Showing papers on "Flow separation published in 1983"
TL;DR: In this paper, the velocity distribution and reattachment length of a single backward-facing step mounted in a two-dimensional channel were measured using laser-Doppler measurements.
Abstract: Laser-Doppler measurements of velocity distribution and reattachment length are reported downstream of a single backward-facing step mounted in a two-dimensional channel. Results are presented for laminar, transitional and turbulent flow of air in a Reynolds-number range of 70 < Re < 8000. The experimental results show that the various flow regimes are characterized by typical variations of the separation length with Reynolds number. The reported laser-Doppler measurements do not only yield the expected primary zone of recirculating flow attached to the backward-facing step but also show additional regions of flow separation downstream of the step and on both sides of the channel test section. These additional separation regions have not been previously reported in the literature.Although the high aspect ratio of the test section (1:36) ensured that the oncoming flow was fully developed and two-dimensional, the experiments showed that the flow downstream of the step only remained two-dimensional at low and high Reynolds numbers.The present study also included numerical predictions of backward-facing step flow. The two-dimensional steady differential equations for conservation of mass and momentum were solved. Results are reported and are compared with experiments for those Reynolds numbers for which the flow maintained its two-dimensionality in the experiments. Under these circumstances, good agreement between experimental and numerical results is obtained.
1,637 citations
TL;DR: In this article, a high-speed video system and hydrogen bubble-wire flow visualization was used to investigate the characteristics of low-speed streaks which occur in the near-wall region of turbulent boundary layers.
Abstract: Employing a high-speed video system and hydrogen bubble-wire flow visualization, the characteristics of the low-speed streaks which occur in the near-wall region of turbulent boundary layers have been examined for a Reynolds-number range of 740 [les ] Reθ < 5830. The results indicate that the statistics of non-dimensional spanwise streak spacing are essentially invariant with Reynolds number, exhibiting consistent values of and remarkably similar probability distributions conforming to lognormal behaviour. Further studies show that streak spacing increases with distance from the wall owing to a merging and intermittency process which occurs for y+ [simg ] 5. An additional observation is that, although low-speed streaks are not fixed in time and space, they demonstrate a tremendous persistence, often maintaining their integrity and reinforcing themselves for time periods up to an order of magnitude longer than the observed bursting times associated with wall region turbulence production. A mechanism for the formation of low-speed streaks is suggested which may explain both the observed merging behaviour and the streak persistence.
621 citations
TL;DR: In this paper, the cross-correlations between surface-pressure and velocity fluctuations are found to be useful for the study of large-scale vortex structure in the separation bubble formed along the sides of a blunt flat plate with right-angled corners.
Abstract: Flow in the separation bubble formed along the sides of a blunt flat plate with right-angled corners has been studied in terms of extensive single- and two-point measurements of velocity and surface-pressure fluctuations. The cross-correlations between the surface-pressure and velocity fluctuations are found to be useful for the study of large-scale vortex structure in the bubble. Large-scale vortices are shed downstream from the separation bubble with a frequency of about 0.6U∞/xR, where U∞ is the approaching velocity and xR is the time-mean length of the bubble. On top of this regular vortex shedding, there exists a large-scale unsteadiness in the bubble. Vortices which are much larger than the regular vortices are shed with frequencies less than about 0.2U∞/xR. The large-scale unsteadiness is accompanied by enlargement and shrinkage of the bubble and also by a flapping motion of the shear layer near the separation line. The intermittent nature of the flow in the bubble is clarified in some detail. The distributions of the cross-correlations between the pressure and velocity fluctuations demonstrate the vortex structure in the reattaching zone. The longitudinal distance between the vortices is estimated to be (0.7–0.8) xR and their convection velocity is about 0.5U∞ near the reattachment line. The cross-correlations also suggest the existence of a longitudinal counter-rotating system in the bubble. The distance between the axes of the rotation is of the order of 0.6xR. Variations of timescales, lengthscales and phase velocities of the vortices are presented and discussed.
454 citations
TL;DR: The turbulent wall jet, even limiting attention to the topologically simple cases beloved of academics, arguably provides more puzzles for those seeking an ordered set of rules to describe turbulence than any other class of turbulent shear layer as discussed by the authors.
Abstract: The turbulent wall jet, even limiting attention to the topographically simple cases beloved of academics, arguably provides more puzzles for those seeking an ordered set of rules to describe turbulence than any other class of turbulent shear layer. Formally, we can regard a wall jet as a boundary layer in which, by virtue of the initially supplied momentum, the velocity over some region in the shear layer exceeds that in the free stream. Wall jets are of great and diverse engineering importance. While some of the best known and most challenging applications lie in the field of advanced airfoil design, these aerodynamic roles are far outweighed in number and, probably, in economic importance by the use of wall jets in problems of heating, cooling or ventilating-areas where traditionally design has proceeded unfettered by any deep concern about the turbu lence structure of the flows in question. Two major industrial applications are the film-cooling of the lining walls of gas-turbine combustion chambers and of the leading stages of
438 citations
349 citations
TL;DR: In this article, the bursting frequency in turbulent boundary layers has been measured over the Reynolds-number range 103 < U∞/ν < 104, and the non-dimensional frequency was constant independent of Reynolds number.
Abstract: The bursting frequency in turbulent boundary layers has been measured over the Reynolds-number range 103 < U∞/ν < 104. When scaled with the variables appropriate for the wall region, the non-dimensional frequency was constant independent of Reynolds number. A strong effect of the sensor size was noted on the measured bursting frequency. Only sensors having a spatial scale less than twenty viscous lengthscales were free from spatial-averaging effects and yielded consistent results. The spatial-resolution problem was apparently the reason for erroneous results reported in the past.
286 citations
TL;DR: In this article, the wall-pressure fluctuations in a turbulent boundary layer were investigated in a wind tunnel designed for flow-acoustic measurements, and it was shown that the dimensionless diameter d+ = 19 of the transducers is sufficient to resolve the essential structures of the turbulent pressure fluctuations.
Abstract: In a wind tunnel designed for flow-acoustic measurements, the wall-pressure fluctuations beneath a turbulent boundary layer have been investigated. The measurements were carried out with variously sized pressure transducers (19 [les ] d+ [les ] 333) and with an array of four small transducers (separation distance Δx+ = 75). It is shown that the dimensionless diameter d+ = 19 of the transducers is sufficient to resolve the essential structures of the turbulent pressure fluctuations. The power spectrum Φ(ω+) measured with the smallest transducer d+ = 19 partly exhibits power-law decay , which has been theoretically predicted for locally isotropic turbulence. By visual analysis and signal averaging in the time domain, pressure structures with high amplitudes could be detected which have the shape of short wavetrains or pulses. Their characteristic frequency and longitudinal wavelength have the mean values ω+ = 0.52 and λ+ = 145 respectively, and their mean convection velocity amounts to uc/u∞ = 0.53. It was calculated from the measured probability density that these characteristic structures play an important role, although the probability of their occurrence is low. The sources of these wall-pressure structures can be located in the buffer layer of the boundary layer.
284 citations
TL;DR: In this article, the Strouhal number for the rectangular prism and the circular cylinder was found to be a power function of the aspect ratio h / w (or h / d ). Here f c is the vortex shedding frequency, U 0 is the free-stream velocity, h is the height, w is the width and d is the diameter.
Abstract: Measurements of the vortex-shedding frequency behind a vertical rectangular prism and a vertical circular cylinder attached to a plane wall are correlated with the characteristics of the smooth-wall turbulent boundary layer in which they are immersed. Experimental data were collected to investigate the effects of (i) the aspect ratio of these bodies and (ii) the boundary-layer characteristics on the vortex-shedding frequency. The Strouhal number for the rectangular prism and the circular cylinder, defined by S = f c w / U 0 and f c d / U 0 respectively, was found to be expressed by a power function of the aspect ratio h / w (or h / d ). Here f c is the vortex-shedding frequency, U 0 is the free-stream velocity, h is the height, w is the width and d is the diameter. As the aspect ratio is reduced, the type of vortex shedding behind each of the two bodies was found to change from the Karman-type vortex to the arch-type vortex at the aspect ratio of 2·0 for the rectangular prism and 2·5 for the circular cylinder.
256 citations
12 Jul 1983
168 citations
TL;DR: In this article, the authors investigated the response of boundary-layer flow to convex curvature and found that curvature effects were clearly apparent one or two boundarylayer thicknesses downstream of the start of curvature, while the disappearance of the curvature effect on a flat wall was an extremely slow process.
Abstract: Although the number of data sets which show the effects of convex curvature on turbulence has increased in the last 10 years, the development of really good calculational models is being held back by a lack of reliable data. The pesent investigation is concerned with a set of experiments which provide a litle more information regarding the response of a boundary-layer flow to convex curvature. Careful measurements were conducted to show the response of the boudnary layer to a sudden change from flat wall to curvature, and from curvature to flat wall. It was found that curvature effects were clearly apparent one or two boundary-layer thicknesses downstream of the start of curvature, while the disappearance of the curvature effects on a flat wall was an extremely slow process.
01 Jan 1983
TL;DR: A review and summary of turbulent drag reduction approaches applicable to external flows can be found in this paper, where the authors focus on the emerging areas of nonplanar geometry and large eddy alteration.
Abstract: Paper presents a review and summary of turbulent drag reduction approaches applicable to external flows. Because relatively recent and exhaustive reviews exist for laminar flow control and polymer (hydrodynamic) drag reduction, the paper focuses upon the emerging areas of nonplanar geometry and large eddy alteration. Turbulent control techniques for air generally result in modest (but technologically significant) drag reductions (order of 20 percent or less) whereas hydrodynamic approaches can yield drag reductions the order of 70 percent. Paper also includes suggestions for alternative concepts and optimization of existing approaches.
TL;DR: In this paper, a wall-pressure fluctuation pattern associated with the burst-sweep cycle of events in the wall region is identified. But the phase relationship with velocity fluctuations close to the wall and the wall shear-stress fluctuations during the burst cycle has not been established.
Abstract: The wall-pressure fluctuations beneath a turbulent boundary layer have been conditionally sampled on a basis of the high-frequency activity of the pressure fluctuations themselves, the high-frequency activity of the streamwise velocity fluctuations in the vicinity of the wall, and the excursions in velocity in the vicinity of the wall. This has led to the identification of a characteristic wall-pressure fluctuation pattern which is associated with the burst–sweep cycle of events in the wall region. The pattern has the form of an overpressure over a streamwise extent of about 1·5−2·0δ * , with a region of underpressure and a pressure minimum to either side of it, the distance between pressure minima being about 3·0−3·5δ * . This pattern is convected at a velocity 0·67 times the freestream velocity. Its phase relationship with velocity fluctuations close to the wall and the wall shear-stress fluctuations during the burst–sweep cycle have been established. It appears to be produced by the inclined shear layer which forms the upstream surface of the large organized structures in the layer, and calculated pressure patterns support this conclusion. The phase relationships indicate that fluid involved in the bursting process is subjected to a favourable streamwise pressure gradient by the characteristic wall-pressure pattern at the time that the lift-up of low-speed streaks in the wall region begins. In addition, order-of-magnitude estimates suggest that the adverse pressure gradients associated with the characteristic pressure pattern, even if their phasing with streak lift-up were appropriate, would be insufficient to initiate the lift-up. It is therefore concluded that the streamwise pressure gradients associated with the pressure patterns do not play an active role in the dynamics of the wall flow and are not the direct cause of the bursting process.
TL;DR: In this article, the breakdown and separation of boundary layers adjoining a mainstream are studied in three related situations (i)-(iii) of the title, for (i) the classical steady boundary laye...
Abstract: The breakdown and separation or reattachment of boundary layers adjoining a mainstream are studied in the three related situations (i)-(iii) of the title. For (i) the classical steady boundary laye...
TL;DR: In this article, the effect of sound on the flow around plates with semicircular or square leading edges and square trailing edges located in a low turbulence open jet has been studied, and the length of the leading edge separation bubbles associated with square leading edge plates was found to oscillate.
TL;DR: In this article, the authors present theoretical results which are offered in an attempt to explain these observations quantitatively Comparison between the predicted and measured net energy loss which occurs upon transmission of high amplitude impulsive acoustic waves through various duct terminations, and also between calculated and measured reflection coefficients in the duct.
TL;DR: In this article, a boundary element method for the solution of non-zero Reynolds number incompressible flows in which the non-linear terms are lumped together to form a forcing function is presented.
Abstract: A formulation of the boundary element method for the solution of non-zero Reynolds number incompressible flows in which the non-linear terms are lumped together to form a forcing function is presented. Solutions can be obtained at low to moderate Reynolds numbers. The method was tested using the flow of a fluid in a two-dimensional converging channel (Hamel flow) for which an exact solution is available. An axisymmetric formulation is demonstrated by examining the drag experienced by a sphere held stationary in uniform flow. Performance of the method was satisfactory. New results for an axisymmetric free jet at zero Reynolds number obtained using the boundary element method are also included. The method is ideal for this type of free-surface problem.
TL;DR: In this paper, the authors calculate the flow induced by a vortex pair in a viscous fluid, which is otherwise at rest, in the presence of a plane boundary, which may be either a no-slip or a stress-free boundary.
Abstract: We calculate the flow induced by a vortex pair in a viscous fluid, which is otherwise at rest, in the presence of a plane boundary. This may be either a no-slip or a stress-free boundary. The phenomenon of rebound of the vortices from the boundary occurs for either type of boundary, and an explanation for this is offered in terms of viscous effects.
Patent•
02 Aug 1983
TL;DR: In this paper, a sensor element in the surface measures certain flow values in the boundary layer, conveyed to an analyzer control circuit which, in turn, conveys a signal to an active vibration transmitter in the body for providing the required control.
Abstract: In controlling the boundary layer of flow over the surface of a body, for reducing resistance and impeding flow separation, a sensor element in the surface measures certain flow values in the boundary layer The measured values are conveyed to an analyzer control circuit which, in turn, conveys a signal to an active vibration transmitter in the surface of the body for providing the required control
TL;DR: In this article, the structure of the laminar-turbulent transition region for the three-dimensional boundary layer along a 30° cone rotating in external axial flow is investigated.
Abstract: The purpose of the present paper is to investigate the structure of the laminar–turbulent transition region for the three-dimensional boundary layer along a 30° cone rotating in external axial flow. Spiral vortices, which were assumed as small disturbances in the present stability analysis, are observed experimentally in the transition region. The process of transition to a turbulent boundary layer is visualized in detail. When the ratio of rotational speed to external axial flow is increased, the critical and transition Reynolds numbers decrease remarkably. The spiral angle and the number of vortices appearing on the cone decrease as the rotational speed ratio is increased.
TL;DR: In this paper, a single analytical expression is proposed to describe the velocity distribution in a two-dimensional, separated, turbulent boundary layer on smooth, impermeable, adiabatic walls over the domain 0 less than or = to y less than infinity.
Abstract: : A single analytical expression is proposed to describe the velocity distribution in a two-dimensional, separated, turbulent boundary layer on smooth, impermeable, adiabatic walls over the domain 0 less than or = to y less than infinity. The expression is an extension of one previously derived for attached flow which depends upon local values of skin friction, shape factor, and Reynolds number based on momentum thickness. Boundary-layer shape factor and local skin friction correlations applicable to separated flows are derived from fitting the proposed analytical expression for separated velocity profiles to available experimental data. These correlations are then available for analytically describing separated velocity profiles without further fitting.
TL;DR: In this article, a global or pressure relaxation formulation for the reduced form of the Navier-Stokes equations, frequently referred to as semi-elliptic or partially parabolized or just “parabolized” Navier Stokes (PNS), is presented.
TL;DR: In this paper, a solution methodology has been employed that enables the fully developed regime in a duct of periodically varying cross section to be determined without dealing with the entrance region of the duct.
Abstract: A solution methodology has been employed that enables the fully developed regime in a duct of periodically varying cross section to be determined without dealing with the entrance region. ...
Book•
01 Jan 1983
TL;DR: The importance of changing the structure of the separated shear layer on the reattachment process itself was examined in this article, where the authors developed a pulsed wall probe for measurement of skin friction in the re-attachment region, thus providing an unambiguous definition of the Reattachment length.
Abstract: The reattachment of a fully turbulent, two dimensional, separated shear layer downstream of a single-sided sudden expansion in a planar duct flow was examined experimentally. The importance of changing the structure of the separated shear layer on the reattachment process itself was examined. For all cases, the Reynolds number based on step height was greater than 20,000, the expansion ratio was 5/3, and the inlet boundary layer was less than one-half step height in thickness. A crucially important phase was the development of a pulsed wall probe for measurement of skin friction in the reattachment region, thus providing an unambiguous definition of the reattachment length. Quantitative features of reattachment - including streamwise development of the mean and fluctuating velocity field, pressure rise, and skin friction - were found to be similar for all cases studied when scaled by the reattachment length. A definition of the reattachment zone is proposed.
10 Jan 1983
TL;DR: In this paper, an active two-layer flow in a rectangular geometry is studied to show that internal flow separation can occur when the flow is accelerated sufficiently by a narrowing channel and almost always this separation accompanies hydraulic control: the slowest upstream moving Kelvin wave is stopped and upstream and downstream states are not symmetric with respect to the channel width.
Abstract: Observations from a recent field experiment in the Vema Channel are briefly described. These show a remarkable change in the configuration of isopycnal surfaces within the channel and the development of thick, nearly homogeneous regions near the bottom which are capped by sharp vertical gradients. Contrary to previous speculation that these “bottom boundary layer” result from enhanced vertical mixing, a dynamical mechanism is explored. This involves the hydraulic adjustment of an inertial, semi-geostrophic flow to the channel geometry. First, an active two-layer flow in a rectangular geometry is studied to show that internal flow separation can occur when the flow is accelerated sufficiently by a narrowing channel. Almost always this separation accompanies hydraulic control: the slowest upstream moving Kelvin wave is stopped and upstream and downstream states are not symmetric with respect to the channel width. An active three-layer flow with a variable bottom profile is then presented as a more ...
TL;DR: In this paper, an improved single-variable correlation is developed for incipient and full detachment, and for reattachment, of turbulent boundary layers on two-dimensional surfaces, which is shown to agree with the wallwake law of Coles; the criterion for detachment of Sandborn and Kline; and new measurements of time-averaged fraction of forward flow.
Abstract: An improved single-variable correlation is developed for incipient and full detachment, and for reattachment ,of turbulent boundary layers on two-dimensional surfaces. The correlation is shown to agree with 1) the wallwake law of Coles; 2) the criterion for detachment of Sandborn and Kline; 3) a large amount of old flowvisualization data; and 4) new measurements of time-averaged fraction of forward flow. Relationships among the correlation, the underlying physics, and the implications for computations are discussed. The results clarify long-standing confusion between incipient and full detachment of turbulent boundary layers with regard to both correlation and computation.
01 Jan 1983
TL;DR: In this article, the authors present a survey of turbulent wall layers subjected to abrupt changes in boundary conditions and identify several methods with potential for altering the turbulence production process in the outer region of the boundary layer and reducing the turbulent skin-friction drag over relatively long downstream distances.
Abstract: Paper reviews large eddy behavior in turbulent boundary layers and presents a survey of turbulent wall layers subjected to abrupt changes in boundary conditions. From this survey several methods have been identified with potential for altering the turbulence production process in the outer region of the boundary layer and reducing the turbulent skin-friction drag over relatively long downstream distances; these include convex longitudinal surface curvature, large-eddy breakup devices, and Emmons spot alteration. Paper also presents results of an ongoing experimental study to parametrically evaluate large-eddy breakup devices as a turbulent drag reduction concept and reproduce the 20 percent net drag reductions found in large-eddy breakup experiments at Illinois Institute of Technology.