摘要:

We conduct a linear stability analysis of Rayleigh-Be´nard convection in an infinite horizontal layer with active control of the lower boundary heat flux. A simple linear proportional control loop uses a shadowgraph of the convection pattern to actively distribute the constant-mean lower boundary heat flux while the upper boundary is kept at a constant temperature. We find it possible to shift the convection threshold by a factor of approximately 3. This is a companion paper to our recent experimental work. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869428

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Two- and three-dimensional (2-D and 3-D) instabilities in the wake of a circular cylinder placed in an electrically conducting fluid and subjected to a constant magnetic field aligned with the freestream are investigated numerically. Increasing magnetic fields suppress 2-D instability (vortex shedding), whereas 3-D instabilities are influenced in a more complex way. In the presence of a magnetic field, 3-D instability has been detected below the 2-D stability threshold. This can lead to a reversal of the order of instabilities, i.e., 3-D instability appears at lowerRethan 2-D instability. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869429

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The existence of absolute instability in a liquid jet has been predicted for some time. The disturbance grows in time and propagates both upstream and downstream in an absolutely unstable liquid jet. The image of absolute instability is captured in the NASA 2.2 sec drop tower, and is reported here. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869422

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A quantity of interest,&bgr;(degree of intermittency between scale hierarchy), which appears in a recent intermittency model by She and Le´ve⁁que [Phys. Rev. Lett.72, 336 (1994)] is computed from turbulent simulations with Taylor-Reynolds number in the order of 100. It is found that the lateral and longitudinal&bgr;values are unequal. For most of the moment hierarchy orderp, lateral&bgr;deviates more from K-41 (i.e., smaller) than the longitudinal.&bgr;is also found to bepdependent. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869464

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

There is a wealth of experimental and computational results available for the motion of Newtonian fluids in the lid-driven cavity geometry, however little is known about the corresponding motion of viscoelastic fluids. We use laser Doppler velocimetry (LDV) and digital particle image velocimetry (DPIV) to probe the dynamics of viscoelastic fluid motion in the classic “lid-driven cavity” problem for a range of industrially important aspect ratios(0.25⩽&Lgr;⩽4)using an ideal elastic fluid as the test material. The magnitude of non-Newtonian effects in the cavity are characterized by the dimensionless Deborah number and the experiments span the range0⩽De⩽0.35. Elastic effects break the symmetry observed in the velocity field of cavity flows of viscous Newtonian fluids at zero Reynolds number. At low De, the flow remains two-dimensional but increasing the imposed velocity causes the center of the primary recirculating vortex in the cavity to shift progressively upstream. At larger Deborah numbers, the fluid motion becomes unstable and a three-dimensional flow develops. Upon cessation of the forcing boundary motion, a pronounced elastic recoil is observed which leads to a rapid reversal in the direction of the recirculating vortex. This transient motion subsequently decays through viscous dissipative effects on the elastic time scale of the fluid. The kinematics of the localized corner flow near the downstream corner are studied in detail and the distinguishing features of the viscoelastic corner flow with respect to the classic knife-edge problem of Taylor are reported. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869430

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The flow of inviscid fluid around a disc in a pipe is computed, and the results are used to determine the added mass of the accelerating disc in the frame in which the mixture velocity is zero. The added mass of an array of discs spaced at regular intervals along the pipe is then computed, and is related to the pressure gradient along the pipe. Some flow profiles are also presented. The results show that the added mass per particle increases as the pipe diameter is reduced relative to the particle size. The added mass per particle decreases as the number density of particles increases, but the added mass per unit length of the pipe nevertheless increases. Thus an increase of either the particle size or number density leads to a tighter coupling between the liquid and the particles; this result should hold for other particle shapes and configurations. Results are also presented for the drift, i.e., the displacement of fluid particles caused by the motion of an isolated disc along the axis of the pipe. If the diameter of the pipe is sufficiently small, the added mass of the disc is modified from that in unbounded fluid, and the background drift at the walls of the pipe can no longer be estimated from the added mass of the disc. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869431

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We numerically investigate the two-dimensional (2-D) convective flow developing in the liquid phase above an alloy growing in the upward Bridgman configuration of directional solidification. Using a time-dependent approach, we are able to describe the various cycles of hysteresis that connect the different branches of stable steady solutions. The main trends of the present results show that the bifurcation diagram, composed of the branches, found in previous works for the partition coefficientk=0.3,remains qualitatively valid fork=1.1:for a small frontal width the leading primary bifurcation is subcritical, while a transcritical bifurcation occurs for larger front. We bring the new complementary feature that the subcritical bifurcation becomes supercritical when the front width tends to zero. Furthermore, for an intermediate frontal width, we address the question of the nature of upper stability limits on various stable steady branches. We show that the limit occurs via either a steady secondary bifurcation or a Hopf bifurcation that initiates an unsteady solution branch which is followed up to chaos. The related route is a subharmonic cascade. When following this chaotic branch, a striking relaminarization process towards a steady secondary branch occurs. Finally we shortly investigate the case of a twice larger frontal width, for which several cycles of hysteresis are equally reported. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869432

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We investigate the spreading of passive tracers in closed basins. If the characteristic length scale of the Eulerian velocities is not very small compared with the size of the basin the usual diffusion coefficient does not give any relevant information about the mechanism of spreading. We introduce a finite size characteristic time&tgr;(&dgr;)which describes the diffusive process at scale&dgr;. When&dgr;is small compared with the typical length of the velocity field one has&tgr;(&dgr;)∼&lgr;−1, where&lgr;is the maximum Lyapunov exponent of the Lagrangian motion. At large&dgr;the behavior of&tgr;(&dgr;)depends on the details of the system, in particular the presence of boundaries, and in this limit we have found a universal behavior for a large class of system under rather general hypothesis. The method of working at fixed scale&dgr;makes more physical sense than the traditional way of looking at the relative diffusion at fixed delay times. This technique is displayed in a series of numerical experiments in simple flows. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869433

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Low Weber number deposition of small molten droplets on cold targets is of importance in certain dropwise buildup processes, but at this time, critical elements are absent from our theoretical understanding of the deposition process, and prediction from basic principles is not possible. This paper lays down a framework for understanding low Weber number deposition in terms of similarity laws and experimentation. Based on experiments from the highly viscous limit to the inertia-dominated limit, correlations are given for the spreading velocity, spreading time scales, post-spreading oscillation amplitudes, and oscillation damping time scales. Molten droplets are arrested, and their final solid shape determined, by contact line freezing. In homologous deposition, where the drop and the target are of the same material, the spreading factor is determined principally by the Stefan number, the dimensionless parameter which measures the temperature difference between the fusion point and the target temperature. Some concluding remarks are offered on what needs to be done to accurately compute such deposition processes. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869434

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The pinch-off of a drop of viscous fluid is observed using high-speed digital imaging. The behavior seen by previous authors is observed here; namely, the filament that attaches the drop to the orifice evolves into a primary thread attached to a much thinner, secondary thread by a slight bulge. Here, we observe that the lengths of the primary and secondary threads are reproducible among experiments to within 3&percent; and 10&percent;. The secondary thread becomes unstable as evidenced by wave-like disturbances. The actual pinch-off does not occur at the point of attachment between the secondary thread and the drop. Instead, it occurs between the disturbances on the secondary thread. After the initial pinch-off, additional breaks occur between the disturbances, resulting in several secondary satellite drops with a broad distribution of sizes. The pinch-off of the thread at the orifice is similar to that at the drop with one main difference: there is no distinct secondary thread. Instead, the primary thread necks down monotonically until wave-like disturbances form, resulting in pinch-off at multiple sites in between. The speed of the tips of the retreating, secondary threads after pinch-off are reported and discussed in the context of various scaling laws. ©1997 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869435

出版商:AIP    

年代:1997

数据来源: AIP