The evolution of the coherent structures in a uniformly distorted plane turbulent wake

1995 ◽  
Vol 291 ◽  
pp. 299-322 ◽  
Author(s):  
G. A. Kopp ◽  
J. G. Kawall ◽  
J. F. Keffer
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Turbulent Wake ◽  
Entrainment Rate ◽  
Rapid Distortion Theory ◽  
Vortex Model ◽  
Pattern Recognition Analysis ◽  
Preservation Theory ◽  
Far Wake ◽  
Plane Turbulent Wake

A plane turbulent wake generated by a flat plate is subjected to a uniform distortion. It is observed that nearly two-dimensional, quasi-periodic coherent structures dominate the distorted wake. Rapid distortion theory, applied to a kinematic vortex model of the coherent structures in the undistorted far wake, predicts many of the effects revealed by a hot-wire anemometry/pattern-recognition analysis of these structures. Specifically, rapid distortion theory predicts reasonably well the observed changes in the ensemble-averaged velocity patterns and the disproportionate amplification of the large-scale coherent structures relative to the smaller-scale ‘isotropic’ eddies. These results are consistent with the view that self-preservation of the distorted wake is not possible because of the selective amplification of the coherent structures, which control the development of the wake. As well, the entrainment rate in the distorted wake increases at a rate greater than that predicted by the self-preservation theory.

Entrainment vortices and interfacial intermittent turbulent bulges in a plane turbulent wake

2002 ◽  
Vol 469 ◽  
pp. 49-70 ◽  
Author(s):  
GREGORY A. KOPP ◽  
FRANCESC GIRALT ◽  
JAMES F. KEFFER
Keyword(s):  
Phenomenological Models ◽  
Coherent Structures ◽  
Large Scale ◽  
Horseshoe Vortex ◽  
Turbulent Wake ◽  
Wake Region ◽  
Hot Wire ◽  
Helmholtz Instability ◽  
Far Wake ◽  
Plane Turbulent Wake

Hot-wire measurements were made simultaneously in two homogeneous ‘horizontal’ planes in the far-wake region of a cylinder. A technique developed using hot-wire data to identify the spatial characteristics of the large-scale bulges at the interface between the internal turbulent motions and the external irrotational flow was used to unambiguously relate these outer intermittent bulges to the inner coherent structures. It was found that a turbulent bulge is made up of a combination of a horseshoe vortex (whose legs form one double-roller eddy) and the straining region present just upstream of this structure. The approach also allowed the evaluation of the two most prominent phenomenological models for the entrainment mechanism in the far-wake region: the Kelvin–Helmholtz instability and Townsend's growth–decay cycle. It was found that the decaying and re-forming of the bulges and entrainment structures is not likely to occur. Rather, the evidence is that the large-scale bulges remain coherent for long streamwise distances in equilibrium with the overall similarity of the flow.

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

2013 ◽  
Vol 731 ◽  
pp. 312-363 ◽  
Author(s):  
Jean-Pierre Hickey ◽  
Fazle Hussain ◽  
Xiaohua Wu
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Initial Conditions ◽  
Entrainment Rate ◽  
Bypass Transition ◽  
Spread Rate ◽  
Instability Modes ◽  
Self Similar ◽  
The Individual ◽  
Far Wake

AbstractWe 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 ∼$\pm 3{3}^{\circ } $ to the downstream, shallower than the preferential $\pm 4{5}^{\circ } $ 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.

Three-dimensionality of organized structures in a plane turbulent wake

1989 ◽  
Vol 206 ◽  
pp. 375-404 ◽  
Author(s):  
Michio Hayakawa ◽  
Fazle Hussain
Keyword(s):  
Cross Sections ◽  
Coherent Structures ◽  
Near Field ◽  
Spatial Coherence ◽  
Three Dimensional ◽  
Quantitative Measure ◽  
Turbulent Wake ◽  
Topological Features ◽  
Plane Wake ◽  
Plane Turbulent Wake

This paper describes a quantitative study of the three-dimensional nature of organized motions in a turbulent plane wake. Coherent structures are detected from the instantaneous, spatially phase-correlated vorticity field using certain criteria based on size, strength and geometry of vortical structures. With several combinations of X-wire rakes, vorticity distributions in the spanwise and transverse planes are measured in the intermediate region (10d [les ] x [les ] 40d) of the plane turbulent wake of a circular cylinder at a Reynolds number of 13000 based on the cylinder diameter d. Spatial correlations of smoothed vorticity signals as well as phase-aligned ensemble-averaged vorticity maps over structure cross-sections yield a quantitative measure of the spatial coherence and geometry of organized structures in the fully turbulent field. The data demonstrate that the organized structures in the nominally two-dimensional wake exhibit significant three-dimensionality even in the near field. Using instantaneous velocity and vorticity maps as well as correlations of vorticity distributions in different planes, some topological features of the dominant coherent structures in a plane wake are inferred.

Evolution of coherent structures in the plane turbulent wake of a porous body

1993 ◽  
Vol 7 (2) ◽  
pp. 151
Author(s):  
Z. Huang ◽  
J.G. Kawall ◽  
J.F. Keffer ◽  
J.A. Ferré
Keyword(s):  
Coherent Structures ◽  
Porous Body ◽  
Turbulent Wake ◽  
Plane Turbulent Wake

scholarly journals Behaviour of small-scale turbulence in the turbulent/non-turbulent interface region of developing turbulent jets

2019 ◽  
Vol 879 ◽  
pp. 187-216 ◽  
Author(s):  
M. Breda ◽  
O. R. H. Buxton
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Direct Action ◽  
Subsequent Development ◽  
Turbulent Jets ◽  
Small Scale ◽  
Entrainment Rate ◽  
Potential Core ◽  
Velocity Gradients ◽  
Canonical State

Tomographic particle image velocimetry experiments were conducted in the near and intermediate fields of two different types of jet, one fitted with a circular orifice and another fitted with a repeating-fractal-pattern orifice. Breda & Buxton (J. Vis., vol. 21 (4), 2018, pp. 525–532; Phys. Fluids, vol. 30, 2018, 035109) showed that this fractal geometry suppressed the large-scale coherent structures present in the near field and affected the rate of entrainment of background fluid into, and subsequent development of, the fractal jet, relative to the round jet. In light of these findings we now examine the modification of the turbulent/non-turbulent interface (TNTI) and spatial evolution of the small-scale behaviour of these different jets, which are both important factors behind determining the entrainment rate. This evolution is examined in both the streamwise direction and within the TNTI itself where the fluid adapts from a non-turbulent state, initially through the direct action of viscosity and then through nonlinear inertial processes, to the state of the turbulence within the bulk of the flow over a short distance. We show that the suppression of the coherent structures in the fractal jet leads to a less contorted interface, with large-scale excursions of the inner TNTI (that between the jet’s azimuthal shear layer and the potential core) being suppressed. Further downstream, the behaviour of the TNTI is shown to be comparable for both jets. The velocity gradients develop into a canonical state with streamwise distance, manifested as the development of the classical tear-drop shaped contours of the statistical distribution of the velocity-gradient-tensor invariants $\mathit{Q}$ and $\mathit{R}$. The velocity gradients also develop spatially through the TNTI from the irrotational boundary to the bulk flow; in particular, there is a strong small-scale anisotropy in this region. This strong inhomogeneity of the velocity gradients in the TNTI region has strong consequences for the scaling of the thickness of the TNTI in these spatially developing flows since both the Taylor and Kolmogorov length scales are directly computed from the velocity gradients.

scholarly journals Interaction of Very Large Scale Motion of Coherent Structures with Sediment Particle Exposure

Water ◽  
2021 ◽  
Vol 13 (3) ◽  
pp. 248
Author(s):  
Sencer Yücesan ◽  
Daniel Wildt ◽  
Philipp Gmeiner ◽  
Johannes Schobesberger ◽  
Christoph Hauer ◽  
...  
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Numerical Study ◽  
Local Maximum ◽  
Wave Number ◽  
Exposure Level ◽  
Sediment Particle ◽  
High Wave Number ◽  
Large Scale Motion ◽  
Scale Motion

A systematic variation of the exposure level of a spherical particle in an array of multiple spheres in a high Reynolds number turbulent open-channel flow regime was investigated while using the Large Eddy Simulation method. Our numerical study analysed hydrodynamic conditions of a sediment particle based on three different channel configurations, from full exposure to zero exposure level. Premultiplied spectrum analysis revealed that the effect of very-large-scale motion of coherent structures on the lift force on a fully exposed particle resulted in a bi-modal distribution with a weak low wave number and a local maximum of a high wave number. Lower exposure levels were found to exhibit a uni-modal distribution.

Holes and Entrainment in Stratocumulus

2005 ◽  
Vol 62 (2) ◽  
pp. 443-459 ◽  
Author(s):  
H. Gerber ◽  
G. Frick ◽  
S. P. Malinowski ◽  
J-L. Brenguier ◽  
F. Burnet
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Evaporative Cooling ◽  
Length Distribution ◽  
Relative Length ◽  
Minimal Amount ◽  
Entrainment Rate ◽  
Conditional Sampling ◽  
Free Atmosphere ◽  
Average Width

Abstract Aircraft flights through stratocumulus clouds (Sc) during the Dynamics and Chemistry of Marine Stratocumulus II (DYCOMS-II) study off the California coast found narrow in-cloud regions with less liquid water content (LWC) and cooler temperatures than average background values. The regions are named cloud holes and are assumed to be a result of water evaporated by the entrainment of dryer air from above the Sc. While such features have been noted previously, this study provided a unique opportunity to investigate in much greater detail the nature of the holes, as well as their relationship to the entrainment rate, because high-speed temperature and LWC probes with maximum spatial resolution of 10 cm were flown together for the first time. Nine long-duration flights were made through mostly unbroken Sc for which conditional sampling was used to identify the location and size of the holes. The holes are concentrated near cloud top, their average width near cloud top is about 5 m, their relative length distribution is nearly constant for all flights, and they can penetrate hundreds of meters deep into the Sc before being lost by mixing. Entrainment velocities at cloud top are estimated from measurements of fluxes of reduced LWC and vapor mixing ratios in holes, the fraction of cloud area covered by holes, and the total water jump between cloud top and the free atmosphere. Rates as large as 10 mm s−1 are found for nocturnal flights, and these rates are about 3 times larger than for daytime flight segments. The rates correlate best with the size of the buoyancy jump above the Sc; the present conditional-sampling approach for measuring the rates gives larger rates than the “flux jump” rates determined by others for the same flights by a factor of about 2. The stability criterion for all Sc predicts thinning and breakup of the Sc, which does not occur. The minimal amount of cloud-top evaporative cooling caused by entrainment contributes little to the top-down convection dominated by radiative cooling during nocturnal flights; however, evaporative cooling caused by the mixing of holes as they subduct with the large-scale eddy circulation in the Sc may contribute, but with an as-of-yet unknown amount.

Large-scale coherent structures as drivers of combustion instability

1987 ◽  
Author(s):  
K. SCHADOW ◽  
E. GUTMARK ◽  
T. PARR ◽  
D. PARR ◽  
K. WILSON
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Combustion Instability
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