摘要:

Using an array of hot wires and wall shear stress probes results are presented which support the hypothesis that an organized structure exists in the boundary layer. From the correlation between wall shear stress and velocity it is shown that this structure is at an oblique angle to the free stream. The inferred angle from this correlation is found to agree remarkably with recent flow visualization photographs. Evidence is found that the large scale motion in the organized structure produces a slowly varying component in the wall shear stress and, importantly, a high frequency large amplitude fluctuation occurring near the maximum in the slowly varying wall shear. It is suggested that this latter fluctuation is associated with the bursting phenomenon. A model which accounts for these results is described. It leads to a description of the bursting process in terms of a rotational instability and while too approximate to give more than good order of magnitude estimates, it gives values of the wall streak spacing which are not inconsistent with previous measurements.

ISSN:0031-9171    

DOI:10.1063/1.861737

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Special purpose high‐speed cameras have been developed which enable the detailed visualization of turbulent water jets discharging in air. The laminar‐turbulent transition and initial turbulent formation on the surface of the jet is followed by amplified disturbances which result in spray detachment. A very large modification to this process is observed when polymers are added to the water.

ISSN:0031-9171    

DOI:10.1063/1.861738

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A spark was used to initiate and mark in time a turbulent spot in an initially laminar boundary layer. This marked spot of turbulence merged and interacted with the natural turbulent boundary layer generated by a row of spherical trips. By using a digital technique to align individual spot signatures, thus correcting for variations in the transit time to a given measurement station, a structure was tracked over a streamwise extent of 70 average turbulent boundary‐layer thicknesses. The scale of the structure is of the order of 10&dgr; in the streamwise direction becoming 2−3&dgr; in the interface region of the boundary layer and is less than 4&dgr; in the spanwise direction, in spite of the fact that no spanwise alignment was performed. The structure is characterized by a convection speed of 0.9U∞. It exhibits features in detailed agreement with those at the outer region of the turbulent boundary layer (interface region) and is consistent with existing two‐ and three‐point space‐time correlations.

ISSN:0031-9171    

DOI:10.1063/1.861739

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A hypothesis is made for the modification of a complete turbulence model to take account of a dilute polymer concentration. The equations are integrated in the wall layer and give the effective slipu*&Dgr;Bas functions of wall roughness and empirical polymer properties, whereu*is the friction velocity and &Dgr;Bis the change of the constant in the logarithmic law of the wall. Results in qualitative agreement with experiment are found.

ISSN:0031-9171    

DOI:10.1063/1.861740

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The turbulent drag reducing abilities of polyethylenoxide polymer additive is shown to depend not only upon the rate of mixing, but upon premixing procedures as well, of parent polymer solution being injected into the water flow at the inlet end of a rectangular experiment pipe via a rotating impeller mixer. Pressure drop measurements over a downstream measuring section along the pipe, correlated with visual observations on the presence of polymer strands in the flow by means of a schlieren arrangement indicate that forced mixing, so that all visible polymer strands dissappear, does not impair the drag reducing abilities of the additive until quite high rates of mixing.

ISSN:0031-9171    

DOI:10.1063/1.861742

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The non‐Newtonian features of onset, saturation, and degradation exhibited by the flow of dilute water solution of linear polymers through a porous medium are described. A physical model for the non‐Newtonian behavior is presented as arising from the gradual extension of the polymer molecules, acting as elongated particles, and their alignment in the direction of flow. Comparisons are made with data from measurements in idealized porous‐media configurations.

ISSN:0031-9171    

DOI:10.1063/1.861743

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

In two visualization experiments in water, Gyr and Mu¨ller1showed that the addition of dilute polymer solutions changes the pattern of the flow structures near the wall. They observed a thickening of the viscous sublayer and of the buffer zone, as well as a stabilization of the shear waves in the direction of flow connected with an increase in the formation of transverse vortices. This observation is related to the burst cycle mechanism as described by Offen and Kline.2Based upon their burst mechanism a hypothesis is proposed that the drag reduction is the result of a higher alignment of the wall near separation vortices. Such a stabilization of the local pattern would produce a higher number of enrolled vortices, which means more elements of larger coherent motion. At the same time it would reduce the chaotic redistribution in the velocity field. The hypothesis is in good agreement with the observation of higher turbulent intensityu′ in the direction of flow in the buffer zone. To describe this higher stabilization it is proposed to use a constitutive relation of the form, &sgr;=2&mgr;[E+&Vthgr;xˆxˆ (xˆExˆ)] which is based on the concept that the polymer molecules are elongated in the direction of flow (x⁁) and can support strain forces in this direction.3To get an idea of the stabilization process the stability of a plane Poiseuille flow is demonstrated for a fluid with such an anisotropic viscosity, although for a complete investigation at least a two‐dimensional flow system should have been taken into consideration. (The full paper is published in Ref. 4.)

ISSN:0031-9171    

DOI:10.1063/1.861745

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Simultaneous measurements have been made of all three components of the fluctuating temperature dissipation in the inner region of a fully developed turbulent boundary layer at a moderate Reynolds number. Measurements are made with a four‐wire arrangement which consists of two parallel vertical wires mounted a small distance upstream of two parallel horizontal wires. Each of the four wires is operated at very low current by a contant current anemometer and is sensitive to only the temperature fluctuation &THgr;. The separation between wires in each parallel pair is kept small, so that the differences between the outputs of each pair are reasonable approximations to ∂&THgr;/∂zand ∂&THgr;/&Lgr;y, the temperature derivatives in the transverse and vertical directions, respectively. The streamwise derivative ∂&THgr;/&Lgr;xwas obtained from the time derivative, through use of Taylor’s hypothesis. Mean square and spectral density measurements show that in the inner region local isotropy is not closely approximated [(∂&THgr;/∂z)2≳ (∂&THgr;/∂y)2≳ (∂&THgr;/∂x)2] and (∂&THgr;/∂x)2is richer in high frequency content than the other two components or the sum. The probability density of the sum &khgr;[= (∂&THgr;/∂x)2+(∂&THgr;/∂y)2+(∂&THgr;/∂z)2] has a lower skewness and flatness factor and is more closely log‐normal than the probability densities of the individual components. This is true regardless of whether &khgr; and its components are unaveraged or locally averaged over a linear dimensionr, in the Obukhov–Kolmogoroff sense. The variance &sgr;2of the logarithm of the locally averaged &khgr; is proportional to logrover a wide range ofr(rmax/rmin≃30), in contrast to the individual components where this ratio may be as small as 3. The value of the Kolmogoroff constant &mgr; determined from the slope of &sgr;2vs logris about 0.35. This is consistent with the slope of the spectral density of &khgr; and is also in agreement with previous best estimates of &mgr; obtained at high Reynolds numbers. Using only one component of &khgr;, the evaluation of &mgr; either from the slope of the spectral density or from the slope of &sgr;2vs logrseems to be highly ambiguous and can lead to erroneous results.

ISSN:0031-9171    

DOI:10.1063/1.861747

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A numerical model of the effect of compliant walls on turbulent boundary layer flows, has been investigated. The model is based on Burton’s1observation that turbulent bursts produce large pressure fluctuations that tend to produce low speed ’’streaks’’ near the wall. These streaks undergo space‐time retardation and a new burst appears when the velocity profile becomes highly inflectional. The idea of the model is that the compliant wall motion interrupts this feedback loop of burst formation and that short wavelength wall motions can possibly delay burst formation long enough for the favorable gradient part of the pressure pulse caused by previous bursts to effect a decrease in the burst frequency. The numerical model involves solution of the two‐dimensional time‐dependent Navier–Stokes equations in a rectangular geometry. A typical calculation is performed using 257 grid points in the downstream direction with inflow‐outflow boundary conditions applied. In the boundary layer direction 33 Chebyshev polynomials are used; linearized boundary conditions are applied at the compliant boundary and a match is made to a logarithmic velocity profile in the outer region. A large moving pressure pulse is applied in the outer region and it is assumed that the background turbulent Reynolds stress between bursts is a small fraction of the mean stress. This code is used to determine mean velocity profiles and has been checked thoroughly by comparison with the velocity profiles measured by Blackwelder. A stability analysis is then made of the resulting mean velocity profiles. The results of the stability analysis are not yet complete and will be reported in detail elsewhere.2The results of our calculations to date indicate that only very small wavelength wall motions have a significant effect upon the stability of the turbulent boundary layer. This result suggests that novel structural dynamics will be an essential component of successful drag reduction by compliant walls. This work was supported by the National Aeronautics and Space Administration Langley Research Center under contract No. NAS1‐14275.

ISSN:0031-9171    

DOI:10.1063/1.861749

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The presence of large scale structures in circular jets is well documented in the literature over a wide range of Reynolds numbers. This presentation explores in more detail the generation and interaction and the role they play in the development of the flow field. It is based on experiments carried out, both in water and air jets, at Reynolds numbers up to 100 000.1,2It is found, as reported by others, that the frequency of generation, as well as the various modes of the large scale vortices, can be predicted by linear stability theory. However, their further development as they are convected downstream is most difficult to study in detail. Conventional correlation measurements or sampling techniques have proven to be inadequate. In fact, the purpose for doing parallel water and jet experiments is to enable one to develop an adequate experimental method that could provide better information on the interaction process between adjacent structures. In a qualitative way, the measurements so far suggest that the induced flow field of the structures must play an important role in the interaction process and in the flow development. In the region of the potential core there is apparently an upstream influence due to the induced pressure field. This affects and, perhaps, controls such flow characteristics as the length of the potential core and the passage frequency of the structures at the end of the core. By introducing artificial disturbances into the flow further work is being carried out to clarify the above conjecture. This research was supported by the National Aeronautics and Space Administration under Grant NASA‐NSG‐1221.

ISSN:0031-9171    

DOI:10.1063/1.861751

出版商:AIP    

年代:1977

数据来源: AIP