Stability of parallel, quasi-parallel and stationary flows, The

CER73-74-RK-RNM12.Prepared under Office of Naval Research, project no. NR 062-414/6-6-68 (Code 438), U.S. Department of Defense.Includes bibliographical references.The methods of linear perturbation theory have been used to study the stability of various flows, among them being (i) The stability of boundary layers along concave heated walls; (ii) The stability of boundary layers along concave walls with suction; (iii) The stability of wall jets along concave and convex walls; (iv) The spin up of a two-dimensional cylinder in an infinite medium; (v) The stability of stationary layers of fluid with arbitrary temperature stratification; (vi) The stability of natural convection flow along inclined plates. During the course of this work, three different solution techniques were employed; one of them was an approximate analytic technique, the remaining two were numerical. Three-dimensional spatially and temporally amplifying disturbances were considered in this study. The results indicated that the normal velocity component of the mean flow in a boundary layer, although much smaller than the stream wise component had a profound effect in reducing the stability of the flow. On the other hand, suction at the wall improved the stability characteristics. For the flow of parallel layers of fluid along heated walls with small curvature, it was found that a unique stability curve for neutral disturbances may be obtained if the quantity plotted along the abscissa is Ra + KsNg2 where Ra is the Rayleigh number, Ng is the Goertler number and Ks is a constant which expresses the relative importance of the mean temperature and velocity profiles. It was demonstrated also that wall jets are unstable on concave as well as convex walls. The results obtained for the stability of the spin up of a cylinder in an infinite medium are in qualitative agreement with experiment. The dependence of the onset of convective overturning in an unstable layer of fluid with a nonlinear basic temperature profile and bounded above by fluid of varying stability on Rayleigh number was established. The angle at which the two-dimensional wave instability passes into the three-dimensional mode in natural convection along an inclined plate was calculated. The result was found to be in good agreement with experiment. Other results obtained for this flow were in good qualitative agreement with experiment. Finally, some simple wind tunnel experiments with boundary layers along curved heated walls were performed. Photographic evidence of longitudinal vortices was obtained together with some qualitative data.Under contract no. N00014-68-A-0493-0001

Longitudinal vortices in concave surface boundary layers

Imperial Users onl

Recent insights into instability and transition to turbulence in open-flow systems

Roads to turbulence in open-flow shear layers are interpreted as sequences of often competing instabilities. These correspond to primary and higher order restructurings of vorticity distributions which culminate in convected spatial disorder (with some spatial coherence on the scale of the shear layer) traditionally called turbulence. Attempts are made to interpret these phenomena in terms of concepts of convective and global instabilities on one hand, and of chaos and strange attractors on the other. The first is fruitful, and together with a review of mechanisms of receptivity provides a unifying approach to understanding and estimating transition to turbulence. In contrast, current evidence indicates that concepts of chaos are unlikely to help in predicting transition in open-flow systems. Furthermore, a distinction should apparently be made between temporal chaos and the convected spatial disorder of turbulence past Reynolds numbers where boundary layers and separated shear layers are formed

Instability in laminar and transitional boundary layers on concave surfaces

Imperial Users onl

Heat/mass transfer in rotating, smooth, high aspect-ratio (4:1) coolant channels with curved walls

The thesis presents an experimental study of heat/mass transfer coefficient in 4:1 aspect ratio smooth channels with non-uniform cross-sections. Curved leading and trailing edges are studied, for two curvatures of 9.06 m-1 (0.23 in-1) and 15.11 m-1 (0.384 in-1) and for two different curvature configurations. One configuration has curved walls with curvature corresponding to the blade profile (positive curvature on both leading and trailing walls), and the other configuration has leading and trailing walls that curve inwards into the coolant passage (negative curvature on the leading surface and positive curvature on the trailing surface). The experiments are conducted in a rotating two-pass coolant channel facility using the naphthalene sublimation technique. Only the radially outward flow is considered for the present study. The span-wise mass transfer distributions of fully developed regions of the channel walls are also presented, to delineate the effect of rotation number. The mass transfer data from the curved wall channels is compared to those from a smooth 4:1 rectangular duct with similar flow parameters. In the first set of experiments Re=10,000 with Ro=0-0.071 and 90 degree orientation, heat transfer enhancement especially in the leading wall is seen for the lower curvature channels, and there is a subsequent reduction in the higher curvature channel, when compared to the 4:1 rectangular smooth channel. This indicates that an optimal channel wall curvature exists at this Reynolds number for which heat transfer is the highest. For Re=20,000 with Ro=0-0.051 and 90 and 45 degree orientation, the heat transfer results show highest enhancements for the higher curvature positive-positive section, with respect to the 4:1 rectangular channel, but this enhancement is reduced with increase in rotation. The lower curvature positive-negative section shows the highest increment in heat transfer enhancement with rotation number for both 90 and 45 degree orientations. The heat transfer ratios are reduced with rotation for the 45 degree orientation for the higher curvature positive-negative section. Thus, all the measurements indicate that an optimum curvature exists for a particular Reynolds number where the heat transfer would be the highest

Aeronautical engineering: A continuing bibliography with indexes (supplement 232)

This bibliography lists 422 reports, articles, and other documents introduced into the NASA scientific and technical information system in October, 1988

Aeronautical engineering: A continuing bibliography with indexes (supplement 250)

This bibliography lists 420 reports, articles, and other documents introduced into the NASA scientific and technical information system in February, 1990. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Aeronautical engineering: A continuing bibliography with indexes (supplement 289)

This bibliography lists 792 reports, articles, and other documents introduced into the NASA scientific and technical information system in Mar. 1993. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

An experimental investigation of a three dimensional wall jet

One and two point statistical properties are measured in the flow fields of a coflowing turbulent jet. Two different confining surfaces (one flat, one with large curvature) are placed adjacent to the lip of the circular nozzle; and the resultant effects on the flow field are determined. The one point quantities measured include mean velocities, turbulent intensities, velocity and concentration autocorrelations and power spectral densities, and intermittencies. From the autocorrelation curves, the Taylor microscale and the integral length scale are calculated. Two point quantities measured include velocity and concentration space-time correlations and pressure velocity correlations. From the velocity space-time correlations, iso-correlation contours are constructed along with the lines of maximum maximorum. These lines allow a picture of the flow pattern to be determined. The pressures monitored in the pressure velocity correlations are measured both in the flow field and at the surface of the confining wall(s)