摘要:

The effects of the flow time scales on the onset and magnitude of drag reduction in turbulent pipe flow were studied using comparatively low molecular weight samples of water soluble polymers in glycerine‐water mixtures and in two different pipe sizes. The results show that onset depends only on the time scale &ngr;u&tgr;2with no Reynolds number dependence. The magnitude of drag reduction after the molecules are extended was found to increase with pipe diameter. A correlation with the lifetime of large eddies or the mean period of turbulence production is suggested.

ISSN:0031-9171    

DOI:10.1063/1.861726

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The fully‐developed turbulent flow generated by a grid is axisymmetric, the intensity of the axial turbulence component being greater than that of the lateral component. Since the mean‐velocity distribution is uniform and consequently, the gradient of the mean velocity is zero everywhere, a local stress‐strain relationship is not possible. Measurements of the turbulence structure in a region up to 50 grid mesh‐sizes downstream of the grid, have revealed that long‐distance memory effects are responsible for a nonlocal relationship. In the local shear layers just downstream of the rods, the principal axes of the stress tensor make an angle of roughly 40 deg with the direction of the main flow. This angle decreases in the developing region, and becomes zero in the fully‐developed region. The plane of maximum shear stress then makes an angle of 45 deg with the direction of the main flow. The transport equations for turbulence are transformed to nonhomogeneous linear relaxation equations, which clearly show the long‐distance memory behavior of the maximum shear stress.

ISSN:0031-9171    

DOI:10.1063/1.861727

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A study is presented of the mechanisms occurring in a turbulent near wake flow at supersonic speed, in which there exist strong pressure and velocity gradients. The role played by the ’’production’’ of turbulent kinetic energy is shown. It is influenced by compressibility and anisotropy. The analysis indicates that compressibility increases production when a sheared flow develops in a positive pressure gradient, and decreases it in the case of an expansion. The experiments give the mean and turbulent characteristics of a near wake flow. The results show that: the levels of vorticity and entropy modes verify the relationship given by the strong Reynolds analogy; the correlation coefficient between modes is not significantly altered by strong gradients and rates of strain; negative production occurs in the expansion zone; as a consequence, there exists a decrease in turbulence measured downstream of this region.

ISSN:0031-9171    

DOI:10.1063/1.861728

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Earlier models for flows with drag reduction are reviewed and compared with a new energy‐dissipation closure which describes the observed effects of drag reduction on the mean velocity, turbulent energy, and turbulent length‐scale distributions, and suggests a new maximum drag reduction law related to the onset of drag reduction.

ISSN:0031-9171    

DOI:10.1063/1.861729

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Essential characteristics of a turbulent boundary layer, bearing silt flocks in suspension, are described and interpreted with the help of a simple steady state model calibrated for the test region of the Math Modelsea project off the Northern Belgian Coast under near critical conditions : no net flux of particles through the bottom boundary. The model emphasizes the existence between the viscous sublayer and the classical Prandtl–Ka´rma´n logarithmic layer, of an ’’elastic sublayer’’ where gravity acting on the suspended load provides the necessary restoring force.

ISSN:0031-9171    

DOI:10.1063/1.861730

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Methods of detecting and characterizing molecular aggregates in solution are discussed and it is shown that several polymer/solvent systems of interest in drag reduction also form aggregates. Drag reduction results obtained with a spinning disk apparatus can most readily be explained by an aggregation hypothesis, as can a number of other results reported in the literature. The relevance of molecular aggregates to the development of drag reduction theory and investigations is considered.

ISSN:0031-9171    

DOI:10.1063/1.861731

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The turbulent boundary layer flow over a flat plate, smooth and hard except for a finite section, is considered. In this section the plate may be compliant, have a prescribed waviness, or consist of an ’’active wall’’, etc. The section thus constitutes a disturbance to the turbulent boundary layer flow over a completely smooth and hard plate. An exact integral equation for the resulting disturbances of the fluid stresses at the plate is derived. To a reasonable approximation, this integral equation reduces to an explicit integral representation of the stress disturbances at the plate in terms of the undisturbed flow and the properties of the disturbing plate section. Further evaluation then requires the use of measured data. For a certain type of compliant wall a drag reduction of 1%–10% is estimated. PFL

ISSN:0031-9171    

DOI:10.1063/1.861732

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

An optical method has been developed by Emmerling, Meier, and Dinkelacker which allows the investigation of the instantaneous structure of the wall pressure field under a turbulent boundary layer. The main component of this method is a pressure transducer consisting of several hundred small elastic membranes, the displacements of which are recorded by interferometric means with a high‐speed camera. The method is explained and examples of measurements in a wind‐tunnel are presented. The measurements show the development and convection of pressure patterns of a wide range of sizes (from larger than one boundary layer thickness &dgr; down to 0.1 &dgr;) and with a wide range of convection velocities (from about 0.9U∞down to about 0.2U∞, withU∞the flow velocity outside the boundary layer). Some speculative considerations are made on what sort of flow configurations might be connected with the observed pressure patterns.

ISSN:0031-9171    

DOI:10.1063/1.861733

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A pattern recognition technique has been applied to data obtained in a turbulent channel flow. Theusignal patterns are recognized using several simple criteria, but simultaneously, other signals are processed. A newly designed probe provides, in addition to simultaneousu,v, andwsignals, vorticity signals, &ohgr;zand &ohgr;y, and a signal which can be interpreted as the instantaneous turbulence production,uv(∂U/∂y). The ensemble averaged results provide insight into the vorticity dynamics and turbulence production characteristics of organized structures. The instantaneous turbulent production results show that ejection‐type motions are the principal turbulent energy producers and that the interesting dynamics occur during the acceleration phase of the recognized pattern. A partial model of the flow which explains the results is offered.

ISSN:0031-9171    

DOI:10.1063/1.861734

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Conditional sampling techniques have been studied in order to better understand the phase relationship between the signal used to detect the events and the sampled data signal. It is shown that whenever there is a mean time delay between these two signals or whenever they are obtained from two different spatial locations, a random phase will exist between them if the signals are obtained from a random environment. This randomness is not accounted for in the usual definitions of conditional sampling, and it can cause a serious degradation of the conditionally averaged results. A corrective method is discussed and illustrated with examples from a turbulent boundary layer.

ISSN:0031-9171    

DOI:10.1063/1.861736

出版商:AIP    

年代:1977

数据来源: AIP