Oblique Bands Research Articles

Occurrence of Stenurus globicephalae (Nematoda: Pseudaliidae) in the blowhole of Globicephala macrorhynchus (Cetacea: Delphinidae) in Tasmania, Australia.

Information about parasites of cetaceans in Australia is scarce and mostly opportunistic. The morphology of specimens of the metastrongyloid Stenurus globicephalae Baylis & Daubney, 1925 (Nematoda: Pseudaliidae), collected from the blowhole of a pilot whale Globicephala macrorhynchus Gray, 1846 (Cetacea: Delphinidae) off northern Tasmania, Australia, were studied. Light and scanning electron microscopical examinations enabled a detailed redescription of this nematode species, including corrections of some inaccuracies in previous species descriptions, particularly those concerning cephalic and caudal structures. The presence of numerous ventrolateral oblique muscle bands, characteristic of the males of S. globicephalae, is reported for the first time. This is the second finding of this nematode parasite, in a different host species, in Tasmania.

Structured input–output analysis of oblique laminar–turbulent patterns in plane Couette–Poiseuille flow

In this work, we employ structured input–output analysis to study the flow patterns in transitional plane Couette–Poiseuille flow (CPF). First, we focus on the well-studied intermediate laminar profile, which balances the shear and pressure effects. We show that the highest structured gain corresponds to perturbations with wavelengths associated with the oblique turbulent bands observed in experiments. In addition, the inclination angles of these structures show a Reynolds number dependence consistent with experimentally observed trends. We then examine the Reynolds number scaling of the maximal structured frequency response as the velocity profile is varied from plane Couette to Poiseuille flow. Our results demonstrate that, as expected, the scaling exponent increases over this range, but this increase is not monotonic. We attribute the variation in the shape of the scaling exponent curve as a function of the velocity profile shape to changes in flow patterns that dominate each associated flow regime. We then focus on the particular case of plane Couette flow and compute the structured response modes. Their behavior and structural features are consistent with results obtained through direct numerical simulation (DNS) studies. Finally, we employ the structured analysis framework to examine the temporal evolution of the dominant structures. For the well-studied cases of plane Couette and plane Poiseuille flows, the computed advection speeds of the oblique structures are consistent with those predicted through DNS.

Description and phylogenetic position of a new species of Rhyacoglanis (Siluriformes: Pseudopimelodidae) from the Jamanxim River basin

Abstract In this study, a new species of Rhyacoglanis is described from the Jamanxim River basin, Tapajós River basin. The new species differs from congeners based on the combination of the following diagnostic characters: two oblique dark bands formed by an agglomerate of melanophores on the predorsal region; dorsal confluence between the dark subdorsal and subadipose bands in large juveniles and adults; ventral confluence between the dark subadipose and caudal peduncle bands; body without conspicuous dark brown spots; complete dark band on caudal peduncle; body with three dark bands; a thin dark caudal-fin band; pectoral-fin spine with anterior serrae distributed along the entire margin; the posterior tip of the post-cleithral process reaching vertical through the base of the dorsal-fin spine; and hypural 5 free of hypural 3 and 4 and pointed caudal-fin lobes. Additionally, our molecular phylogenetic results using ultraconserved elements (UCEs) corroborate the new species as Rhyacoglanis and sister to an undescribed species of Rhyacoglanis from the Xingu River basin. Moreover, as pointed out in previous studies, we confirm Cruciglanis as a sister group to Pseudopimelodus plus Rhyacoglanis.

3D numerical modelling and simulation of the impact of fault zones on fluid flow in sandstones of the Rio do Peixe Basin, NE Brazil

Deformation bands are usually responsible for a reduction in permeability perpendicular to the structure planes of up to three orders of magnitude, while the fault core represent a reduction of up to seven orders of magnitude in cross-fault permeability, imposing large anisotropies to fluid flow. As deformation bands occur distributed along the damage zone, they impact not only the across-fault flow but also the along-fault flow. The fault core is usually represented using fault-transmissibility multipliers (TMs) along the fault planes, using well-established workflows. However, there is a lack of methods to represent fault damage zones in any direction and grid-cell sizes. In this context, we proposed new methods to: (1) estimate the deformation intensity in damage zones; (2) calculate their most representative value within the cell domain; and (3) calculate the equivalent permeability of a cell containing oblique deformation bands. The workflow is applied to a 3D numerical model for the Santa Helena High in the Rio do Peixe Basin, NE Brazil. We performed streamline simulations in four models to evaluate the impact of fault damage zones and the fault core in fluid flow. Our models show that the fault core and damage zone negatively affected the performance of the reservoir. Supplementary material : A description of the method developed to estimate the deformation intensity in damage zones and of the method developed to calculate the equivalent permeability are available at https://doi.org/10.6084/m9.figshare.c.6251469

Structured input–output analysis of stably stratified plane Couette flow

We employ a recently introduced structured input–output analysis (SIOA) approach to analyse streamwise and spanwise wavelengths of flow structures in stably stratified plane Couette flow. In the low-Reynolds-number ( $Re$ ) low-bulk Richardson number ( $Ri_b$ ) spatially intermittent regime, we demonstrate that SIOA predicts high amplification associated with wavelengths corresponding to the characteristic oblique turbulent bands in this regime. SIOA also identifies quasi-horizontal flow structures resembling the turbulent–laminar layers commonly observed in the high- $Re$ high- $Ri_b$ intermittent regime. An SIOA across a range of $Ri_b$ and $Re$ values suggests that the classical Miles–Howard stability criterion ( $Ri_b\leq 1/4$ ) is associated with a change in the most amplified flow structures when the Prandtl number is close to one ( $Pr\approx 1$ ). However, for $Pr\ll 1$ , the most amplified flow structures are determined by the product $PrRi_b$ . For $Pr\gg 1$ , SIOA identifies another quasi-horizontal flow structure that we show is principally associated with density perturbations. We further demonstrate the dominance of this density-associated flow structure in the high $Pr$ limit by constructing analytical scaling arguments for the amplification in terms of $Re$ and $Pr$ under the assumptions of unstratified flow (with $Ri_b=0$ ) and streamwise invariance.

Sidewall effect on turbulent band in subcritical transition of high-aspect-ratio duct flow

We numerically studied high-aspect-ratio channel flows with spanwise sidewalls, that is, wide duct flows, in its subcritical turbulent transitional regime. The infinite channel flow is known to form large-scale intermittency of turbulent–laminar coexistence and undergo two-stage transition (or crossover transition) process: a second-order phase transition with a critical Reynolds number Rec≈1000 and a deviation from it to maintain turbulence down to the global critical value Reg≈700. However, a real channel must have spatial finiteness, and its effect on transition phenomena is nontrivial. With the objective of understanding the turbulence maintenance limitations in the real channel flow, we investigated the effect of spanwise finiteness on the localized turbulence and its criticality, using direct numerical simulation. In our widest duct with an aspect ratio of 1:96 in the flow cross section, turbulent bands colliding with sidewalls above Re = 1069 often stochastically reflected or reversely traveled, keeping two-dimensional intermittencies with oblique bands, similar to the channel flow, whereas, in a narrower duct of 1:24, the critical value was higher as 1151 in the steeper transition profile, forming a quasi-one-dimensional intermittency dominantly. The transition in the high-aspect-ratio duct flow was converged to Reg≈1000 as the sidewall distance was increased. The critical phenomenon differs significantly from the channel flow for all duct flows, even for high aspect ratios. Due to spatial finiteness, the duct flows become fully laminar within a finite time for Re≲1000, unlike the channel flow. Possible causes of the difference in Reg between the two systems with fixed pressure gradient and fixed flow rate are discussed.

A New Species of Parrot-Snake of the Genus Leptophis Bell, 1825 (Serpentes, Colubridae) from the Semi-Arid Region of Brazil

We describe a new species of Leptophis from the Caatinga ecoregion encompassing the semi-arid region of Brazil. Phylogenetic analysis of 16S rDNA sequences indicate that the new species is nested deep within the L. ahaetulla complex. The new species differs from all other congeners in the following unique character combination: two dorsolateral Light Emerald Green (142) to Robin's Egg Blue (161) stripes separated from each other by Pale Buff (1) to Smoke Gray (266) vertebral stripe (continuous to tail) present; loreal scale absent; maxillary teeth 21–24; ventrals 158–177; subcaudals 137–162; black spots on head absent; supracephalic plates of head not edged with black pigment; adult color pattern lacking dark oblique bands; keels absent on first dorsal scale rows; hemipenis unilobed, capitate, with undivided sulcus spermaticus. The new species is distinguished from L. ahaetulla ahaetulla by the Light Emerald Green (142) to Robin's Egg Blue (161) (in life) dorsolateral stripes separated from each other by a Pale Buff (1) to Smoke Gray (266) vertebral stripe (at least anteriorly), and by having white to Pale Sulphur Yellow (92) scales on the first (on anterior region of body) to fourth (midbody region) scale rows [vs. Light Grass Green (109) to Light Emerald Green (142) with Yellow Ocher (14), Cinnamon-Drab (50) or Sulphur Yellow (80) vertebral stripe; second to third—occasionally the fourth—scale rows Sulphur Yellow (80), at least anteriorly], wider snout (vs. narrow), postocular stripe wider (vs. narrow), basal region of hemipenis with 10–14 spines and first row of hemipenial body with 8–9 spines (vs. 18–22 in the basal region and 5–8 in the first row). The new species differs from L. a. liocercus, which is also distributed in the Northeast Region Brazil with a small overlap in distribution, by the dorsolateral stripes (vs. dorsum unstriped), and 18–22 spines in the fourth row of the hemipenial body (vs. 11–18). Recognition of the new species is also consistent with uncorrected pairwise distances between 16S rDNA sequences.

Structured input–output analysis of transitional wall-bounded flows

Input–output analysis of transitional channel flows has proven to be a valuable analytical tool for identifying important flow structures and energetic motions. The traditional approach abstracts the nonlinear terms as forcing that is unstructured, in the sense that this forcing is not directly tied to the underlying nonlinearity in the dynamics. This paper instead employs a structured-singular-value-based approach that preserves certain input–output properties of the nonlinear forcing function in an effort to recover the larger range of key flow features identified through nonlinear analysis, experiments and direct numerical simulation (DNS) of transitional channel flows. Application of this method to transitional plane Couette and plane Poiseuille flows leads to not only the identification of the streamwise coherent structures predicted through traditional input–output approaches, but also the characterization of the oblique flow structures as those requiring the least energy to induce transition, in agreement with DNS studies, and nonlinear optimal perturbation analysis. The proposed approach also captures the recently observed oblique turbulent bands that have been linked to transition in experiments and DNS with very large channel size. The ability to identify the larger amplification of the streamwise varying structures predicted from DNS and nonlinear analysis in both flow regimes suggests that the structured approach allows one to maintain the nonlinear effects associated with weakening of the lift-up mechanism, which is known to dominate the linear operator. Capturing this key nonlinear effect enables the prediction of a wider range of known transitional flow structures within the analytical input–output modelling paradigm.

Flow visualization and skin friction determination in transitional channel flow

The present study experimentally determines the transitional Reynolds number range for plane channel flow and characterizes its transitional state. The pressure along the channel is measured to determine the skin friction coefficient as function of Reynolds number from the laminar state, through the transitional region into the fully turbulent state. The flow structure was studied through flow visualisation which shows that as the Reynolds number increases from the laminar state the transitional region starts showing randomly occurring turbulent spots. With increasing Reynolds number the spots shift into oblique patches and bands of small scale turbulence that form across the channel width, together with large-scale streaky structures found in areas between the turbulent regions. An image analysing technique was used to determine the intermittency factor, i.e. the turbulence fraction in the flow, as function of Reynolds number. It is found that the skin friction coefficient reaches its turbulent value before the flow is fully turbulent (the intermittency factor is still below one). This suggests that the observed streaky structures in non-turbulent regions contribute to the enhancement of the wall-normal transfer of momentum. Also above the Reynolds numbers where the turbulent skin friction coefficient has been established large-scale features consisting of irregular streaky structures are found. They have an oblique shape similar to the non-turbulent and turbulent patches in the transitional flow indicating that the transition process is not fully complete even above the Reynolds number where the skin friction reaches its turbulent level.Graphic abstractProcessing of flow visualization images to determine the turbulenceintermittency in the transitional region. Flow is from left to right, (a)unprocessed, (b) filtered (c) standard deviation of (b), (d) binary picture

Intermittency and Critical Scaling in Annular Couette Flow.

The onset of turbulence in subcritical shear flows is one of the most puzzling manifestations of critical phenomena in fluid dynamics. The present study focuses on the Couette flow inside an infinitely long annular geometry where the inner rod moves with constant velocity and entrains fluid, by means of direct numerical simulation. Although for a radius ratio close to unity the system is similar to plane Couette flow, a qualitatively novel regime is identified for small radius ratio, featuring no oblique bands. An analysis of finite-size effects is carried out based on an artificial increase of the perimeter. Statistics of the turbulent fraction and of the laminar gap distributions are shown both with and without such confinement effects. For the wider domains, they display a cross-over from exponential to algebraic scaling. The data suggest that the onset of the original regime is consistent with the dynamics of one-dimensional directed percolation at onset, yet with additional frustration due to azimuthal confinement effects.

Direct numerical simulations of spiral Taylor–Couette turbulence

We perform direct numerical simulations of spiral turbulent Taylor–Couette (TC) flow for $400\leqslant Re_{i}\leqslant 1200$ and $-2000\leqslant Re_{o}\leqslant -1000$ , i.e. counter-rotation. The aspect ratio $\unicode[STIX]{x1D6E4}=\text{height}/\text{gap width}$ of the domain is $42\leqslant \unicode[STIX]{x1D6E4}\leqslant 125$ , with periodic boundary conditions in the axial direction, and the radius ratio $\unicode[STIX]{x1D702}=r_{i}/r_{o}=0.91$ . We show that, with decreasing $Re_{i}$ or with decreasing $Re_{o}$ , the formation of a turbulent spiral from an initially ‘featureless turbulent’ flow can be described by the phenomenology of the Ginzburg–Landau equations, similar as seen in the experimental findings of Prigent et al. ( Phys. Rev. Lett. , vol. 89, 2002, 014501) for TC flow at $\unicode[STIX]{x1D702}=0.98$ an $\unicode[STIX]{x1D6E4}=430$ and in numerical simulations of oblique turbulent bands in plane Couette flow by Rolland & Manneville ( Eur. Phys. J. , vol. 80, 2011, pp. 529–544). We therefore conclude that the Ginzburg–Landau description also holds when curvature effects play a role, and that the finite-wavelength instability is not a consequence of the no-slip boundary conditions at the upper and lower plates in the experiments. The most unstable axial wavelength $\unicode[STIX]{x1D706}_{z,c}/d\approx 41$ in our simulations differs from findings in Prigent et al. , where $\unicode[STIX]{x1D706}_{z,c}/d\approx 32$ , and so we conclude that $\unicode[STIX]{x1D706}_{z,c}$ depends on the radius ratio $\unicode[STIX]{x1D702}$ . Furthermore, we find that the turbulent spiral is stationary in the reference frame of the mean velocity in the gap, rather than the mean velocity of the two rotating cylinders.

Patterns in Wall-Bounded Shear Flows

Experiments and numerical simulations have shown that turbulence in transitional wall-bounded shear flows frequently takes the form of long oblique bands if the domains are sufficiently large to accommodate them. These turbulent bands have been observed in plane Couette flow, plane Poiseuille flow, counter-rotating Taylor–Couette flow, torsional Couette flow, and annular pipe flow. At their upper Reynolds number threshold, laminar regions carve out gaps in otherwise uniform turbulence, ultimately forming regular turbulent–laminar patterns with a large spatial wavelength. At the lower threshold, isolated turbulent bands sparsely populate otherwise laminar domains, and complete laminarization takes place via their disappearance. We review results for plane Couette flow, plane Poiseuille flow, and free-slip Waleffe flow, focusing on thresholds, wavelengths, and mean flows, with many of the results coming from numerical simulations in tilted rectangular domains that form the minimal flow unit for the turbulent–laminar bands.

Bifurcations to turbulence in transitional channel flow

In wall-bounded parallel flows, sustained turbulence can occur even while laminar flow is still stable. Channel flow is one of such flows and displays spatio-temporal fluctuating patterns of localized turbulence along its way from/to featureless turbulence. By direct numerical simulation, we study the observed inconsistency between turbulence decay according to a two-dimensional directed-percolation (2D-DP) scenario and the presence of sustained oblique localized turbulent bands (LTBs) below the DP critical point. Above Reynolds number Reg \sim 700 sustained LTBs are observed; most LTBs have the same orientation so that the spanwise symmetry of the LTB pattern is broken below Re2 \sim 1000. The frequency of transversal splitting, by which an LTB generates another one with opposite obliqueness, so that turbulence spreading becomes intrinsically two dimensional, increases in the range Reg < Re < Re2. It reaches a critical rate at Re2 beyond which symmetry is restored. 2D-DP behavior is retrieved only above Re2. A mean-field model is proposed which qualitatively accounts for the above symmetry-restoring bifurcation by considering interactions between space-averaged densities of LTBs propagating in either direction.

Non-Native Neotropical Nesting Whitefly, &lt;i&gt;Paraleyrodes minei&lt;/i&gt; Iaccarino on Coconut Palms in India and its Co-Existence with Bondar’s Nesting Whitefly, &lt;i&gt;Paraleyrodes bondari&lt;/i&gt; Peracchi

Field occurrence of the exotic neotropical nesting whitefly, Paraleyrodes minei Iaccarino in association with Bondar’s nesting whitefly, Paraleyrodes bondari Peracchi on coconut leaflets is reported from Kerala, India. These coconut palms were previously infested by the rugose spiralling whitefly, Aleurodicus rugioperculatus Martin, which was reported from Kerala and Tamil Nadu during 2016. P. minei closely resembles P. bondari , but is devoid of the oblique grey bands on the wings and it constructs loosely woven, woolly wax nests. Female P. minei are white, but males are smoky grey. Cockhead-like male aedeagus with two thin appendixes projected downwards is the unique feature for species-level identification of P. minei . Detection of three non-native whiteflies of neotropical origin infesting coconut palms in India within a span of two years suggests their simultaneous introduction. Invasive potential of P. minei due to its polyphagous nature and short lifecycle calls upon strict policy frameworks in exchange of planting materials. Domestic quarantine should be strictly enforced in the country to avoid spread of this pest to other coconut-growing regions.

Investigating the use of Paleolithic perforated batons: new evidence from Gough\u2019s Cave (Somerset, UK)

Perforated batons, usually made from a segment of antler and formed of a sub-cylindrical shaft and at least one perforation, have been documented across Europe from sites throughout the Upper Paleolithic and Mesolithic. The function of perforated batons is still debated. We present here three Magdalenian perforated batons from the site of Gough’s Cave (Somerset, UK); these are unique to Britain and represent an important northern example of this artifact type. Our technological analysis revealed that the Gough’s Cave perforated batons did not have a purely symbolic purpose, but were clearly used as tools as demonstrated by extensive use-wear on the perforations’ edges and ancient fractures across both the distal parts and the shafts. The reconstruction of the chaîne opératoire suggests that the engraving of the deep curved lines within the perforation of each baton was a functional re-adjustment following the significant distortion of the perforation by use. Additionally, oblique bands of incisions were engraved on two of the batons’ shafts possibly to provide grip on the smooth antler surface. Altogether, the modifications of the perforations and shafts of the three batons support the hypothesis that the Gough’s Cave batons were used in a task associated with ropes and subjected to considerable forces. Their extensive use may be due to the rarity of the raw material (reindeer antler) in the Cheddar Gorge area during the Magdalenian. Extensive usage aside, the Gough’s Cave batons fit typologically and share a number of features with other Magdalenian perforated batons. They can, therefore, add significant insight to the debate about the use of perforated batons.

First record of the invasive Bondar’s nesting whitefly, Paraleyrodes bondari Peracchi on coconut from India

First incidence of the neotropical invasive Bondar’s Nesting Whitefly (BNW), Paraleyrodes bondari Peracchi (Hemiptera: Aleyrodidae) in India is reported on coconut palms from Kerala. P. bondari constructs unique woolly wax nests on abaxial surface of palm leaflets and possesses “X”-shaped oblique greyish bands on wings. Presence of flower-petal like ovoid facets on abdominal compound pores (33–35 μm) of the puparium and conspicuous single dorsal and ventral horn with a pair of apicolateral processes on the adult male aedeagus confirms the typical morphological identification features. Partial mitochondrial cytochrome oxidase 1 sequences (675 bp) from the Indian specimens shared 100% nucleotide identity with BNW, thus confirming its identity as P. bondari. Co-existence of P. bondari and Aleurodicus rugioperculatus on coconut palms indicate probable simultaneous introduction of both these pests from the New World. The polyphagous nature of the non-native BNW warrants stringent quarantine protocols to prevent its further spread to other coconut growing areas.

Formation of spanwise vorticity in oblique turbulent bands of transitional plane Couette flow, part 2: Modelling and stability analysis

This article presents a modelling of the formation of spanwise vorticity in the turbulent streaks of the oblique bands and spots of transitional plane Couette flow. A functional model is designed to mimic the coherent flow in the streaks. The control parameters of the model are extracted from Direct Numerical Simulations (DNS) statistical data. A Reynolds stress is proposed to study the effect on the instability of this additional force maintaining the baseflow. Local (quasi-parallel) temporal stability analysis is performed on that model to investigate the linear development of the spanwise vorticity. Results show that average profiles, even if they have an inflection, are stable: the shear layers inside the velocity streaks are responsible for the vorticity formation. Emphasis is put on the convective or absolute nature of the instability, depending on the location in the band. This shows that a transition from a convective to an absolute instability occurs in the zone in between fully turbulent and laminar flow. The group velocity of the most unstable modes compares well to the advection velocity of spanwise vorticity measured in DNS. This investigation is completed by the global (non-parallel) stability analysis of a typical band case. Eventually, the possible cycles of sustainment of localised low Reynolds number turbulence of shear flows are discussed in the light of these results.

Stochastic analysis of the time evolution of laminar-turbulent bands of plane Couette flow.

This article is concerned with the time evolution of the oblique laminar-turbulent bands of transitional plane Couette flow under the influence of turbulent noise. Our study is focused on the amplitude of modulation of turbulence (the bands). In order to guide the numerical study of the flow, we first perform an analytical and numerical analysis of a Stochastic Ginzburg-Landau (GL) equation for a complex order parameter. The modulus of this order parameter models the amplitude of modulation of turbulence. Firstly, we compute the autocorrelation function of said modulus once the band is established. Secondly, we perform a calculation of average and fluctuations around the exponential growth of the order parameter. This type of analysis is similar to the Stochastic Structural Stability Theory (S3T). We then perform numerical simulations of the Navier-Stokes equations in order to confront these predictions with the actual behaviour of the bands. Computation of the autocorrelation function of the modulation of turbulence shows quantitative agreement with the model: in the established band regime, the amplitude of modulation follows an Ornstein-Uhlenbeck process. In order to test the S3T predictions, we perform quench experiments, sudden decreases of the Reynolds number from uniform turbulence, in which modulation appears. We compute the average evolution of the amplitude of modulation and the fluctuations around it. We find good agreement between numerics and modeling. The average trajectory grows exponentially, at a rate clearly smaller than that of the formation of laminar holes. Meanwhile, the actual time evolution remains in a flaring envelope, centered on the average, and expanding at the same rate. These results provide further validation of the stochastic modeling for the time evolution of the bands for further studies. Besides, they stress on the difference between the oblique band formation and the formation of laminar holes.

Mechanical and statistical study of the laminar hole formation in transitional plane Couette flow

This article is concerned with the numerical study and modelling of two aspects the formation of laminar holes in transitional turbulence of plane Couette flow (PCF). On the one hand, we consider quenches: sudden decreases of the Reynolds number R which force the formation of holes. The Reynolds number is decreased from featureless turbulence to the range of existence of the oblique laminar-turbulent bands [R g;R t]. The successive stages of the quench are studied by means of visualisations and measurements of kinetic energy and turbulent fraction. The behaviour of the kinetic energy is explained using a kinetic energy budget: it shows that viscosity causes quasi modal decay until lift-up equals it and creates a new balance. Moreover, the budget confirms that the physical mechanisms at play are independent of the way the quench is performed. On the other hand we consider the natural formation of laminar holes in the bands, near R g. The direct numerical simulations (DNS) show that holes in the turbulent bands provide a mechanism for the fragmented bands regime and orientation fluctuations near R g. Moreover the analysis of the fluctuations of kinetic energy toward low values demonstrates that the disappearance of turbulence in the bands can be described within the framework of large deviations. A large deviation function is extracted from the probability density function of the kinetic energy.

Formation of spanwise vorticity in oblique turbulent bands of transitional plane Couette flow, part 1: Numerical experiments

This article investigates the formation of spanwise vorticity in the velocity streaks of the oblique laminar–turbulent bands of plane Couette flow (PCF) by mean of Direct Numerical Simulations (DNS). The spanwise vorticity is created by a roll-up type development of the streamwise-wall normal shear layer of the velocity streaks. It is advected by the large scale flow along the bands. We propose a criterion on spanwise vorticity which detects these events in order to perform systematic measurements. Besides the streamwise and spanwise correlation lengths of the rolls, their advection velocity is measured and shown to match the large scale flow along the band near the turbulent region. Eventually, we discuss the possible relation between ejection of vorticity away from the bands near the laminar region and the size of the said laminar region.