摘要:

The problem of steady convection rolls and their instabilities in a fluid layer heated from below is studied numerically in the case of a highly conducting, rigid lower and a nearly insulating, stress‐free upper boundary. A Galerkin method is used to obtain two‐dimensional solutions in dependence on the Rayleigh numberRand the wave number &agr; for different values of the Prandtl numberP. Their stability is analyzed through the superposition of general three‐dimensional infinitesimal disturbances. Most instabilities correspond qualitatively to those found in the case of symmetric highly conducting, rigid boundaries. A new instability, the subharmonic varicose instability, is found, however, which restricts the region of stable rolls toward a higher Rayleigh number in the case of moderate Prandtl numbers. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868768

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The response of a heated, nearly homogeneous and isotropic turbulent field to a nonuniform rapid distortion upstream of two‐dimensional objects placed in a homogeneous grid generated flow is investigated experimentally. In particular, the effect of the nonuniform distortion on the single‐point statistical properties of the velocity and a passive scalar (temperature), the axial heat flux and higher‐order cross moments are presented. The nearly homogeneous, turbulent flow is produced by a biplane grid of rods and a square mesh grid of electrically heated wire that is placed downstream of the turbulence producing grid. Spatially and temporally resolved, simultaneous measurements of the streamwise turbulent velocity and temperature are obtained upstream of several two‐dimensional objects along the mean stagnation streamline. The effects of the blocking and vortex stretching mechanisms on the root‐mean‐square (RMS) velocity for various ratios of the integral length scale to the characteristic length of the object,L0/D, are in agreement with previous results, i.e., forL0/D≪1, the velocity intensity is increased and for,L0/D≫1, it is decreased. In contrast, to within a distance equal to about one integral scale from the object, the distorted RMS temperature remains nearly a constant. WhenL0/D≪1, where vortex stretching is important, the axial heat flux and the diffusion of temperature variance increase as the object is approached. In contrast, whenL0/D≫1, where the blockage effect is dominant, they decrease as the object is approached. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868740

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Spin‐stabilized projectiles with liquid payloads may experience different types of flight instabilities caused by the fluid motion in the payload cylinder. The first type is known to occur in low‐viscosity fluids, i.e., at high Reynolds numbers, owing to resonance with inertial waves at critical frequencies. The second type originates from a forced secondary flow at arbitrary frequency, and is most pronounced for fluids of high viscosity, i.e., relatively low Reynolds numbers. For cylinders completely filled with a single fluid, these instabilities were analyzed by eigenfunction expansion developed by Selmi, Li, and Herbert [Phys. Fluids A4, 1998 (1992)]. The method permits unified analysis of both types of instability, since it can be used for flows at moderate as well as high Reynolds numbers. Often in practice, cylinders are made to include a central rod to alter resonance properties or are partially filled during production, to ensure safety as the liquid payload may expand under different conditions. In this paper, the eigenfunction approach is extended to analyze the moments caused by the flow in a spinning and nutating cylinder, containing a partial fill or a central rod. The analysis shows that the fill ratio (defined as the ratio of the volume of the fluid to the volume of the cylinder) affects resonance with inertial waves. The inviscid flow equations are solved analytically to provide criteria for the onset of resonance in the two configurations. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868731

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Recent experiments of thermal convection in finite containers of intermediate and large aspect ratios have shown the presence of flows spanning the largest dimension of the container [R. Krishnamurti and L. N. Howard, Proc. Natl. Acad. Sci.78, 1985 (1981); J. Fluid Mech.170, 385 (1986)]. Large‐scale flows of this kind computed from two‐dimensional (2‐D) numerical simulations are presented. The marginal stability curves for the bifurcations are computed in the range of aspect ratiosL=1,...,6 and for Prandtl number &sgr; =10. The nonlinear dynamics of the bifurcated solution is explored for containers with aspect ratiosL=1,2,4. By increasing the Rayleigh number from criticality the system produces different sequences of symmetry breaking, Hopf‐type bifurcations, which finally result in large scale flows, oscillatory net mass flux and chaos. The bifurcation involves different mode resonances with vertical and horizontal couplings, which are modeled using formal group theoretical techniques. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868732

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The principal subject of this paper is analysis and modeling of turbulent wing tip vortex flows in a far‐field region of the vortex evolution. The choice of a Reynolds stress closure (RSC) to model the vortex turbulence is shown to be indispensable for representation of the flow rotation effects on turbulence. The principal result reported is the model–experiment comparison of the vortex growth rates for different vortex Reynolds numbers. The mean vortical flow generated by the wing tip very effectively suppresses the Reynolds shear stress, which mediates the extraction of energy from the mean flow by turbulence. In consequence, the vortex‐core growth rate is controlled only by molecular viscosity and the vortex turbulence decays since the turbulence production rate is very nearly zero. This rather unexpected result is shown to be supported by experiments. Finally, it is shown that the computed turbulence structure is consistent with experimental data at the NASA Ames Research Center. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868734

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A new methodology to use mapping closures for the mixing of several scalars in homogeneous turbulence is explained. The main idea is the unidimensional mapping for each scalar, with the cross‐dissipation handled by a joint reference field. A restricted and a general closure are described. A Monte Carlo code based on a fractional step technique has been developed. As an example, the segregated double‐delta two‐scalar mixing is analytically and numerically solved and predictions are shown. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868735

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The horseshoe vortex system resulting from the interaction between a laminar boundary layer and a round transverse jet was studied over a range of Reynolds numbers and velocity ratios using hydrogen bubble wire visualization in a water channel. The study shows that the horseshoe vortex system can be steady, oscillating, or coalescing, depending on the flow conditions. Topological concepts are used to interpret the observed flow patterns and compare these patterns with those observed and computed upstream of wall‐mounted circular cylinders. The Strouhal numbers of the observed oscillating and coalescing systems agree reasonably well with those appearing in the literature for wall‐mounted circular cylinders. The relationship between the unsteady horseshoe vortex motions and the unsteady vortex motions in the wake is studied for a velocity ratio of 4. Here it is shown that the oscillating regime occurs at the same frequency as the wake and the coalescing regime occurs at approximately double the frequency of the wake. The results indicate that the wake intermittently becomes coupled to the horseshoe vortex motions and that this occurs either at the horseshoe vortex frequency in the case of the oscillating system or a subharmonic in the case of the coalescing system. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868736

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Detailed mean velocity and turbulence data have been obtained with a laser Doppler velocimeter for two axisymmetric shear layers downstream of rapid expansions of different strengths. A comparison of the data in the near field (immediately downstream of separation) and far field (shear layer approaching self‐similarity) is presented, and the fluid dynamic effects of the rapid expansion are ascertained for each regime. In general, the rapid expansion was found to distort the initial mean velocity and turbulence fields in the shear layer, in a manner similar to that in rapidly expanded, attached supersonic boundary layers; namely, two distinct regions were found in the initial shear layer: an outer region, where the turbulent fluctuations are quenched primarily due to mean compressibility effects (bulk dilatation), and an inner region, where turbulence activity is magnified due to the interaction of organized large‐scale structures in the shear layer with low‐speed fluid at the inner edge. With increasing strength of the rapid expansion, the effects in both regions become more pronounced, especially in the inner region, where turbulent fluctuations and mass entrainment rates are greatly magnified. Farther downstream, the turbulence activity of the large‐scale eddies remains elevated, due to the rapid expansion, even though the relative distribution of the turbulence energy between the Reynolds stress components (structure of the turbulence) is independent of expansion strength. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868737

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Numerical simulations of a shock interacting with a compressible vortex are presented for shocks and vortices of various relative strengths. The simulations show the effects of the vortex on the shock structure and the structure of the acoustic field generated by the shock–vortex interaction. A relatively weak vortex perturbs the transmitted shock only slightly, whereas a strong vortex leaves the transmitted shock with a structure corresponding to either a regular or Mach reflection. The acoustic wave generated by the interaction consists of two components: a ‘‘quadrupolar’’ component produced by the initial shock–vortex interaction and the complex reflected shock system. When these waves merge, they form the asymmetric structure seen in experiments. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868738

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This paper describes the spatial stability characteristics of compressible elliptic jets. Solutions are obtained to the compressible, inviscid, linearized equations of motion; the compressible Rayleigh equation. Separable forms of solution are obtained in the jet potential core and outside the jet in terms of series of Mathieu and modified Mathieu functions. These solutions are matched using a shooting method that integrates the Rayleigh equation through the region of nonuniform velocity and density. Four classes of instability modes are studied; modes that are odd or even about the jet’s major and minor axes. Their stability characteristics are documented for a range of jet aspect ratios, jet Mach numbers and temperatures, and azimuthal distributions of jet shear layer thickness. The growth rates of the modes are found to depend on their class and the jet thickness on the major and minor axes. The mode that ‘‘flaps’’ about the jet major axis is found to be the most unstable as the jet Mach number or aspect ratio increases. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868739

出版商:AIP    

年代:1995

数据来源: AIP