摘要:

We study the two-dimensional convective patterns in a long horizontal porous layer heated from below, where a nonzero cross-flow is imposed. Indeed experiments show that time-periodic planar flows are found at moderate values of the flow rate. Within the framework of the Darcy law, above the absolute threshold, varied end-conditions lead to oscillatory patterns, which are more or less similar to each other in the bulk of the device but present differences near the extremities. Depending on the boundary-conditions, the numerical simulation may produce patterns which are space-periodic traveling rolls or waves of amplitude modulated within a stationary region, with envelopes in the form of fronts. Space-periodic boundary-conditions yield wavelengths linked to the total length of the device, which sets the frequency. Input boundary-conditions breaking translational invariance along the direction of the main flow yield different structures and select the temporal period. Most attention is paid to inlet-conditions imposing a linear profile of temperature (at the entrance of the device). We study the variations of the frequency vs the seeping flow rate and the filtration Rayleigh number. The length of the resulting front is also considered. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869741

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Magnetic levitation was used to stabilize cylindrical columns of a paramagnetic liquid in air between two solid supports. The maximum achievable length to diameter ratioRmaxwas ∼(3.10±0.07), very close to the Rayleigh–Plateau limit of &pgr;. For smallerR, the stability of the column was measured as a function of the Bond number, which could becontinuouslyvaried by adjusting the strength of the magnetic field. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869742

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A method has been developed to drive a cylindrical liquid jet unstable for deformations with axial wavelengths shorter than the circumference of the jet and azimuthal mode numbers greater than0.The benefit of this method is that a cylindrical liquid jet can be broken into a spray with an average diameter smaller than the diameter of the initial jet. The higher-order instabilities were created by establishing initial conditions for the jet in space and time at the nozzle. An electromechanical transducer creates the applied temporal initial condition which is a sinusoidally varying velocity perturbation added to the steady velocity of the jet. The amplitude of the velocity perturbation can be as large as the jet’s steady velocity and the energy in the applied velocity perturbation drives the instability. The spatial perturbation is created by placing perturbations in the circumference of the nozzle. As the velocity perturbation travels on the jet, its leading edge steepens and the trailing edge broadens in a manner analogous to the steepening of a pressure pulse in a compressible gas. If the driven velocity perturbation is sufficiently large, a shock or jump forms on the leading edge of the velocity pulse and the jet may break up into higher-order modes. A theoretical analysis of the breakup process, based on an adaptation of compressible fluid shock theory, is used to derive a fundamental lower bound on the spray’s Sauter mean diameter as a function of the velocity perturbation amplitude. Techniques for approaching the theoretical minimum spray diameter by using the higher-order modes to atomize liquid jets are discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869743

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Two-dimensional nonlinear equilibrium solutions for the swept Hiemenz flow attachment line boundary layer are directly computed by solving the full Navier-Stokes equations as a nonlinear eigenvalue problem. The equations are discretized using the two-point fourth order compact scheme and the resulting nonlinear homogeneous equations are solved using the Newton-Raphson iteration technique. It is found that for Reynolds numbers larger than the linear critical Reynolds number of 583, the nonlinear neutral surfaces form open curves. The results showed that the subcritical instability exists near the upper branch neutral curve and supercritical equilibrium solutions exist near the lower branch. These conclusions are in agreement with the weakly nonlinear theory. However, at higher amplitudes away from the linear neutral points the nonlinear neutral surfaces show subcritical instability at lower and higher wave number regions. At Reynolds numbers lower than the critical value, the nonlinear neutral surfaces form closed loops. By reducing the Reynolds number, we found that the nonlinear critical point occurs at a Reynolds number of 511.3, below which all the two-dimensional disturbances will decay. The secondary instability of these equilibrium solutions is investigated using the Floquet theory. The results showed that these two-dimensional finite amplitude neutral solutions are unstable to three-dimensional disturbances. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869744

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Hot-wire anemometry, smoke visualization and nonlinear calculations were used to study the wall jet on a cylinder with rotation. It was found that streamwise vortices are amplified in convex wall jet flow without rotation and that the maximum amplitude was higher than for the concave case. Furthermore, the transition region was located downstream compared with the concave wall jet. Rotation was found to destabilize the convex wall jet, i.e., the transition region appeared upstream compared with nonrotation. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869745

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In nearly isothermal, compressible turbulence of a nondense gas the mass fraction of an embedded passive scalar satisfies the same formal conservation equation as a passive scalar in incompressible turbulence. Direct numerical simulation of this system shows that the compressible turbulence modes are less efficient than the incompressible in transporting scalar spectral content from large to small scales. It is argued that the cause of this outcome is the reduced size of the integral length scale of the compressible velocity components vis a´ vis that of the incompressible velocity components, and this also explains the experimentally observed ineffectiveness of the dilatational velocity modes in determining scalar flux in homogeneous, compressible turbulence with a uniform mean scalar gradient. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869746

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The alignment of vorticity and scalar gradient with the eigendirections of the rate of strain tensor is investigated in turbulent buoyant nonpremixed horizontal and vertical flames. The uniqueness of a buoyant nonpremixed flame is that it contains regions with distinct alignment characteristics. The strain-enstrophy angle &PSgr; is used to identify these regions. Examination of the vorticity field and the vorticity production in these different regions indicates that &PSgr; and consequently the alignment properties near the flame surface identified by the mixture fraction bandF≈Fstdiffer from those in the fuel region,F>Fstand the oxidizer region,F<Fst.TheF≈Fstband shows strain-dominance resulting in vorticity/&agr; alignment whileF>Fst(andF<Fstfor the vertical flame) band(s) show(s) vorticity/&bgr; alignment. The implication of this result is that the scalar dissipation,&egr;F,attains its maximum value always nearF≈Fst.These results are also discussed within the framework of recent dynamical results [Galanti &etal;, Nonlinearity10, 1675 (1997)] suggesting that the Navier-Stokes equations evolve towards an attracting solution. It is shown that the properties of such an attracting solution are also consistent with our results of buoyant turbulent nonpremixed flames. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869747

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We report experimental measurements of Lagrangian accelerations in a turbulent water flow between counter-rotating disks for Taylor–Reynolds numbers900<R&lgr;<2000.Particle tracks were recorded by imaging tracer particles onto a position sensitive photodiode, and Lagrangian information was obtained from fits to the position versus time data. Several challenges associated with extracting Lagrangian statistical quantities from particle tracks are addressed. The acceleration variance is obtained as a function of Reynolds number and shows good agreement with Kolmogorov (1941) scaling. The Kolmogorov constant for the acceleration variance is found to bea0=7±3.©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869748

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A nominally plane turbulent jet is synthesized by the interactions of a train of counter-rotating vortex pairs that are formed at the edge of an orifice by the time-periodic motion of a flexible diaphragm in a sealed cavity. Even though the jet is formed without net mass injection, the hydrodynamic impulse of the ejected fluid and thus the momentum of the ensuing jet are nonzero. Successive vortex pairs are not subjected to pairing or other subharmonic interactions. Each vortex of the pair develops a spanwise instability and ultimately undergoes transition to turbulence, slows down, loses its coherence and becomes indistinguishable from the mean jet flow. The trajectories of vortex pairs at a given formation frequency scale with the length of the ejected fluid slug regardless of the magnitude of the formation impulse and, near the jet exit plane, their celerity decreases monotonically with streamwise distance while the local mean velocity of the ensuing jet increases. In the far field, the synthetic jet is similar to conventional 2D jets in that cross-stream distributions of the time-averaged velocity and the corresponding rms fluctuations appear to collapse when plotted in the usual similarity coordinates. However, compared to conventional 2D jets, the streamwise decrease of the mean centerline velocity of the synthetic jet is somewhat higher(∼x−0.58),and the streamwise increase of its width and volume flow rate is lower(∼x0.88 and ∼x0.33, respectively).This departure from conventional self-similarity is consistent with the streamwise decrease in the jet’s momentum flux as a result of an adverse streamwise pressure gradient near its orifice. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869828

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Results from large eddy simulations (LES) and direct numerical simulations (DNS) of a two-dimensional, spatially developing, compressible planar free jet undergoing an idealized, exothermic, chemical reaction of the typeF+rOx→(1+r)Pare presented in order to assess several subgrid-scale (SGS) combustion models. Botha priorianda posterioriassessments are conducted. The SGS turbulence model used is the dynamic Smagorinsky model (DSM). Two classes of SGS combustion models are employed in this study. These include the conserved scalar approach and the direct closure approach. Specifically, the SGS combustion models involve several forms of direct filtered reaction rate closures, including ascale similarity filtered reaction rate model(SSFRRM), and a mixing controlledstrained laminar flamelet model(SLFM) in the form of thermochemical state relationships, obtained from the DNS, and two assumed forms for the subgrid mixture fraction filtered density function (FDF). In general, LES results are in reasonable agreement with DNS results and highlight the performance of the various SGS combustion models. In particular, in the context of the present study, it is found that: (1) the SLFM cases overpredict product formation due to their inability to capture finite-rate chemistry effects; (2) due to the relatively low values of the SGS mixture fraction variance in the flow under study, the SLFM results are not sensitive to the form of the assumed FDF; and (3) in comparison to the other models investigated, the SSFRRM combustion model provides the best agreement with the DNS for product formation. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869749

出版商:AIP    

年代:1998

数据来源: AIP