摘要:

Using direct numerical simulations of turbulent channel flow, we present a new method for skin friction reduction, enabling large-scale flow forcing without requiring instantaneous flow information. As proof-of-principle,x-independent forcing, with azwavelength of 400 wall units and an amplitude of only 6&percent; of the centerline velocity, produces a significant sustained drag reduction: 20&percent; for imposed counterrotating streamwise vortices and 50&percent; for colliding,z-directed wall jets. The drag reduction results from weakened longitudinal vortices near the wall, due to forcing-induced suppression of an underlying streak instability mechanism. In particular, the forcing significantly weakens the wall-normal vorticity&ohgr;yflanking lifted low-speed streaks, thereby arresting the streaks’ sinuous instability which directly generates new streamwise vortices in uncontrolled flows. These results suggest promising new drag reduction techniques, e.g., passive vortex generators or colliding spanwise jets fromx-aligned slots, involving durable actuators and no wall sensors or control logic. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869789

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The Rayleigh capillary instability of a cylindrical interface between two immiscible fluids is one of the most fundamental in fluid dynamics. As Plateau observed from energetic considerations and Rayleigh clarified through hydrodynamics, such an interface is linearly unstable to fission due to surface tension. In traditional descriptions of this instability it occurs everywhere along the cylinder at once, triggered by infinitesimal perturbations. Here we explore in detail a recently conjectured alternate scenario for this instability: front propagation. Using boundary integral techniques for Stokes flow, we provide numerical evidence that theviscousRayleigh instability can indeed spread behind a front moving at constant velocity, in some cases leading to a periodic sequence of pinching events. These basic results are in quantitative agreement with the marginal stability criterion, yet there are important qualitative differences associated with the discontinuous nature of droplet fission. A number of experiments immediately suggest themselves in light of these results. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869650

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Experimental observations of a purely elastic flow instability occurring in the lid-driven cavity flow of two semi-dilute polymer solutions are reported and the effect of cavity aspect ratio on the dynamical structure of the unstable flow is quantitatively investigated. The spatial and temporal characteristics of the secondary flow are measured using flow visualization, laser Doppler velocimetry, and digital particle image velocimetry. At the onset conditions the disturbances appear in the form of spatially periodic flow cells which propagate along the neutral direction of the cavity. The secondary flow structure is analogous to the Taylor–Go¨rtler vortices observed in inertially driven hydrodynamic instabilities. The critical onset conditions for two elastic test fluids and five different aspect ratios correlate with a recently proposed dimensionless stability criterion which incorporates measures of the local streamline radius of curvature and the non-Newtonian normal stresses in the flow domain. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869631

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Nonlinear oscillations of viscous liquid drops with and without initial internal circulation are investigated. The full Navier–Stokes equations are solved for a liquid drop surrounded by a dynamically inactive ambient gas. The Galerkin/finite element technique along with the spine-flux method for the advection of the free boundaries are used. Internal circulations are generated by imposing a constant velocity at the surface of the drop and obtaining the steady state velocity field for a fixed drop shape. Oscillations of drops subject to small to large amplitude, and for the second-, third-, fourth-, and fifth-mode disturbances are considered. New data on the period and the decay factor of the oscillations are reported. The internal circulation in a drop released from an even-mode shape results in the transfer of energy mainly between the even modes. The internal circulation in a drop released from an odd-mode shape results in the transfer of energy between both odd and even modes. In general, the internal circulations generated by a constant surface velocity tend to transfer energy from any mode of drop oscillations to the second mode. This tendency increases as the strength of the internal circulation increases. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869632

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A systematic series of experiments on the instantaneous release of two-dimensional, heavy, particle-driven gravity currents has been conducted. High-concentration currents propagated in a qualitatively different way than low concentration currents. In particular, beyond a critical initial volume fraction of particles, the resulting dense current came to an abrupt halt at some point down the channel, depositing the bulk of its initial sediment load as a relativly thick layer of fairly constant thickness, characterized by a pronounced, steep snout. A very much thinner layer of sediment extended for some distance beyond the arrest point. This layer was deposited from the subsequent propagation of a slower moving, low concentration residual cloud. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869633

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We consider the 2:1 internal resonances (such that&OHgr;1>0and&OHgr;2≃2&OHgr;1are natural frequencies) that appear in a nearly inviscid, axisymmetric capillary bridge when the slenderness &Lgr; is such that0<&Lgr;<&pgr;(to avoid the Rayleigh instability) and only the first eight capillary modes are considered. A normal form is derived that gives the slow evolution (in the viscous time scale) of the complex amplitudes of the eigenmodes associated with&OHgr;1and&OHgr;2,and consists of two complex ODEs that are balances of terms accounting for inertia, damping, detuning from resonance, quadratic nonlinearity, and forcing. In order to obtain quantitatively good results, a two-term approximation is used for the damping rate. The coefficients of quadratic terms are seen to be nonzero if and only if the eigenmode associated with&OHgr;2is even. In that case the quadratic normal form possesses steady states (which correspond to mono- or bichromatic oscillations of the liquid bridge) and more complex periodic or chaotic attractors (corresponding to periodically or chaotically modulated oscillations). For illustration, several bifurcation diagrams are analyzed in some detail for an internal resonance that appears at&Lgr;≃2.23and involves the fifth and eighth eigenmodes. If, instead, the eigenmode associated with&OHgr;2is odd, and only one of the eigenmodes associated with&OHgr;1and&OHgr;2is directly excited, then quadratic terms are absent in the normal form and the associated dynamics is seen to be fairly simple. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869634

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Numerical stability analysis and general flow characterization were performed for the flow induced by transverse rotating magnetic fields in cylindrical containers. Starting from Richardson’s theoretical critical value for the generation of Taylor vortices in infinite cylinders, overstability in finite cylinders with different aspect ratios was investigated. Decreasing aspect ratio leads to a more stable configuration. For small cylinders and reasonable frequencies of the magnetic field, instabilities arise at low magnetic inductions, e.g.,Bc≈0.5 mT[gallium as model liquid, with radius of the cylinderR=12.5 mm,aspect ratio(h/d)=2and frequencyf=50 Hz]. As a source of instabilities Taylor vortices have been identified. They are transported by the secondary flow, emerging in finite cylinders due to the imbalance of pressure gradient and centrifugal force close to the top and bottom, thus generating time-dependent flow. Their generation is statistical and manifests itself in, e.g., nonperiodic temperature oscillations with small amplitudes in a classical Be´nard configuration. Furthermore the intensity of the secondary flow was investigated for the isothermal case to give hints on its general influence on the overall mass transport. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869635

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The dynamics of inviscid, steady, two dimensional flows is examined for the case of a hyperbolic sine functional relation between the vorticity and the stream function. The 2-soliton solution of the sinh-Poisson equation with complex wavenumbers will reproduce the Mallier-Maslowe pattern, a row of counter-rotating vortices. A special 4-soliton solution is derived and the corresponding flow configuration is studied. By choosing special wavenumbers complex flows bounded by two rigid walls can result. A conjecture regarding the number of recirculation regions and the wavenumber of the soliton expansion is offered. The validity of the new solution is verified independently by direct differentiation with a computer algebra software. The circulation and the vorticity of these novel flow patterns are finite and are expressed in terms of well defined integrals. The questions of the linear stability and the nonlinear evolution of a finite amplitude disturbance of these steady vortices are left for future studies. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869636

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The effect of swirl on jets and wakes is investigated by analyzing the inviscid spatiotemporal instability of the Rankine vortex with superimposed plug flow axial velocity profile. The linear dispersion relation is derived analytically as a function of two nondimensional control parameters: the swirl ratioSand the external axial flow parametera(a>−0.5for jets,a<−0.5for wakes). For each azimuthal wave numberm,there exists a single unstable Kelvin–Helmholtz mode and an infinite number of neutrally stable inertial waveguide modes. Swirl decreases the temporal growth rate of theaxisymmetricKelvin–Helmholtz mode(m=0),which nonetheless remains unstable for all axial wave numbers. Forhelicalmodes(m≠0),small amounts of swirl lead to the widespread occurrence of direct resonances between the unstable Kelvin–Helmholtz mode and the inertial waveguide modes. Such interactions generate, in the low wave number range, neutrally stable wave number bands separated by bubbles of instability. AsSincreases above a critical value, all bubbles merge to give rise to unstable wave numbers throughout, but the growth rate envelope decreases continuously with increasing swirl. In the high wave number range, negative helical mode growth rates are enhanced for small swirl and decrease continuously for large swirl, while positive helical mode growth rates monotonically decrease with increasing swirl. For a given positive swirl, negative modes are more unstable than their positive counterparts, although their growth rates may not necessarily be larger than in the nonrotating case. The absolute/convective nature of the instability in swirling jets and wakes is determined in ana−Scontrol parameter plane by numerical implementation of the Briggs–Bers criterion. In the absence of swirl,jets(a>−0.5)become absolutely unstable (AI) to the axisymmetric modem=0only for a sufficiently large external axial counterflowa<−0.15.AI is found to be significantly enhanced by swirl: forS>1.61,AI occurs, even for coflowing jets(a>0).AsSis gradually increased, the transitional mode to AI successively becomesm=0,−1,−2,−3,etc. In the absence of swirl,wakes(a<−0.5)become absolutely unstable to the bending modesm=±1only for sufficiently large counterflow1+a>0.091.ForS>0.47,AI occurs even for coflowing wakes(a<−1)and, asSincreases, the transitional mode to AI successively becomesm=−1,−2,−3,etc. This instability analysis is found to provide a preliminary estimate of the critical Rossby number for the onset of vortex breakdown: for zero external axial flow jets(a=0),absolute/convective transition first takes place at a Rossby numberRo≡S−1∼0.62,which very favorably compares with available experimental and numerical threshold values for vortex breakdown onset. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869637

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Experimental data on differential diffusion between two species with large and quite disparate Schmidt numbers were obtained in a turbulent water jet by optically measuring the two species concentrations simultaneously. Experimental conditions were chosen so that the species were dilute and did not affect the water density thereby avoiding inertial effects. Differential diffusion was found to be significant in magnitude, even in the absence of these effects. Schmidt number ratios of 4 and 18 were considered. Differential diffusion was found to be statistically significant and to manifest at scales larger than the computed Batchelor scale. In some instances the concentration signal for the species with larger diffusivity was simply a blurred version of the other, while in other instances structures present in one signal were completely absent from the other. This second observation, presumably a more complex effect due to diffusion across velocity gradients, is discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869638

出版商:AIP    

年代:1998

数据来源: AIP