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Flow separation

About: Flow separation is a research topic. Over the lifetime, 16708 publications have been published within this topic receiving 386926 citations.


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TL;DR: In this article, the growth of a line vortex with time and the spread of a trailing vortex behind a wing due to turbulence are considered, and it is shown that the eddy viscosity for this type of motion may be taken to be proportional to the circulation round the vortex and the solution is then similar to the solution for the growing of a vortex in laminar flow.
Abstract: The growth of a line vortex with time and the spread of a trailing vortex behind a wing due to turbulence are considered. It is shown that the eddy viscosity for this type of motion may be taken to be proportional to the circulation round the vortex and the solution is then similar to the solution for the growth of a vortex in laminar flow. The method is applied to calculate the distance behind a wing for which the trailing vortices will touch one another.

187 citations

Journal ArticleDOI
TL;DR: In this article, the stability of the Ekman boundary-layer flow was studied in a rotating system with a slow relative flow compared to the basic speed of rotation, and the initial instability was similar to that which occurs in the boundary layer on a rotating disk.
Abstract: This study concerns the stability of the steady laminar boundary-layer flow of a homogeneous fluid which occurs in a rotating system when the relative flow is slow compared to the basic speed of rotation. Such a flow is called an Ekman boundary-layer flow after V. W. Ekman who considered the theory of such flows with application to the wind-induced drift of the surface waters of the ocean.Ekman flow was produced in a large cylindrical rotating tank by withdrawing water from the centre and introducing it at the rim. This created a steady-state symmetrical vortex in which the flow from the rim to the centre took place entirely in the shallow viscous boundary layer at the bottom. This boundary-layer flow became unstable above the critical Reynolds number is the characteristic depth of the boundary layer, v is the kinematic viscosity, and Ω is the basic speed of rotation. The initial instability was similar to that which occurs in the boundary layer on a rotating disk, having a banded form with a characteristic angle to the basic flow and with the band spacing proportional to the depth of the boundary layer.

187 citations

Journal ArticleDOI
TL;DR: In this paper, the authors used single dielectric barrier plasma actuators for flow separation on turbine blades in the low-pressure turbine stage at low Reynolds numbers typical of high-altitude cruise.
Abstract: This is a continuation of our work on the use of single dielectric barrier plasma actuators for controlling flow separation on turbine blades in the low-pressure turbine stage at low Reynolds numbers typical of high-altitude cruise. This used a linear cascade of Pratt & Whitney "PakB" shaped blades to provide generic low-pressure turbine conditions. The flow over one of the blades was documented through surface pressure, laser-Doppler velocimetry, and hot-wire measurements. These were used to define the location and size of the separated flow region on the suction side of the blade. Both steady and unsteady plasma actuators were implemented and found to be effective in separation control. For the unsteady actuators, there was an optimum excitation frequency to reattach the flow that corresponded to a Strouhal number, based on the length of the separated zone and the local freestream velocity, equal to unity. The unsteady actuator was more effective than the steady actuator in reattaching the flow while also requiring less power. It was suggested by the experimental results that the mechanism for the steady actuators was turbulence tripping, whereas the mechanism for the unsteady actuators was to generate a train of spanwise structures that promoted mixing.

186 citations

01 Apr 1991
TL;DR: In this paper, the authors define the current state of boundary layer structure knowledge and utilize direct numerical simulation results to help close the unresolved issues identified in part A and to unify the fragmented knowledge of various coherent motions into a consistent kinematic model of boundary layers.
Abstract: The long history of research into the internal structure of turbulent boundary layers has not provided a unified picture of the physics responsible for turbulence production and dissipation. The goals of the present research are to: (1) define the current state of boundary layer structure knowledge; and (2) utilize direct numerical simulation results to help close the unresolved issues identified in part A and to unify the fragmented knowledge of various coherent motions into a consistent kinematic model of boundary layer structure. The results of the current study show that all classes of coherent motion in the low Reynolds number turbulent boundary layer may be related to vortical structures, but that no single form of vortex is representative of the wide variety of vortical structures observed. In particular, ejection and sweep motions, as well as entrainment from the free-streem are shown to have strong spatial and temporal relationships with vortical structures. Disturbances of vortex size, location, and intensity show that quasi-streamwise vortices dominate the buffer region, while transverse vortices and vortical arches dominate the wake region. Both types of vortical structure are common in the log region. The interrelationships between the various structures and the population distributions of vortices are combined into a conceptual kinematic model for the boundary layer. Aspects of vortical structure dynamics are also postulated, based on time-sequence animations of the numerically simulated flow.

185 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
2023177
2022333
2021361
2020394
2019403
2018371