摘要:

In this paper we deal with self‐similar thermal and electrohydrodynamic (EHD) plumes. The former arises from hot lines or points, whereas the latter arises when sharp metallic contours submerged in nonconducting liquids support high electrostatic potential, resulting in charge injection. Although the motive force is buoyancy in one case and Coulomb force in the other, it is shown that the solution for EHD plumes is the same as for thermal plumes in the limit of large Prandtl numbers. We present the analysis of axisymmetric plumes for large values of Prandtl number, and this analysis is subsequently applied to EHD plumes. The validity of the approximations for EHD plumes is discussed in the light of experimental data. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868983

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This experimental study is devoted to visualisation and ultrasonic velocity measurement of the wakes formed behind a row of parallel cylinders placed side by side, perpendicular to an incoming flow at low Reynolds numbers. When the distance separating the cylinders is small compared to their diameter, two instability mechanisms, associated with different patterns and dynamics compete. A first spatial symmetry breaking appears when the stationary wakes behind each cylinder are deviated towards one side or the other and form large clusters containing from two to sometimes more than ten wakes. These clusters are separated by intense recirculating zones. When the Reynolds number is increased, the wakes belonging to the widest clusters experience a secondary temporal oscillatory bifurcation. Classical Be´nard‐Von Ka´rma´n vortex streets are thus shed in phase by these cylinders (acoustic mode), by contrast with the wakes outside these cells which stay stationary. Finally, the flow around far apart cylinders is also investigated. The primary instability does not occur in this case and a perfect optical mode of vortex shedding, with neighbours in phase opposition, takes place in the flow. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868984

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A horizontal cylinder intersecting a free surface is subjected to controlled vertical perturbations, and the consequent vortex formation is characterized by high‐image‐density particle image velocimetry, which leads to instantaneous patterns of velocity, vorticity, and streamlines. For the limiting case of the stationary cylinder, the near wake does not exhibit rapid formation of organized vortical structures in a manner similar to Ka´rma´n vortices. Application of perturbations, however, generates phase‐locked vortex formation over a wide range of excitation frequencies, even at relatively low amplitudes, indicating that the near wake in presence of a free surface is convectively, rather than absolutely, unstable. At a sufficiently high value of excitation frequency, the formation of the initial vortex undergoes an abrupt change in timing, which is analogous to that occurring for Ka´rma´n vortex formation from a completely submerged cylinder. All of these features of the near wake are interpreted in terms of foci, saddle points, and reattachment points of the streamwise topology. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868985

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The wake of a circular cylinder is investigated for Reynolds numbers between 160 and 500 by means of particle image velocimetry (PIV). For the first time cross‐stream velocity fields are determined for two classes of secondary vortices (A‐mode and B‐mode). The circulation of the A‐mode secondary vortices in this plane is approximately twice the circulation of the B‐mode secondary vortices. The spanwise wavelength of the secondary vortices is four to five cylinder diameters for the A‐mode and one diameter for the B‐mode. The spatio‐temporal development of the wake is analyzed by acquiring a time sequence of PIV images covering several Ka´rma´n periods. On the basis of the vorticity field, the A‐ and B‐modes can be identified as topologically different vortex structures. Two vortex models are developed to explain the differences between these modes. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868986

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Large eddy simulations (LES) of spatially developing circular jets were carried out. The subgrid scale (SGS) model was of a dynamic type and was based on an assumed asymptotic behaviour of the SGS‐stress. This assumption is valid only for adequate spatial and temporal resolutions. The effects of the SGS‐model were studied by comparing simulations with and without SGS‐model. LES with different spatial resolutions were performed to study the effects of the spatial resolution on the numerical solution. The numerical results were compared with experimental data. Simulations were performed for the Reynolds numbers 1⋅104, 5⋅104and 50⋅104to study the Reynolds numbers effects in the proximal region of the jet. The turbulent intensity increases from a low initial level, given by a low amplitude white noise disturbance in the inlet, to a high level in the studied proximal region of the jet. For the lower Reynolds numbers certain amplified frequencies were found, at Strouhal numbers about 0.3 and the corresponding first two harmonics, which agree well with experimental observations. The spatial resolution was found to be adequate to support the longitudinal and transversal Taylor length scales. A new bound for the dynamic model parameter is proposed and it is studiedaprioriusing the computed flow fields. This bound is based on the non‐negativity of the total dissipation, i.e. an entropy condition for the sum of viscous‐, SGS‐ and numerical‐ dissipation, in the discrete transport equation of the resolved scale‐energy. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868987

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A framework, which combines mathematical analysis, closure theory, and phenomenological treatment, is developed to study the spectral transfer process in turbulent flows that are subject to rotation. First, we outline a mathematical procedure that is particularly appropriate for problems with two disparate time scales. The approach that is based on the Green’s method leads to the Poincare´ velocity variables and the Poincare´ transformation when applied to rotating turbulence. The effects of the rotation are now conveniently included in the momentum equation as the modifications to the convolution of nonlinear term. The Poincare´ transformed equations are used to obtain a time‐dependent Taylor–Proudman theorem valid in the asymptotic limit when the nondimensional parameter &mgr;≡&OHgr;t→∞ (&OHgr; is the rotation rate andtis the time). The ‘‘split’’ of the energy transfer in both direct and inverse directions is established. Second, we apply the Eddy‐Damped‐Quasinormal‐Markovian (EDQNM) closure to the Poincare´ transformed Euler/ Navier–Stokes equations. This closure leads to expressions for the spectral energy transfer. In particular, a unique triple velocity decorrelation time is derived with an explicit dependence on the rotation rate. This provides an important input for applying the phenomenological treatment of Zhou [Phys. Fluids7, 2092 (1995)]. In order to characterize the relative strength of rotation, another nondimensional number, a spectral Rossby number, which is defined as the ratio of rotation, and turbulence time scales, is introduced. Finally, the energy spectrum and the spectral eddy viscosity are deduced. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868988

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Heat flow and temperature and density distributions in a rarefied gas between two parallel plates at rest with different uniform temperatures are analyzed numerically on the basis of the full nonlinear Boltzmann equation for hard‐sphere molecules and the Maxwell‐type boundary condition by a finite difference method where the collision term is computed direct numerically. The accurate results are presented for the case in the density measurement by Teagan and Springer [Phys. Fluids11, 497 (1968)], where the temperature ratio is 1.326, the value of the accommodation coefficient is 0.826, and the ratio of mean free path to plate spacing (Knudsen number Kn) is 0.0658≤Kn≤0.7582. It is found that there is a considerable difference between the present density distribution and the experimental data. The reason for this discrepancy is also discussed. The accurate numerical results of the linearized problem are also presented for comparison. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868989

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We employ direct numerical simulations to study the mixing of two passive scalars in stationary, homogeneous, isotropic turbulence. The present work is a direct extension of that of Eswaran and Pope from one scalar to two scalars and the focus is on examining the evolution states of the scalar joint probability density function (jpdf) and the conditional expectation of the scalar diffusion to motivate better models for multi‐scalar mixing. The initial scalar fields are chosen to conform closely to a ‘‘triple‐delta function’’ jpdf corresponding to blobs of fluid in three distinct states. The effect of the initial length scales and diffusivity of the scalars on the evolution of the jpdf and the conditional diffusion is investigated in detail as the scalars decay from their prescribed initial state. Also examined is the issue of self‐similarity of the scalar jpdf at large times and the rate of decay of the scalar variance and dissipation. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868990

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Effects of acceleration on turbulent jets were investigated in a series of flow visualization experiments. Prior to the initiation of acceleration, a steady jet with a Reynolds number of 3000 was established. Three distinct acceleration schemes of linear, quadratic, and exponential were utilized to increase the nozzle exit velocity by an order of magnitude. As the flow accelerated, a discernible ‘‘front’’ was established. The parcels constituting the front were less diluted than the steady jet parcels at the same location. For each acceleration scheme, the temporal evolution of the front position had the same functional form as the nozzle velocity. The front velocity increased linearly with the acceleration rate for the linear and quadratic cases. In comparison with a steady jet, the front’s lateral growth rate was reduced by 16% in the linear case and by 25% in the two nonlinear cases, even though the linear cases had generally larger acceleration rates. A model, based on the scaling of centerline velocity in steady jets, appears to correctly predict the time dependence of the front position. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868991

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The vorticity spectrum has been inferred, using local isotropy, from measurements of the lateral vorticity components in a turbulent wake over a small Reynolds number range. The high wavenumber part of the spectrum supports Kolmogorov’s [C. R. Akad. Sci. USSR30, 301 (1941)] similarity theory. Among the different published analytical expressions for the three‐dimensional energy spectrum, the model of Kida and Murakami [Phys. Fluids30, 2030 (1987)] is closest to the present data. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868992

出版商:AIP    

年代:1996

数据来源: AIP