simulation (LES) technique with the existing probability density function (PDF) approach for turbulent reacting flows and provides a closed form representation for all terms that are involved in the simulations. Some other issues related to this problem are also discussed.

ISSN:0899-8213    

DOI:10.1063/1.858617

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Vorticity is deposited due to baroclinic effects on the surface of a density‐stratified interface accelerated by a shock. An analytical expression is presented, derived from shock polar analysis, for circulation per unit length on a fast–slow planar density interface inclined at an angle to the incident shock. The analytical expression is integrated to yield total circulation on nonplanar interfaces (sinusoidal and semicircular interfaces) accelerated by shocks. The analytical results agree well with diagnostics from numerical experiments using a second‐order Godunov code for the Euler equations.   

ISSN:0899-8213    

DOI:10.1063/1.858618

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In order to measure the diffusion coefficient of a heated plane airstream, a single laser beam is passed through the jet, perpendicularly to the flow direction. The thermic turbulence in the airstream causes random fluctuations of the refractive index. Consequently, the beam direction undergoes, in the flow, random perturbations. After having traversed the jet, the beam produces a luminous trace on a photoelectric cell placed outside the jet. An experimental setup for measuring the probabilities of the beam impact positions on the cell is described. From the Markovian process model, applied along the whole random path of the beam, it has been possible to compute these probabilities by solving the Einstein–Fokker–Kolmogorov equation. The diffusion coefficient can be determined by adjusting the numerical solution to agree with the experimental results. In addition, the calculation procedure gives the order of magnitude of an integral scale, characterizing the dimension of the turbulent structures in which the propagation of light can be considered rectilinear. A good agreement between the results and the published data obtained by means of the cold‐wire anemometer technique proves the validity of the method.

ISSN:0899-8213    

DOI:10.1063/1.858565

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

This work considers the effects of finite mass concentrations on the force and the heat transfer rate for a small particle in a dusty gas. The particular flow studied is a plane, monochromatic sound wave, where the particle velocity and temperature slips are very small. Assuming that the fluid’s temperature and velocity near a test particle are changed by amounts which depend on the mass loading and on the respective slips, it is shown that the heat transfer rate to a sphere in the suspension changes from the pure conduction limitq˙k0, applicable to an isolated sphere, toq˙k0(1+Cmcpp/cpf), whereCmis the mass loading,cppis the specific heat of the particle material, andcpfis the constant pressure specific heat of the gas. Another but less significant effect is to change the corresponding viscous force from the Stokes value,F&ngr;0, toF&ngr;0&ggr;m/&ggr;f, where &ggr;mand &ggr;fare the specific heat ratios for the suspension and for the dust‐free gas, respectively.

ISSN:0899-8213    

DOI:10.1063/1.858566

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

equal‐diameter solid disks subjected to an axial gravity field of arbitrary intensity is analyzed for all possible liquid volumes. The boundary of the stability region for axisymmetric shapes (considering both axisymmetric and nonaxisymmetric perturbations) have been calculated. It is found that, for sufficiently small Bond numbers, three different unstable modes can appear. If the volume of liquid is decreased from that of an initially stable axisymmetric configuration the bridge either develops an axisymmetric instability (breaking in two drops as already known) or detaches its interface from the disk edges (if the length is smaller than a critical value depending on contact angle), whereas if the volume is increased the unstable mode consists of a nonaxisymmetric deformation. This kind of nonaxisymmetric deformation can also appear by decreasing the volume if the Bond number is large enough. A comparison with other previous partial theoretical analyses is presented, as well as with available experimental results.

ISSN:0899-8213    

DOI:10.1063/1.858567

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The steady‐state deformation of a bubble rising in polymeric liquid has been investigated using a general numerical technique for the solution of free‐boundary problems in non‐Newtonian fluid mechanics. The technique is based on a finite‐difference solution of the governing equations on an orthogonal curvilinear coordinate system, which is constructed numerically and adjusted to fit the boundary shape exactly at any time. The problem was analyzed based on the constitutive equation proposed by Chilcott and Rallison [J. Non‐Newtonian Fluid Mech.29, 381 (1988)], which models a dilute polymer solution as a suspension of dumbbells with finite extensibility. Computations were carried out to investigate the effect of conformation change of polymer chains on the bubble deformation for various values of the Deborah number, maximum chain extensibility (i.e., roughly proportional to molecular weight), capillary number, and the Reynolds number. Numerical results show good agreement with existing experimental findings reported elsewhere. Especially, the tendency of transition from a prolate shape to a cusped shape has been observed in the creeping flow limit.

ISSN:0899-8213    

DOI:10.1063/1.858568

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Numerical results are presented on the hydrodynamic dispersion of passive tracer in flow through a two‐dimensional channel bounded by parallel plates, one having substantial rugosities perpendicular to the flow. The simulation is based on the numerical solution of the microscopic Stokes and convention‐diffusion equations, over a range of Pe´clet numbers and of degrees of rugosity of the channel. One observes a tenfold increase of the effective dispersion between the smooth plate case (classical Taylor dispersion) and the maximum rugosity case, attributable to the trapping of tracer inside deep rugosities. The results are analyzed in terms of tracer exchange models, involving a characteristic exchange time between the low‐ and high‐velocity zones and their relative volume fractions, as a function of the Pe´clet number and the relative depth of the rugosities. The simulations are compared to ‘‘echo’’ dispersion experiments in which the tracer is first injected into the model and then, upon reversal of the fluid flow, drawn back through the inlet.

ISSN:0899-8213    

DOI:10.1063/1.858569

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The interface region between two fluids of different densities and viscosities in a porous medium in which gravity is directed at various angles to the interface is analyzed. Under these conditions, base states exist that involve both tangential and normal velocity components. These base states support traveling waves. In the presence of a normal displacement velocity, the amplitude of these waves grows according to the viscous fingering instability. For the immiscible case, it can easily be shown that the growth rate is not affected by the tangential velocities, while surface tension results in the usual stabilization. For the case of two miscible fluids, the stability of the base states using the quasi‐steady‐state approximation is investigated. The resulting equations are solved analytically for timet=0 and a criterion for instability is formulated. The stability of the flow for timest≳0 is investigated numerically using a spectral collocation method. It is found that the interaction of pressure forces and viscous forces is modified by tangential shear as compared to the classical problem, resulting in a stabilizing effect of the tangential shear. The key to understanding the physical mechanism behind this stabilization lies in the vorticity equation. While the classical problem gives rise to a dipole structure of the vorticity field, tangential shear leads to a quadrupole structure of the perturbation vorticity field, which is less unstable. This quadrupole structure is due to the finite thickness of the tangential base state velocity profile, i.e., the finite thickness of the dispersively spreading front, and hence cannot emerge on the sharp front maintained in immiscible displacements.

ISSN:0899-8213    

DOI:10.1063/1.858570

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The effect of a nonmonotonic viscosity profile on the stability of miscible displacements in porous medium is studied. A linear theory using the quasi‐steady‐state approximation is employed to find the growth rate of the perturbations in the flow. The resulting eigenvalue problem has both a discrete and continuous spectrum. It is possible to obtain an analytical solution for a step base state concentration profile. The step profile result indicates that a nonmonotonic viscosity profile can be stable even when the viscosity contrast, measured by the end‐point viscosities, is unfavorable, and vice versa. Asymptotic expansions of the growth rate for short times and for small wave numbers of the disturbances are obtained. The short‐time expansion shows that the diffusion of the base state does not always mitigate the instabilities and the small wave‐number expansion gives a sufficient condition for the flow to be unstable. Finally, the eigenvalue problem is solved numerically for diffused concentration profiles using finite difference methods. A model viscosity profile is chosen to parametrically study the stability of nonmonotonic profiles. The parametric study shows that the diffusion of the base state can have a destabilizing effect, with the surprising result that the nonmonotonic profiles that are predicted to be stable by the step profile analysis eventually become unstable by the destabilizing effect of diffusion. The ‘‘critical’’ time at which a stable flow turns unstable is obtained as a function of the parameters of the problem. A physical mechanism is proposed to explain the effects of diffusion on the stability of the flow.  

ISSN:0899-8213    

DOI:10.1063/1.858571

出版商:AIP    

年代:1993

数据来源: AIP