Condensates in rotating turbulent flows

Seshasayanan, Kannabiran; Alexakis, Alexandros

doi:10.1017/jfm.2018.106

Cited by 49 publications

(46 citation statements)

References 96 publications

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“…Such mechanisms have been demonstrated for rotating turbulence, where a flux-loop mechanism has been identified (cf. Bartello et al 1994;Alexakis 2015;Seshasayanan & Alexakis 2018). Similar condensates have also been observed in 3D fast rotating convection (Favier et al 2014;Rubio et al 2014;Guervilly et al 2014).…”

Section: Introductionsupporting

confidence: 76%

Condensates in thin-layer turbulence

Kan

Alexakis

2019

J. Fluid Mech.

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We examine the steady state of turbulent flows in thin layers using direct numerical simulations. It is shown that when the layer thickness is smaller than a critical height, an inverse cascade arises which leads to the formation of a steady state condensate where most of the energy is concentrated in the largest scale of the system. For layers of thickness smaller than a second critical height, the flow at steady state becomes exactly twodimensional. The amplitude of the condensate is studied as a function of layer thickness and Reynolds number. Bi-stability and intermittent bursts are found close to the two critical points. The results are interpreted based on a mean-field three-scale model that reproduces some of the basic features of the numerical results.

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Section: Introductionsupporting

confidence: 76%

Condensates in thin-layer turbulence

Kan

Alexakis

2019

J. Fluid Mech.

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“…Finally, we note that in rotating Newtonian fluids, transitions to condensate states have been observed as a function of the rotation rate (Rossby number) [38][39][40]. This suggests that the appearance of a condensate may be connected with a phase transition also in other flows.…”

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confidence: 80%

Phase Transition to Large Scale Coherent Structures in Two-Dimensional Active Matter Turbulence

2019

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The collective motion of microswimmers in suspensions induce patterns of vortices on scales that are much larger than the characteristic size of a microswimmer, attaining a state called bacterial turbulence. Hydrodynamic turbulence acts on even larger scales and is dominated by inertial transport of energy. Using an established modification of the Navier-Stokes equation that accounts for the small scale forcing of hydrodynamic flow by microswimmers, we study the properties of a dense supensions of microswimmers in two dimensions, where the conservation of enstrophy can drive an inverse cascade through which energy is accumulated on the largest scales. We find that the dynamical and statistical properties of the flow show a sharp transition to the formation of vortices at the largest length scale. The results show that 2d bacterial and hydrodynamic turbulence are separated by a subcritical phase transition. PACS numbers: 47.52.+j; 05.40.JcThin layers of bacteria in their planctonic phase form vortical structures that are reminiscent of vortices in turbulent flows [1][2][3]. This state has been called "bacterial turbulence" [1] because of the shape and form of the patterns, and has been seen in many swimming microorganisms [1-3] and in active nematics [4][5][6]. Bacterial turbulence usually appears on scales much smaller than those of hydrodynamic turbulence, with its inertial range dynamics and the characteristic energy cascades [7]. A measure of this separation is the Reynolds number, which is of order 10 −4 − 10 −6 for an isolated swimmer in a fluid at rest [8] and typically several tens of thousands in hydrodynamic turbulence. Recent studies of the rheology of bacterial suspensions have indicated, however, that the active motion of pusher-type bacteria can lower considerably the effective viscosity of the suspension [9][10][11][12][13][14], to the point where it approaches zero, reaching an active-matter induced "superfluid" phase where the energy input from active processes compensates viscous dissipation [15,16]. In such a situation the collective action of microswimmers can produce a dynamics that may reach the transition to the inertial range in fluid flow, as evidenced by the breaking of helical symmetry in 3d [17]. In two dimensions, a possible connection to hydrodynamic turbulence is particularly intriguing because the energy cascade proceeds from small to large scales and can result in an accumulation of energy at the largest scales admitted by the domain, thereby forming a so-called condensate [18][19][20]. If bacterial turbulence can couple to hydrodynamic turbulence, then the inverse cascade in 2d provides a mechanism by which even larger scales can be driven. We here discuss the conditions under which such a coupling between bacterial and hydrodynamic turbulence can occur.It has recently been shown that the pattern-formation process associated with bacterial turbulence can be captured by minimal models where activity in encoded in suitable forcing terms in the dynamical equations [17,21].Most previo...

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“…In particular, recent experiments were able to investigate the presence of the inverse cascade (Campagne et al 2014, 2015, 2016; Yarom & Sharon 2014). The transition from a forward to an inverse cascade in rotating turbulence was studied systematically using numerical simulations by Smith, Chasnov & Waleffe (1996), Deusebio et al (2014) and Pestana & Hickel (2019, 2020), while the transition to a condensate regime was studied by Alexakis (2015), Yokoyama & Takaoka (2017) and Seshasayanan & Alexakis (2018).…”

Section: Introductionmentioning

confidence: 99%