摘要:

Longitudinal turbulence intensities measured by split‐film anemometry in degraded polymer solutions in pipe flow peaked much farther from the wall than longitudinal turbulence intensities in a Newtonian oil at the same Reynolds numbers. The peak values were at about the same level for the degraded solutions as for the oil, but were higher and nearer the wall for fresh polymer solutions. Radial turbulence intensities were lower for the polymer solutions at all locations under all conditions. Drag reduction for the polymer solutions ranged from 1 to 21% based on Newtonian fluid of equal viscosity. For the fresh solutions the Reynolds stresses dropped to zero or low values much farther from the wall than normal, indicating a region of almost complete turbulence shear strain recovery near the wall (positive uv excursions balanced negative uv excursions). Numerical computations with a turbulence model involving balance equations for turbulence energy and turbulence energy dissipation rate (thek‐&egr; model) show that the location of the peak turbulence intensity (energy) is sensitive to the model for Reynolds stress production. It was possible to model the velocity and turbulence intensity profiles and predict the correct level of drag reduction using thek‐&egr; model with an eddy viscosity as a function of shear relaxation time.

ISSN:0031-9171    

DOI:10.1063/1.861764

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The velocity profile, longitudinal turbulent intensity profile, and bursting period in drag reducing pipe flow were measured using a laser Doppler technique. The temperature distribution and heat transfer coefficient were measured with thermocouples in the same system. The results of the decrease in eddy diffusivities of momentum and heat near the wall, the gross flow measurements, and the velocity distribution were shown to be in a good agreement with the predictions of the eddy diffusivity model for viscoelastic fluids previously proposed. A simple method based on the eddy diffusivity model to calculate the rate of heat transfer in the fully developed pipe flow of viscoelastic fluids is proposed, and is ascertained to be effective by the previous experiments.

ISSN:0031-9171    

DOI:10.1063/1.861717

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A theory is presented which predicts the spreading in time of a cylindrical turbulent cloud, allowing an estimation of the interface entrainment coefficient. In light of the theory, previously published experiments are analyzed and the effect of polymer concentration on entrainment is deduced; the entrainment is substantially reduced by concentrations as low as 25 ppm. It is suggested that this reduction accompanies a thickening of the interfacial superlayer of the same magnitude as the sublayer thickening which accompanies substantial drag reduction.

ISSN:0031-9171    

DOI:10.1063/1.861718

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

An electrochemical method, which is the mass transfer analog of the hot film anemometer, has been developed to study flow fluctuations in the immediate vicinity of a wall. Electrodes mounted flush with the wall are the cathode of an electrolysis cell. At high enough voltages, the electric current is controlled by the rate of mass transfer to the cathode and the concentration of the reacting species at the surface is zero. These techniques allow the measurement of two components of the fluctuating velocity gradient at the wall. Results have been obtained on the structure of turbulence close to a wall and on the influence of drag reducing polymers on this structure.

ISSN:0031-9171    

DOI:10.1063/1.861719

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Results from drag reduction experiments with polystyrene in toluene are presented. Measurements were performed with a polymer sample with a molecular weight of 2×106at different concentrations (c). Assuming a new sublayer portion of the velocity profile of the same form as that proposed by Virk in his elastic sublayer model, it is found that the thickness of this layer is proportional toc0.76. The dependence of the onset wall shear stress on molecular weight was investigated for molecular weights of 2×106, 4.1×106, and 7.1×106. Because of degradation, the molecular weight range used was not sufficient to determine whether onset of drag reduction correlates with length or time scale.

ISSN:0031-9171    

DOI:10.1063/1.861720

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The technique of simultaneous hot‐wire anemometry and flow visualization has been used to determine the average characteristics of two important scales of motion in the outer region of turbulent boundary layers: large scale motions (average length 1.6&dgr;), and ’’typical eddy’’ motions (average streamwise length approximately 200 &ngr;/u&tgr;). Results showed that the Reynolds number dependent ’’typical eddies’’ produced most of the Reynolds stress in the outer half of the layer atR&Vthgr;≈1200, and that they are formed on the upstream side of large scale motions at all Reynolds numbers investigated. This phase relationship explains the scaling of the frequency of occurrence of outer layer bursts (which are identified with ’’typical eddies’’) on the free stream velocity and overall boundary layer thickness, although it is found that the lengths of the ’’typical eddies’’ scale on inner layer variables. In the log region, roughly one‐half of the large scale motions sampled had zone‐averaged streamwise velocity defects. Ensemble averaged results showed that they were associated with significant Reynolds stress contributions. A structural model showing the phase relationship of ’’typical eddies’’ and large scale motions is presented.

ISSN:0031-9171    

DOI:10.1063/1.861721

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The effect of dilute, long‐chain, polymer solutions upon the spatial structure of the viscous sublayer in fully developed, turbulent, two‐dimensional channel flows was determined. The longitudinal, low‐speed streaks characteristic of the sublayer were made visible by seeping dye through a transverse slot in one wall. Motion pictures yielded a detailed description of the streak formation process and revealed two major differences in the sublayer structure when drag reduction occurs. At a given nondimensional distance from the wall, the average transverse spacing of the streaks increases as the amount of drag reduction increases. The average streak spacing within the viscous sublayer is not a function of the distance from the wall in water flows and flows at lower levels of drag reduction. At high levels of drag reduction, the average spacing varies within the sublayer increasing as the wall is approached.

ISSN:0031-9171    

DOI:10.1063/1.861722

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

It is shown that the rheological behavior of dilute polymeric solutions can be described, at least semi‐quantitatively, by means of an equation containing a single viscosity coefficient and a single time constant. This expression and the Batchelor equation for suspension rheology are used to predict the major drag‐reduction phenomena observed experimentally. It is shown that mixed polymer‐fiber systems may be much more effective in their drag reduction capabilities than is either kind of additive when used alone. This latter prediction finds its qualitative verification in drag coefficients which areseveral‐foldlower than those observed at the asymptote of ’’maximum’’ drag reduction in polymer solutions.

ISSN:0031-9171    

DOI:10.1063/1.861723

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The presence of a rough wall does not fundamentally alter the intermittent maintenance of Reynolds shear stress as found for turbulent flows over a smooth wall, be it boundary layers or pipe flows. Ejections, and to a lesser extent sweeps, are still the dominant events contributing to the shear stress. The mean period of occurrence of the violent ejections (those whose amplitude exceeds a characteristic threshold) is, however, larger for the rough pipe than it is for the smooth pipe at the same Reynolds number. A scaling based on specific parameters of the largest scale motions is proposed. The mean length scale of the violent ejections is larger than those encountered for a smooth pipe, but still seems to be of the order of the integral length scale of the transverse component of the velocity fluctuation.

ISSN:0031-9171    

DOI:10.1063/1.861724

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Small scale turbulent temperature and velocity measurements were made in a variety of turbulent shear flows in the atmosphere and laboratory, in air and in water. Nonzero skewness values of the streamwise temperature gradient were observed in every case, with a sign equal to that of the gradient dotted with the cross product of the mean temperature gradient and mean shear vectors. Large temperature jumps were found across sharp shear zones at the outer boundaries of large scale eddies rotating with the mean vorticity of the shear flow, giving a distinctive ramp‐like structure to the temperature signals which seems to be a characteristic feature of scalar fields mixed by sheared turbulence. The streamwise gradient of the vertical velocity in the atmospheric boundary layer over the ocean was found to have positive skewness. Thus, both velocity and temperature fields are locally anisotropic in the same sense as the anisotropic mean quantities.

ISSN:0031-9171    

DOI:10.1063/1.861725

出版商:AIP    

年代:1977

数据来源: AIP