Laminar Superlayer at the Turbulence Boundary

Holzner, Markus; Lüthi, Beat

doi:10.1103/physrevlett.106.134503

Cited by 120 publications

(166 citation statements)

References 18 publications

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“…The magnitudes of the velocity differences (u c − u), (v c − v) and (w c − w) are comparable to the Kolmogorov velocity scale, 0.013U e at the present height in the boundary layer, or u τ = 0.041U e . This observation seems consistent with the work of Holzner & Lüthi (2011), who considered the velocity of the turbulent non-turbulent interface relative to the local flow velocity. Like the present vortical structures, this interface is associated with small-scale turbulent motions (Westerweel et al 2009), and it typically moves at speeds of the order of the Kolmogorov velocity scale relative to the local flow velocity (Holzner & Lüthi 2011).…”

Section: Dispersion Of Vortices 41 Convection Of Individual Vorticessupporting

confidence: 81%

Tracking of vortices in a turbulent boundary layer

et al. 2012

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The motion of spanwise vortical elements and large-scale bulges has been tracked in the outer region between wall-normal distance z/δ = 0.11 and 0.30 of a turbulent boundary layer at Re θ = 2460. The experimental dataset of time-resolved threedimensional velocity fields used has been obtained by tomographic particle image velocimetry. The tracking of these structures yields their respective average trajectories as well as the variations thereof, quantified by the root mean square of the trajectory coordinates as a function of time. It is demonstrated that the variation in convection can be described by a dispersion model for infinitesimal particles in homogeneous turbulence, which suggests that these vortical structures and bulges are transported passively by the external velocity field without significant changes in their topology, at least over the present observation time of 1.2δ/U e . However, this does not mean that the structure's convection velocity is equal to the local flow velocity at each instant. Differences of the order of the Kolmogorov or wall friction velocity have been observed for the spanwise vortical elements. In addition, the simultaneous detection and tracking of multiple structures allows an evaluation of the relative velocity between two spanwise vortex elements, which are approximately aligned along the streamwise direction. The typical streamwise distance between such neighbouring structures is found to be around 0.2δ. Their relative velocities are small, especially the streamwise component, which shows less variation as may be expected based on the relative flow velocity statistics for the same separation distance. This appears consistent with the hairpin packet model, which comprises a set of streamwise aligned hairpins travelling coherently. In exceptional cases, however, the structures approach each other rapidly, forcing an interaction on a time scale of the order of 1δ/U e . It is shown that the measured variation in convection velocity can further be used successfully to predict the temporal development of space-time correlation functions starting from the instantaneous correlation map. In this prediction the structures are assumed to convect without change, following our observations.

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Section: Dispersion Of Vortices 41 Convection Of Individual Vorticessupporting

confidence: 81%

Tracking of vortices in a turbulent boundary layer

et al. 2012

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“…locally occurring at the interface between turbulent and surrounding flow. It has been demonstrated recently that E can be understood from small-scale processes [6,10,11] since the global entrainment comes about through small scale viscous diffusion of vorticity that is augmented by the strongly convoluted interface separating turbulent from surrounding flow regions. However, our advancements in the understanding of the turbulent/ nonturbulent interface (TNTI) have been mostly limited to flows without density contrast [12].…”

Section: Introductionmentioning

confidence: 99%

Small-scale entrainment in inclined gravity currents

2017

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We investigate the effect of buoyancy on the small-scale aspects of turbulent entrainment by performing direct numerical simulation of a gravity current and a wall jet. In both flows, we detect the turbulent/nonturbulent interface separating turbulent from irrotational ambient flow regions using a range of enstrophy iso-levels spanning many orders of magnitude. Conform to expectation, the relative enstrophy isosurface velocity v n in the viscous superlayer scales with the Kolmogorov velocity for both flow cases. We connect the integral entrainment coefficient E to the small-scale entrainment and observe excellent agreement between the two estimates throughout the viscous superlayer. The contribution of baroclinic torque to v n is negligible, and we show that the primary reason for reduced entrainment in the gravity current as compared to the wall-jet are 1) the reduction of v n relative to the integral velocity scale u T ; and 2) the reduction in the surface area of the isosurfaces.

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“…3 Turbulent entrainment has been recently studied in relation to the interfacial layer which separates turbulent from irrotational (non-turbulent) flow: the so-called turbulent/non-turbulent interface (TNTI). 4,5 The long-standing question of turbulent entrainment has been the understanding of what are the physical mechanisms causing it. Turbulent motions with wide range of scales have been observed near TNTIs, from small-scale eddies, which are statistically universal, to large-scale structures, which are flow dependent, while both small and large-scale motions are expected to contribute to the turbulent entrainment by nibbling and engulfment, respectively.…”

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confidence: 99%

“…6 As the turbulent entrainment takes place, irrotational fluid crosses the TNTI due to some mechanism. The TNTI is often detected using a constant vorticity magnitude isosurface, 6 and for this reason turbulent entrainment has been investigated by analyzing this isosurface propagation, whose velocity is of the order of the Kolmogorov velocity v η ≡ (νε) 1/4 , reflecting the importance of small-scale features, 5 where ε is the dissipation rate of turbulent kinetic energy and ν is the kinematic viscosity. However, recent studies on the structure of TNTI showed that it displays a finite thickness, 7 consisting of two layers: the viscous superlayer (VSL) and the turbulent sublayer (TSL).…”

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confidence: 99%

“…Subsequently, the marker is also tracked using the velocity of the irrotational boundary u I = u 0 + u P , which is the sum of the fluid velocity u 0 and the propagation velocity of the irrotational boundary u P = v E n, 5 where n = −∇ω 2 /|∇ω 2 | is the unit normal vector of the irrotational boundary, and v E = (2ω i S i j ω j + 2νω i ∇ 2 ω i )/|∇ω 2 | (ω i is the vorticity and S i j is the rate-of-strain tensor). 5 The mean propagation velocity ⟨v E ⟩ at t = t 0 is shown in Table I. This marker represents the location of the isosurface of |ω| = ω th (ω 2 /2 = ω 2 th /2).…”

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confidence: 99%

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Lagrangian properties of the entrainment across turbulent/non-turbulent interface layers

et al. 2016

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Lagrangian statistics obtained from direct numerical simulations of turbulent planar jets and mixing layers are reported for the separation distance between the tracer particles at the outer edge of the turbulent/non-turbulent interface layer, and the entrained fluid particles. In the viscous superlayer (VSL) the mean square particle distance exhibits a ballistic evolution, while the Richardson-like scaling for relative dispersion prevails inside the turbulent sublayer (TSL). The results further support the existence of two different regimes within the interface layer, where small-scale outward enstrophy diffusion governs the entrained particles in the VSL, while inviscid small-scale motions govern the TSL.

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Laminar Superlayer at the Turbulence Boundary

Cited by 120 publications

References 18 publications

Tracking of vortices in a turbulent boundary layer

Tracking of vortices in a turbulent boundary layer

Small-scale entrainment in inclined gravity currents

Lagrangian properties of the entrainment across turbulent/non-turbulent interface layers

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