On the dynamical relevance of coherent vortical structures in turbulent boundary layers

2010 ◽  
Vol 648 ◽  
pp. 325-349 ◽  
Author(s):  
SERGIO PIROZZOLI ◽  
MATTEO BERNARDINI ◽  
FRANCESCO GRASSO
Keyword(s):  
Viscous Sublayer ◽  
Momentum Balance ◽  
Substantial Contribution ◽  
Vortex Sheets ◽  
Vortical Structures ◽  
Buffer Region ◽  
Non Local ◽  
Low Speed Streaks ◽  
Vortex Tubes ◽  
Stream Direction

The dynamical relevance of vortex tubes and vortex sheets in a wall-bounded supersonic turbulent flow at Mach numberM= 2 and Reynolds numberReθ≈ 1350 is quantitatively analysed. The flow in the viscous sublayer and in the buffer region is characterized by intense, elongated vorticity tongues forming a shallow angle with respect to the wall, whose characteristic length isO(200) wall units and whose size in the cross-stream direction isO(50) wall units. The formation of vortex tubes takes place starting fromy+≈ 10, and it is mainly associated with the roll-up and the interaction of vortex sheets. The analysis of the non-local dynamical effect of tubes and sheets suggests that the latter have a more important collective effect, being closely associated with low-speed streaks, and being responsible for a substantial contribution to the mean momentum balance and to the production of turbulence kinetic energy and enstrophy.

On the near-wall characteristics of acceleration in turbulence

2010 ◽  
Vol 659 ◽  
pp. 405-419 ◽  
Author(s):  
K. YEO ◽  
B.-G. KIM ◽  
C. LEE
Keyword(s):  
Turbulent Flows ◽  
Streamwise Vortex ◽  
Reynolds Numbers ◽  
Viscous Sublayer ◽  
Centripetal Acceleration ◽  
Vortical Structures ◽  
Turbulent Channel Flows ◽  
Background Shear ◽  
Kolmogorov Constant ◽  
Near Wall

The behaviour of fluid-particle acceleration in near-wall turbulent flows is investigated in numerically simulated turbulent channel flows at low to moderate Reynolds numbers, Reτ = 180~600). The acceleration is decomposed into pressure-gradient (irrotational) and viscous contributions (solenoidal acceleration) and the statistics of each component are analysed. In near-wall turbulent flows, the probability density function of acceleration is strongly dependent on the distance from the wall. Unexpectedly, the intermittency of acceleration is strongest in the viscous sublayer, where the acceleration flatness factor of O(100) is observed. It is shown that the centripetal acceleration around coherent vortical structures is an important source of the acceleration intermittency. We found sheet-like structures of strong solenoidal accelerations near the wall, which are associated with the background shear modified by the interaction between a streamwise vortex and the wall. We found that the acceleration Kolmogorov constant is a linear function of y+ in the log layer. The Reynolds number dependence of the acceleration statistics is investigated.

Spatial and statistical analysis of coherent vortical structures within a stress-driven free-surface turbulent shear flow

2020 ◽  
Author(s):  
Po-Chen Chen ◽  
Wu-ting Tsai
Keyword(s):  
Free Surface ◽  
High Speed ◽  
Surface Friction ◽  
Turbulent Shear ◽  
Wind Condition ◽  
Turbulent Shear Flow ◽  
Streamwise Vortices ◽  
Vortical Structures ◽  
Horseshoe Vortices ◽  
Low Speed Streaks

<p>The water surface under high wind condition is characterized by elongated high-speed streaks and randomly emerged low-speed streaks, which are attributed to underneath coherent vortical motions. These vortical structures within aqueous turbulent boundary layer plays a critical role in turbulent exchange, their characteristics and statistics are therefore of interest in this study. Direct numerical simulation of an aqueous turbulent flow bounded by a stress-driven flat free surface was performed. Simulation results of cases with high wind condition (surface friction velocity = 1.22 cm/s) as well as weak wind condition (surface friction velocity = 0.71 cm/s) are analyzed. To identify the underlying vortical structures, an indicator of swirling strength derived from local velocity-gradient tensor is adopted. A formal classification scheme, based on the topological geometry of the vortex core, is then applied to classify the identified structures. Surface layers with the two wind conditions reveal similar results in statistics and spatial distribution of vortical structures. Two types of characteristic vortices which induce the surface streaks are extracted, including quasi-streamwise vortex and reversed horseshoe vortex (head pointing upstream), most inclining at about 10 to 20 degrees. Quasi-streamwise vortices are the dominant structure, and both high- and low-speed streaks are fringed with such vortices; they adjoin the surface streaks as counter-rotating arrays in either staggered or side-by-side spatial arrangement. The length of quasi-streamwise vortices, however, are significantly shorter than the corresponding surface streaks, only 10% of the extracted quasi-streamwise vortices are longer than 150 wall units. Reversed horseshoe vortices, associated with downwelling motions and surface convergence, are located beneath the high-speed streaks. In contrast to the turbulent boundary layer next to a flat wall, typical forward horseshoe vortices (head pointing downstream) associated with upwelling motions are barely found within the free-surface turbulent shear flow.</p><p>This work was supported by the Taiwan Ministry of Science and Technology (MOST 107-2611-M-002 -014 -MY3).</p>

Identifying the tangle of vortex tubes in homogeneous isotropic turbulence

2019 ◽  
Vol 874 ◽  
pp. 952-978 ◽  
Author(s):  
Shiying Xiong ◽  
Yue Yang
Keyword(s):  
Isotropic Turbulence ◽  
Time Instant ◽  
Small Scale ◽  
Homogeneous Isotropic Turbulence ◽  
Complex Flow ◽  
Time Step ◽  
Vortex Sheets ◽  
Helical Geometry ◽  
Vortex Tubes

We extend the vortex-surface field (VSF), whose isosurface is a vortex surface consisting of vortex lines, to identify vortex tubes and sheets in homogeneous isotropic turbulence. The VSF at a time instant is constructed by solving a pseudo-transport equation. This equation is convected by a given instantaneous vorticity obtained from direct numerical simulation. In each pseudo-time step, we develop a novel local optimization algorithm to minimize a hybrid VSF constraint, balancing the accuracy and smoothness of VSF solutions. This key improvement makes the numerical construction of VSFs feasible for arbitrarily complex flow fields, as a general flow diagnostic tool. In the visualization of VSF isosurfaces in decaying homogeneous isotropic turbulence, the initial curved vortex sheets first evolve into vortex tubes, and then the vortex tubes are stretched and tangled, constituting a complex network. Some vortex tubes exhibit helical geometry, which suggests the important role of vortex twisting in the generation of small-scale structures in energy cascade.

A study of streamwise vortex structure in a stratified shear layer

1994 ◽  
Vol 281 ◽  
pp. 247-291 ◽  
Author(s):  
David G. Schowalter ◽  
Charles W. Van Van Atta ◽  
Juan C. Lasheras
Keyword(s):  
Shear Layer ◽  
Three Dimensional ◽  
Streamwise Vortex ◽  
Streamwise Vorticity ◽  
Vortical Structures ◽  
Flow Evolution ◽  
Spanwise Vorticity ◽  
Unstable Part ◽  
Dimensional Instability ◽  
Vortex Tubes

The existence of an organized streamwise vortical structure, which is superimposed on the well known coherent spanwise vorticity in nominally two-dimensional free shear layers, has been studied extensively. In the presence of stratification, however, buoyancy forces contribute to an additional mechanism for the generation of streamwise vorticity. As the spanwise vorticity layer rolls up and pulls high-density fluid above low-density fluid, a local instability results. The purpose of the current investigation is to force the three-dimensional instability in the stratified shear layer. In this manner, we experimentally observe the effect of buoyancy on the streamwise vortex tube evolution, the evolution of the buoyancy-induced instability, and the interaction between these two vortical structures. A simple numerical model is proposed which captures the relevant physics of the flow evolution. It is found that, depending on the location, streamwise vortices resulting from vortex stretching may be weakened or enhanced by the stratification. Buoyancy-induced vortex structures are shown to form where the unstable part of the interface is tilted by the streamwise vortex tubes. These vortices strengthen initially, then weaken downstream, the timescale for this process depending upon the degree of stratification. For initial Richardson numbers larger than about 0.03, the baroclinically weakened vortex tubes eventually disappear as the flow evolves downstream and the baroclinically generated vortices dominate the three-dimensional flow structure.

On the structure of vortex rings from inclined nozzles

2011 ◽  
Vol 686 ◽  
pp. 451-483 ◽  
Author(s):  
Trung Bao Le ◽  
Iman Borazjani ◽  
Seokkoo Kang ◽  
Fotis Sotiropoulos
Keyword(s):  
Vortex Ring ◽  
Vortex Sheet ◽  
Vortex Rings ◽  
Cylinder Wall ◽  
Primary Vortex ◽  
Vortical Structures ◽  
Ambient Flow ◽  
Piston Cylinder ◽  
Axial Stretching ◽  
Vortex Tubes

AbstractWe carry out numerical simulations to investigate the vortex dynamics of laminar, impulsively driven flows through inclined nozzles in a piston–cylinder apparatus. Our simulations are motivated by the need to provide a complete description of the intricate vortical structures and governing mechanisms emerging in such flows as documented in the experiments of Webster & Longmire (Phys. Fluids, vol. 10, 1998, pp. 400–416) and Troolin & Longmire (Exp. Fluids, vol. 48, 2010, pp. 409–420). We show that the flow is dominated by the interaction of two main vortical structures: the primary inclined vortex ring at the nozzle exit and the secondary stopping ring that arises due to the entrainment of the flow into the cylinder when the piston stops moving. These two structures are connected together with pairs of vortex tubes, which evolve from the continuous vortex sheet initially connecting the primary vortex ring with the interior cylinder wall. In the exterior of the nozzle the key mechanism responsible for the breakup of the vortical structure is the interaction of the stronger inclined primary ring with the weaker stopping ring near the longest lip of the nozzle. In the interior of the nozzle the dynamics is governed by the axial stretching of the secondary ring and the ultimate impingement of this ring on the cylinder wall. Our simulations also clarify the kinematics of the azimuthal flow along the core of the primary vortex ring documented in the experiments by Lim (Phys. Fluids, vol. 10, 1998, pp. 1666–1671). We show that the azimuthal flow is characterized by a pair of two spiral saddle foci at the long and short lips of the nozzle through which ambient flow enters and exits the primary vortex core.

scholarly journals Particle-driven gravity currents

1993 ◽  
Vol 250 ◽  
pp. 339-369 ◽  
Author(s):  
Roger T. Bonnecaze ◽  
Herbert E. Huppert ◽  
John R. Lister
Keyword(s):  
Particle Concentration ◽  
Local Density ◽  
Gravity Current ◽  
Pressure Head ◽  
Viscous Sublayer ◽  
Gravity Currents ◽  
Momentum Balance ◽  
Exchange Flow ◽  
Mean Flow ◽  
Ambient Fluid

Gravity currents created by the release of a fixed volume of a suspension into a lighter ambient fluid are studied theoretically and experimentally. The greater density of the current and the buoyancy force driving its motion arise primarily from dense particles suspended in the interstitial fluid of the current. The dynamics of the current are assumed to be dominated by a balance between inertial and buoyancy forces; viscous forces are assumed negligible. The currents considered are two-dimensional and flow over a rigid horizontal surface. The flow is modelled by either the single- or the two-layer shallow-water equations, the two-layer equations being necessary to include the effects of the overlying fluid, which are important when the depth of the current is comparable to the depth of the overlying fluid. Because the local density of the gravity current depends on the concentration of particles, the buoyancy contribution to the momentum balance depends on the variation of the particle concentration. A transport equation for the particle concentration is derived by assuming that the particles are vertically well-mixed by the turbulence in the current, are advected by the mean flow and settle out through the viscous sublayer at the bottom of the current. The boundary condition at the moving front of the current relates the velocity and the pressure head at that point. The resulting equations are solved numerically, which reveals that two types of shock can occur in the current. In the late stages of all particle-driven gravity currents, an internal bore develops that separates a particle-free jet-like flow in the rear from a dense gravity-current flow near the front. The second type of bore occurs if the initial height of the current is comparable to the depth of the ambient fluid. This bore develops during the early lock-exchange flow between the two fluids and strongly changes the structure of the current and its transport of particles from those of a current in very deep surroundings. To test the theory, several experiments were performed to measure the length of particle-driven gravity currents as a function of time and their deposition patterns for a variety of particle sizes and initial masses of sediment. The comparison between the theoretical predictions, which have no adjustable parameters, and the experimental results are very good.

Vorticity dynamics in a breaking internal gravity wave. Part 2. Vortex interactions and transition to turbulence

1998 ◽  
Vol 367 ◽  
pp. 47-65 ◽  
Author(s):  
DAVID C. FRITTS ◽  
STEVE ARENDT ◽  
ØYVIND ANDREASSEN
Keyword(s):  
Gravity Wave ◽  
Internal Gravity Wave ◽  
Companion Paper ◽  
Transition To Turbulence ◽  
Single Vortex ◽  
Vortex Sheets ◽  
Initial Formation ◽  
Vortex Interactions ◽  
Vorticity Dynamics ◽  
Vortex Tubes

A companion paper (Part 1) employed a three-dimensional numerical simulation to examine the vorticity dynamics of the initial instabilities of a breaking internal gravity wave in a stratified, sheared, compressible fluid. The present paper describes the vorticity dynamics that drive this flow to smaller-scale, increasingly isotropic motions at later times. Following the initial formation of discrete and intertwined vortex loops, the most important interactions are the self-interactions of single vortex tubes and the mutual interactions of multiple vortex tubes in close proximity. The initial formation of vortex tubes from the roll-up of localized vortex sheets gives the vortex tubes axial variations with both axisymmetric and azimuthal-wavenumber-2 components. The axisymmetric variations excite axisymmetric twist waves or Kelvin vortex waves which propagate along the tubes, drive axial flows, deplete the tubes' cores, and fragment the tubes. The azimuthal-wavenumber-2 variations excite m=2 twist waves on the vortex tubes, which undergo strong amplification and unravel single vortex tubes into pairs of intertwined helical tubes; the vortex tubes then burst or fragment. Reconnection often occurs among the remnants of such vortex fragmentation. A common mutual interaction is that of orthogonal vortex tubes, which causes mutual stretching and leads to long-lived structures. Such an interaction also sometimes creates an m=1 twist wave having an approximately steady helical form as well as a preferred sense of helicity. Interactions among parallel vortex tubes are less common, but include vortex pairing. Finally, the intensification and roll-up of weaker vortex sheets into new tubes occurs throughout the evolution. All of these vortex interactions result in a rapid cascade of energy and enstrophy toward smaller scales of motion.

scholarly journals Quantized vortices in pionic superfluid

2022 ◽  
Vol 258 ◽  
pp. 10008
Author(s):  
Oleg Teryaev ◽  
Valentin Zakharov
Keyword(s):  
Experimental Investigation ◽  
Heavy Ion ◽  
Quantized Vortices ◽  
The Other ◽  
Vortex Rings ◽  
Vortex Sheets ◽  
Ion Collisions ◽  
Local Polarization ◽  
Vortex Tubes

The interplay between classical vorticity being the main undisputed source of polarization in heavy-ion collisions (HIC) and quantized vortices is considered. The vortex tubes emerging in the rotating pionic (super) fluid polarize the baryons in their cores and explain the emerging global polarization. The appearance of vortices in the region separating participants and spectators in non-central HIC is similar to that for sliding layers of liquid helium. From the other side, it is also the region where the classical vorticity was earlier found to be large forming the vortex sheets. The formation of tubes manifests a threshold at certain critical vorticity implying the vanishing polarization at lower energies. For central HIC the compact jet-like flows may lead to formation of vortex rings related to local polarization. The P-odd momentum correlations for their experimental investigation are suggested. The role of shear and viscosity in the emergence of polarization is discussed.

Coherent structures near the wall in a turbulent channel flow

1997 ◽  
Vol 332 ◽  
pp. 185-214 ◽  
Author(s):  
J. Jeong ◽  
F. Hussain ◽  
W. Schoppa ◽  
J. Kim
Keyword(s):  
Channel Flow ◽  
Coherent Structures ◽  
Turbulent Channel Flow ◽  
Normal Velocity ◽  
Reynolds Stresses ◽  
Hairpin Vortices ◽  
Vortical Structures ◽  
Instantaneous Flow Field ◽  
Strong Shear ◽  
Low Speed Streaks

Coherent structures (CS) near the wall (i.e.y+ ≤ 60) in a numerically simulated turbulent channel flow are educed using a conditional sampling scheme which extracts the entire extent of dominant vortical structures. Such structures are detected from the instantaneous flow field using our newly developed vortex definition (Jeong & Hussain 1995) - a region of negativeλ2, the second largest eigenvalue of the tensorSikSkj+ ΩikΩkj- which accurately captures the structure details (unlike velocity-, vorticity- or pressure-based eduction). Extensive testing has shown thatλ2correctly captures vortical structures, even in the presence of the strong shear occurring near the wall of a boundary layer. We have shown that the dominant near-wall educed (i.e. ensemble averaged after proper alignment) CS are highly elongated quasi-streamwise vortices; the CS are inclined 9° in the vertical (x, y)-plane and tilted ±4° in the horizontal (x, z)-plane. The vortices of alternating sign overlap inxas a staggered array; there is no indication near the wall of hairpin vortices, not only in the educed data but also in instantaneous fields. Our model of the CS array reproduces nearly all experimentally observed events reported in the literature, such as VITA, Reynolds stress distribution, wall pressure variation, elongated low-speed streaks, spanwise shear, etc. In particular, a phase difference (in space) between streamwise and normal velocity fluctuations created by CS advection causes Q4 ('sweep’) events to dominate Q2 ('ejection’) and also creates counter-gradient Reynolds stresses (such as Ql and Q3 events) above and below the CS. We also show that these effects are adequately modelled by half of a Batchelor's dipole embedded in (and decoupled from) a background shearU(y). The CS tilting (in the (x, z)-plane) is found to be responsible for sustaining CS through redistribution of streamwise turbulent kinetic energy to normal and spanwise components via coherent pressure-strain effects.

Impacts of global re-/afforestation and deforestation on large scale atmospheric circulation

2020 ◽  
Author(s):  
Iris Manola ◽  
Dim Coumou ◽  
Andrea Alessandri ◽  
Edouard Davin ◽  
Suqi Guo ◽  
...  
Keyword(s):  
Atmospheric Circulation ◽  
Large Scale ◽  
Crop Yields ◽  
Dominant Role ◽  
Momentum Balance ◽  
Boreal Summer ◽  
Earth System ◽  
Model Experiments ◽  
Non Local ◽  
Energy And Momentum

<p>Land cover and land management (LCLM) changes have a high potential to influence the biogeophysical and biogeochemical earth system processes. The interaction of soil and vegetation with the atmosphere alternates the water, energy and momentum balance, in turn affecting the climate locally, as well as the climate of distant regions through teleconnection pathways. This, among others, might benefit or oppose risks to local and global breadbasket regions, impacting the crop yields.</p> <p>In this study, we conduct model experiments to assess the local and remote impact of LCLM changes, in particular global re-/afforestation and deforestation, with a focus on the large-scale boreal summer atmospheric circulation. We hypothesize that due to the dominant role of land-atmosphere feedbacks in this season, robust dynamical transformations take place due to the LCLM changes. The idealized model experiments consist of three fully coupled Earth System Models (EC-EARTH, MPI-ESM and CESM) that run under constant 2015 greenhouse forcing for 150 years. Globally the LCLM changes go through a sequence of unchanged grid boxes in a checkerboard approach as recent studies have done, in order to accurately separate the local from the non-local effects.</p>

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