摘要:

The development of streamwise‐oriented, symmetric, two‐dimensional vortices in curved channels of large aspect ratios is studied near the threshold of the first centrifugal instability. The nonlinear equations of motion governing the two‐dimensional, stationary flows are solved numerically over a range of parameter values. The dynamical parameter is the normalized streamwise pressure gradient defined as &egr;=[(∂p/∂x)−(∂p/∂x)c]/(∂p/∂x)c, where (∂p/∂x)cis the critical value for the infinite geometry. The development of interior cells and the selection of the wavelength as &egr; is gradually increased through zero is quite different from that observed in Taylor vortex flow. For pressure gradients of 0.5% and 1.0% (&egr;=0.005, 0.01) above critical, the interior cells begin to grow spontaneously and are strongest in the middle of the channel. Unlike the interior cells in Taylor vortex flow, they are only weakly coupled to the end cells. The end cells (or Ekman vortices) are also found to be anomalously long.As &egr; is increased further to 0.04 and 0.07, the amplitude and wavelength of the interior cells are more uniform. There is, however, a complex interaction between the Ekman vortices and their neighboring interior cells, often resulting in the formation of additional cells in that region. Next, a simple Ginzburg–Landau (GL) model is tested for weakly nonlinear, two‐dimensional vortices. The coefficient in the steady form of this equation is evaluated for a wide parameter range using high accuracy calculations of infinite aspect ratio neutral stability curves. (When suitably normalized, neutral stability curves are found to vary only a little with radius ratio.) For large aspect ratio curved channels, predictions from the model are compared with results from numerical simulation. The variation with &egr; of vortex amplitude near the center of the duct is correctly predicted by the Ginzburg–Landau equation. For given &egr;, however, agreement of the spanwise variation of vortex amplitude and spacing between the numerical simulation and the model is not obtained. The development of consistent amplitude equations and boundary conditions that link the interior flow to the boundary is expected to be a challenging task.

ISSN:0899-8213    

DOI:10.1063/1.857617

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The boundary between absolute and convective (linear) instability of two‐dimensional inertial jets and wakes is determined as a function of the ratio of jet/wake to ambient density, as well as the ratio of mixing layer thickness to jet/wake width, the velocity ratio, and the Reynolds number. For this, a viscous, heat‐conducting ideal gas is taken as the fluid, a zero Mach number, no buoyancy and a parallel basic flow are assumed, and the density variation is achieved by specifying a mean temperature profile similar to the velocity profile. Considering both ‘‘varicose’’ and ‘‘sinuous’’ disturbances, results are obtained for the inviscid top‐hat jet/wake bounded by two vortex sheets, the inviscid jet with continuous velocity and density profiles, and the viscous wake. For the latter, both constant and temperature‐dependent viscosity are investigated. In all the cases it is found that low density of thehigh‐speed fluid promotes absolute instability, while low density of thelow‐speed fluid has the opposite effect. By comparison with experiments it is shown that the present results provide useful information about the parameter range in which flow oscillations are self‐excited.

ISSN:0899-8213    

DOI:10.1063/1.857618

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Stability of a rotating film flow attached inside a circular cylinder was analyzed on a linear basis, where the cylinder is at rest and the basic flow is assumed to be kept rotating under the imposition of an artificial body force acting in the azimuthal direction. A traveling‐wave mode that is intrinsic for a film flow on a vertical or an inclined flat plate and a centrifugal instability mode are obtained as unstable modes. The effect of a superimposed shear flow in the axial direction was also examined and it was shown that the shear flow exerts a stabilizing effect on the centrifugal instability mode.

ISSN:0899-8213    

DOI:10.1063/1.857619

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The effect of turbulence on particle concentration fields and the modification of turbulence by particles has been investigated using direct numerical simulations of isotropic turbulence. The particle motion was computed using Stokes’ law of resistance and it was also assumed the particle volume fraction was negligible. For simulations in which the particles do not modify the turbulence field it was found that light particles collect preferentially in regions of low vorticity and high strain rate. For increased mass loading the particle field attenuated an increasing fraction of the turbulence energy. Examination of the spatial energy spectra showed that the fraction of turbulence kinetic energy in the high wave numbers was increased relative to the energy in the low wave numbers for increasing values of the mass loading. It was also found that the turbulence field was modified differently by light particles than by heavy particles because of the preferential collection of the light particles in low‐vorticity, high‐strain‐rate regions. Correlation coefficients between the second invariant of the deformation tensor and pressure showed little sensitivity to increased loading while correlations between enstrophy and pressure were decreased more by the light particles than by the heavy particles for increased mass loading.

ISSN:0899-8213    

DOI:10.1063/1.857620

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The closure problem generated by the molecular mixing term in the turbulent convection of scalars is studied. The statistical average of this term both in moment formulations and in the probability density function (pdf) approach implicitly encloses the turbulence straining action on scalar gradients leading to a significant enhancement of the molecular dissipative effects. Previous pdf model equations are examined in terms of cumulants evolution and reasons for their failure are diagnosed. A new noninteractive model is proposed, combining a linear mean square estimation (LMSE) deterministic subprocess affecting all the Monte Carlo particles, used to represent the pdf, and a binomial sampling acting on a fraction of them. The scalar lower and/or upper bounds are naturally considered in the formulation. For unbounded scalars, or when the scalar standard deviation is much smaller than the absolute value of the difference between the bounds and the scalar mean, the binomial sampling tends to a Gaussian one. The extension of this model to investigate the joint statistics of velocity and scalars is also considered. Numerical results for the homogeneous turbulent mixing of one scalar with initial values of either 0 or 1 are presented. The evolution of the first four moments and the pdf is qualitatively reasonable. The correct asymptotic Gaussian state is predicted.

ISSN:0899-8213    

DOI:10.1063/1.857621

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The characteristics of isenthalpic planar premixed flames in counterflowing, high Reynolds number streams with counter‐ and corotating swirl are analyzed by activation energy asymptotics for a one‐step Arrhenius reaction with small departures from unity Lewis numbers. Density changes resulting from heat release are fully taken into account. For moderate rates of rotation, the situation considered in the present study, there is only one stagnation plane that is coincident with the plane of symmetry separating the inviscid outer flows and that involves a viscous layer with an embedded flame in some circumstances. The analysis of a flame in the viscous layer is relevant to the experimentally observed variation of the lean flammability limit with the rotation rate. Comparison with the existing experimental data is carried out.

ISSN:0899-8213    

DOI:10.1063/1.857622

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The formation and evolution of a front between turbulent and nonturbulent fluid in a two‐dimensional flow are studied using a combined experimental‐numerical technique. Turbulence was created by towing a grid of vertical bars partway through a tank that was filled with a two‐layer stratified fluid. The velocity field at several times during the evolution of the turbulence was obtained by photographing neutrally buoyant particles suspended in the sharp density interface. Each realization was used to compute the advance of lines of particles released in the flow near the initial location of the grid. The computed location and shape of the turbulent/nonturbulent front, representing the distribution of a passive scalar in the flow, correlate well with the ‘‘zero contour’’ of the absolute‐vorticity field. It is shown that the turbulent/nonturbulent front, in the most distorted cases, may be described by a fractal dimension over a restricted range of length scales. Certain geometric characteristics of the frontal region, in particular the probability distribution of a passive scalar, the frontal width, and length are also examined; the latter appears to grow with a power of time related to the fractal dimension of the interface.

ISSN:0899-8213    

DOI:10.1063/1.857623

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

High Reynolds number compressible free shear layers were studied experimentally to explore the effects of compressibility on the turbulence field. Previous preliminary results reported by the authors showed that the level and the lateral extent of turbulence fluctuations are reduced as the compressibility, which is characterized by a convective Mach number, is increased. The two convective Mach numbers used in the previous study were relatively close,Mc=0.51 and 0.64, and as a result the conclusions were not concrete. The present results withMc=0.86 strongly support the earlier results, showing even higher reductions in the level and the lateral extent of Reynolds stresses. The higher‐order moments of turbulence fluctuations such as skewness and flatness are reported, which show that the intermittency resulting from the excursion of large‐scale structures into the free streams at the edge of shear layers was significantly reduced (both in the level and the extent) because of increasedMc. In the developing region of shear layers, development of mean flow and turbulence fluctuation profiles are reported that have similar trends seen in incompressible shear layers.

ISSN:0899-8213    

DOI:10.1063/1.857816

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The effect of suction on the wall region of a turbulent boundary layer over a slightly heated wall has been quantified in terms of several aspects of the motion that are related to bursts and sweeps. These events are detected by the modifiedu‐level method (with alignment on the ends of bursts), theuv‐quadrant 2 method, and theuv‐quadrant 4 method. The mean periods of bursts and sweeps are increased by suction. The regions upstream fromu‐level detected bursts are more coherent than the bursts themselves. Suction substantially increases the relative contributions from the organized motion to momentum and heat fluxes, ∼(uv), ∼(u&thgr;), and ∼(v&thgr;), and has a lesser effect on contributions to ∼(u2), ∼(v2), and ∼(&thgr;2).

ISSN:0899-8213    

DOI:10.1063/1.857624

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A theory is developed for a selection rule governing collisions in a particle simulation of rarefied gas‐dynamic flows. The selection rule leads to an algorithmic form that is highly compatible with fine grain parallel decomposition, allowing for very efficient utilization of supercomputers having vector or massively parallel single instruction multiple data (SIMD) architectures. A comparison of shock‐wave profiles obtained using both the selection rule and Bird’s direct simulation Monte Carlo (DSMC) method show excellent agreement. This serves to establish the validity of the method, as the DSMC method is known to compare well with experimentally determined shock‐wave profiles. In addition, the equation on which the selection rule is based is shown to be directly related to the time‐counter procedure in the DSMC method, further establishing their equivalence. The results of several example simulations of representative rarefied flows are presented, for which the number of particles used ranged from 106to 107, demonstrating the greatly improved computational efficiency of the method.

ISSN:0899-8213    

DOI:10.1063/1.857625

出版商:AIP    

年代:1990

数据来源: AIP