摘要:

A young turbulent spot develops in a spatially developing direct numerical simulation of an incompressible constant‐pressure boundary layer exposed to a strong localized disturbance. The data are analyzed to determine both the gross characteristics of the spot and the substructures that develop inside the spot. The calculations confirm that hairpinlike vortices are added near the trailing edge of the spot. However, the computations also suggest that the importance of large spanwise vorticity structures may have been overestimated by previous experiments. The current simulation data reveal that streamwise vortices are more intense and more numerous. In addition, the streamwise vortices provide at least one route for the entrainment of near‐wall fluid into the turbulent spot near the leading edge. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868804

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A diffusion model is applied to calculate the turbulent mixing width at an ablating surface. It is proposed that the general model be tested first on well‐determined and easily accessible stabilizing mechanisms such as surface tension, viscosity, density gradient, or finite thickness. In this model the turbulent mixing widthhis directly correlated with the growth rate &ggr; of the perturbations in the presence of stabilizing mechanisms:h/hclass=(&ggr;/&ggr;class)1/2, wherehclass=0.07Ag&tgr;2and &ggr;class=&sqrt;Agk(whereAis the Atwood number,gis the acceleration, &tgr; is the time, andk=2&pgr;/&lgr; =2&pgr;/(&ohgr;hclass), &ohgr; being a dimensionless constant in the model). The method is illustrated with several examples forhablation, each based on a different &ggr;ablation. Direct numerical simulations are presented comparinghwith and without density gradients. In addition to mixing due to the Rayleigh–Taylor instability, the diffusion model is applied to the Kelvin–Helmholtz and the Richtmyer–Meshkov mixing layers. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868805

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Recently, one of the authors, studying a model for turbulent diffusion involving a large‐scale velocity field rapidly fluctuating in time, rigorously demonstrated intermittency in a diffusing scalar field by exhibiting broader than Gaussian tails in the scalar PDF. Here, we explore this model further with exact formulas within the context of general initial data possessing both a mean and a fluctuating component. Several new phenomena due to the presence of a nonzero scalar mean are documented here. We will establish that the limiting long time scalar PDF has long tails, as well as persisting skewness. Further, we show that the limiting PDF depends on the large‐scale energy of initial temperature fluctuations and exhibits long time memory of the initial data. Additionally, we will exhibit an explicit phase transition occurring in the scalar PDF as this large scale energy is varied, whereby the limiting PDF switches between states arising from deterministic initial data and states dominated by fluctuation. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868806

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this paper the behavior of a turbulent boundary layer perturbed from its equilibrium state, due to the presence of an obstacle on an otherwise smooth surface, was investigated. The data of the equilibrium turbulent boundary layers are presented in terms of dimensionless integral parameters, where the flow features can be simply estimated. It was found that the rate of recovery to equilibrium, at highly adverse pressure gradients, is almost instantaneous in the inner part of the new boundary layer initiated by the reattachment process. The flow recovery downstream of an obstacle is correlated in term of Clauser’s profile parameter ratio,G/G′. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868807

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A cinematographic system allows acquisition of high‐image‐density PIV images in the cross‐flow plane of the near‐wake of a cylinder at a Reynolds number of 10 000. Images were acquired at a temporal resolution corresponding to 1% of the period of formation of the large‐scale spanwise (Ka´rma´n) vortices. Such a sequence of images leads to three‐dimensional space–time representations, which show the relationship between the instantaneous concentrations of streamwise vorticity and the phase of formation of the large‐scale Ka´rma´n vortices. The first spatial correlations of instantaneous streamwise vorticity, taken over the cross‐flow plane, reveal that the predominant concentrations of streamwise vorticity maintain a spanwise wavelength of approximately one cylinder diameter. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868808

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Two different large eddy simulations are statistically equivalent if they produce the same statistical representation. In this paper this basic definition is discussed in detail and a new formulation of the subgrid scale dynamic model that should force the statistical equivalence of different simulations at different resolution levels is proposed. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868841

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We propose new dynamical equations to describe fully developed turbulence. We begin with the Wyld equations (WE), which are exact solutions of the NSE. The WE, and their Langevin‐like representation, show that nonlinearities induce a turbulent forceft(k) and a turbulent viscosity &ngr;t(k), which are given by an infinite series of Wyld diagrams. The series for &ngr;t(k) is renormalizable, and its sum can be found using RNG methods. The result, Eq. (2a), holds for stirring forcesfextwith an arbitrary correlation function &fgr; and generalizes previous RNG results, which neglectedftand were limited to power law &fgr;∼k1−2&egr;. To recover Kolmogorov law, these earlier RNG‐based theories were forced to introduce anadhocstirring force with a prescribed &fgr;∼k−3. By contrast, we show that ∼k−3belongs to &fgr;˜, which is the correlation function offt, and that in the inertial rangeft≫fext. The series for &fgr;˜ cannot be summed because of a nonrenormalizable infrared divergence (IR) with an infinite number of divergent irreducible diagrams. To overcome this difficulty, we use the well‐accepted notion of local energy transfer and we derive an expression for the energy flux &Pgr;(k), Eq. (2d), as well as a dynamical equation for the energy spectrumE(k), Eq. (2b). We also construct the dynamical equations for Reynolds stress spectra (solved in papers II and III). An analogous approach is developed for the temperature field. The model contains no free parameters. Some of its predictions are Kolmogorov spectrumE(k)∼k−5/3with Ko=5/3, in agreement with recent data; temperature spectrum in the inertial‐convective regionE&thgr;∼Ba &egr;¯−1/3&egr;&thgr;k−5/3, in agreement with the data; Batchelor constant Ba=&sgr;t Ko. In addition, in papers II and III we carry out extensive comparisons with the laboratory, DNS, LES data, and phenomenological models. The model can be used to construct a subgrid model for LES calculations. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868842

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Using the formalism developed in paper I, we treat the case of shear‐driven flows.First, we derive the dynamic equations for the Reynolds stress. The equations are expressed in both tensorial and scalar forms, that is, as a set of coupled differential equations for the functions that enter the expansion of the Reynolds stress in terms of basic tensors. We specialize the general results to (a) axisymmetric contraction, (b) plane strain, and (c) homogeneous shear, for which there is a wealth of DNS, LES, and laboratory data to test the predictions of our model.Second, for homogeneous shear, in the inertial range, the equations for the Reynolds stress spectral function can be solved analytically,E12(k)=−C&egr;1/3Sk−7/3, which is in excellent agreement with recent data. Since the model has no free parameters, we stress that the model yields a numerical coefficientC, which is also in agreement with the data.Third, we derive the general expressions for the rapid and slow parts of the pressure–strain correlation tensors &Pgr;rijand &Pgr;sij. Within the second‐order closure models, the closure of &Pgr;sij(third‐order moment) in terms of second‐order moments continues to be particularly difficult. The general expression for &Pgr;ijare then specialized to the three flows discussed above. When &Pgr;sijis written in the form first suggested by Rotta, we show that the Rotta constant is a nonconstant tensor.Fourth, we discuss the dissipation tensor &egr;ij. In standard turbulence models, one not only assumes that &egr;ij=2/3&egr;&dgr;ij+f(uiuj), wheref(x) is a empirical function of the one‐point Reynolds stressuiuj, but, in addition, one employs a highly parametrized equation for &egr;. In the present model, neither of the two assumptions is required nor adjustable parameters are needed since &egr;ijis computed directly. The model provides thek‐dependentRij(k) as one of the primary quantities. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868843

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present the numerical solutions of the equations for turbulence developed in papers I and II. (1) The model predicts the Kolmogorov law and Ko=5/3, in accord with recent data; (2) in the inertial‐conductive regime, the model predicts the Corrsin spectrum for the temperature variance and the Batchelor constant Ba=&sgr;t Ko, where &sgr;t=0.72 is the turbulent Prandtl number; (3) the predicted energy spectrum in the dissipation region is in agreement with recent laboratory measurements; (4) in the inertial‐convective region, the temperature variance spectrum is closer to the spectrum (−11/3) obtained by LES when the velocity field is rapidly stirred at all scales than(−17/3), which holds when the velocity field is frozen in time and has a Gaussian statistics; (5) for freely decaying turbulence, the power law spectra for energy and temperature variance, as well as the velocity and temperature integral scales, agree with the most recent LES data; (6) after a few evolutionary times, the skewnessSreachesS=0.5, in accord with a variety of data; (7) for shear‐driven flows, the Reynolds stress spectrumE12(k) has an inertial regime with a power −7/3, in accord with recent data; (8) for two shear‐driven flows, plane strain and axisymmetric contraction, turbulent kinetic energy, Reynolds stress tensor, and dissipation rate &egr;ijversus time compare very well with DNS data; (9) the slow and rapid parts of the pressure–strain correlation tensor compare with DNS data better than with the three most widely used phenomenological models. The rapid parts are also in excellent agreement with the DNS data; (10) for homogeneous shear, turbulent kinetic energy and Reynolds stress tensor versus time match quite closely LES data. We recall that the model does not contain any free parameters. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868844

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An experimental investigation of the Richtmyer–Meshkov instability is carried out in a shock tube. The purpose of this study is to obtain information on the growth in the thickness of the turbulent mixing zone, which is induced by the impulsive acceleration of the interface between two gases of different densities. The turbulent phase of the evolution of this instability is of interest here. The thickness of the turbulent mixing zone is inferred from two different diagnostic techniques: measurements of infrared emission of CO2and black‐and‐white or color schlieren photographs. Following an assessment of the diagnostic techniques, discussions of the main experimental difficulties as the presence of membrane fragments and the disturbances induced by the wall boundary layers, are given. Comparisons of the thickness and the thickness growth rate of the turbulent mixing zones obtained in the present experiments, with both experimental and theoretical results, are made. A tentative picture of the evolution in time of the turbulent mixing zone thickness has been developed. Results fall between the predictions of the linear and thet2/3power law theories, with a tendency toward the latter one. ©1996 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868845

出版商:AIP    

年代:1996

数据来源: AIP