摘要:

We report results of time‐dependent numerical simulation of spatially developing free square jets initialized with a thin square vortex‐sheet with slightly rounded corner‐regions. The studies focus on the near field of jets with Mach number 0.3–0.6 and moderately high Reynolds numbers. A monotonically‐integrated large‐eddy‐simulation approach is used, based on the solution of the unfiltered inviscid equations and appropriate inflow/outflow open boundary conditions. The simulations show that the initial development of the square jet is characterized by the dynamics of vortex rings and braid vortices. Farther downstream, strong vortex interactions lead to the breakdown of the vortices, and to a more disorganized flow regime characterized by smaller scale elongated vortices and spectral content consistent with that of the Kolmogorov (K41) inertial subrange. Entrainment rates significantly larger than those for round jets are directly related to the enhanced fluid and momentum transport between jet and surroundings determined by the vortex dynamics underlying the axis‐rotation of the jet cross‐section. The first axis‐rotation of the jet cross‐section can be directly correlated with self‐induced vortex‐ring deformation. However, subsequent jet axis‐rotations are the result of strong interactions between ring and braid vortices, rather than being correlated with successive self‐induced vortex‐ring deformations, as previously conjectured based on laboratory observations. The interaction between braid and ring vortices has the effect of inhibiting the periodic self‐induced axis‐rotations observed in the case of isolated square vortex rings. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868895

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A powerful feature‐tracking tool is applied to several high‐resolution, very long‐duration, regularized contour dynamics (contour surgery) simulations of unforced nearly‐inviscid two‐dimensional turbulence (2DT) on the surface of a sphere. Particularly low density gases of vortices (i.e. on average, very widely separated vortices) are examined to ascertain the nature of their interactions. The simplest (minimal) model system is studied, namely a set of vortex patches of just two vorticity values, ±&ohgr;0, whose total circulation is zero. The areas of the patches are selected initially from a pre‐assigned, stable (nearly time invariant) power‐law distribution. When the vorticity occupation fractionf≳0.01, often more than three vortices are found relatively close together at the onset of a strong interaction. But, whenf≲0.01, all such interactions involve only three nearby vortex patches, not all having the same sign of vorticity. This is related to the well‐known collapse of three singular (point) vortices. Thus, under these conditions, isolated two‐vortex interactions, which have figured in recentadhoctheories and models for decaying 2DT, cannot occur. Taking into account these results, we propose an asymptotically‐motivated and computationally‐efficient ‘‘reduced’’ model. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868896

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

It has recently been recognized that the non‐normality of the dynamical operator obtained by the linearization of the equations of motion about the strongly sheared background flow plays a central role in the dynamics of both fully developed turbulence and laminar/turbulent transition. This advance has led to the development of a deterministic theory for the role of coherent structures in shear turbulence as well as a stochastic theory for the maintenance of the turbulent state. In this work the theory of stochastically forced non‐normal dynamical systems is extended to explore the possibility of controlling the transition process and of suppressing fully developed shear turbulence. Modeling turbulence as a stochastically forced non‐normal dynamical system allows a great variety of control strategies to be explored and their physical mechanism understood. Two distinct active control mechanisms have been found to produce suppression of turbulent energy by up to 70%. A physical explanation of these effective control mechanisms is given and possible applications are discussed. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868897

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Direct numerical simulations of Richtmyer–Meshkov instabilities in shocked fluid layers are reported and compared with analytic theory. To investigate new phenomena such as freeze‐out, interface coupling, and feedthrough, several new configurations are simulated on a two‐dimensional hydrocode. The basic system is anA/B/Acombination, whereAis air andBis a finite‐thickness layer of freon, SF6, or helium. The middle layerBhas perturbations either on its upstream or downstream side, or on both sides, in which case the perturbations may be in phase (sinuous) or out of phase (varicose). The evolution of such perturbations under a Mach 1.5 shock is calculated, including the effect of a reshock. Recently reported gas curtain experiments [J. M. Budzinskietal., Phys. Fluids6, 3510 (1994)] are also simulated and the code results are found to agree very well with the experiments. A new gas curtain configuration is also considered, involving an initially sinuous SF6or helium layer and a new pattern, opposite mushrooms, is predicted to emerge. Upon reshock a relatively simple sinuous gas curtain is found to evolve into a highly complex pattern of nested mushrooms. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868898

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A new dissociation model for the direct simulation Monte Carlo (DSMC) method is formulated using the threshold line concept. This model considers in some detail the coupling between the vibrational energy distribution of molecules in a gas and the rate of dissociation. For a particular reaction, the new threshold line model only requires determination of a leading multiplicative constant and is therefore significantly less deterministic than existing DSMC chemistry models. The new model is evaluated in a heat bath under equilibrium and nonequilibrium conditions. It is also employed to calculate hypersonic shock waves of nitrogen and air. Comparison of the results generated with the new model is made with existing experimental data and with previous computational results. The more detailed DSMC implementation reveals properties of the threshold line dissociation model not apparent in the previous continuum implementation. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868899

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The purpose of this study is to theoretically and numerically investigate the behavior of different subgrid scale (SGS) models for large eddy simulations (LES) of compressible fluids in engineering applications. Various models have been analyzed including eddy viscosity, scale similarity, mixed and dynamic models and finally the concept of monotone integrated large eddy simulation (MILES). The theoretical investigation concerns primarily the demonstration and motivation of the frame indifference constraint and the application of this physical requirement on the various SGS models. LES’ utilizing the presented models have been carried out in a configuration corresponding to a bluff body flow in a rectilinear duct. Experimental data, obtained by LDV, are used for investigating the simulation results. It is found that the results from the numerical simulations are fairly resilient to the SGS models selected when the computational grid is fine enough. However, the different SGS models reproduce the energy dissipation or the interscale energy transfer −B⋅D˜and the SGS fluxes div Bin highly dissimilar ways. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868900

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effect of anisotropic turbulence on the vertical transport of a tracer in a fluid layer is investigated numerically. It is found that for a fixed vertical turbulence spectrum an increase in the intensity of horizontal turbulence reduces the efficiency of the vertical transport. The reduction factor is inversely proportional to the anisotropy (defined as the ratio of horizontal to vertical rms velocities) for large Peclet numbers. This effect had been overlooked in studies of turbulent particle transport in stratified media, where the reduction in vertical transport coefficients was entirely attributed to vertical mode suppression by stratification. Our study is based on Gaussian velocity fields. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868912

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effect of boundary geometry on the properties of a rarefied vapor flow emerging from a narrow and infinitely long source was studied. The boundary consisted of walls, along both sides of the source which were parallel to the bulk velocity direction and of a collimator which plane was perpendicular to this direction. Two‐dimensional numerical solutions for the flow field were obtained based on the Bhatnagar–Gross–Krook model. It was found that the rarefied flow is affected mainly by the size of the collimator aperture and by the walls separation. The streamwise length of the walls and the location of the collimator have a minor influence on the far field density. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868901

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Numerical calculations employing the fully three‐dimensional, time‐dependent Navier–Stokes equations have been utilized to compute a transition from spiral‐ to bubble‐type vortex breakdown for an unconfined longitudinal vortex. The transition was initiated through a small increase in the magnitude of the free‐stream axial velocity deceleration. The resulting bubble structure, which consisted of a single toroidal recirculation cell, ultimately grew unstable. Transition back to a stronger spiral breakdown followed. Results are in qualitative agreement with experimental observations. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868902

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Multi‐point hot‐wire measurements in grid turbulence have been used to conditionally average the dissipation over inertial‐scale features of the turbulent field. Velocities were measured at 5 separate spatial points obtaining six simultaneous velocity components, at turbulent Rey‐ nolds numbersRe&lgr;of 160 and 230. The dissipation was approximated with pseudo‐ or surro‐ gate dissipation &egr;r′based on one or two velocity gradients. The conditioned dissipation &egr;r′inside the most intense large‐scale rotational regions of the flow is more than twice as large as the expected value of &egr;r′over the whole domain. Conditional averaging was also carried out over strong large‐scale fronts showing smaller excess in dissipation. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868903

出版商:AIP    

年代:1996

数据来源: AIP