Numerical investigation of the generation and growth of coherent flow structures in a triggered turbulent spot

2014 ◽  
Vol 759 ◽  
pp. 257-294 ◽  
Author(s):  
Joshua R. Brinkerhoff ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Numerical Study ◽  
Quantitative Description ◽  
Shear Layers ◽  
Vortical Flow ◽  
Flow Structures ◽  
Test Surface ◽  
Turbulent Spot ◽  
Hairpin Vortices ◽  
Coherent Flow Structures

AbstractMultiple mechanisms for the regeneration of hairpin-like coherent flow structures in transitional and turbulent boundary layers have been proposed in the published literature, but a complete understanding of the typical topologies of coherent structures observed in the literature has not yet been achieved. To contribute to this understanding, a numerical study is performed of a turbulent spot triggered in a zero-pressure-gradient laminar boundary layer by a pulsed, transverse jet. Two direct numerical simulations (DNS) capture the growth of the spot into a mature turbulent region containing a large number of coherent vortical flow structures. The boundary-layer Reynolds number based on the test-surface streamwise length is $\mathit{Re}_{L}=309\,200$. The internal structure of the spot is characterized by densely spaced packets of hairpin vortices. Lateral growth of the spot occurs as new hairpin vortices form along the spanwise edges of the spot. The formation of these hairpin vortices is attributed to unstable shear layers that develop in the streamwise–spanwise plane due to the wall-normal motions induced by the streamwise oriented legs of hairpin vortices within the spot. Results are presented that highlight the mechanism by which the instability of such shear layers forms wavepackets of hairpin vortices; how the formation of these vortices produces a flow environment that promotes the creation of new hairpin vortices; and how the newly created hairpin vortices impact the production of turbulence kinetic energy in the flow region surrounding the spot. A quantitative description of the hairpin-vortex regeneration mechanism based on the transport of the instantaneous vorticity vector is presented to illustrate how the velocity and vorticity fields interact with the local strain rates to promote the growth of coherent vortical structures. The simulation results also shed light on a mechanism that seems to have a dominant influence on the formation of the calmed region in the wake of the turbulent spot.

scholarly journals Optimal wave packets in a boundary layer and initial phases of a turbulent spot

2010 ◽  
Vol 656 ◽  
pp. 231-259 ◽  
Author(s):  
S. CHERUBINI ◽  
J.-C. ROBINET ◽  
A. BOTTARO ◽  
P. DE PALMA
Keyword(s):  
Boundary Layer ◽  
Wave Packet ◽  
Scaling Law ◽  
Initial Perturbation ◽  
Three Dimensional ◽  
Plate Boundary ◽  
Optimal Time ◽  
Spanwise Direction ◽  
Turbulent Spot ◽  
Hairpin Vortices

The three-dimensional global optimal dynamics of a flat-plate boundary layer is studied by means of an adjoint-based optimization in a spatial domain of long – but finite – streamwise dimension. The localized optimal initial perturbation is characterized by a pair of streamwise-modulated counter-rotating vortices, tilted upstream, yielding at the optimal time elongated streaks of alternating sign in the streamwise direction. This indicates that perturbations with non-zero streamwise wavenumber have a role in the transient dynamics of a boundary layer. A scaling law is provided, describing the variation of the streamwise modulation of the optimal initial perturbation with respect to the streamwise domain length and to the Reynolds number. For spanwise-extended domains, a near-optimal three-dimensional perturbation is extracted during the optimization process; it is localized also in the spanwise direction, resulting in a wave packet of elongated disturbances modulated in the spanwise and streamwise directions. The nonlinear evolution of the optimal and near-optimal perturbations is investigated by means of direct numerical simulations. Both perturbations are found to induce transition at lower levels of the initial energy than local optimal and suboptimal perturbations. Moreover, it is observed that transition occurs in a well-defined region of the convected wave packet, close to its centre, via a mechanism including at the same time oscillations of the streaks of both quasi-sinuous and quasi-varicose nature. Hairpin vortices are observed before transition; they have an active role in the breakdown of the streaks and result in a turbulent spot which spreads out in the boundary layer.

Substructures in a turbulent spot

1988 ◽  
Vol 197 ◽  
pp. 389-414 ◽  
Author(s):  
R. Sankaran ◽  
M. Sokolov ◽  
R. A. Antonia
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Leading Edge ◽  
Simple Relation ◽  
Sufficient Evidence ◽  
Streamwise Direction ◽  
Geometrical Construction ◽  
Turbulent Spot ◽  
Hairpin Vortices ◽  
Streamwise Distance

Substructures within a turbulent spot which develops in a slightly heated laminar boundary layer have been identified using arrays of cold wires aligned in either a streamwise direction or in a direction normal to the wall. At any given streamwise distance from the spot origin, histograms of the number of detected substructures exhibit a peak, defining the most probable spot or the spot with the most likely number of substructures. The number of substructures in the most probable spot increases with streamwise distance but all substructures are convected at approximately the same velocity for any given distance from the wall. This velocity is approximately equal to that of the leading edge of the spot and increases slightly with distance from the wall. The increase in the number of substructures accounts for the streamwise growth of the spot. A simple relation is derived for determining the number of substructures at a particular streamwise station and a geometrical construction is proposed for identifying the origin of a new substructure. There is sufficient evidence for suggesting that the new substructures are formed near the trailing edge of the spot. The convection velocity, inclination and lengthscales of the substructures compare favourably with the corresponding characteristics of hairpin vortices.

Numerical Study of Unsteady Cavitating Flow in an Inducer with Omega Vortex Identification

2022 ◽  
Author(s):  
Yan Longlong ◽  
Bo Gao ◽  
Dan Ni ◽  
Ning Zhang ◽  
Wenjie Zhou
Keyword(s):  
Numerical Study ◽  
Pressure Fluctuations ◽  
Cavitating Flow ◽  
Vortical Flow ◽  
Flow Structures ◽  
Clear Correlation ◽  
Vortex Identification ◽  
Design Point ◽  
Identification Method ◽  
Unsteady Cavitating Flow

Abstract To accurately capture the behaviors of cavitation and reveal the unsteady cavitating flow mechanism, a condensate pump inducer is numerically analyzed in a separate numerical experiment with LES at critical cavitation number sind,c under the design point. Based on the new Omega vortex identification method, the correction between the flow structures and cavities is clearly illustrated. Besides, the pressure fluctuations around the inducer are analyzed. Special emphasis is put on the analysis of the interactions between the cavities, turbulent fluctuations, and vortical flow structures. The Omega vortex identification method could give an overall picture of the whole cavitating flow structures to present a clear correlation between the vortices and cavities. The results show that the shear cavitation dominant the cavitation characteristics under the design point. The pure rigid rotation region mainly concentrates at the edge of the cavities while the other sheet-like cavities near the casing walls are characterized by strong turbulence fluctuations. Besides, based on the analysis of the correlation between the cavities and flow structures, the rotating cavitation under the design point may mainly attribute to the interaction between the tip leakage vortex cavitation and the next blade.

Numerical study of the motions within a slowly precessing sphere at low Ekman number

2001 ◽  
Vol 437 ◽  
pp. 283-299 ◽  
Author(s):  
JÉRÔME NOIR ◽  
D. JAULT ◽  
P. CARDIN
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Numerical Study ◽  
Ekman Layer ◽  
Shear Layers ◽  
Nonlinear Interactions ◽  
Geostrophic Velocity ◽  
Ekman Number ◽  
Geostrophic Shear ◽  
Good Agreement

A geostrophic circulation and a pair of oblique oscillating shear layers arise in a spherical uid cavity contained in a slowly precessing rigid body. Both are caused by the breakdown of the Ekman boundary layer at two critical circles. We rely on numerical modelling to characterize these motions for very small Ekman numbers. Both the O(E1/5) amplitude of the velocity in the oscillating shear layer and the width (also O(E1/5)) of these oblique layers are the result of in ux into the interior from the regions where the Ekman layer breaks down. The oscillating motions are confined to narrow shear layers and their amplitude decays exponentially away from the characteristic surfaces. Nonlinear interactions inside the boundary layer drive the geostrophic shear layer attached to the critical circles. This steady layer, again of O(E1/5) thickness, contains O(E−3/10) velocities. Our results are in good agreement with the experimental measurement by Malkus of the geostrophic velocity arising in a slowly precessing spheroid.

Measurements of the Effects of Streamwise Riblets on Boundary Layer Turbulence

2017 ◽  
Vol 139 (11) ◽  
Author(s):  
Derek B. Ancrum ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Coherent Structures ◽  
Wave Packets ◽  
Effective Control ◽  
Turbulent Spot ◽  
Hairpin Vortices ◽  
Cross Sectional ◽  
Vortical Structures ◽  
Turbulent Fluctuations ◽  
Boundary Layer Turbulence

This study presents experimental results on the effects of riblets on the coherent structures of turbulence within a turbulent spot. The riblet spacings of the study correspond to 0.5 and 1.5 times the natural spacing of the low-speed streak. The cross-sectional dimensions of the riblets were chosen to control the spatial distribution of wave packets consisting of streamwise-aligned hairpin vortices. Both riblet spacings demonstrated effective control on the spanwise positioning of the wave packets. The wider-spaced riblets reduced spanwise mutual interaction between wave packets. The closer-spaced riblets promoted this interaction via spanwise-oriented vortical structures which produced stronger turbulent fluctuations.

Hairpin vortices and highly elongated flow structures in a stably stratified shear layer

2019 ◽  
Vol 878 ◽  
pp. 37-61
Author(s):  
Tomoaki Watanabe ◽  
James J. Riley ◽  
Koji Nagata ◽  
Keigo Matsuda ◽  
Ryo Onishi
Keyword(s):  
Turbulent Mixing ◽  
Reynolds Numbers ◽  
Shear Layers ◽  
Flow Structures ◽  
Late Time ◽  
Large Contribution ◽  
Turbulent Structures ◽  
Hairpin Vortices ◽  
Shear Parameter ◽  
O 10

Turbulent structures in stably stratified shear layers are studied with direct numerical simulation. Flow visualization confirms the existence of hairpin vortices and highly elongated structures with positive and negative velocity fluctuations, whose streamwise lengths divided by the layer thickness are $O(10^{0})$ and $O(10^{1})$, respectively. The flow at the wavelength related to these structures makes a large contribution to turbulent kinetic energy. These structures become prominent in late time, but with small buoyancy Reynolds numbers indicating suppression of turbulent mixing. Active turbulent mixing associated with the hairpin vortices, however, does occur. The structures and the vertical profile of the integral shear parameter show connections between stable stratified shear layers and wall-bounded shear flows.

Noise generation mechanisms for a supersonic jet impinging on an inclined plate

2016 ◽  
Vol 797 ◽  
pp. 802-850 ◽  
Author(s):  
Christoph Brehm ◽  
Jeffrey A. Housman ◽  
Cetin C. Kiris
Keyword(s):  
Sound Pressure ◽  
Acoustic Noise ◽  
Vortical Flow ◽  
Flow Structures ◽  
Wall Jet ◽  
Far Field ◽  
Noise Generation ◽  
Inclined Plate ◽  
Coherent Flow Structures ◽  
Generation Mechanisms

Noise generation mechanisms for a perfectly expanded supersonic Mach number $M=1.8$ turbulent jet impinging on a $45^{\circ }$ inclined plate are investigated for a Reynolds number of $1.6\times 10^{6}$ employing a large-eddy simulation. Excellent comparisons with experimental acoustic far-field measurements and pressure measurements on the impingement plate are obtained. Two local maxima are identified in the far-field overall sound pressure levels in the $75^{\circ }$ and $120^{\circ }$ observer directions, which are associated with different noise generation mechanisms. The peak frequencies in the spectra with Strouhal numbers of $St=0.2$ for $75^{\circ }$ and $St=0.5$ for $120^{\circ }$ match the experimental measurements. The jet-impingement region generates pressure waves that propagate predominantly in the $120^{\circ }$ observer direction. The noise generation in this region is attributed to vortex stretching and tearing during shear-layer impingement, and shock oscillations that are induced by the motion of downstream convected vortical flow structures. The second peak in the overall sound pressure distribution at $75^{\circ }$ is associated with noise sources located in the wall jet. The noise generation in the wall jet is associated with supersonically convecting large-scale coherent flow structures that also interact with tail shocks in the wall jet causing large localized pressure fluctuations. Strongly coherent flow structures are identified by applying proper orthogonal decomposition (POD) to the unsteady flow field. The frequency characteristics of the most energetic POD modes are distinctly different based on which energy norm is chosen. The most energetic entropy-based POD modes contain a peak frequency of approximately $St=0.4{-}0.6$, while the most energetic turbulent kinetic-energy-based POD modes appear to be dominated by lower-frequency content. The causality method, based on Lighthill’s acoustic analogy, is used to link the acoustic noise signature to the relevant physical mechanisms in the source region. A differentiation is made between the application of normalized and non-normalized cross-correlation functions for noise source identification and characterization.

Coherent structures in wave boundary layers. Part 1. Oscillatory motion

2010 ◽  
Vol 646 ◽  
pp. 169-206 ◽  
Author(s):  
STEFAN CARSTENSEN ◽  
B. MUTLU SUMER ◽  
JØRGEN FREDSØE
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Boundary Layers ◽  
Flow Structures ◽  
Bed Shear Stress ◽  
Turbulent Spots ◽  
Onset Of Turbulence ◽  
Stress Signal ◽  
Coherent Flow Structures ◽  
Layer Flow

This work concerns oscillatory boundary layers over smooth beds. It comprises combined visual and quantitative techniques including bed shear stress measurements. The experiments were carried out in an oscillating water tunnel. The experiments reveal two significant coherent flow structures: (i) Vortex tubes, essentially two-dimensional vortices close to the bed extending across the width of the boundary-layer flow, caused by an inflectional-point shear layer instability. The imprint of these vortices in the bed shear stress is a series of small, insignificant kinks and dips. (ii) Turbulent spots, isolated arrowhead-shaped areas close to the bed in an otherwise laminar boundary layer where the flow ‘bursts’ with violent oscillations. The emergence of the turbulent spots marks the onset of turbulence. Turbulent spots cause single or multiple violent spikes in the bed shear stress signal, which has profound implications for sediment transport (in both the laboratory and the field). The experiments also show that similar coherent flow structures exist in the case of combined oscillatory flow and current.

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