摘要:

Combination modes appear in the spectrum of the surface pressure signals when a circular cylinder is forced to oscillate at a frequency different from the von Karman vortex shedding frequency. The spatial symmetry of the sum and difference modes depends on the direction of cylinder oscillation, and is predictable with a simple set of symmetry relations. As a result of the symmetry relations, cross-flow oscillations of the cylinder aimed at enhancing fluctuating lift also channel energy into the fluctuating drag component through the combination modes. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869651

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The conditional moment closure with second-order approximation for the reaction rate and an equation for the conditional fluctuations of the temperature increments before autoignition of a turbulent nonpremixed flow has been developed for one-step chemistry. The explicit incorporation of conditional variances is necessitated due to the temperature fluctuations induced by heat losses from the reaction zone before ignition, as indicated by recent direct numerical simulations (DNS). Predicted ignition times and reaction zone structure are in very good agreement with DNS data and the differences between the first- and second-order closure are discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869652

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Here we investigate the effect of a steady pressure disturbance moving at a constant velocity along the interface of a thin viscous film flowing down an inclined plane. Both steady and unsteady solutions are determined for the interface. We find that it is possible for the steady solutions to have surface waves, either preceding the disturbance or behind it, depending on the values of the Bond and capillary numbers. In addition, for fixed Bond number, disturbance amplitude, and constant film heights both far upstream and downstream, there exists a finite range of capillary numbers for which no steady solutions exist. Within this range of capillary numbers, the transient solution develops a shock like profile which adjusts the heights upstream and downstream of the disturbance and grows in width. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869653

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The heat transfer rate and the hydrodynamic forces experienced by a single vapor bubble of variable radius moving in a superheated or subcooled liquid are studied by means of numerical simulation. For that purpose the full Navier–Stokes equations and the temperature equation are solved in a frame of reference where the bubble surface is steady. The time evolution of the bubble radius is determined by solving the energy balance at the bubble surface. The numerical method is first validated by comparing present predictions with previous asymptotic or numerical results in the case where no relative motion between the liquid and the bubble exists. Then the situation where a constant relative velocity exists is considered. Effects of the mean flow on the heat transfer rate and on the bubble radius evolution are first discussed. Two different stages are generally observed in the computations. First, the radial motion induced by the displacement of the bubble surface dominates and the bubble evolution is essentially identical to the one observed in a liquid at rest. Then the ratio between the radial velocity and the translatory velocity decreases and the heat transfer rate becomes governed by streamwise advection effects. In this second stage a substantial increase of the growth or collapse rate of the bubble is observed, compared to the case of a liquid at rest. For a growing bubble it is shown that the complete process is successively described by the analytical solutions given by Scriven [Chem. Eng. Sci.10, 1 (1959)] and Ru¨ckenstein [Chem. Eng. Sci.10, 22 (1959)]. The situation is much less simple for a collapsing bubble and the reasons of this increased complexity are discussed. It is found that, when the heat transfer mechanism is dominated by streamwise advection, the bubble evolution and the collapse time predicted by the simulations agree well with the experimental results obtained by Chen and Mayinger [Int. J. Multiphase Flow18, 877 (1992)]. Based on the present results, a general correlation giving the collapse time as a function of the characteristic parameters of the problem is proposed. The second contribution of the present work concerns the hydrodynamic force experienced by the bubble. Using a general decomposition procedure, the added mass effect and the viscous contribution are separately identified. It is first shown that the added mass coefficient is strictly constant and equal to one half, whatever the Reynolds number and the relative magnitude of the radial velocity. The viscous drag is then systematically compared with the quasisteady viscous drag corresponding to the instantaneous value of the Reynolds number. In situations of boiling, effects due to unsteadiness are found to exist during the first stages of the motion if the initial Reynolds number is not very large. In contrast, for a collapsing bubble, such effects remain significant all along the process because the relative importance of viscous phenomena increases in time. In both cases it is shown that the time variations of the bubble radius may affect deeply the viscous drag force. For example, when the radial velocity is high enough, the viscous drag force is found to be identical to the one corresponding to a potential flow, even if the instantaneous Reynolds number is low. These effects are discussed with the help of two asymptotic expressions of this force derived recently by Magnaudet and Legendre [Phys. Fluids10, 550 (1998)] for a bubble with a time-dependent radius. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869654

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A temperature gradient is applied along a fluid filled slot with a flat upper interface, establishing flow via thermocapillarity and/or buoyancy. There is a known parallel flow along the slot, in which the fluid velocity varies vertically, and there is a known convected temperature profile. This parallel flow is then subjected to gravitational modulation or “jitter” which is applied at low frequency and in various directions. For gravity modulations in the plane of the basic flow, analytic solutions for velocity and temperature profiles are obtained for jitter of arbitrary amplitude. These solutions involve modifications to the earlier parallel flow solutions. Jitter in the vertical direction generates vorticity due to coupling with the applied horizontal temperature gradient. This alternately cooperates or competes with the steady basic flow over a cycle of the modulation, but does not qualitatively change the flow or temperature profiles. Jitter applied along the slot produces vorticity only when coupled to vertical convected temperature gradients and so is important when the basic flow is sufficiently strong (large Marangoni and/or Rayleigh number). Various cases are considered for the basic flow, which may be driven by thermocapillarity alone, by vertical gravity alone or by a mixture of thermocapillarity and vertical gravity. When strong streamwise jitter is added to any of these cases, the flow profile alternates during the modulation cycle between boundary layer structures and vertically stacked cells. The type of structure selected depends on the sense of the horizontal thermal stratification with respect to the jitter, and in that part of the cycle where this stratification is unstable, there are particular amplitudes of jitter which can give strong cellular motions or runaways. These runaways represent a resonant interaction with stationary Rayleigh-Be´nard cells. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869655

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A temperature gradient is applied along a fluid filled slot. A basic state is considered where the slot is subject to thermocapillary forces and vertical mean gravity, each of which produces a parallel flow and a vertical advected temperature gradient, and is also subject to streamwise mean gravity, which will make the applied temperature stratification either stable or unstable. When this basic state is perturbed by jitter imposed in thespanwisedirection, normal to the plane of the basic flow, the resulting fluid motion is three dimensional. The flow and temperature fields are found to have a simple functional dependence on streamwise and spanwise coordinates, but retain a complicated dependence on vertical coordinate. Perturbation equations describing the vertical variation of these fields are derived when the jitter is weak. At first order in the spanwise jitter, there is a time periodic spanwise-streamwise circulation around the slot. As this circulation also advects heat, it produces spanwise temperature gradients, enabling thermocapillarity and vertical gravity to generate subsidiary spanwise flows. At next order in the weak spanwise jitter, parallel streamwise flows are encountered, along with streamwise and vertical temperature gradients. In most parameter regimes these are opposed to the flow and temperature fields in the basic state. A thorough parametric investigation is performed where the weak spanwise jitter equations are solved, assuming for simplicity that streamwise gravity is absent. This leads to comparatively simple polynomial solutions in vertical coordinate for the various fields. A large number of parameters can still affect the solutions, however, and a detailed parametric investigation is performed. Interesting behavior is found at small Biot number, with trapping of heat producing large temperatures in the slot and large subsidiary flows. The spanwise to streamwise aspect ratio is another influential parameter, since geometric constraints encountered at extreme values of this ratio suppress certain velocity components of the flow and enhance others, thereby suppressing or enhancing temperature advection. These advected temperature fields themselves produce subsidiary velocities and subsidiary temperatures, which can exhibit a subtle and often counterintuitive dependence on the spanwise-streamwise aspect ratio. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869656

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We report laboratory measurements of nonlinear parasitic capillary waves generated by longer waves in a channel. The experiments are conducted for three frequencies of longer waves (4, 5, and 6 Hz), corresponding to wavelengths of approximately 11, 7, and 5 cm. For these wavelengths we apply a model developed recently by Fedorov and Melville [J. Fluid Mech.354, 1 (1998)] to predict the wave profile. Based on a viscous boundary layer approximation near the surface, the model enables us to efficiently calculate gravity-capillary waves. We present direct comparisons that show good agreement between the measurements and numerical predictions over a range of parameters. Finally, we give some simple estimates for a sharp cutoff in the wave number spectra observed in both the numerical solutions and the laboratory measurements of short gravity-capillary waves. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869657

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A Revised Enskog theory, valid to second order in the Enskog approximation, is used to characterize the amount of thermal diffusion in a steady, fully developed flow of a binary mixture of hard, inelastic spheres. The dependence of the thermal diffusion factor on mixture volume fraction, mole fraction, radii ratio, and material density ratio is explored in the presence and absence of gravity. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869658

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Stability and bifurcation analyses of a partially melted or solidified material heated from below and cooled from above in a cavity, the so-called two-phase Rayleigh–Benard problem, are conducted by a finite-volume/Newton’s method. Bifurcation analysis techniques using a numerical Jacobian and an iterative matrix solver suitable to this large complicated system are adopted. The onset and evolution of melt flows coupling with the heat conduction in the solid and a deformable melt/solid interface are illustrated through detailed bifurcation diagrams, and the linear stability of each flow family is carefully examined. Some comparison with the one-phase system is performed. Results are presented for a variety of parameters of interest, including the Rayleigh number, aspect ratio, and tilt angle. Although most calculations are presented for the melt with a Prandtl number of one, the effects of Prandtl number on the onset of cellular convection and the sensitivity of symmetry breaking by tilting are examined. Furthermore, the dynamic responses of an unstable static state to stable solutions after small disturbances are illustrated, and the effect of heat of fusion is discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869659

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The treatment of chemical reactions and nonequilibrium energy exchange in Direct Simulation Monte Carlo calculations is examined. Details of a Maximum Entropy chemical reaction model are presented that is based on the classical scheme devised by Levine and Bernstein. Data are given for all of the significant reactions that occur in hypersonic reentry flight into the atmospheres of the Earth, Mars, and Venus. The method is an extension of that described and used previously by the authors (Gallis and Harvey [J. Fluid Mech.312, 149 (1996); AIAA J.34(7), 1378 (1996)]) and now includes carbon dioxide/nitrogen and ionic reactions. The model allows an appropriate dependence of each reaction on its controlling energy mode and avoids inappropriate use of equilibrium distributions to determine the reaction probabilities and post-collision energy reallocation. Sample flow solutions are given and comparisons are made with results obtained using continuum solvers. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869660

出版商:AIP    

年代:1998

数据来源: AIP