摘要:

The near‐field of an azimuthally excited round jet was investigated in a combined computational/experimental study. The reaction zones in the jet were visualized using OH Planar‐Laser‐ Induced‐Fluorescence (PLIF) diagnostics. Both axisymmetric and azimuthal modes of the jet were excited to stabilize its spatial structure. Three‐dimensional flame visualization of the laboratory jet reconstructed from multiple two‐dimensional images acquired at constant phase angle, reveal a complex structure of the reaction zone. Time‐dependent numerical simulations provided insight into the underlying fluid‐dynamical processes leading to this flame structure. Simulations of reactive and non‐reactive free jets used a Monotonically Integrated Large‐Eddy‐Simulation (MILES) approach, multi‐species diffusive transport, global finite‐rate chemistry and appropriate inflow/outflow boundary conditions. The flow visualizations of the experimental and computational jets strongly resemble each other, revealing tight coupling between axisymmetric vortex rings and braid (rib) vortices. The jet vorticity evolution is dominated by the dynamics of vortex‐ring self‐deformation induced by the azimuthal excitation imposed at the jet exit, the dynamics of rib vortices forming in the braid regions between undulating vortex rings, and strong interactions between rings and ribs. The observed topological features of the flow are directly related to the nearly‐inviscid jet vorticity dynamics. These processes affect the mixing pattern of the jet, resulting in localized regions of high fuel concentration leading to combustion inactive regions in the flame, and other regions with enhanced mixing and a proper air‐to‐fuel ratio in the flame where the combustion process is intense. The vorticity dynamics and ensuing mixing processes determine the regions of combustion within the flame and thus the overall heat release pattern. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868927

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The interaction between small‐amplitude surface waves and a wavy bed described by two sinusoids is considered. Based on the method of multiple scales, an analytical theory for subharmonic Bragg resonances caused by second‐order interactions between the surface wave and the seabed is derived. Comparisons with experiments and with a numerical theory are also discussed. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868928

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Results from a series of simulations of unforced turbulence evolving within a shallow layer of fluid on a rotating sphere are presented. Simulations show that the turbulent evolution in the spherical domain is strongly dependent on numerical and physical conditions. The independent effects of (1) (hyper)dissipation and initial spectrum, (2) rotation rate, and (3) Rossby deformation radius are carefully isolated and studied in detail. In the nondivergent and nonrotating case, an initially turbulent flow evolves into a vorticity quadrupole at long times, a direct consequence of angular momentum conservation. In the presence of sufficiently strong rotation, the nondivergent long‐time behavior yields a field dominated by polar vortices—as previously reported by Yoden and Yamada. In contrast, the case with a finite deformation radius (i.e., the full spherical shallow‐water system) spontaneously evolves toward a banded configuration, the number of bands increasing with the rotation rate. A direct application of this shallow‐water model to the Jovian atmosphere is discussed. Using standard values for the planetary radius and rotation, we show how the initially turbulent flow self‐organizes into a potential vorticity field containing zonal structures, where regions of steep potential vorticity gradients (jets) separate relatively homogenized bands. Moreover, Jovian parameter values in our simulations lead to a strong vorticity asymmetry, favoring anticyclonic vortices—in further agreement with observations. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868929

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The simultaneous effects of curvature, rotation and heating/cooling in channel flow complicate the flow and heat transfer characteristics beyond those observed in the channels with simple curvature or rotation. The phenomena encountered are examined for steady, hydrodynamically and thermally fully developed flow in square channels. The governing equations are solved numerically by using a finite‐volume method. Certain hitherto unknown flow patterns are found. And the results show both the nature of the flow transition and the effect of this transition on the distributions of temperature, friction factor and Nusselt number in a square channel. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868930

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A Boundary Element Method (BEM) has been developed to investigate the nonlinear evolution of the surface of liquid jets injected from circular orifices under unsteady inflow conditions. For fixed wavelength perturbations, the model predicts the formation of main and satellite droplets. The size of the droplets is affected by changes in the perturbation wavelength, perturbation magnitude and Weber number. Satellite droplet velocities are less than main droplet velocities due to the sequential shedding of droplets from the orifice. Using this information, one can determine the likelihood of droplet recombinations downstream of the initial pinching event. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868931

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The average properties of many turbulent flows are remarkably reproducible, yet the detailed flows are never the same. Here necessary conditions are found for the stability of a statistically steady state with ideal reproducibility. The examples explored in this paper are the symmetry breaking large scale flows observed in high Rayleigh number Boussinesq convection. A necessary condition found is that an integral function of convective and advective local averages must be a maximum, selecting among the possible solutions. Although these many solutions may not be determined in this century, an estimate from above of this maximum can be sought among the class of vector fields compatible with derived integral constraints and the boundary conditions. The condition for statistical stability found here permits a first determination of a unique relation between the large scale mean flows and the heat flux without the introduction of empirical parameters. It is found that statistical stability at large Rayleigh number is achieved by that solution with maximum momentum transport, and not by the solution with maximum heat transport. The past limited data supports this deduction that the large scale flow approaches its maximum possible value. New experiments, with a stress free boundary condition, are proposed to explore these and other unanticipated theoretical results. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868932

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Dynamic equations for the scalar autocorrelation and scalar‐velocity cross correlation spectra have been derived for a passive scalar with a uniform mean gradient using the Eddy Damped Quasi Normal Markovian (EDQNM) theory. The presence of a mean gradient in the scalar field makes all correlations involving the scalar axisymmetric with respect to the axis pointing in the direction of the mean gradient. Equivalently, all scalar spectra will be functions of the wave numberkand the cosine of the azimuthal angle designated as &mgr;. In spite of this complication, it is shown that the cross correlation vector can be completely characterized by a single scalar functionQ(k). The scalar autocorrelation spectrum, in contrast, has an unknown dependence on &mgr;. However, this dependency can be expressed as an infinite sum of Legendre polynomials of &mgr;, as first suggested by Herring [Phys. Fluids17, 859 (1974)]. Furthermore, since the scalar field is initially zero, terms beyond the second order of the Legendre expansion are shown to be exactly zero. The energy, scalar autocorrelation, and scalar‐velocity cross correlation were solved numerically from the EDQNM equations and compared to results from direct numerical simulations. The results show that the EDQNM theory is effective in describing single‐point and spectral statistics of a passive scalar in the presence of a mean gradient. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868933

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The principle of minimum Fisher information (MFI) is used to work out the joint distribution function of the density and velocity in homogeneous, isotropic, stationary, nearly incompressible turbulence. We assume that the Mach number is very low. It is shown that simple constraints on the minimization may be chosen to give a good fit to the pressure distribution function found in recent direct numerical simulations and experiments, where the PDF is exponential for negativepand roughly exp[−(p/p0)3/2]p−1/2for positivep. The appropriate constraints in the MFI problem are on the skewness of the PDF both by itself and weighted by the internal energy density of the fluid. MFI then predicts that in the limit of very low Mach number, the pressure and the fluid velocityuare independent of each other, as random variables; i.e.,P(p,u)→P(p)P(u). Also,P(u) is a Gaussian. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868934

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

This study concerns turbulent jets with density variations. The local statistical properties, obtained for the velocity field and the mean mass fraction, are needed both for better understanding of the density effects and for modeling of such flows. Special attention is paid to the near‐field region (x/Dj≤20), where there is a lack of experimental results. A vertical axisymmetric turbulent jet emitted from a fully developed pipe flow, weakly confined in an air coflow, is investigated. Apart from the constant density case (air/air), two situations with variable density are treated with a density ratio varying from 0.14 [He/air] to 1.5 [CO2/air], with the momentum fluxMjmaintained constant. Even though there is no exact similarity for jets when density varies, some pseudosimilarity laws can be established, and they are experimentally well confirmed. It is also found that the values of the correlations ⟨u2v⟩, ⟨uv2⟩, and ⟨uw2⟩ are negative in the central region (r/Lu<0.5), and this is valid for the three gases considered here. On the other hand, the use of the results for calculating balances of some of the flow conservation equations (i.e., continuity, momentum, and kinetic energy) allows us to confirm some measurements and particularly to estimate some terms that are not, and cannot be, measured at present in variable density flows, such as the dissipation rate of turbulent kinetic energy. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868935

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Recent statistical analyses of field trials and wind tunnel experiments show a new feature of fluctuations of tracer concentration dispersing in turbulent shear flow. For an ensemble of releases, the scatter‐plot of the skewnessSversus kurtosisKcollapses to a quadratic curve lying aboveK=S2+1. Several explanations for this collapse have been put forth in the literature, including a model for cloud meander in instantaneous releases that does not treat the effects of turbulence on the internal structure of the cloud. In this paper, a model is proposed for the creation of internal striations within the cloud due to convection by the ABC‐velocity field that is a paradigm for three‐dimensional helical motions. The numerical simulations conducted here show that the collapse of theK‐Sscatter plot obtains under this more complicated situation where the smaller than cloud size structure is created by chaotic advection. Further, these simulations complement a recent multiscale analysis for eddy diffusivity of ABC flows. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868936

出版商:AIP    

年代:1996

数据来源: AIP