摘要:

The behavior of two‐dimensional periodic disturbances introduced artificially into turbulent shear flows is examined in a wind tunnel using flow visualization methods. It is found that when the wavelength of the disturbances is about 20 &dgr;* in the case of the boundary layer on a flat plate and about 2.4 &dgr;* in the case of the wake behind a circular cylinder, the disturbances are strongly amplified in the downstream direction, where &dgr;* is the displacement thickness of the shear layer. The present experiments suggest that the large‐scale coherent motions in natural turbulent shear flows originate from the instability of the mean velocity distributions.

ISSN:0031-9171    

DOI:10.1063/1.864046

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

One of the interesting aspects of transitional and turbulent boundary layers is the development of counter‐rotating streamwise vortices near the wall. The most regular pattern is found in a boundary layer on a concave wall where the generation mechanism is known to be the Go¨rtler instability. The origin of these vortices in other translational and turbulent boundary layers is presently unknown. Since the counter‐rotating vortices are located in a region of strong shear, low‐speed fluid is pumped away from the wall which coalesces into regions of low momentum lying between the vortices. As this pumping action continues, localized inflectional velocity profiles become apparent in the transitional and turbulent boundary layers. The oscillations which develop upon these profiles scale with the local thickness and velocity difference in the same manner as the two‐dimensional steady free shear layer stability problems. The oscillations grow to large amplitude and break down into new turbulence in both the transitional and turbulent boundary layers.

ISSN:0031-9171    

DOI:10.1063/1.864047

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The nature and significance of large‐scale coherent structures in turblent shear flows are addressed. A definition for the coherent structure is proposed and its implications discussed. The characteristic coherent structure properties are identified and the analytical and experimental constraints in the eduction of coherent structures are examined. Following a few comments on coherent motions in wall layers, the accumulated knowledge from a number of recent and ongoing coherent structure investigations in excited and unexcited free shear flows in the author’s laboratory is reviewed. Also briefly addressed are effects of initial conditions, the role of coherent structures in jet noise production and broadband noise amplification, the feedback effect of coherent structures, the use of the Taylor hypothesis in coherent structure description, negative production, turbulence suppression via excitation, validity of the Reynolds number similarity hypothesis, etc. From the detailed quantitative results, a picture of the state of the art in coherent structure studies emerges. While coherent structures are highly interesting characteristic features of (perhaps all) turbulent shear flows, it is argued that their dynamical significance has been overemphasized. These are predominant only in their early stages of formation following instability, or in resonant situations and excited flows, or in regions adjacent to a wall of a turbulent boundary layer. The coherent Reynolds stress, vorticity, and production are comparable to (and not an order of magnitude larger than) the time‐average Reynolds stress, vorticity, and production, respectively, in fully developed states of turbulent shear flows, where incoherent turbulence is also important and cannot be ignored. The concept and importance of coherent structures are here to stay; understanding and modeling of turbulent shear flows will be incomplete without them; but they are not all that matter in turbulent shear flows.

ISSN:0031-9171    

DOI:10.1063/1.864048

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Following a scientific cooperation beginning in 1954 with L. S. G. Kovasznay, and in his memory, in this paper is presented a survey of two of the main areas of research to which he contributed: (1) turbulence properties deduced through statistical conditional sampling and space‐time measurements of correlations and contingencies; these include celerities, optimum correlations, memories, and coherent structures; and (2) the study of turbulence in supersonic flows initiated by Kovasznay; this include the use of mass‐weighted averages, decrease of turbulence in expansion, increase in compression, strong Reynolds analogy extension, and temperature–velocity correlation conservation.

ISSN:0031-9171    

DOI:10.1063/1.864049

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

The Brinkman equation purports to describe low‐Reynolds‐number flow in porous media in situations where velocity gradients are non‐negligible. The equation involves modifying the usual Darcy law by the addition of a standard viscosity term whose coefficient is usually identified with the pure‐fluid viscosity. It is argued instead that the porous medium induces a renormalization of viscosity, which is calculated in the dilute limit and separately in a self‐consistent approximation. The effective Brinkman viscosity is found todecreasefrom the pore‐fluid value. The calculation fails at low porosity but agrees at least in part with experiment. In addition, the relationship between the Brinkman equation and the phenomenological boundary condition of Beavers and Joseph is discussed and it is pointed out that their experimental configuration provides a simple means of measuring viscosity renormalization.

ISSN:0031-9171    

DOI:10.1063/1.864050

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

A theory is presented for the movement of a vesicle in response to a gradient in concentration of an impermeable molecular solute. The vesicle is a fluid sphere bounded by a rigid semipermeable membrane which prevents solute from crossing it. The external solute gradient acts as an osmotic driving force for solvent exchange across the membrane; thus, the membrane converts the solute gradient into mechanical stresses which propel the vesicle toward regions of lower solute concentration. The speed of the vesicle is essentially independent of the fluid viscosity, only weakly dependent on the vesicle size, and directly proportional to the solute concentration gradient in the external fluid.

ISSN:0031-9171    

DOI:10.1063/1.864051

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

An axisymmetric liquid bridge is subjected to an axial temperature difference. Thermal variations in surface tension drive a motion that is described using lubrication theory for slender bridges. At leading order in aspect ratio, a class of similarity solutions is obtained valid in the core region away from the endwalls of the bridge. These solutions describe the flows, temperature fields, and interfacial shapes of bridges that may sustain substantial interfacial deflections.

ISSN:0031-9171    

DOI:10.1063/1.864052

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

In this work a momentum source model is provided to describe the flow pattern of an incompressible fluid with a magnetic field. A restricted class of analytical solutions for a one‐sided jet, a two‐sided symmetric jet, and a two‐sided asymmetric jet are obtained. Four kinds of analytical solutions for a two‐sided jet are classified by the jet cone angle which depends on two parameters related to the Alfve´nic Mach number and the momentum flux. The cone angle and the boundary between the two sides of a two‐sided asymmetric jet are determined by the ratio of the momentum flux parameters on each side. From this model the mass flux, momentum flux, and energy flux entrained by the jet are calculated and the differences between the above properties on each side of a two‐sided asymmetric jet are obtained as well. The rotation of the jet and the restrictions of incompressibility are discussed.

ISSN:0031-9171    

DOI:10.1063/1.864053

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

High Prandtl number convection possesses a square flow pattern that is steady and is apparently stable to infinitesimal disturbances. This pattern is unstable to finite‐amplitude disturbances, however, because a more chaotic (in time and space) spoke pattern of convection eats its way into the squares from the lateral boundaries. Experiments are described in which the breakup of the squares is initiated by dislocating one square in the middle of the apparatus with the use of a small, heated resistor. Once a critical heating rate and time is exceeded, the dislocation initiates a spoke cell which then systematically destroys neighboring square cells, resulting in the more chaotic spoke pattern. If the critical rate is not exceeded, the cell becomes severely deformed during the heating, but will relax back to a square convection cell after heating has ceased.

ISSN:0031-9171    

DOI:10.1063/1.864054

出版商:AIP    

年代:1983

数据来源: AIP

摘要:

Internally generated vertical vorticity enters the lowest‐order amplitude equations for free‐slip boundaries in an essential way when the Prandtl numberPis finite and the flow three dimensional. For parallel rolls the band of stable wavenumbers is substantially modified from what was previously believed to be correct. In particular there are no stable states forP<0.301. Numerical simulations for free boundaries and largerPsuggest a mechanism through which the box size determines the critical Rayleigh number for noisy time dependent convection. A stability analysis of model amplitude equations for rigid boundaries agrees qualitatively with the numerical results of Cleaver or Busse forP≲O(1). There is now considerable continuity between the stability diagrams for rigid and free boundaries.

ISSN:0031-9171    

DOI:10.1063/1.864055

出版商:AIP    

年代:1983

数据来源: AIP