ISSN:0031-9171    

DOI:10.1063/1.861757

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A survey of recent measurements of turbulent structure near the wall which are applicable to the phenomenon of drag reduction by polymer additives is presented. Knowledge of the vorticity in the wall region provides a framework for understanding turbulent structure and the effects of polymer additives. New measurements using a small, calibrated streamwise vorticity probe are described. Near the wall where the turbulent production and dissipation are a maximum the streamwise vorticity is a maximum and is highly intermittent. New measurements of the Reynolds stress have also been made with a very small, computer calibrated hot‐wire array. Intermittent small scale structures producing rapid oscillations of the hot‐wire signals are detected near the wall which are too small to be resolved by the array. Their spatial scale is much less than the Kolmogoroff length. It is suggested that they are associated with streamwise vorticity.

ISSN:0031-9171    

DOI:10.1063/1.861765

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

In strong flows, such as turbulence, it is suggested that randomly coiled macromolecules might become nearly fully extended. To investigate this extended state, four models are considered which show the importance of the inextensibility of the polymer chain and the variation of the friction coefficient with the extension. The rheological consequences of these processes are then explored in an appropriately modified dumb‐bell model. A dilute polymer solution is predicted to have a high extensional viscosity but a low shear viscosity, a stress hysteresis, and a slow stress relaxation. The action of the novel rheology is shown in two simple flows. Finally, speculations are made on the possible way dilute polymer solutions can modify turbulence.

ISSN:0031-9171    

DOI:10.1063/1.861735

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A critical analysis of available compliant wall data which indicated drag reduction under turbulent boundary layers is presented. Detailed structural dynamic calculations suggest that the surfaces responded in a resonant, rather than a compliant, manner. Alternate explanations are given for drag reductions observed in two classes of experiments: (1) flexible pipe flows and (2) water−backed membranes in air. Analysis indicates that the wall motion for the remaining data is typified by short wavelengths in agreement with the requirements of a possible compliant wall drag reduction mechanism recently suggested by Langley.

ISSN:0031-9171    

DOI:10.1063/1.861756

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A series of experiments is described using solutions of various concentrations of Polyox coagulant flowing through porous beds of spherical particles. Results are reported, similar to those obtained by James and McLaren, which show a substantial increase in drag. The onset of this drag increase from solvent flow seems to occur at a constant Deborah number multiplied by the square root of the concentration. The flow in a bundle of symmetrical convergent‐divergent channels which may serve as a model for a porous medium is analyzed. The presence of elongated particles in the fluid will alter the flow pattern in such a channel from that given by the Stokes’ solution. Assuming the particles to be aligned parallel to the mean flow direction, this flow re‐arrangement will attenuate the effect of the particles on the dissipation of mechanical energy. Extended molecules may be misaligned with the streamlines in the diverging section of the channel, which could explain the large increase in drag observed with dilute polymer solutions in porous beds.

ISSN:0031-9171    

DOI:10.1063/1.861758

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The dynamics of the fluctuating velocity field in a turbulent boundary layer is discussed on the basis of a simplified analysis of localized unsteady perturbations in a parallel shear flow. Three classes of such disturbances may be distinguished, namely: (i) large‐scale propagating disturbances which may be represented by a superposition of shear waves; (ii) convected disturbances which decay only slowly through the action of viscosity; and (iii) small‐scale disturbances resulting from the secondary instability of the large‐scale motion. Coupling between large and small scales is incorporated in a two‐scale model in which the large scales are considered driven by the Reynolds stresses produced by the small scales. Shearing by the mean flow is shown to cause intensification of internal shear layers in the flow and could lead to local inflection in the instantaneous velocity profile, thus making secondary instability possible. Drag reducing additives, as modeled by some simple constitutive relations are found to be able to inhibit secondary instability, and would hence lower turbulent stress production according to the model employed.

ISSN:0031-9171    

DOI:10.1063/1.861759

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The basic dynamics of turbulent boundary layers of several media is described qualitatively: suspensions of several types and polymer solutions. Despite the considerable differences in these media, it is argued that a number of the flows are affected only in the buffer layer, and drag reduction can result if behavior in the sublayer and buffer layer differ. In polymer solutions, it is argued that molecular expansion is responsible for the difference, and experimental evidence of this expansion is presented, and compared with calculations.

ISSN:0031-9171    

DOI:10.1063/1.861760

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Based on an assumption that drag reduction occurs due to the interaction of an elongational type of secondary flow with an elongational viscosity of a polymer solution, the effect of a polymer additive on Taylor–Go¨rtler vortices was calculated. The results indicate that the Go¨rtler parameter for the neutral stability always increases by increasing elongational viscosity and that the critical wavenumber decreases.

ISSN:0031-9171    

DOI:10.1063/1.861761

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Available drag reduction correlations are presented and their utility for engineering scale‐up to a scale‐up from a correlation based on friction velocity and the availability of some turbulent flow data with the process fluid of interest is compared. A further purpose is to investigate the consequences of polymer modification of the mean flow profile in the outer layer. Emphasis is placed on accuracy and ability to portray the drag reducing activity of complex systems. In the case of very dilute polymeric systems in large conduits, very accurate predictions may be needed to assess the performance of a particular additive. Similarly, equations capable of detailed predictions of observed behavior can provide needed insight into the mechanism of drag reduction. Inasmuch as the semi‐logarithmic resistance law, e.g., the Seyer and Metzner model of turbulent viscoelastic flow, provides a scale‐up criterion significantly more complicated to use than a number of other models, an engineering approximation to this model is described. It is shown that for accurate predictions to be made at modest levels of drag reduction activity, the model must be modified to compensate for thickening of the sublayer.

ISSN:0031-9171    

DOI:10.1063/1.861762

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Laser Doppler anemometry is employed to measure the mean velocity and turbulence intensity in the early turbulence regime of the flow of a dilute polymer solution through a small‐diameter tube. (This regime is initiated at an onset Reynolds number in the subcritical range, above which the polymer additive increases drag, and continues through the transitional Reynolds number range, where a continuous transition to turbulent flow with reduced drag is observed.) When the onset flow rate for early tubulence is exceeded, the mean velocity profile becomes progressively flatter with increasing flow rate. That is, a gradual transition from a laminar profile at onset to a more ’’turbulent‐like’’ profile takes place. Turbulence intensities at the tube centerline are about one half those of a fully developed turbulent flow. The inference is drawn from these results that the onset of eary turbulence corresponds to the flow becoming hydrodynamically unstable due to viscoelastic effects introduced by the polymer additive and that the character of the flow is intermediate between laminar and fully developed turbulent flow.

ISSN:0031-9171    

DOI:10.1063/1.861763

出版商:AIP    

年代:1977

数据来源: AIP