摘要:

In the present work, a fine, dry powder was sheared in a Couette device: i.e., sheared between concentric vertical cylinders. The torque generated on the rough, inner cylinder was measured as this inner wall was rotated. Our experiments provided evidence that, in a column of granular material undergoing continuous shearing, normal and shear stresses increase linearly with depth. In other words, Janssen’s analysis ceases to apply if granular material is continuously sheared. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869525

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Experiments compare the chaotic mixing of miscible and immiscible impurities in a two-dimensional flow composed of a chain of alternating vortices. Periodic time dependence is imposed on the system by sloshing the fluid slowly across the stationary vortices, mimicking the even oscillatory instability of Rayleigh–Be´nard convection. The transport of a miscible impurity is diffusive with an enhanced diffusion coefficientD*that depends on the size of “lobes” which are, in turn, dependent on the oscillation amplitude. The lobes play an important role in the transport ofimmiscibleimpurities as well. In this case, the impurity is broken into a distribution of droplets, whose areas determine the nature of the transport. If the characteristic long-term droplet areas are appreciably smaller than the lobe areas, then there is long-range transport withD*equal to that for the miscible case with the same flow conditions. If the droplet areas remain larger than the lobe areas, then there is no long-range transport. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869526

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The strong dependence of the rheology of a fluid on temperature has a great impact on the style of thermally driven convection. When the viscosity contrast is sufficiently large, the viscosity of the coldest fluid at the top of a bottom-heated box is so high that this fluid layer becomes very stiff and a so-called cold “stagnant lid” develops on top of a hot convecting layer. Studying this style of convection is relevant for planetary mantles since the rheology of mantle material is likely to be very strongly temperature dependent. In this paper, the Rayleigh number dependence of stagnant-lid convection with a viscosity contrast of106is studied numerically in two and three dimensions in wide Cartesian domains. Like in constant-viscosity cases, the convection in the layer underneath the stagnant lid undergoes the typical transition from steady to time-dependent with the thinning of plumes and with the appearance of boundary layer instabilities as the Rayleigh number increases. A stagnant-lid style of convection was obtained in 2D and 3D for all supercritical Rayleigh numbers considered and the interior temperature appeared not to depend on the Rayleigh number. We have compared our results with other theoretical and numerical results and we found a close agreement. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869527

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this paper we study several problems involving two dimensional and axisymmetric jets issuing in a medium with negligible dynamical effects: (1) a two-dimensional jet entirely bounded by free surfaces, (2) a two-dimensional jet bounded by a free surface and a wall, (3) an axisymmetric jet and (4) a hollow annular jet in a tube. The theoretical study is based on a new technique in which, on the basis of a boundary layer approximation, the velocity components are expanded into a suitable set of eigenfunctions. The deformation of the jet, that is assumed to be a slow function of the distance from the jet exit, is handled by a scaled coordinate system. For the case of the axisymmetric jet, which is the only one previously considered in the literature, this technique is found to be in excellent agreement with previously published, much more computationally demanding results. An experiment involving a vertical water jet has been carried out to further verify the accuracy of the current technique. In particular the behavior very near the exit was found to be in excellent agreement with the experiment at high Reynolds numbers. The fact that the jet relaxation time is much shorter than the time obtained from dimensional arguments is explained with the aid of the semi-analytical structure of the method. In the case of the annular wall jets, the length of the inner gaseous region is found to be finite depending on the Froude number. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869528

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

This paper aims at investigating the viscous corrections to mode selection and associated growth rate in the inflectional instability of shear layers. While, in the inviscid limit, the most unstable mode and its growth rate are fully determined by the initial thickness of the layer2Land the velocity jump2uit experiences, we show here that these quantities are modified by a factor{1+[(a/0.2)/Re]2}−1/2in the small Reynolds numberRe=uL/&ngr;limit, withaa constant depending on the detailed shape of the initial velocity profile. This result agrees well with early numerical computations of Betchov and Szewczyk [Phys. Fluids6, 1391 (1963)] and its interest is discussed in several different contexts. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869529

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this paper we report a laboratory investigation of the motion within a rotating cylinder of fluid subject to internal heating and to cooling at the outer cylindrical sidewall. The internal heating is supplied by ohmic dissipation as an electric current passes between the outer sidewall and an axial wire. The experiments focus on the formation of eddy features and the associated zonal jets. To identify how the flow regimes generated in this continuously forced system are modified by the presence of a radial depth gradient, experiments have been performed with both horizontal(f-plane) and oppositely sloping boundaries. Endwall configurations which cause the fluid depth(D)to increase with radius(∂D/∂r>0)and to decrease with radius(∂D/∂r<0)have been studied, as the former is applicable to the terrestrial atmosphere and oceans, while the latter may be relevant to deep atmospheres such as those of the giant planets and even planetary interiors. Stable, coherent, regular eddy features are observed over a wide range of rotation rates(&OHgr;)in both horizontal and oppositely sloping endwall experiments. In thef-plane experiments regular modes with azimuthal wavenumbersm=1andm=2are observed. The regular wave regime of the∂D/∂r>0endwall experiments consists of modes with azimuthal wavenumbersm=1to 5. Only the modem=1is seen in the regular wave regime of the∂D/∂r<0endwall experiments. The regular eddy structures produced in the∂D/∂r>0(∂D/∂r<0)endwall experiments are seen to be “vertically trapped” close to the bottom (top) boundary, whilst the amplitude of thef-plane modes is found not to vary substantially with depth. Further effects of the radial depth gradient are the observed reduction in the lateral scale of the eddy features in the∂D/∂r>0endwall experiments, which leads to the formation of between two and three independent trains of eddies within the lateral domain at sufficiently high values of&OHgr;.Within the non-axisymmetric flow regimes of all three endwall configurations the number of zonal jets observed in the lateral domain of the experiment is larger than expected from the form of the background thermal forcing. The radial scale of the multiple zonal jets seen in the oppositely sloping boundary experiments is, however, much larger than a barotropic Rhines scaleL&bgr;,suggesting thatL&bgr;cannot be used to predict the radial wavenumber of the zonal mean flow in this continuously forced system. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869530

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

An experimental investigation of the dynamics of horizontal soap films stretched over circular or square boundaries undergoing periodic transverse oscillations at frequencies in the range 20–200 Hz is reported. Concomitant with modes of transverse flexural oscillations, it was observed that two-dimensional vortices in the plane of the film are excited. The vortices may be either (i) large, scaling with the size of the cavity or (ii) small, localized at a wavelength or half-wavelength of the membrane modes. In the experiments a stable generation of one, two, &ellip;, ten pairs of counter-rotating vortices were observed in finite regions of amplitude-frequency parameter space. The circulation strength of vortices in a given vortex pattern increases with increasing external forcing and with decreasing soap film thickness. A theoretical model based on the wave-boundary interaction of excited Marangoni waves reveals a vorticity generation mechanism active in vibrating soap films. This model shows that vorticity is generated throughout the entire liquid volume by viscous diffusion, and qualitatively reproduces many steady vortex patterns observed in the experiment. However, the model cannot explain the existence of the sometimes intense vortices observed far from the film boundary that do not appear to be generated by diffusive processes. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869571

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A typical experimental vortex generator was perturbed by inclining the exit orifice. Instantaneous velocity fields were measured with particle image velocimetry at a Reynolds number,&Ggr;0/&ngr;, of 2800, which falls in the laminar regime for the axisymmetric case. Despite the nearly uniform velocity of the axisymmetric piston, the velocity exiting the cylinder is spatially and temporally non-uniform. Specifically, the exit velocity and the entrainment are larger on the short cylinder side. This fluid motion leads to an initial vortex roll-up with maximum and minimum circulation at the shortest and longest cylinder locations, respectively. A highly complex vortex structure forms, consisting of a primary vortex ring with varying circulation and branched vortex tubes that initially extend from the primary ring upstream toward the cylinder. The variation of the circulation in the primary ring and the strength of the branched vortex tubes increase with incline angle. The branched vortex tubes induce a strong cross-stream sweep of fluid toward the long cylinder side. The branched tubes convect across the cylinder exit with the sweep, break free of the cylinder, and pass through the primary ring. Beyond this time, the vortex structure consists of two closed-loop branches connected on the short cylinder side. As the flow progresses, the center of momentum moves toward the short cylinder side. As the cylinder incline angle is increased, the migration away from the centerline increases, and the flow becomes increasingly disorganized. The propagation speed and penetration distance are reduced because of the loss of coherent circulation. Qualitatively similar velocity fields and flow visualization photographs are presented for a larger (nominally turbulent) Reynolds number of 23000.©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869531

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

In this paper we are concerned with a linear problem of hydroelastic instability in a turbulent boundary layer over a compliant coating of a ribbon-like material. A dispersion relation for hydroelastic waves is written in terms of complex stiffness coefficients that characterize the deformation of the compliant surface and the turbulent flow response to the wavy surface flexure. A membrane-type approximation is proposed for the stiffness coefficient of solid coating at sufficiently low frequencies. In order to evaluate the turbulent flow response, the eddy-viscosity model in a curvilinear coordinate system is employed. The quasipotential approximation for the description of flow stiffness is confirmed on the basis of the computations. The critical flow speed and the parameters of the hydroelastic waves at the instability threshold are determined both within general definitions for the stiffness coefficients and for their approximations. The computational results are compared with available experimental observations. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869572

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We present an experimental study of drag reduction by polymer additives in a closed turbulent flow with a zero mean velocity at large Reynolds numbers. The study is performed using two different forcing schemes: a smooth forcing, for which the fluid is brought into movement through viscous boundary layers, and a very rough forcing where the fluid is stirred inertially by baffles. For a polymer solution of 30 wppm, a steady drag reduction is only observed for the smooth forcing. In the other case, absolutely no decrease of the turbulent energy dissipation is observed. When drag reduction is observed, the turbulent intensity in the bulk is unaltered upon addition of the polymer. This result demonstrates that, in our experimental system, the reduction of the energy dissipation does not only occur mainly in the boundary layers (as is well known for a long time), but takes place solely in the boundary layers. We also study heterogeneous drag reduction by local injection of a concentrated polymer solution. During and just after the injection, similar effects on the measured pressure histograms are observed for the two forcing schemes. However, these effects are much stronger and last much longer, for the “smooth” forcing than for the “inertial” one. With the use of a hot film probe as a tool to detect the homogeneity of the solution, we can show that these effects are due to the mixing of the concentrated polymer solution. The measured pressure histograms also provide information about coherent structures in the flow. We show that during the mixing of the polymer solution, coherent low-pressure filaments are partially suppressed, also when no drag reduction is present: the coherent structures are thus not directly related to the energy dissipation. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869532

出版商:AIP    

年代:1998

数据来源: AIP