Masaki Shimizu
email: shimizu@me.es.osaka-u.ac.jp

Paul Manneville

Yohann Duguet

Genta Kawahara

Splitting of a turbulent puff in pipe flow

Keywords: Transition to turbulence, Circular pipe, Puffs

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Introduction

In 1883, Osborne Reynolds experimentally discovered that the state of the flow in a pipe depends on a single parameter, now called Reynolds number Re, and that it changes qualitatively from laminar to turbulent at some critical value about 2000, under sufficiently large disturbances [START_REF] Reynolds | An experimental investigation of the circumstances which determine whether the motion of water shall be direct or sinuous, and of the law of resistance in parallel channels[END_REF]. As is now well recognized, he was confronted to the most complicated case of transition to turbulence, the subcritical one, that occurs when the base flow profile is free from inflection point and is typical of wallbounded flows. In contrast to inflectional shear flows that become turbulent in a more continuous supercritical way via inertial instabilities of Kelvin-Helmholtz type, wallbounded flows turn out to be possibly linearly unstable against Tollmien-Schlichting waves at high Reynolds numbers only. Pipe flow appears to be an extreme case where steady laminar pipe flow happens to be linearly stable for all Reynolds numbers [START_REF] Salwen | Linear stability of Poiseuille flow in a circular pipe[END_REF]. Plane Couette flow (PCF), the simple shear flow between counter-sliding plates, is another such example [START_REF] Romanov | Stability of plane-parallel Couette flow Functional Analysis and Its Applications[END_REF]) to be evoked later. In these cases, finite amplitude perturbations are indeed needed to trigger the transition to turbulence. Accordingly, there exists a whole range of Reynolds numbers, called the transitional range, where laminar flow can coexist with turbulence, both in parameter space and in physical space, i.e. with localized turbulent domains separated from laminar flow by front-like interfaces. A global stability threshold Re g can be defined, below which laminar flow prevails in the long time limit whatever the amplitude and shape of the initial perturbation. The determination of this threshold turns out to be particularly difficult, due to the fact that, below it, these patches of localized turbulence may persist for very long times before they ultimately decay.

In pipe flow axially localized turbulent plugs [START_REF] Reynolds | An experimental investigation of the circumstances which determine whether the motion of water shall be direct or sinuous, and of the law of resistance in parallel channels[END_REF][START_REF] Coles | Mécanique de la Turbulenc[END_REF], called puffs (Wygnanski andChampagne 1973, Wygnanski et al 1975), can be observed in the lowest part of the transitional range. Figure 1 displays such a state as obtained in our numerical simulations. When Re increases, weakly turbulent puffs become growing turbulent slugs filled with small scale turbulence of regularly increasing length tending to invade the pipe (Wygnanski andChampagne 1973, Duguet et al 2010). In contrast, the length of the puffs is statistically constant, which led to think that they were "equilibrium" states [START_REF] Wygnanski | On transition in a pipe. Part 2. The equilibrium puff[END_REF]. Their structure and sustainment mechanism have been much studied. Laminar flow enters the puff upstream, through an abrupt trailing edge, and leaves it downstream, through a smoother leading edge [START_REF] Bandyopadhyay | Aspects of the equilibrium puff in transitional pipe flow[END_REF][START_REF] Shimizu | A driving mechanism of a turbulent puff in pipe flow Fluid[END_REF][START_REF] Duguet | Slug genesis in cylindrical pipe flow[END_REF][START_REF] Holzner | A Lagrangian approach to the interface velocity of turbulent puffs in pipe flow[END_REF] Recent experimental and numerical studies have shown that, although puffs have long lifetimes, they may decay to laminar flow within a finite time with some probability function of Re, pointing to a super-exponential increase of the lifetime with Re [START_REF] Hof | Repeller or attractor? Selecting the dynamical model for the onset of turbulence in pipe flow[END_REF]. As Re gets larger, the turbulent activity grows via an increase of the length of individual puffs, while their decay probability becomes vanishingly small. While trying to equilibrate at a longer length, the puffs may split [START_REF] Lindgren | Liquid flow in tubes II. The transition process under less disturbed inlet flwo conditions[END_REF], progressively yielding trains of puffs [START_REF] Nishi | Laminar-to-turbulent transition of pipe flows through puffs and slugs[END_REF][START_REF] Avila | The onset of turbulence in pipe flow[END_REF]. As Re continues to increase, successive puffs turn into slugs with more abrupt leading edges (Wygnanski andChampagne 1973, Duguet et al 2010). Slugs tend to merge while the turbulent fraction -the fraction of the pipe occupied by developed turbulence -increases [START_REF] Rotta | Experimental contributions to the development of turbulent flow ina pipe Ing[END_REF][START_REF] Sreenivasan | Transition intermittency in open flows, and intermittency routes to chaos[END_REF][START_REF] Moxey | Distinct large-scale turbulent-laminar states in transitional pipe flow[END_REF], Avila and Hof 2013).

Puff splitting is a stochastic process with an exponential distribution of waiting times before splitting. The mean waiting time has been shown to decrease superexponentially as Re increases [START_REF] Avila | The onset of turbulence in pipe flow[END_REF]. A competition between spontaneous decay of turbulence and propagation by splitting thus takes place, and a threshold can be defined when the mean waiting times for decay and splitting are equal, which was found for Re g ≃ 2040 [START_REF] Avila | The onset of turbulence in pipe flow[END_REF]. A simplified model accounting for the phenomenon has been devised by Barkley ((Barkley 2011b, Barkley 2011a)).

The present contribution is dedicated, not to the statistics of splitting, but to its local mechanics, i.e. a description of how the flow passes from one parent puff to a parent-and-child pair of puffs with some laminar gap between them, next to clusters of puffs at higher Re, and to a discussion of the statistical nature of the processes involved.

Equations and numerical resolution

We consider the flow of an incompressible viscous fluid driven by an external force imposing a time-independent bulk speed U in a straight circular pipe of radius a. All physical quantities are non-dimensionalized by a for length and the centerline speed 2U of Hagen-Poiseuille flow for velocity. The Reynolds number is defined as Re ≡ 2aU/ν, ν being the kinematic viscosity. The continuity and Navier-Stokes equations governing the flow then read:

∇ • u = 0 , (1) ∂u ∂t + (u • ∇)u = -∇p + 1 Re ∇ 2 u (2)
whare u and p are velocity and pressure respectively. No-slip boundary conditions are imposed at the wall of the pipe and periodic conditions with period L in the z (streamwise) direction:

u(1, θ, z) = 0 , u(r, θ, z + L) = u(r, θ, z) .
The reduced pressure p includes an external force which is determined at each time to keep the bulk speed constant in time. The solenoidal field u can be expressed as

u = ∇ × (ψ ẑ) + ∇ × (∇ × (ϕ ẑ)) .
Using a spectral method, we numerically solved the set of evolution equations for (ψ, ϕ) straightforwardly derived from (1) and (2). These scalar functions were approximated by truncated expansions:

(ψ, ϕ) = K ∑ k=-K M ∑ m=-M N ∑ n=|m| n+m=even ( ψmk n , φmk n ) Φ m n (r) exp i[mθ + 2πkz/L]
where K, M and N (≥ M ) are positive integers and Φ m n (r) are Zernike circular polynomials. The time stepping algorithm combines a Crank-Nicholson scheme with a second order Adams-Bashforth scheme for the nonlinear terms, and the timestep is fixed to ∆t = 0.005; see [START_REF] Shimizu | Structure of a turbulent puff in pipe flow[END_REF] for details. Here, a very long computational domain of length L = 400 has been considered in order to reduce the influence of the periodic boundary conditions as much as possible. We have taken (N, M, K) = (84, 42, 1535), which is thought to be well resolved. Preliminary work with (N, M, K) = (40, 21, 1535) showed the same phenomenology, apart from a downward shift of Reynolds numbers relevant to the transitional range of about 200. A similar consequence of under-resolution has been reported for PCF [START_REF] Manneville | On modelling transitional turbulent flows using under-resolveddirect numerical simulations: the case of plane Couette flow[END_REF].

Splitting events

Puff splitting has been studied by means of simulations all starting from single puff states at Re = 1900 or 2000. The Reynolds number was then set to its working value, from Re = 2100 to 2500. Turbulent regions were unambiguously identified by using |u z -u lam | ≳ 0.15 as a criterion, u z and u lam being the perturbed and laminar axial speed measured at relevant distance from the centerline, r = 0 in figure 2, and r = 0.8 in the other spatiotemporal diagrams, see below. Computations were continued as long as a significant laminar interval was preserved, modulo the axial periodic boundary conditions.

For Re = 2100 and 2200 (not illustrated) the mean splitting time is so long that no splitting could be expected, and indeed not detected, within observation times of order a few thousand time units. These results are consistent with Fig. 5 in [START_REF] Avila | The onset of turbulence in pipe flow[END_REF]. Figure 2 shows the birth and growth of puffs at Re = 2300, 2400, and 2500. It displays spatiotemporal diagrams of the centerline streamwise velocity component in a reference frame moving with the bulk velocity u m = 1 2 u cl = 0.5. These individual puffs have statistically well defined lengths as expected, the one at Re = 2200 being about 25-30% longer than the one at Re = 2100.

As seen in figure 2 (left), at Re = 2300 the puff -the isolated turbulent patch featured as a dark segment at given time t -splits irregularly, a first time at t ≈ 4700 and a second time at t ≈ 7500. The newborn puffs are separated from their parents by significant laminar gaps. A third splitting takes place from of the parent puff at t ≈ 8300, but the corresponding child puff is hardly observable on the left border of the graph, i.e. for (z -u m t) ≃ 0. The result of this third splitting is better visible in figure 3 which displays the final state at t = 9000. Here the azimuthally-unfolded streamwise component of the velocity close to the wall is displayed. This more readable representation was chosen after scrutinizing the velocity field at various distances from the centerline. The value r = 0.8 offered the clearest discrimination and will be kept in forthcoming figures. The laminar gaps between successive puffs are less masked by the elongated but weak perturbations of the velocity field along the centerline, as can be understood from the consideration of figure 1.

At larger Re (figure 2, right), the turbulent region expands quite regularly. The whole region between the most upstream front and the leading edge is rather messy and can definitely not be considered as a train of pseudo-equilibrium puffs, though laminar intervals of significant length can still be transiently detected.

For all the values of Re that we have studied, the speed of the most upstream laminar-turbulent front is well defined. In the case of pseudo-equilibrium puffs at Re = 2100 we measured a trailing edge speed u te ≈ 0.485, which is slightly smaller than the bulk speed u m = 0.5, as expected; see for example Hof et al. (2005) who display the relative trailing and leading edge speeds u te /u m and u le /u m in their Fig. 4. As soon as the splitting sets in, turbulence expands due to a gentle decrease of the trailing edge speed, u te = 0.463, 0.454, and 0.444, for Re = 2300, 2400, and 2500 respectively, while the leading edge speed u le remains close to 1 and increases slowly. We measure u le = 0.494 and 0.505 for Re = 2400 and 2500, respectively, but no reliable value could be extracted for Re = 2300, due to the discrete distribution of splitting events. Returning to the case Re = 2300, three trailing edges corresponding to the three puffs can be identified near the end of the simulation for t ≳ 7500, the parent puff being separated from its children by a substantial laminar interval, while the grand-child is still somewhat locked to the child. Measured speeds from upstream to downstream are u te = 0.463, 0.486 and 0.493, in qualitative agreement with results displayed in [START_REF] Avila | The onset of turbulence in pipe flow[END_REF], Fig. 1C. This confirms that the amount and quality of the laminar flow upstream of a puff in a given train, i.e. the distance between puffs, plays an important role in fixing its speed [START_REF] Samanta | Experimental investigation of laminar turbulent intermittency in pipe flow[END_REF].

In all the cases considered, the final simulation was fixed by the condition that the leading edge of the puff or of the cluster of puffs remains sufficiently far from its own trailing edge, when taking periodic boundary conditions into account (see figure 2). We can thus safely consider the numerical domain as sufficiently elongated for the perturbations to be regarded as evolving along an infinitely long pipe.

Let us now illustrate the processes through which puffs split. Figures 4 and 5 display the first and second splitting events observed at t ≈ 4750 and t ≈ 7100 for Re = 2300 using the same representation as in figure 3. In both cases, before splitting, several streaks, alternately rapid and slow, can be observed at the leading edge. The second splitting, to be examined first (figure 5), shows a more intense pair protruding into the laminar region ahead (t = 7200, 7240, 7280) for θ ≈ π/2. The disturbance then rapidly spreads azimuthally (t = 7320, 7360). Turbulence next begins to collapse just upstream of this new puff head (t = 7400), progressively forming a clear trailing edge (visible from t = 7600 on). This description is consistent with former experimental observations [START_REF] Lindgren | Liquid flow in tubes II. The transition process under less disturbed inlet flwo conditions[END_REF][START_REF] Avila | The onset of turbulence in pipe flow[END_REF]. In fact the second puff (B in figure 2-5) continues to emit streak structures ahead of itself (t = 7880), that tend to fill the laminar gap (t = 7960). A sizable permanent laminar gap between the two puffs (B and C) is then present for t ≥ 8080 and persists later as testified by the snapshot at t = 9000 displayed in figure 3. Figure 6 gives an illustration of the local intensity of the self-sustainment process (Waleffe 1997) zoomed on the region of splitting. It displays iso-surfaces of the fluctuation component u z -⟨u z ⟩ θz = -0.1 (gray) and ω θ -⟨ω θ ⟩ θz = 1.5 (black). The former locate the slow speed streaks and the latter the high intensity spanwise vortices. The top and middle frames, at t = 7200 and 7280 illustrate an intense streaky pattern induced by lift-up and its subsequent breakdown around θ = 2π/3 and z = 100. The bottom frame, at a later time, t = 7400, shows that the apparently laminar space generated by the on-going splitting is filled with diametrically opposed streaky structures around θ = 0, π ahead of z -u m t ≈ 92 forming the leading edge of the upstream puff, while the trailing edge of the downstream puff is not yet well formed.

Let us now focus on the first splitting event, around t ≈ 4750 and z -u m t ≈ 100 (figure 4). Again at start (t = 4600) an azimuthally periodic streaky disturbance with wavenumber ≈ 3 is observed. The parent puff now emits two diametrically opposite strong low-speed streaks (t = 4720, 4760) that, not surprisingly, quickly contaminate the full perimeter of the pipe (t = 4800). However, the collapse of turbulence allowing for the identification of two separate puffs seems more difficult to achieve. Premises of a laminar gap between parent and child are visible at t = 4920 but they are well separated only for t ≥ 5000. This gap is never observed to close (see t = 5200) but instead progressively widens due to the velocity difference between the two different trailing edge speeds (see also figure 2, left). Turning to larger Re, here Re = (figure 7), it is clear that "puff splitting" has a little changed its meaning. Downstream of the leading edge, one can still recognize the emission of streaky protuberances and their possible failing at nucleating a puff (see the pair of streaks at t = 710 and π < θ < 3π/2 that collapses around t = 910, and also consider figure 2, top-right for 700 < t < 1000 where the widening and next narrowing of the puff is clearly visible). This aborted event results in a conspicuous temporal fluctuation of the length of the puff. When the azimuthal expansion of the protuberance is successful and turbulence spreads downstream, there may also be no opening of a gap behind the leading edge, yielding a compact anomalously elongated puff that can last for a substantial amount of time (1180 ≲ t ≲ 1420). Such longer puffs appear however to be unstable, and turbulence can collapse locally, somewhere strictly between the leading edge and the trailing edge (Avila and Hof 2013). An example of collapse close to the leading edge is given for 3150 ≲ t ≲ 3350. Another one close to the trailing edge can be detected at t ≈ 3700 in figure 2, top-right, yielding a separate puff at the rear of the turbulent domain. The last example is a general collapse of turbulence all over the pipe's cross-section over a significant downstream length, observed near the end of our simulation (t = 4900, 5000).

This limited set of illustrations cannot claim to give a complete catalogue of the processes observable in the upper transitional range, where the probability of puff splitting is no longer negligible and the puffs make their way to becoming "slugs." Let us notice already at this stage that macroscopic features of the flow pattern at the scale of the puff itself (turbulence breakdown, flow reorganization within the laminar gap) seem to result from the combination/interplay of microscopic high-frequency processes at the scale of the local streak structures (emission of streaky protuberances, azimuthal expansion), which we discuss further in the next section.

Discussion

In this paper, by means of well-resolved but computationally demanding numerical simulations extending previous work [START_REF] Shimizu | A driving mechanism of a turbulent puff in pipe flow Fluid[END_REF], we have investigated the part of the transitional range of pipe flow where turbulence is sustained with essentially full probability, > Re g [START_REF] Avila | The onset of turbulence in pipe flow[END_REF], while isolated gentle pseudo-equilibrium puffs change into expanding trains of puffs [START_REF] Nishi | Laminar-to-turbulent transition of pipe flows through puffs and slugs[END_REF] before turning into fastergrowing slugs [START_REF] Duguet | Slug genesis in cylindrical pipe flow[END_REF].

At the lowest end of the transitional range (Re < Re g ) the much studied decay of puffs -see [START_REF] Hof | Repeller or attractor? Selecting the dynamical model for the onset of turbulence in pipe flow[END_REF] and references therein -has been interpreted using the familiar concept of transient chaos within the framework of deterministic dynamical systems theory [START_REF] Eckhardt | Dynamical systems and the transition to turbulence in linearly stable shear flows Phil[END_REF], Avila et al 2013). This approach straightforwardly explains the exponentially decreasing lifetime distributions observed experimentally and happens to be quite acceptable only well below Re g where puffs are short and, spatially speaking, relatively coherent.

However, as we have reported, the area occupied by turbulent fluctuations statistically elongate when Re increases. Spatial coherence supporting the dynamical systems approach then becomes questionable. A concurrent interpretation of the decay in terms of extreme events [START_REF] Clusel | Global fluctuations in physical systems: a subtle interplay between sum and extreme value statistics[END_REF] has indeed been proposed [START_REF] Goldenfeld | Extreme fluctuations and the finite lifetime of the turbulent state[END_REF], able to account for both the distribution of lifetimes and the variation of their means with Re. A similar approach has been put forward to explain the decay of turbulent bands in PCF below its own global stability threshold [START_REF] Manneville | On the decay of turbulence in plane Couette flow Fluid[END_REF][START_REF] Faranda | On using extreme values to detect global stability thresholds in multi-stable systems: The case of transitional plane Couette flow Chaos[END_REF] and further adapted to deal with the growth of the oblique laminar-turbulent pattern conspicuous in its transitional range [START_REF] Manneville | On the growth of laminar-turbulent patterns in plane Couette flow Fluid[END_REF] and to which the puff splitting regime closely corresponds.

The general context is that of spatiotemporal dynamics appropriate to extended systems [START_REF] Pomeau | Front motion, metastability and subcritical bifurcations in hydrodynamics[END_REF]) with two facets: (i) at the local scale, a growth-decay problem treated as a manifestation of spatiotemporal intermittency [START_REF] Chaté | Spatiotemporal intermittency NATO ASI series[END_REF], a deterministic but spatiotemporally chaotic variant of a fully stochastic process named directed percolation [START_REF] Hinrichsen | Non-equilibrium critical phenomena and phase transitions into absorbing states[END_REF], and (ii ) at the global scale, a nucleation problem [START_REF] Bergé | L'espace chaotique[END_REF] as encountered in the theory of first-order phase transitions in thermodynamics, viz. the liquid-gas transition. The first facet was shown to be relevant to the transition in pipe flow, at least in the model discussed in [START_REF] Avila | The onset of turbulence in pipe flow[END_REF], Barkley 2011b); see also [START_REF] Sipos | Directed percolation describes lifetime and growth of turbulent puffs and slugs[END_REF]. Large deviations [START_REF] Clusel | Global fluctuations in physical systems: a subtle interplay between sum and extreme value statistics[END_REF] are directly connected to the second facet: Breakdown of turbulence over large enough turbulent segment, be it a long puff [START_REF] Goldenfeld | Extreme fluctuations and the finite lifetime of the turbulent state[END_REF] or a portion of oblique turbulent band as in Couette flow, is indeed easily understood as resulting from the spontaneous nucleation of critical laminar trough [START_REF] Manneville | On the decay of turbulence in plane Couette flow Fluid[END_REF][START_REF] Faranda | On using extreme values to detect global stability thresholds in multi-stable systems: The case of transitional plane Couette flow Chaos[END_REF]. The growth problem of interest here fits this framework well, in much the same way as in PCF for which probabilities introduced in the directed percolation approach have been computed [START_REF] Duguet | Stochastic and deterministic motion of a laminar-turbulent front in a spanwisely extended Couette flow[END_REF]. Here, from the fine oblique lines displayed in the spatiotemporal diagrams of figure 2 and corresponding to the traces of the streaks emitted by the puff near the centerline, a dominant frequency of streak emission of about 0.1 per unit time, can be inferred, which corresponds to a period of ≈ 10a/2U . The order of magnitude of this period is comparable to that of invariant solutions relevant to the pipe [START_REF] Duguet | Relative periodic orbits in transitional pipe flow[END_REF], Avila et al 2013[START_REF] Kawahara | The significance of simple invariant solutions in turbulent flows[END_REF], typical of "microscopic" processes alluded to above. Furthermore, the irregularly varying length and gray intensity of the obliques traces is a clear indication of the stochastic character of this process and quantitative support needs further scrutiny. The large-deviation consequences at the "macroscopic" scale of local processes at the scale of the streak thus seem a key element in order to account for the catalogue of events involved in puff expansion, as highlighted in the previous section. Morever, at the microscopic level, perturbations flushed downstream and potentially nucleating new puffs are advected with a mean velocity larger than that of the parent puff. It would thus be interesting to examine how large scale flows, that appear to play an important role in plane flows [START_REF] Manneville | On the growth of laminar-turbulent patterns in plane Couette flow Fluid[END_REF][START_REF] Duguet | Oblique laminar-turbulent interfaces in plane shear flows[END_REF][START_REF] Lemoult | Turbulent spots in a channel: large-scale flow and selfsustainability[END_REF], contribute to the splitting process in pipe flow. At any rate, statistical physics [START_REF] Pomeau | Front motion, metastability and subcritical bifurcations in hydrodynamics[END_REF][START_REF] Bergé | L'espace chaotique[END_REF]) and its derivatives, non-equilibrium phase transitions [START_REF] Hinrichsen | Non-equilibrium critical phenomena and phase transitions into absorbing states[END_REF], large deviations and extreme values theory [START_REF] Clusel | Global fluctuations in physical systems: a subtle interplay between sum and extreme value statistics[END_REF], have provided thoughtful insights in the transition to turbulence of pipe flow and PCF and will certainly continue to enlighten the dynamics of transitional wall-bounded flows.

Figure 1 .

 1 Figure 1. Pseudo-equilibrium puff at Re = 2000. The absolute vorticity distribution in a plane through the pipe's centerline is represented using a linear gray scale. Only a part of the solution is displayed, the length of the computational domain being L = 400.

Figure 2 .

 2 Figure 2. Spreading of turbulence for Re = 2300 (left), Re = 2400 (top-right), and Re = 2500 (bottom-right). Spatiotemporal diagrams of the centerline streamwise velocity component are displayed in a reference frame moving at the mean velocity u m = 0.5. White corresponds to laminar flow u cl = 1, shades of gray to disturbed flow with lower values inside the puffs.

Figure 3 .

 3 Figure 3. State reached at t = 9000 for Re = 2300 showing four puffs. The streamwise component of the velocity at distance r = 0.8, i.e. close to the wall, is represented in shades of gray in the (θ, z) plane, with the angle θ ∈ [0, 2π] along the vertical axis as a function of the axial coordinate z ∈ [0, 400] along the horizontal axis.

Figure 4 .

 4 Figure 4. Snapshots taken during the first puff splitting observed for Re = 2300. Same representation as in figure 3. From top to bottom: t = 4600,4640, 4680, 4720, 4760, 4800, 4840, 4880, 4920, 5000, 5200.

Figure 5 .

 5 Figure 5. Snapshots taken during the second puff splitting observed for 2300. Same representation as in figure 3. From top to bottom, t = 7120, 7200, 7240, 7280, 7320, 7360, 7400, 7600, 7880, 7960, 8080. Periodic boundary conditions have been used to display the solution in the most appropriate window (compare with figure 3).

Figure 6 .

 6 Figure 6. Disturbances during the second puff splitting observed for Re = 2300.Inward view from the pipe wall in cylindrical coordinate. Gray and black iso-surfaces represent u z -⟨u z ⟩ θz = -0.1 and ω θ -⟨ω θ ⟩ θz = 1.5, respectively. From bottom, t = 7200, 7280, 7400.

Figure 7 .

 7 Figure 7. Snapshots of different cases observed for Re = 2400. Same representation as in figure 3. From top to bottom, t = 710, 910 (aborted splitting), t = 1180, 1240, 1420 (extension without separation), t = 3150, 3250, 3350 (separation of leading puff), t = 4900, 5000 (breakdown of last but one puff).

Acknowledgments

Part of this work has been performed within the framework of the EGIDE-PHC SAKURA project #25714NH funded by the Japan Society for the Promotion of Science and the French Ministry of Foreign Affairs. The authors are grateful to the referees for their constructive remarks and YD would like to thank Prof. Avila for interesting discussions.