摘要:

A joint numerical and experimental investigation has been performed to study a vortex ring normally impinging on a solid plane surface. Specific emphasis is placed upon the vortex structure and the associated surface force at Reynolds numbers between 500 and 2000. Along the line of Chang [Proc. R. Soc. London Ser. A437, 517 (1992)], the force was viewed to be contributed by fluid elements with nonvanishing vorticity in the flow. This viewpoint enables the paper to provide a unique opportunity to shed light on the detailed mechanism which causes a small net surface force during the impingement process. It is found that the small net surface force results from the cancellation of a force pushing on the surface, due to the primary vortex ring, and a suction force, due to the induced boundary layer. Nevertheless, both the pushing and suction forces are much larger than that of the net surface force by several orders of magnitude. This fact provides proper directions for flow manipulation regarding forces exerted on the surface. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868527

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Monte Carlo extrapolation methods and approximate evaluations of the partition function are used to obtain the phase transition properties and spatial probability distribution exponents for the Hamiltonian system. A second condensation from dipole pairs to a liquid‐like phase is found on cooling the system to a temperature approximately one‐tenth the pair condensation temperature. In the condensed dipole pair temperature range the Metropolis algorithm yields dipole configurations similar to those observed in previous numerical integrations of the motion. In the region of the pair condensation transition the distribution of distances between plus and minus vortices follow Le´vy probability laws over spatial scales extending from the vortex core diameter to the radius of the container enclosing the flow. The joint space‐time probability density function for the travel times and lengths of the dipole translation paths between collisions is derived for thermodynamic equilibrium and yields Le´vy flight like enhanced diffusion. Theoretical values for the diffusion exponent and its temperature dependence are obtained and found to be in agreement with diffusion rates measured during numerical integrations of the vortex motion. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868528

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Numerical simulations of wake flow behind an equilateral triangular obstacle are presented. The form of global modes and their dependence on the Reynolds number found in this study are in accordance with recent experimental results of Goujon–Durandetal. [Phys. Rev. E50, 308 (1994)]. A scaling law of the amplitude oscillating with the fundamental frequency corresponding to themaximumof the global mode is found to agree with the Landau model in a range of ReynoldsRenumbers larger than in previous studies. The position of the maximum amplitude of the fundamental modes scales as (Re−Rec)−1/2. The amplitude of the second harmonic of the longitudinal component of the velocity as well as the correction to the mean flow have different critical behavior than the velocity components oscillating with fundamental frequency. During linear growth the position of the maximum of the global modes is constant and moves only in the nonlinear regime. The effects of the blockage and the boundary conditions on the side walls on the form of the global modes are discussed. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868529

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A new computational model for the return to isotropy is presented. In order to reproduce the significant role of the third invariant IIIb(=bijbjkbki) of the Reynolds stress anisotropybij[=uiuj/(2k)−(1/3)&dgr;ij] in the return‐to‐isotropy process, a nonlinear return‐to‐isotropy model is formulated by a Taylor series expansion up to fifth power ofbij. Then the strong realizability condition for non‐negativity of the component energies is utilized to reduce the number of model constants produced. Correction for the low Reynolds number effect is then included by investigating an energy‐weighted average time scale of eddies over the three‐dimensional energy spectrum. Superiority of the proposed model performance is exemplified by a number of test computations of homogeneous relaxing turbulence in a wide range of turbulence Reynolds number and IIIb. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868760

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The restricted Euler equation captures many important features of the behavior of the velocity gradient tensor observed in direct numerical simulations (DNS) of isotropic turbulence. However, in slightly more complex flows the agreement is not good, especially in regions of low dissipation. In this paper, it is demonstrated that the Reynolds‐averaged restricted Euler equation violates the balance of mean momentum for most homogeneous turbulent flows with only two major exceptions: isotropic and homogeneously sheared turbulence. A new model equation which overcomes this shortcoming and is more widely applicable is suggested. This model is derived from the Navier–Stokes equation with a restricted Euler‐type approximation made on the fluctuating velocity gradient field. Analytical solutions of the proposed modified restricted Euler equation appear, at this point, to be difficult to obtain. Hence, a strategy for numerically calculating the velocity gradient tensor is developed. Preliminary calculations tend to indicate that the modified restricted Euler equation captures many important aspects of the behavior of the fluctuating velocity gradients in anisotropic homogeneous turbulence. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868530

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Heat release effects on laminar flame propagation in partially premixed flows are studied. Data for analysis are obtained from direct numerical simulations of a laminar mixing layer with a uniformly approaching velocity field. The structure that evolves under such conditions is a triple flame, which consists of two premixed wings and a trailing diffusion flame. Heat release increases the flame speed over that of the corresponding planar premixed flame. In agreement with previous analytical work, reductions in the mixture fraction gradient also increase the flame speed. The effects of heat release and mixture fraction gradients on flame speed are not independent, however; heat release modifies the effective mixture fraction gradient in front of the flame. For very small mixture fraction gradients, scaling laws that determine the flame speed in terms of the density change are presented. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868531

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this paper we report measurements of differential diffusion in nonreacting jet flows. Pulsed laser Raman scattering spectroscopy is used to measure species concentrations in nonreacting jets of H2/CO2into air over a range of Reynolds numbers from 1000 to 64 000. Measurements are also made in strained laminar opposed flows of H2/CO2against air, where differential diffusion effects on relative species concentrations are observed to be independent of strain rate. In laminar opposed flows and in a laminar jet (Re=1000), measurements of average species concentrations show significant differential diffusion effects. In jet flows of higher Reynolds numbers, only instantaneous species concentrations are affected by differential diffusion; measured species concentrations are the same,onaverage, as the species concentrations that would have been observed if all species diffusivities were equal. Instantaneous differential diffusion effects in turbulent jets are quantified by the variance of a differential diffusion variablez, wherezis the difference between the normalized mole fractions of H2and CO2(and normalization refers to dividing each species mole fraction by the mole fraction of that species in pure jet fluid). Measurements show thatzRMSdiminishes with increasing Reynolds number. Histograms ofzshow negative skewness in the low Reynolds number jet. At higher Reynolds numbers the histograms become more symmetric, although asymmetries do remain at higher Reynolds numbers in the data obtained near the jet edge, where there is an interface between jet fluid and coflowing air. ©1995 American Institute of Physics. 

ISSN:1070-6631    

DOI:10.1063/1.868532

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This paper concerns electroconvectional stability of a conduction state in an electrolyte layer flanked by cation‐permselective walls (electrodialysis membranes, electrodes) under constant current conditions. It is shown through a numerical finite difference solution of the linear stability problem that above a certain current threshold the basic conduction state becomes electroconvectionally unstable. Marginal stability curves in the current/wave number plane are calculated and the dependence of the critical threshold characteristics on the system’s parameters (ionic diffusivities ratio, electroconvectional Pe´clet number) studied. Electroconvectional instability is shown to occur for an arbitrary ionic diffusivities ratio. A one‐dimensional model of electroconvection in a loop is developed and the respective problem solved explicitly for a steady state. It is shown that above a certain current threshold, the quiescent conduction in the loop bifurcates into a pair of electroconvectional steady‐state circulations. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868533

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Results of a combined numerical and experimental investigation of the near field of low‐subsonic air square jets are presented. The study focuses on examining the role of initial conditions and other features of the jet dynamics in determining the nature and frequency of occurrence of axis switching and the related mechanisms which enhance entrainment, mixing, and turbulence production. Three different experimental square jet facilities were utilized, including orifice jets with low and high initial turbulence level, and pipe jets. Unsteady, spatially developing jets were investigated computationally using direct and monotonically‐integrated large‐eddy simulation approaches, and appropriate inflow/outflow boundary conditions. Insight on the axis‐switching process was obtained using the detailed database from the simulations to investigate how the unsteady vorticity dynamics reflects on the time‐averaged properties of the jet cross sections. The different experimental jets were chosen such that important parameters affecting the initial conditions could be tested. Depending on the particular initial conditions of the subsonic jets studied, several or no axis switchings were observed in the first few diameters of jet development. Observed trends reported, include the effects of initial conditions such as, ratio of equivalent diameter to characteristic‐momentum‐thickness, turbulence level, nonuniform azimuthal momentum‐thickness distributions, and Reynolds number. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868534

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The mixing of a passive scalar field by turbulence that is generated by buoyancy forces acting on an initial random density field is considered. Various asymptotic similarity states of the passive scalar field with and without a uniform mean passive scalar gradient are determined by dimensional arguments based on exact or near invariants of the density and passive scalar fields. The results of large‐eddy numerical simulations are shown to support the derived scaling laws. The large‐eddy simulations also demonstrate the different mixing properties of an active and passive scalar field. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868535

出版商:AIP    

年代:1995

数据来源: AIP