摘要:

The formation of periodic waves in the near wake of straight circular cylinders with arbitrary aspect ratios in crosswise flows at low Reynolds numbers (Re) is analyzed. Using a semiempirical model it is shown that raising the Reynolds number beyond a critical value of about Re(1)=22 yields a supercritical Hopf bifurcation preceding the known onset of the von Ka´rma´n vortex flow at about Re(2)=46.7. In agreement with experimental trends a stable periodic, secondary transversal motion arises in the previously steady crosswise flow.

ISSN:0899-8213    

DOI:10.1063/1.858848

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Three‐dimensional, incompressible Euler calculations of the interaction of perturbed antiparallel vortex tubes using smooth initial profiles in a bounded domain with bounded initial vorticity are discussed. A numerical method that uses symmetries and additional resolution in the direction and location of maximum compression is used to simulate periodic boundary conditions in all directions. For an initial condition that yields singular behavior, the growth of the peak vorticity, the peak axial strain, and the enstrophy production obey (tc−t)−1, and the enstrophy grows logarithmically. The enstrophy growth is associated with the energy spectrum approachingk−3. Self‐similar development and equal rates of collapse in all three directions are shown.

ISSN:0899-8213    

DOI:10.1063/1.858849

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Using a velocity field obtained in a direct numerical simulation of isotropic turbulence at moderate Reynolds number the subgrid‐scale energy transfer in the spectral and the physical space representation is analyzed. The subgrid‐scale transfer is found to be composed of a forward and an inverse transfer component, both being significant in dynamics of resolved scales. Energy exchanges between the resolved and unresolved scales from the vicinity of the cutoff wave number dominate the subgrid‐scale processes and the energetics of the resolved scales are unaffected by the modes with wave numbers greater than twice the cutoff wave number. Correlations between the subgrid‐scale transfer and the large‐scale properties of the velocity field are investigated.

ISSN:0899-8213    

DOI:10.1063/1.858850

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Numerical simulations of statistically steady two‐dimensional (2‐D) turbulence are analyzed to determine the relative importance of the types of wave‐vector triad interactions that transfer energy and enstrophy in the both the energy and enstrophy inertial ranges. In the enstrophy inertial range, it is found(in agreement with previous studies [J. Fluid Mech.72, 305 (1975); Phys. Fluids A2, 1529 (1990)])that the important triads (i.e., those associated with the highest transfer rates) are typically very elongated. On the average, nearly all of the enstrophy transfer within these triads is directed from the intermediate to the largest wave‐number mode (i.e., downscale transfer). Energy, too, is transferred downscale in this manner, but is also transferred upscale due to the interaction of the intermediate with the smallest wave‐number mode of the triad, resulting in no net flux of energy in the enstrophy inertial range. Analysis of the geometry of the important triads indicates they are not of similar shapes at all scales, and that the enstrophy transferring triads generally consist of one wave vector near the scale of the energetic peak, no matter how large the other wave vectors are. In the energy inertial range, elongated triads are also important. As in the enstrophy inertial range, there is downscale transfer of energy and enstrophy due to the interaction of the intermediate with the largest wave‐number mode. There is also upscale transfer of both energy and enstrophy due to a very nonlocal interaction involving the smallest wave‐number modes. The result is a net upscale flux of energy and no net flux of enstrophy in the energy inertial range. Comparison of the transfer functions from the simulations with those calculated by an eddy‐damped quasinormal closure show agreement in the gross functional forms, but display certain quantitative differences in integrated quantities such as total transfer into and flux past a given wave number.  

ISSN:0899-8213    

DOI:10.1063/1.858851

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Turbulent channel flow and homogeneous shear flow have served as basic building block flows for the testing and calibration of Reynolds stress models. In this paper, a direct theoretical connection is made between homogeneous shear flow in equilibrium and the log layer of fully developed turbulent channel flow. It is shown that if a second‐order closure model of the standard type is calibrated to yield good equilibrium values for homogeneous shear flow, it will also yield good results for the log layer of channel flow provided that the Rotta coefficient is not too far removed from one. Most of the commonly used second‐order closure models introduce anadhocwall reflection term in order to mask deficient predictions for the log layer of channel flow that arise either from an inaccurate calibration of homogeneous shear flow or from the use of a Rotta coefficient that is too large. Illustrative model calculations are presented to demonstrate this point which has important implications for turbulence modeling.  

ISSN:0899-8213    

DOI:10.1063/1.858852

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A pattern recognition technique is used to detect and identify patterns and structures embedded in the velocity field of a turbulent wake behind a cylinder. Two component velocitiesuand &ngr; oruandware measured with eight X‐wire anemometers aligned with the vertical and spanwise directions in the wake atx/D=420 and Re=1600. The velocity patterns educed as averaged patterns or instantaneous structures are consistent with the presence of double‐roller eddies, whose legs are continuously stretched by the rate of strain. Experimentaluand &ngr; data suggest that the rollers are connected at the top, with vorticity parallel to the cylinder axis, forming a horseshoe vortex. The fine‐scale activity, estimated by the envelope of the second derivatives, with respect to time of the velocity signals, confirms that this kind of structure can account for both theu&ngr; correlation and the continuous entrainment of potential flow into the wake.

ISSN:0899-8213    

DOI:10.1063/1.858853

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The modification of decaying homogeneous turbulence due to its interaction with dispersed small solid particles (d/&eegr;<1), at a volumetric loading ratio &fgr;v≤5×10−4, is studied using direct numerical simulation. The results show that the particles increase the fluid turbulence energy at high wave numbers. This increase of energy is accompanied by an increase of the viscous dissipation rate, and, hence, an increase in the rate of energy transferT(k) from the large‐scale motion. Thus, depending on the conditions at particle injection, the fluid turbulence kinetic energy may increase initially. But, in the absence of external sources (shear or buoyancy), the turbulence energy eventually decays faster than in the particle‐free turbulence. In gravitational environment, particles transfer their momentum to the small‐scale motion but in ananisotropicmanner. The pressure‐strain correlation acts to remove this anisotropy by transferring energy from the direction of gravity to the other two directions,butatthesamewavenumber, i.e., to the small‐scale motion in directions normal to gravity. This input of energy in the two directions with lowest energy content causes areversecascade. This reverse cascade tends to build up the energy level at lower wave numbers, thus reducing the decay rate of energy as compared to that of either the particle‐free turbulence or the zero‐gravity particle‐laden flow.

ISSN:0899-8213    

DOI:10.1063/1.858854

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A model for passive mixing of multiple scalars in homogeneous turbulence is developed. This model is based on the time‐evolving mapping closure methodology. Calculations from the model are compared against heat conduction simulations (HCS) data. The agreement is excellent at the early stages of mixing and at the later stages it is fair.

ISSN:0899-8213    

DOI:10.1063/1.858855

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The structure of a normal shock wave is calculated on the basis of the hydrodynamics of fast processes [A. D. Khonkin, Fluid Mech. Sov. Res.9, 93 (1980)] to investigate the validity of the theory for this classical problem. The analysis shows that the theory predicts the continuous shock profile to exist only forM≤1.27. The shock thicknesses and the asymmetry factors resulting from the Mott‐Smith method, the Navier–Stokes and the Burnett equations, and also from the experiments and simulations are compared to the present results for a gas of Maxwell molecules.

ISSN:0899-8213    

DOI:10.1063/1.858856

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

It is shown that for a binary mixture of dilute gases characterized by the Boltzmann equation the Burnett contribution for the fluxes can be evaluated without solving the integrodifferential equations for the corrections to the distribution function to third order in the Chapman–Enskog method. The conservation equations can be closed knowing only the second‐order solution (in the Chapman–Enskog method) to the Boltzmann equation which is well known. By considering a trace component of light particles in a heavy gas (Lorentz approximation) all the higher‐order transport coefficients for the soft‐sphere model can be evaluated and from these results an estimation of the Burnett corrections to the Navier–Stokes regime is given. Other pertinent results of this calculation are also discussed.

ISSN:0899-8213    

DOI:10.1063/1.858857

出版商:AIP    

年代:1993

数据来源: AIP