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  • 1
    Electronic Resource
    Electronic Resource
    On linear evolution of unstable disturbances in a plane turbulent wake (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 665-674 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The evolution of sinuous perturbation waves in the turbulent wake of a flat plate is investigated. The Strouhal number of the perturbations is chosen so that the waves remain amplified over the entire range of measurements. Detailed comparisons between linear stability theory and the phase-averaged measurements of the coherent velocity field are presented. Initially, before significant amplification of the perturbation amplitude occurs, the agreement between the linear theory and the measurements is good. The measured amplitude and phase distributions of the streamwise and lateral components of the coherent or wave-induced velocity field as well as the coherent Reynolds stress show excellent agreement with their linear theory counterparts. The coherent Reynolds stress, which is generated through a nonlinear interaction between the fundamental mode and the mean flow, augments the turbulent Reynolds stress causing the spreading rate of the wake to increase. However, this nonlinear interaction does not affect the shape of the mean velocity profile in the early stages of amplification. The linear theory predictions deteriorate with increasing downstream distance because of nonlinearity and the stronger interaction with the turbulent field as the neutral point of the perturbation is approached. For the unforced flow, the peak in the measured spectrum of the cross-stream (turbulent) velocity fluctuations at any downstream location (in the far wake) corresponds to the local neutral frequency from linear, spatial stability theory for inviscid, parallel flow.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.858072
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Experiments on the formation of a recirculation zone in swirling coaxial jets (2000)
    Springer
    Experiments in fluids 29 (2000), S. 494-504 
    Details
    ISSN: 1432-1114
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract  The near flow field of coaxial air jets, with swirl in the outer one, was studied using flow visualization and hot-wire anemometry. The flow is sensitive to both the swirl number and the mass flow ratio between the outer and inner jets. A necessary condition for the formation of an internal recirculation zone (IRZ) is that the swirl number must exceed a minimum value which depends on the mass flow ratio. Spectral analysis of the velocity fluctuations indicates that the formation of an IRZ in the present flow does not appear to be related to the growth of convective flow instabilities. Analysis of the flow visualization and X-wire data indicates that the vorticity dynamics model for vortex breakdown proposed by Brown and Lopez [J Fluid Mech (1990) 222: 553] provides a plausible mechanism for the formation of an IRZ in this flow.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003480000118
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  • 3
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    Experiments on the formation of a recirculation zone in swirling coaxial jets (2000)
    Springer
    Details
    Publication Date: 2000-11-08
    Print ISSN: 0723-4864
    Electronic ISSN: 1432-1114
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Published by Springer
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  • 4
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    On linear evolution of unstable disturbances in a plane turbulent wake (1991)
    American Institute of Physics
    Details
    Publication Date: 1991-04-01
    Print ISSN: 0899-8213
    Topics: Physics
    Published by American Institute of Physics
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  • 5
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    The laminar wall-jet over a curved surface (1968)
    Cambridge University Press
    Details
    Publication Date: 1968-02-26
    Description: The laminar flow of a wall jet over a curved surface is considered. A unique similarity solution is obtained for both concave and convex surfaces when the local radius of curvature is proportional to x3/4. This solution satisfies a similar invariant condition to the one derived by Glauert for the wall jet over a plane surface. The variation of the shape of the velocity profile, the skin friction, and the surface pressure as a function of curvature is given.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 6
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    On the two-dimensional mixing region (1976)
    Cambridge University Press
    Details
    Publication Date: 1976-03-23
    Description: An experimental investigation of the two-dimensional incompressible mixing layer was carried out. The measurements provide new information on the development of the mean and turbulent fields towards a self-preserving state and on the higher-order statistical characteristics of the turbulent field. The relevance of initial conditions to the development of the flow is discussed in the light of both present and previous data. Measurements of spectra, probability densities and moments to eighth order of all three velocity-component fluctuations at various transverse positions across the flow were carried out using an on-line digital data acquisition system. The probability density distributions of the derivative and the squared derivative of the longitudinal and lateral velocity fluctuations were also determined. Direct measurements of moments to eighth order of the velocity derivatives were attempted and are discussed in the light of the simultaneously measured histograms. The problems in obtaining higher-order statistical data are considered in some detail. Estimates of the integral time scale of many of the higher-order statistics are presented. The high wave-number structure was found to be locally anisotropic according to both spectral and turbulent velocity-gradient moment requirements. Higher-order spectra to fourth order of the longitudinal velocity fluctuations were measured and are discussed. Finally the lognormality of the squared longitudinal and lateral velocity-derivative fluctuations was investigated and the universal lognormal constant μ was evaluated. © 1976, Cambridge University Press. All rights reserved.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 7
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    Effect of travelling waves on the growth of a plane turbulent wake (1992)
    Cambridge University Press
    Details
    Publication Date: 1992-02-01
    Description: The results of experimental studies on the nonlinear evolution of perturbation waves in the turbulent wake behind a flat plate are presented. Sinuous perturbations at several amplitudes and frequencies were introduced into the wake by oscillating a small trailing-edge flap. The Strouhal numbers of the perturbations were specially chosen so that the downstream location of the neutral point (where the spatial amplification rate obtained from linear theory vanishes) was well within the range of measurements. The streamwise evolution of the waves and their effect on the growth of the turbulent wake was investigated. The amplitude of the coherent Reynolds stress varied significantly with x and changed sign downstream of the neutral point. This resulted in rather strong changes in the spreading rate of the mean flow with x. At high forcing levels, dramatic deviations from the square-root behaviour of the unforced wake occurred. Although the development of the mean flow depended strongly on the forcing level, there were some common features in the overall response, which are discussed. The measured coherent Reynolds stress changed sign in the neighbourhood of the neutral point as predicted by linear theory. The normalized mean velocity profiles changed shape as a result of nonlinear interactions but relaxed to a new self-similar shape far downstream from the neutral point. Detailed measurements of the turbulent and coherent Reynolds stresses are presented and the latter are compared to linear stability theory predictions. © 1992, Cambridge University Press. All rights reserved.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 8
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    Modal decomposition of velocity signals in a plane, turbulent wake (1989)
    Cambridge University Press
    Details
    Publication Date: 1989-01-01
    Description: The Orr-Sommerfeld equation admits two solution modes for the two-dimensional plane wake. These are the sinuous mode with antisymmetric stream wise fluctuations and the varicose mode with symmetric streamwise fluctuations. The varicose mode is often ignored because its amplification rates are considerably less than those of the sinuous mode. An experimental investigation of the varicose mode in a two-dimensional turbulent wake was undertaken to determine if this mode of instability agrees as well with linear stability theory, as did the sinuous mode in previous experiments (Wygnanski, Champagne & Marasli 1986). The experiments demonstrated that, although it is possible to generate a nearly pure symmetric disturbance wave, it is very difficult to do as the flow is very sensitive to the slightest asymmetries which might be present in the experiments. These asymmetries are preferentially amplified, resulting in the eventual distortion of an initially prominent symmetric wave. It was therefore necessary to decompose phase-averaged measurements of the streamwise component of the velocity fluctuations into their symmetric and antisymmetric parts, and the results were compared with the appropriate theoretical eigenfunctions from linear stability theory. The lateral distribution of the amplitude and the phase of each mode agree reasonably well with their theoretical counterparts from the Orr-Sommerfeld equation. Slowly diverging linear theory predicts the streamwise variation of the sinuous mode quite well, but fails to do so for the varicose mode. An eddy-viscosity model, coupled with the slowly diverging linear equations, predicts the streamwise variation of both modes reasonably well and describes the transverse distributions of the perturbation amplitudes for both modes, but it fails to predict the distribution of phase for the varicose mode. © Copyright Cambridge University Press 1989. All rights reserved.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 9
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    Turbulence measurements with inclined hot-wires Part 2. Hot-wire response equations (1967)
    Cambridge University Press
    Details
    Publication Date: 1967-04-12
    Description: Hot-wire response equations to include the effects of the tangential velocity component as well as the non-linearities caused by high intensity turbulence are derived for linearized constant temperature operation. For low intensity turbulence similar equations are derived for constant current operation. The equations are applied to an X-wire array to determine the errors in selected turbulence quantities which arise from the assumption of cosine law cooling. The error depends upon the quantity measured, the method of operation, and [lscr ]/d. For [lscr ]/d = 200 the error ranges from 0 to 17%.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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  • 10
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    Experiments on nearly homogeneous turbulent shear flow (1970)
    Cambridge University Press
    Details
    Publication Date: 1970-03-26
    Description: With a transverse array of channels of equal widths but differing resistances, we have generated an improved approximation to spatially homogeneous turbulent shear flow. The scales continue to grow with downstream distance, even in a region where the mean velocity gradient and one-point turbulence moments (component energies and shear stress) have attained essentially constant values. This implies asymptotic non-stationarity in the basic Eulerian frame convected with the mean flow, behaviour which seems to be inherent to homogeneous turbulent shear flow. Two-point velocity correlations with space separation and with space-time separation yield characteristic departures from isotropy, including clear ‘upstream–downstream’ unsymmetries which cannot be classified simply as axis tilting of ellipse-like iso-correlation contours. The high wave-number structure is roughly locally isotropic although the turbulence Reynolds number based on Taylor ‘microscale’ and r.m.s. turbulent velocity is only 130. Departures from isotropy in the turbulent velocity gradient moments are measurable. The approximation to homogeneity permits direct estimation of all components of the turbulent pressure/velocity-gradient tensor, which accounts for inter-component energy transfer and helps to regulate the turbulent shear stress. It is found that its principal axes are aligned with those of the Reynolds stress tensor. Finally, the Rotta (1951, 1962) linear hypothesis for intercomponent energy transfer rate is roughly confirmed. © 1970, Cambridge University Press. All rights reserved.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Cambridge University Press
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