摘要:

In this paper we report the results of the first laboratory study on the relationship between the initial growth of the short wind waves and the simultaneous development of the wind-induced drift current. The phenomenon of the first visible ripples appearing “suddenly” on the water surface and forming V-shaped streaks aligned with the wind is explained. We show that the laminar–turbulent transition of the near surface water flow causes an explosive growth of the initial wind-generated ripples. The grown ripples become visible and thus mark the surface of the well-localized V-shaped turbulent zones forming the streaks. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869600

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We study the dynamics of a compound liquid drop which is comprised of an outer membrane surface, a shell layer, and a core. The deformation due to an imposed extensional flow and the subsequent recovery are investigated computationally employing a combined Eulerian–Lagrangian technique. The numerical method allows for large viscosity and capillarity differences between layers. The present study reports several findings which provide direct insight into developing a dynamic model for leukocytes. A compound drop behaves like a homogeneous, simple liquid drop if the core is sufficiently deformed and the time scale of the core, related to the combination of its viscosity and capillarity, is comparable to that of the shell layer. Disparate time scales between the core and shell layer result in a rapid initial recoil of the drop during which the shell fluid is the primary participant in the hydrodynamics, followed by a slower relaxation period during which the core and shell layer interact with each other. Consequently, the apparent viscosity of the drop depends not only on the rheological properties of the drop, but also on the flow dynamics surrounding it. The findings obtained with the three-layer compound drop model can explain several main characteristics of leukocytes reported in the literature. Furthermore, our study suggests that unless the presence and possible deformation of the nucleus are explicitly accounted for, neither Newtonian nor non-Newtonian models for leukocytes can adequately predict the hydrodynamics of leukocytes. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869601

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We consider the propagation of chemical fronts in a Hele-Shaw flow where the front is assumed to propagate with a curvature dependent velocity. The motivation is to model some recent experiments that employ aqueous autocatalytic chemical reactions in such a device. The density change across the front in such experiments is quite small so the Boussinesq approximation can be used, and the flow field generated is exclusively due to buoyancy effects. We derive a free boundary formulation based on Darcy’s law and potential theory, and describe the evolution in terms of the&thgr;−Lformulation, in which the tangent angle and the perimeter of the closed front are followed in time. Numerical solutions are obtained for this formulation with a rising and expanding bubble. As observed in the experiments, a fingering phenomenon which is different from the surface tension associated phenomenon appears in our calculations. The mechanisms that control the wavelength selection of the fingers, and a comparison with the result of a linear stability analysis for flat fronts are discussed. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869602

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

For the problem of one fluid displacing another on a solid surface, Dussan V. &etal; (1991) proposed a one-parameter analytical solution (the DRG solution) to describe the dynamic interface shape in the overlap region of the intermediate and the outer regions, for small capillary numbers. In the present study we examined the validity of the DRG solution with both experimental and numerical approaches. Our experiments consisted of displacing air with paraffin oil in parallel (Hele–Shaw) glass cells. The slope of the air–oil interface was measured at distances from the contact line, ranging between 5 and 200 &mgr;m. The displacement speeds corresponded to capillary numbers ranging between4.7×10−6and2.6×10−4.Excellent agreement was obtained among the DRG solution, the numerical, and the experimental results in the region>10 &mgr;mfrom the contact line, but systematic deviation was observed in the region close to the contact line. This deviation was confirmed by the numerical simulations that used the finite element method. The measured dynamic contact angle increases with the displacing speed and can be correlated with a power law in Ca, which is similar to Tanner’s law. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869603

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We show that the equation of quasigeostrophic (QG) potential vorticity conservation in geophysical fluid dynamics follows from Hamilton’s principle for stationary variations of an action for geodesic motion in thef-plane case or its prolongation in the&bgr;-plane case. This implies a new momentum equation and an associated Kelvin circulation theorem for QG motion. We treat the barotropic and two-layer baroclinic cases, as well as the continuously stratified case. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869623

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The stationary, periodic solution to the problem of oscillating flow of a conducting fluid, in a duct closed at one end and with periodical mass flow rate at the other end, is known to exhibit a singularity in the mean temperature at the closed end when transverse conduction in the fluid and the duct wall is taken into account, but longitudinal conduction is neglected. For instance, a solution of that type was originally suggested by Gifford and Longsworth as a prototype for pulse-tube refrigeration. Whether these stationary singular solutions are physical or mere theoretical curiosities depend upon the existence of a scenario leading to such a limit cycle. To address that question, a transient theory is formulated, using the narrow duct approximation. The results show that at least for constant fluid thermal conductivity, all singular profiles generated by the mechanism under study are linearly stable. For round tubes, the temperature profile is shown to evolve, from an arbitrary initial value, toward an equilibrium profile which results in balanced energy fluxes. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869604

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We study the linear stability of a fluid to double-diffusive convection in the case for which the background temperature field is linear, but the mean salt distribution is a nonlinear function. A variety of idealized limits amenable to perturbative treatment are examined, and a detailed numerical study of a hyperbolic tangent profile is presented. It is shown that nonlinearity in the background salinity profile leads to pronounced competition and interaction between the linear normal modes. ©1998 American Institute of Physics. 

ISSN:1070-6631    

DOI:10.1063/1.869605

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

We examine non-linear spiral flow in the Taylor–Couette problem for a wide gap with axially periodic conditions. We present a highly efficient computational method adapted to this problem, based on continuation methods applied to a pseudospectral discretization of the Navier–Stokes equations in a rotating frame of reference. The spiral flow is computed in a wide range of parameters, and different features are explored in detail: domain of existence of the flow, behavior for high Reynolds number, appearance of axial flows, dependency on parameters, and stability against helical disturbances. A first integral is obtained and used to describe the particle trajectories in the fluid. This description shows that the axial and radial motion of the particles is mainly confined within an internal boundary layer. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869606

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The aim of the present paper is to study three-dimensional elliptic instability in two-dimensional flattened Taylor–Green vortices, which constitutes a model problem for the topics of wake vortex dynamics. Shortwave asymptotics and classical linear stability theory are developed. Both approaches show that the flow is unstable. In particular, the structure of the most amplified growing mode is the same as that obtained in unbounded elliptical flows. The limits of the linear regime and the effects of the nonlinear interactions are characterized by means of a spectral Direct Numerical Simulation (DNS). ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869607

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The onset of convection in double diffusion with equal and opposite thermal and solutal buoyancy forces is studied. Numerical linear stability analyses and integration of the full Boussinesq equations are performed in an infinite vertical fluid layer and in closed rectangular cavities bounded by rigid walls. Detailed study of the subsequent nonlinear evolution is carried out forLe=1.2andPr=1.In the infinite vertical layer, the onset of convection is found to correspond to a subcritical circle pitchfork bifurcation. The finite-amplitude branch of steady states in turn loses stability to traveling waves via a supercritical drift pitchfork bifurcation. In a square cavity the bifurcation is transcritical and the full branch of stable and unstable solutions is constructed. With increasing cavity aspect-ratio, we observe alternating transcritical and pitchfork bifurcations, depending on the symmetry of the most unstable eigenvector. ©1998 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.869608

出版商:AIP    

年代:1998

数据来源: AIP