Turbulent vortex streets and the entrainment mechanism of the turbulent wake

Papailiou, D. D.; Lykoudis, Paul S.

doi:10.1017/s0022112074000553

Cited by 74 publications

(23 citation statements)

References 32 publications

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“…The authors do remark that the difference between Townsend's 'nibbling' mechanism and their 'entrainment by the rotation of large eddies' may only be a matter of scale and that 'the swirling action of the smallest eddies is probably similar to that of the largest'. Papailou & Lykoudis (1974) confirm the findings of Bevilaqua & Lykoudis (1971), but in the context of the near wake of a cylinder. They observe that the turbulent vortex street is the origin of the convolutions on the wake surface and that these vortices draw ambient fluid into the wake because of their rotation.…”

Section: Introductionsupporting

confidence: 83%

Detached shear-layer instability and entrainment in the wake of a flat plate with turbulent separating boundary layers

Mohan

2015

J. Fluid Mech.

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The near and very near wake of a flat plate with a circular trailing edge, with vigorous vortex shedding, is investigated with data from direct numerical simulations (DNS). Computations were performed for four different combinations of the Reynolds numbers based on plate thickness (D) and momentum thickness near the trailing edge (θ ). Unlike the case of the cylinder, these Reynolds numbers are independent parameters for the flat plate. The objectives of the study are twofold, to investigate the entrainment process when the separating boundary layers are turbulent and to better understand the instability of the detached shear layers (DSLs). A visualization of the entrainment process, the effect of changing the ratio θ /D on entrainment and wake-velocity statistics, and a way of understanding entrainment in a phase-averaged sense via distributions of the turbulent transport rate are provided here. The discussion on shear-layer instability focuses on the role of log-layer eddies in the destabilization process, the effect of high-speed streaks in the turbulent boundary layer in the vicinity of the trailing edge on shear-layer vortex generation rates, and a relationship between the prevalence of shear-layer vortex generation and shedding phase that is a result of an interaction between the shedding process and the shear-layer instability mechanism. A power-law relationship between the ratio of shear-layer and shedding frequencies and the Reynolds numbers mentioned above is obtained. A discussion of the relative magnitudes of the exponents is provided. A second power-law relationship between shed-vortex strength and these two Reynolds numbers is also proposed.

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Section: Introductionsupporting

confidence: 83%

Detached shear-layer instability and entrainment in the wake of a flat plate with turbulent separating boundary layers

Mohan

2015

J. Fluid Mech.

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“…would be a manifestation of hydrodynamic instability associated with local inflectional and, what appears now, wavelike structures in this class of flows (Corrsin 1943, Townsend 1947, Roshko 1954, Grant 1958, Bradshaw et al 1964, Mollo-Christensen 1967, Brown and Roshko 1974, Papailiou and Lykoudis 1974. The present impetus about the existence and importance of large-scale coherent structures in free turbulent shear flows is essentially brought about by optical observations of such flows (e.g.…”

Section: Sknimsted 1948)mentioning

confidence: 96%

Contributions to the Understanding of Large-Scale Coherent Structures in Developing Free Turbulent Shear Flows

Liu

1988

Advances in Applied Mechanics

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“…It has long been assumed that wakes, like mixing layers, jets and boundary layers, spread by entraining ambient, irrotational fluid via a two-step process: engulfment and nibbling. The entrainment process has been thought of as a 'pumping' (Papailiou & Lykoudis 1974) mechanism by which the large-scale turbulent motion near the TNTI draws in irrotational fluid parcels, engulfing them into the folds of the TNTI; unlike the jet, there is no induced flow component to drive the large-scale entrainment in the wake (Philip & Marusic 2012). Vorticity is then efficiently imparted to the irrotational engulfed fluid by viscous diffusion.…”

Section: Statistical Multiplicity Structural Plurality and Entrainmentmentioning

confidence: 99%

Role of coherent structures in multiple self-similar states of turbulent planar wakes

Hickey

Hussain

2013

J. Fluid Mech.

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We study the nature of archetypal, incompressible, planar splitter-plate wakes, specifically the effects of the exit boundary layer state on multiple approximate self-similarity. Temporally developing direct numerical simulations, at a Reynolds number of 1500 based on the volume-flux defect, are performed to investigate three distinct wake evolution scenarios: Kelvin-Helmholtz transition, bypass transition in an asymmetric wake, and an initially fully turbulent wake. The differences in the evolution and far-wake statistics are analysed in detail. The individual approximately self-similar states exhibit a relative variation of up to 48 % in the spread rate, in second-order statistics, and in peak values of the energy budget terms. The multiplicity of self-similar states is tied to the non-universality of the large-scale coherent structures. These structures maintain the memory of the initial conditions. In the far wake, two distinct spanwise-coherent motions are identified: (i) staggered, segregated spanwise rollers on either side of the centreplane, dominant in wakes transitioning via anti-symmetric instability modes; and, (ii) larger spanwise rollers spanning across the centreplane, emerging in the absence of a near-wake characteristic length scale. The latter structure is characterized by strong spanwise coherence, cross-wake velocity correlations and a larger entrainment rate caused by deep pockets of irrotational fluid within the folds of the turbulent/non-turbulent interface. The mid-sized structures, primarily vortical rods, are generic for all initial conditions and are inclined at ∼±33 • to the downstream, shallower than the preferential ±45 • inclination of the vorticity vector. The spread rate is driven by the inner-wake dynamics, more specifically the advective flux of spanwise vorticity across the centreplane, which depends on the large-scale coherent structures.

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Turbulent vortex streets and the entrainment mechanism of the turbulent wake

Cited by 74 publications

References 32 publications

Detached shear-layer instability and entrainment in the wake of a flat plate with turbulent separating boundary layers

Detached shear-layer instability and entrainment in the wake of a flat plate with turbulent separating boundary layers

Contributions to the Understanding of Large-Scale Coherent Structures in Developing Free Turbulent Shear Flows

Role of coherent structures in multiple self-similar states of turbulent planar wakes

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