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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 authors investigated the properties of the time and span-averaged mean wake recirculation region in separated flows over several different two-dimensional bluff bodies and found that the contribution from Reynolds normal stress to the force balance of the wake bubble is significant.
Abstract: The properties of the time- and span-averaged mean wake recirculation region are investigated in separated flows over several different two-dimensional bluff bodies Ten different cases are considered and they divide into two groups: cylindrical geometries of circular, elliptic and square cross-sections and the normal plate A wide Reynolds number range from 250 to 140000 is considered, but in all the cases the attached portion of the boundary layer remains laminar until separation The lower Reynolds number data are from direct numerical simulations, while the data at the higher Reynolds number are obtained from large-eddy simulation and the experimental work of Cantwell & Coles (1983), Krothapalli (1996, personal communication), Leder (1991) and Lyn et al (1995) Unlike supersonic and subsonic separations with a splitter plate in the wake, in all the cases considered here there is strong interaction between the shear layers resulting in Karman vortex shedding The impact of this fundamental difference on the distribution of Reynolds stress components and pressure in relation to the mean wake recirculation region (wake bubble) is considered It is observed that in all cases the contribution from Reynolds normal stress to the force balance of the wake bubble is significant In fact, in the cylinder geometries this contribution can outweigh the net force from the shear stress, so that the net pressure force tends to push the bubble away from the body In contrast, in the case of normal plate, owing to the longer wake, the net contribution from shear stress outweighs that from the normal stress At higher Reynolds numbers, separation of the Reynolds stress components into incoherent contributions provides more insight The behaviour of the coherent contribution, arising from the dominant vortex shedding, is similar to that at lower Reynolds numbers The incoherent contribution to Reynolds stress, arising from small-scale activity, is compared with that of a canonical free shear layer Based on these observations a simple extension of the wake model (Sychev 1982; Roshko 1993a, b) is proposed

136 citations

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate that substantial changes in a reattaching flow can be produced by controlled forcing techniques, and the forcing apparently works by affecting the vortex merging process in a fashion similar to that observed in forced mixinglayer experiments.
Abstract: Recent experimental observations have shown that large-scale organized vortices are produced in reattaching separated flows. Interactions between these vortices are important in the development of these flows downstream. Experimental studies from a downstream-fac ing step flow are presented to demonstrate that substantial changes in a reattaching flow can be produced by controlled forcing techniques. The forcing apparently works by affecting the vortex merging process in a fashion similar to that observed in forced mixinglayer experiments. The separated mean flow spreading rate could be increased most effectively by forcing at a nondimensional frequency (based on step height and freestream velocity) between 0.2 and 0.4. This result was found to be relatively independent of step Reynolds numbers over the range (26,000-76,000) studied. A significant decrease in the reattachment length accompanied the increased growth of the separated shear layer. Considerable changes in the turbulence energy and the Reynolds stress levels were also observed for the forced flows.

136 citations

Journal ArticleDOI
01 Jan 1975-Nature
TL;DR: In this article, the authors draw attention to examples of flow separation in natural meander bends and attempt to define an empirical criterion for predicting the onset of separation, which can be used to predict sedimentation and erosion in sinuous channels.
Abstract: IT is generally assumed that downstream flow through meander bends is helicoidal and is accompanied by a transverse, bottom flow component directed towards the inner bank1–3. Thus particles of sediment on a point bar are transported at some angle inwards from the generalised local downstream flow vector4,5. As pointed out by Bagnold2, however, “… a stage must be reached at which the flow along the inner boundary becomes unstable and breaks away from the boundary, leaving an intervening space occupied by a zone of unstable and confused motion …”. The experimental results of Leopold et al.5 leave no doubt that this phenomenon of flow separation (Fig. 1) can be a highly important feature in river hydraulics. Since 1960, however, no workers have extended these initial experimental ideas into field situations. Existing models of sedimentation and erosion in sinuous channels4,5 ignore any possible effects of flow separation. Here we draw attention to examples of flow separation in natural meander bends and attempt to define an empirical criterion for predicting the onset of separation.

136 citations

Journal ArticleDOI
TL;DR: In this article, the authors examined the voltage requirements for the plasma actuators to reattach the flow at the leading edge of airfoils at poststall angles of attack for a range of flow parameters in order to establish scaling between laboratory and full flight conditions.
Abstract: We present experimental results to yield insight into the scalability and control effectiveness of single-dielectricbarrier-discharge plasma actuators for leading-edge separation control on airfoils. The parameters investigated are chord Reynolds number, Mach number, leading-edge radius, actuator amplitude, and unsteady frequency. This includes chord Reynolds numbers up to 1:0 � 106 and a maximum freestream speed of 60 m=s corresponding to a Mach number of 0.176. The main objective of this work is to examine the voltage requirements for the plasma actuators to reattach the flow at the leading edge of airfoils at poststall angles of attack for a range of flow parameters in order to establish scaling between laboratory and full-flight conditions. For the full range of conditions, an optimum unsteady actuator frequency f is found to minimize the actuator voltage needed to reattach the flow, such that F� � fLsep=U1 � 1. At the optimum frequencies, the minimum voltage required to reattach the flow is weakly dependent on chord Reynolds number and strongly dependent on the poststall angle of attack and leading-edge radius. The results indicate that the voltage required to reattach the flow scales as the square of the leading-edge radius.

136 citations

Journal ArticleDOI
TL;DR: In this article, the separation point of the flow around a circular cylinder has been numerically and experimentally investigated in the regime of Reynolds number less than 280, and the results reveal that the long-existing discrepancy in the data concerning the time-averaged separation angles reported in the literature results mainly from the oscillating characteristics of flow separation on the cylinder surface and the experimental methodologies rather than the commonly mentioned blockage-ratio effect.
Abstract: The separation point of the flow around a circular cylinder has been numerically and experimentally investigated in the regime of Reynolds number less than 280. The present results reveal that the long-existing discrepancy in the data concerning the time-averaged separation angles reported in the literature results mainly from the oscillating characteristics of the flow separation on the cylinder surface and the experimental methodologies rather than the commonly mentioned blockage-ratio effect. In the present experiment, the time-averaged separation angles are obtained by averaging the instantaneous images from a soap-film flow visualization instead of from the commonly used streakline images from finite time exposures. Excellent agreement has been achieved between the present experimental results and numerical simulations by the spectral element method. Particle-streak visualization in a towing tank has also been conducted to compare with that of the two-dimensional soap-film experiments. It reveals that the separation angle is insensitive to the three-dimensional effect. Variations of the time-averaged separation angles with Reynolds number can be represented by a four-term -range. The blockage effect on the separation angle has also been quantitatively analysed.

136 citations


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