摘要:

Reported herein is a cone‐shaped turbulent vortex breakdown innoncavitatingswirling flows at high Reynolds numbers in a slightly diverging cylindrical tube. The turbulent conical form is in addition to the well‐known double‐helix, spiral, and nearly axisymmetric or ‘‘bubble’’‐type breakdowns. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868742

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Experiments were conducted to investigate how heating only the nozzle exit boundary layer of an axisymmetric jet, with an unheated potential core, affects disturbance growth in the initial shear layer. The exit boundary layer had a minimum density ratio of 0.74, was laminar, and had a constant momentum thickness (&thgr;) for all levels of nozzle heating used in this study. The fluctuating velocity (u′) and temperature (t′) in the exit boundary layer increased monotonically with increasing nozzle temperature. Low‐amplitude acoustic excitation produced a more rapid growth of coherent velocity fluctuations for the heated case than for the unheated. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868743

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this Letter, we demonstrate the coexistence of two distinct systems of streamwise vortices in a bluff body wake. It appears that there exist conditions to amplify streamwise vorticity in bluff body wakes, by vortex stretching, in both the separating shear layers from the sides of the body and also in the vortex street wake. The length scale governing the streamwise vortices in the shear layer has a 1/&sqrt;Re dependence, whereas the scale of such structures in the wake is independent of Reynolds number, Re (over a large range of Re). The proposition that there should exist two distinct, and possibly disparate, spanwise length scales in the cylinder wake is well supported by compiled measurements, particularly those of Williams and co‐workers (Mansyetal. [J. Fluid Mech.270, 277 (1994)]), as well as those from Chyu and Rockwell (submitted to J. Fluid Mech.). ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868744

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Collision rates of two nondeformable, freely suspended drops (or particles) subject to Brownian motion in a simple shear at low Reynolds number are calculated from the solution of the full Fokker–Plank equation for the pair distribution function. Unlike previous studies on shear‐induced collisions, the solution is presented for arbitrary Pe´clet number (Pe), thus covering a broad range of drop sizes. An efficient numerical technique includes a mixed Galerkin/finite‐difference approximation and the ideas of analytical continuation, to represent the solution of the discrete problem as a convergent series for all real Pe. The mobility functions are provided from exact two‐drop hydrodynamics and near‐contact asymptotics. Extensive calculations are presented for the collision efficiency as a function of the size ratio, drop‐to‐medium viscosity ratio (&mgr;ˆ), and Pe≤O(102), for the case of no interdroplet forces. For &mgr;ˆ≳0, the correction to the collision efficiency for Pe≫1 isO(Pe−1/2). For bubbles (&mgr;ˆ=0), there is also anO(Pe−2/3) correction of opposite sign, resulting in a local minimum for the collision efficiency. The asymptotic analysis for the opposite limit of Pe≪1 is in excellent agreement with the numerical calculations. For intermediate Pe, the exact numerical solution is compared with different ‘‘additive approximations.’’ The simple two‐term additivity approximation is generally unsuccessful, whereas a modified, three‐term approximation provides reasonable results except at small size ratios and large viscosity ratios. The effect of the van der Waals attractions on the collision efficiency for typical emulsion drops of 1–10 micron size with &mgr;ˆ=O(1) is relatively small, of the order 10% in the Brownian regime. As a limiting case of drops, the collision efficiency for equal‐sized solid spheres with van der Waals attractions is calculated for Pe≤200; this limit shows a stronger dependence on the Hamaker constant and the retardation parameter. The solution for solid spheres is in excellent agreement with reported experimental data on flocculation dynamics for suspensions with moderate Pe´clet numbers. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868745

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The two‐dimensional, free‐surface flow of a Newtonian fluid exiting from a planar die is computed by finite element analysis using quasiorthogonal mesh generation and local mesh refinement with irregular, embedded elements to obtain extreme resolution of the velocity and pressure fields near the die edge, where the fluid sheet attaches to the solid boundary. Calculations for the limit of large surface tension, the stick‐slip problem, reproduce the singular behavior near the die edge expected from asymptotic analysis using a self‐similar form for the velocity field. Results for finite capillary number (Ca) predict that the meniscus separates from the die at a finite contact angle and suggest that the capillary force enters the dominant normal stress balance at the die edge through an infinite curvature, as previously suggested by Schultz and Gervasio. The size of this region with large positive curvature increases with increasing Ca, and the strength of the singularity is in good agreement with theoretical predictions for a straight meniscus attached to the die at the appropriate contact angle predicted by the simulations. The contact angle appears to be determined from matching of the inner solution structure valid near the singularity with the bulk flow, in agreement with arguments made by Ramalingam; increasing the Reynolds number decreases the contact angle, corroborating this effect. Introducing fluid slip along the surface of the die changes the structure of the singularity in the pressure and stresses, but does not alleviate the singular behavior. In fact, the calculations with slip coefficients small enough not to change the bulk solution are more difficult than calculations with the no‐slip boundary condition. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868746

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A linear stability analysis is performed for two‐ and three‐dimensional steady source and sink flows. Cases studied include inviscid compressible and incompressible fluids. For two‐dimensional flows viscous incompressible fluid is also examined. The one‐dimensional nature of the unperturbed base flow suggested taking the vorticity as a perturbation in order to reduce the number of variables and to simplify the analysis. It is shown that source flows are always unstable. Sink flows are found unstable for inviscid compressible fluid and also for two‐dimensional flow of viscous incompressible fluid for low Reynolds numbers. The different modes of instability existing in perturbed flow are obtained. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868747

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Theoretical studies have shown that compliant walls are able to attenuate the Tollmien–Schlichting waves that lead to conventional two‐dimensional boundary‐layer transition. This phenomenon was demonstrated in towing‐tank tests conducted by Gasteretal. The results of these experiments also featured a different and very dramatic form of boundary‐layer breakdown. We contend that this type of breakdown was due to a hydroelastic mode of instability, namely traveling‐wave flutter. In this paper we model the two‐layer viscoelastic compliant wall of Gasteretal. and its interaction with the boundary‐layer flow using the asymptotic theory of Carpenter and Gajjar;en‐type calculations are carried out for the traveling‐wave flutter. Excellent agreement is found between the stability characteristics of the TWF mode and the measurements of the new form of breakdown found in the experiments; thus a complete understanding of the physical features found in the experiments is now available. Such understanding is essential for progress to be made in the technological development of compliant panels for transition delay. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868748

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We present the results of an experimental study of dynamical phenomena in the fluid flow through a symmetric nominally two‐dimensional channel expansion. The particular phenomena of interest are observed as a result of modulating the flow rate through the system. In the absence of modulation, the flow downstream of the expansion is found to be steady at low Reynolds numbers. It then loses stability to a high‐dimensional dynamical state for Reynolds numbers above a critical value. However, when a small periodic modulation is added to the flow rate new low‐dimensional dynamical phenomena emerge. In some parameter ranges this novel temporal behavior arises at Reynolds numbers below those at which the transition to irregular fluid flow occurs in the unforced system. In other regions of parameter space the low‐dimensional dynamics suppress the irregular flow of the unforced system. Moreover, the dynamics are organized by the underlying solution structure and show evidence for a subharmonic resonance, quasiperiodicity, and homoclinicity. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868749

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A three‐dimensional theory of vorticity dynamics on an incompressible viscous and immiscible fluid–fluid interface, orinterfacialvorticitydynamicsfor short, is presented as a counterpart of the vorticity dynamics on an arbitrarily curved rigid wall [J. Fluid Mech.254, 183 (1993)]. General formulas with arbitrary Reynolds numbers Re are derived for determining (1) how much vorticityexistson an interfaceS, (2) how much vorticityiscreatedfromSand sent into the fluid per unit area in per unit time, and (3) the force and moment acted on a closed interface by the created vorticity thereon. The common feature and fundamental difference between interfacial vorticity dynamics and its rigid‐wall counterpart are analyzed. In particular, on a free surface, the primary driving mechanism of vorticity creation is the balance between the shear stress (measured by tangent vorticity) and the tangent components of the surface‐deformation stress alone, which results in a weak creation rate ofO(Re−1/2) at large Re. Therefore, the exact form of the theory with its full complexity is of importance mainly at low Reynolds numbers, especially in understanding the small‐scale coherent structures of interfacial turbulence. The vorticity creation rate at high‐Re approximations, including an interfacial boundary layer of finite thickness and the limit of Re→∞ (the so‐called Euler limit), is also studied, both allowing for a rotational inviscid outer flow. While for the former this leads to a generalization of Lundgren’s theory [inMathematicAspectsofVortexDynamics, edited by R. E. Caflish (SIAM, Philadelphia, PA, 1989), pp. 68–79] and amounts to solving a linear boundary‐layer problem, for the latter the creation rate can be directly obtained from an inviscid solution, leading to a dynamic evolution equation of interfacial vortex sheet. In three dimensions, a vortex sheet may bifurcate into a normal vorticity field, upon which the dependence of the sheet velocity is determined. A few examples are examined to illustrate different aspects and approximation levels of the general theory. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868750

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A numerical coupled model of air–sea–wave interaction is developed to study the influence of ocean wind waves on dynamical, turbulent structures of the air–sea system and their impact on coupled modeling. The model equations for both atmospheric and oceanic boundary layers include equations for: (1) momentum, (2) ak‐&Vegr; turbulence scheme, and (3) stratification in the atmospheric and oceanic boundary layers. The model equations are written in the same form for both the atmosphere and ocean. In this model, wind waves are considered as another source of turbulent energy in the upper layer of the ocean besides turbulent energy from shear production. The dissipation &Vegr; at the ocean surface is written as a linear combination of terms representing dissipation from mean flow and breaking waves. The &Vegr; from breaking waves is estimated by using similarity theory and observed data. It is written in terms of wave parameters such as wave phase speed, height, and length, which are then expressed in terms of friction velocity. Numerical experiments are designed for various geostrophic winds, wave heights, and wave ages, to study the influence of waves on the air–sea system. The numerical simulations show that the vertical profiles of &Vegr; in the atmospheric and oceanic boundary layers (AOBL) are similar. The magnitudes of &Vegr; in the oceanic surface zone are much larger than those in the atmospheric surface zone and in the interior of the oceanic boundary layer (OBL). The model predicts &Vegr; distributions with a surface zone of large dissipation which was not expected from similarity scaling based on observed wind stress and surface buoyancy. The simulations also show that waves have a strong influence on eddy viscosity coefficients (EVC) and momentum fluxes, and have a dominated effect on the component of fluxes in the direction of the wind. The depth of large changes in flux magnitudes and EVC in the ocean can reach to 10–20 m. The simulations of surface drift currents confirm that the currents are overestimated if the surface waves are not considered. ©1995 American Institute of Physics.

ISSN:1070-6631    

DOI:10.1063/1.868751

出版商:AIP    

年代:1995

数据来源: AIP