NBS Special Publication


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Report


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NASA Technical Note


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Turbulent Flows


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obtained are still severely limited to low Reynolds numbers (about only one decade better than direct numerical simulations), and the interpretation of such calculations for complex, curved geometries is still unclear. It is evident that a lot of work (and a very significant increase in available computing power) is required before such methods can be adopted in daily's engineering practice. I hope to l"Cport on all these topics in a near future. The book is divided into six chapters, each· chapter in subchapters, sections and subsections. The first part is introduced by Chapter 1 which summarizes the equations of fluid mechanies, it is developed in C~apters 2 to 4 devoted to the construction of turbulence models. What has been called "engineering methods" is considered in Chapter 2 where the Reynolds averaged equations al"C established and the closure problem studied (§1-3). A first detailed study of homogeneous turbulent flows follows (§4). It includes a review of available experimental data and their modeling. The eddy viscosity concept is analyzed in §5 with the l"Csulting ~alar-transport equation models such as the famous K-e model. Reynolds stl"Css models (Chapter 4) require a preliminary consideration of two-point turbulence concepts which are developed in Chapter 3 devoted to homogeneous turbulence. We review the two-point moments of velocity fields and their spectral transforms (§ 1), their general dynamics (§2) with the particular case of homogeneous, isotropie turbulence (§3) whel"C the so-called Kolmogorov's assumptions are discussed at length.




DTNSRDC.


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A Study of the Structure of the Turbulent Boundary Layer with and Without Longitudinal Pressure Gradients


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The existence of three regions of the turbulent boundary layer, each correlating with a distinct part of the non-dimensional mean velocity profile, was confirmed; these are: (a) wake or intermittent region (b) fully turbulent region (c) wall layer region. Details of the flow structure of these regions were further verified using a new hot-wire anemometer and an improved combined-time-streak marker hydrogen-bubble technique. Instantaneous spanwise velocity profiles over a large extent of the flow at many fixed y-positions across the layer were obtained. The study of the flow structure was extended to include both positive and negative pressure gradient flows, including a relaminarization flow. The hot-wire anemometer provided a means of obtaining detailed mean velocity profiles well within the sublayer region. The hydrogen-bubble combined-time streak marker visualization technique was shown to be a useful tool for quantitative measurement of time-dependent velocity fields. (Author).




Some Characteristics of Turbulent Boundary Layers in Rapidly Accelerated Flows


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An analysis of time-mean-turbulent boundary layer velocity profiles measured in a rapidly accelerating flow suggests that the outer region of the velocity profiles consists of essentially inviscid, rotational flow. The extent of this inviscid outer region was observed in some cases to exceed 90 percent of what is ordinarily thought of as the turbulent boundary layer thickness. On the other hand, the inner frictional region of these velocity profiles appears to have turbulent characteristics similar to those of more conventional turbulent boundary layers. Hence, the outer edge boundary condition for this inner region is more properly the external rotational flow region than the free stream.