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See detailTwo-dimensional dynamics of elasto-inertial turbulence and its role in polymer drag reduction
Sid, Samir ULiege; Dubief, Yves; Terrapon, Vincent ULiege

in Physical Review Fluids (2018), 3(1),

Direct numerical simulations of a FENE-P fluid in both two- and three-dimensional straight periodic channels find that elasto-inertial turbulence is fundementally two-dimensional. The spurious effect of ... [more ▼]

Direct numerical simulations of a FENE-P fluid in both two- and three-dimensional straight periodic channels find that elasto-inertial turbulence is fundementally two-dimensional. The spurious effect of artificial diffusion of the polymer is demonstrated. [less ▲]

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See detailRole of Elasto-Inertial Turbulence in Polymer Drag Reduction
Dubief, Yves; Sid, Samir ULiege; Terrapon, Vincent ULiege

Conference (2017, November 20)

Elasto-Inertial Turbulence (EIT) is a peculiar state of turbulence found in dilute polymer solutions flowing in parallel wall flows over a wide range of Reynolds numbers. At subcritical Reynolds numbers ... [more ▼]

Elasto-Inertial Turbulence (EIT) is a peculiar state of turbulence found in dilute polymer solutions flowing in parallel wall flows over a wide range of Reynolds numbers. At subcritical Reynolds numbers, appropriate boundary conditions trigger EIT, a self-sustaining cycle of energy transfers between thin sheets of stretched polymers and velocity perturbations, which translates into an increase of friction drag. For critical and supercritical Reynolds numbers, polymer additives may lead to significant drag reduction, bounded by the asymptotic state known as Maximum Drag Reduction (MDR). The present research investigates the role of EIT in the dynamics of critical and supercritical Reynolds number wall flows. Using high-fidelity direct numerical simulations of channel flows and the FENE-P model, we establish that (i) EIT is two-dimensional, (ii) the scales essential to the existence of EIT are sub-Kolmogorov, and (iii) EIT drives MDR at low and possibly moderate Reynolds number turbulent flows. These findings were validated in two different codes and using unprecedented resolutions for polymer flows. [less ▲]

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See detailEffects of Natural Convection on the Near-Wall Turbulence in Unstably Stratified Turbulent Channel Flows
Sid, Samir ULiege; Terrapon, Vincent ULiege; Dubief, Yves

Conference (2015, November 23)

Results of direct numerical simulation of turbulent channel flows under unstable stratification are reported. Two Reynolds number are considered: Reτ = 180 , 395 and the Rayleigh number ranges between Ra ... [more ▼]

Results of direct numerical simulation of turbulent channel flows under unstable stratification are reported. Two Reynolds number are considered: Reτ = 180 , 395 and the Rayleigh number ranges between Ra = [106 -109 ] . The Prandtl number is set to 1. The channel is periodic in both streamwise and spanwise directions and non-slip/isothermal boundary conditions are imposed at the walls. The temperature difference between the walls is set so that the stratification is unstable and the coupling between temperature and momentum is achieved using the Boussinesq approximation. The dependency of the typical large scale convective structures on both Reynolds and Rayleigh numbers are investigated through cross flow sectional statistics and instantaneous flow field visualizations. Moreover, the effects of the natural convection on the coherent structures associated to the cycle of wall-bounded turbulence (Jimenez, et al. JFM 1999), namely velocity streaks and streamwise vortices, are examined. Finally, macroscopic quantities such as friction coefficient and Nusselt number are reported as a function of the Rayleigh number and are compared for both Reynolds numbers. [less ▲]

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See detailElasto-Inertial Turbulence: From Subcritical Turbulence to Maximum Drag Reduction
Dubief, Yves; Sid, Samir ULiege; Terrapon, Vincent ULiege

Conference (2015, November 22)

Elasto Inertial Turbulence (EIT) is a turbulence state found so far in polymer solutions. Upon the appropriate initial perturbation, an autonomous regeneration cycle emerges between polymer dynamics ... [more ▼]

Elasto Inertial Turbulence (EIT) is a turbulence state found so far in polymer solutions. Upon the appropriate initial perturbation, an autonomous regeneration cycle emerges between polymer dynamics, pressure and velocity fluctuations. This cycle is best explained by the Poisson equation derived from viscoelastic flow models such as FENE-P (used in this study). This presentation provides an overview of the structure of EIT in 2D channel flows for Reynolds numbers ranging from Reτ = 10 to 100 and for 3D simulations up to Ret au = 300 . For flows below the Newtonian critical Reynolds number, EIT increases the drag. For higher Reynolds numbers, EIT is surmised to be the energetic bound of Maximum Drag Reduction (MDR), the asymptotic state of drag reduction in polymer solutions. The very existence of EIT at low Reynolds numbers (Reτ < 60) highlights a backward energy transfer from the small scale polymer dynamics to larger flow scales. Similar dynamics is identified at higher Reynolds numbers, which could explain why polymer flows never become fully laminar. [less ▲]

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See detailDirect Numerical Simulation of Mixed Convection in Turbulent Channel Flow: On the Reynolds number dependency of momentum and heat transfer under unstable stratification
Sid, Samir ULiege; Dubief, Yves; Terrapon, Vincent ULiege

in Proceedings of the 8th International Conference on Computational Heat and Mass Transfer, ICCHMT 2015 (2015, May 27)

Direct numerical simulations of unstably stratified turbulent channel flow have been performed in order to investigate the Reynolds number effect on mixed convection. Six different cases are considered ... [more ▼]

Direct numerical simulations of unstably stratified turbulent channel flow have been performed in order to investigate the Reynolds number effect on mixed convection. Six different cases are considered with friction Reynolds number Re_\tau =180 and 395 and friction Richardson number Ri_\tau = 0, 100 and 1000. It is shown that both friction coefficient and Nusselt number increase under unstable stratification for a sufficiently large Richardson number. At low Richardson number, the friction coefficient can either increase or decrease depending on the Reynolds number. The drag reduction is associated with an increase of mean velocity due to an enhanced dissipation of Reynolds shear stress by pressure strain in the buffer region. The breakdown of the Reynolds analogy is demonstrated as the turbulent Prandtl number exhibits a non-constant behavior due to buoyancy. [less ▲]

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See detailDirect numerical simulations of mixed convection in a turbulent channel flow
Sid, Samir ULiege; Terrapon, Vincent ULiege; Dubief, Yves

Conference (2014, November 23)

Wall-bounded turbulence has been extensively studied by the scientific community during the last decades. Much effort has been devoted to identify the role that coherent structures and energy exchanges ... [more ▼]

Wall-bounded turbulence has been extensively studied by the scientific community during the last decades. Much effort has been devoted to identify the role that coherent structures and energy exchanges play in turbulent channel flows. However, in many engineering applications, wall-bounded flows are subjected to additional physical phenomena. For instance, applying a temperature differential to the channel walls leads to a modified turbulent state which results from a balance between buoyancy, inertia and viscosity effects. Although, forced and natural convection have been widely studied separately, the coupling between both and its consequences on turbulence features are still not fully understood. In the present work, direct numerical simulations of a buoyant turbulent channel flow are reported for different values of the Reynolds and Richardson numbers. The energy exchanges between potential and kinetic energy and their impact on coherent structures are investigated. Macroscopic quantities (e.g.: Nusselt number) and statistics are compared with those obtained in forced convection flows. Finally, the influence of the ratio between inertia and buoyancy effects (i.e. Richardson number) is discussed. [less ▲]

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See detailSimulation of elastic and elasto-inertial turbulence in straight channel flows
Dubief, Yves; Terrapon, Vincent ULiege; Sid, Samir ULiege

Conference (2014, November 23)

Elastic turbulence (ET, Nature 410, 905, 2000) is a chaotic flow state generated and sustained by polymer additives at vanishing Reynolds numbers. It is generally accepted that elastic turbulence occurs ... [more ▼]

Elastic turbulence (ET, Nature 410, 905, 2000) is a chaotic flow state generated and sustained by polymer additives at vanishing Reynolds numbers. It is generally accepted that elastic turbulence occurs when the mean flow streamlines are curved. Elasto-inertial turbulence (EIT, PNAS 220, 10557, 2013) is a similar state of turbulence that happens in inertial flows with mean straight flow streamlines at Reynolds numbers for which the flow is laminar in the absence of polymers. A recent experiment (PRL 110, 174502, 2013) has shown that ET generated by the insertion of cylinders at the inlet of a low Reynolds number channel flow is sustained downstream of the perturbation. This experiment suggests a possible relation between ET and EIT. Our study will first confirm that sustained ET can be triggered in low-Reynolds number channel flows. ET is shown to exist in two- and three-dimensional simulations for Reynolds numbers of the order of 100 or less. Much like the aforementioned experiment, the initial conditions triggering ET cause the flow streamlines to be curved for a short duration at the beginning of the simulation. Our study will then discuss the similarities and differences between ET and EIT. [less ▲]

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See detailOn the role of pressure in elasto-inertial turbulence
Terrapon, Vincent ULiege; Dubief, Yves; Soria, Julio

in Journal of Turbulence (2014), 16(1), 26-43

The dynamics of elasto-inertial turbulence is investigated numerically from the perspective of the coupling between polymer dynamics and flow structures. In particular, direct numerical simulations of ... [more ▼]

The dynamics of elasto-inertial turbulence is investigated numerically from the perspective of the coupling between polymer dynamics and flow structures. In particular, direct numerical simulations of channel flow with Reynolds numbers ranging from 1000 to 6000 are used to study the formation and dynamics of elastic instabilities and their effects on the flow. Based on the splitting of the pressure into inertial and polymeric contributions, it is shown that the polymeric pressure is a non-negligible component of the total pressure fluctuations, although the rapid inertial part dominates. Unlike Newtonian flows, the slow inertial part is almost negligible in elasto-inertial turbulence. Statistics on the different terms of the Reynolds stress transport equation also illustrate the energy transfers between polymers and turbulence and the redistributive role of pressure. Finally, the trains of cylindrical structures around sheets of high polymer extension that are characteristics of elasto-inertial turbulence are shown to be correlated with the polymeric pressure fluctuations. [less ▲]

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See detailElasto-inertial turbulence in polymeric flows
Terrapon, Vincent ULiege; Dubief, Yves; Soria, Julio

Conference (2013, November 26)

The dynamics of elasto-inertial turbulence (EIT) is investigated numerically from the perspective of the coupling between polymer dynamics and flow structures. In particular, direct numerical simulations ... [more ▼]

The dynamics of elasto-inertial turbulence (EIT) is investigated numerically from the perspective of the coupling between polymer dynamics and flow structures. In particular, direct numerical simulations of channel flow with Reynolds numbers ranging from 1000 to 6000 are used to study the formation and dynamics of elastic instabilities and their effects on the flow. Based on the splitting of the pressure into inertial and polymeric contributions, it is shown that the trains of cylindrical structures around thin sheets of high polymer extension that are characteristics to elasto-inertial turbulence are mostly driven by polymeric contributions. [less ▲]

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See detailOn the mechanism of elasto-inertial turbulence
Dubief, Yves; Terrapon, Vincent ULiege; Soria, Julio

in Physics of Fluids (2013), 25(110817), 1-16

Elasto-inertial turbulence (EIT) is a new state of turbulence found in inertial flows with polymer additives. The dynamics of turbulence generated and controlled by such additives is investigated from the ... [more ▼]

Elasto-inertial turbulence (EIT) is a new state of turbulence found in inertial flows with polymer additives. The dynamics of turbulence generated and controlled by such additives is investigated from the perspective of the coupling between polymer dynamics and flow structures. Direct numerical simulations of channel flow with Reynolds numbers ranging from 1000 to 6000 (based on the bulk and the channel height) are used to study the formation and dynamics of elastic instabilities and their effects on the flow. The flow topology of EIT is found to differ significantly from Newtonian wall-turbulence. Structures identified by positive (rotational flow topology) and negative (extensional/compressional flow topology) second invariant Qa isosurfaces of the velocity gradient are cylindrical and aligned in the spanwise direction. Polymers are significantly stretched in sheet-like regions that extend in the streamwise direction with a small upward tilt. The Qa cylindrical structures emerge from the sheets of high polymer extension, in a mechanism of energy transfer from the fluctuations of the polymer stress work to the turbulent kinetic energy. At subcritical Reynolds numbers, EIT is observed at modest Weissenberg number (Wi, ratio polymer relaxation time to viscous time scale). For supercritical Reynolds numbers, flows approach EIT at large Wi. EIT provides new insights on the nature of the asymptotic state of polymer drag reduction (maximum drag reduction), and explains the phenomenon of early turbulence, or onset of turbulence at lower Reynolds numbers than for Newtonian flows observed in some polymeric flows. [less ▲]

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See detailDynamics of Elasto-Inertial Turbulence in Flows with Polymer Additives
Terrapon, Vincent ULiege; Dubief, Yves; Soria, Julio

in Proceedings of the 8th International Symposium on Turbulence and Shear Flow Phenomena (TSFP-8) (2013, August 30)

The dynamics of elasto-inertial turbulence is investigated numerically from the perspective of the coupling between polymer dynamics and flow structures. In particular, direct numerical simulations of ... [more ▼]

The dynamics of elasto-inertial turbulence is investigated numerically from the perspective of the coupling between polymer dynamics and flow structures. In particular, direct numerical simulations of channel flow with Reynolds numbers ranging from 1000 to 6000 are used to study the formation and dynamics of elastic instabilities and their effects on the flow. Based on the splitting of the pressure into inertial and polymeric contributions, it is shown that the trains of cylindrical structures around sheets of high polymer extension that are characteristics to elasto-inertial turbulence are mostly driven by polymeric contributions. [less ▲]

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See detailDynamics of Elasto-Inertial Turbulence in Flows with Polymer Additives
Terrapon, Vincent ULiege; Dubief, Yves; Soria, Julio

Scientific conference (2012, December 07)

Elasto-inertial turbulence is a new state of turbulence that may occur in certain viscoelastic flows, in particular flows with polymer additives. The dynamics of elasto-inertial turbulence is here ... [more ▼]

Elasto-inertial turbulence is a new state of turbulence that may occur in certain viscoelastic flows, in particular flows with polymer additives. The dynamics of elasto-inertial turbulence is here investigated numerically from the perspective of the coupling between polymer dynamics and flow structures. The resulting mechanism helps resolve a long standing controversy in the understanding of polymer drag reduction and explains the phenomenon of early turbulence, or onset of turbulence at lower Reynolds numbers than for Newtonian flows, previously observed in polymeric flows. In particular, we show that the introduction of small perturbations into the polymeric flow excites the unstable nature of the nonlinear advection term, resulting in the formation of sheets or cliffs of polymer stretch. These sheets of high polymer stretch, hosting a significant increase in extensional viscosity, create a strong local anisotropy, with a formation of local low-speed jet-like flow. The response of the flow is through pressure, whose role is to redistribute energy across components of momentum, resulting in the formation of waves, or trains of alternating rotational and straining motions. The mechanism shares some similarity with the Kelvin-Helmholtz instability, except that the thickness of these sheets is too close to the Kolmogorov scale for vortices to be created. Once triggered, EIT is self-sustained since the elastic instability creates the very velocity fluctuations it feeds upon. [less ▲]

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See detailStudying the Topology and Dynamics of Elasto-inertial Channel Flow Turbulence Using the Invariants of the Velocity Gradient Tensor and Dynamic Mode Decomposition
Soria, Julio; Terrapon, Vincent ULiege; Dubief, Yves

Conference (2012, November 20)

Direct numerical simulations (DNS) of the transition to and fully developed elasto-inertial turbulence (EIT) of a polymer solution in a channel flow has been used as a basis for the study of the topology ... [more ▼]

Direct numerical simulations (DNS) of the transition to and fully developed elasto-inertial turbulence (EIT) of a polymer solution in a channel flow has been used as a basis for the study of the topology and dynamics of these flows. The Reynolds number in these DNS ranged from 500 to 5000. The topology of these flows was studied through the joint probability density functions (JPDFs) of the second and third invariants of the velocity gradient tensor (VGT), $Q_A$ and $R_A$ respectively and the JPDFs of the second invariants of the rate-of-strain tensor and the rate-of-rotation tensor, $Q_S$ and $Q_W$ respectively. The results suggest that these transitional and fully developed EIT flows are predominantly made up of vortex sheets. Dynamic mode decomposition has been undertaken on the second invariant of the VGT, $Q_A$, which reveals that the most amplified mode is a two-dimensional structure located in the near-wall region. A ``discontinuity'' is observed close to the wall, which corresponds closely to the location of extrema of the mean polymer extension and is hypothesized to be a critical layer. [less ▲]

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See detailA New State of Turbulence: Elasto-Inertial Turbulence
Dubief, Yves; Samanta, Devranjan; Holzner, Markus et al

Conference (2012, November 20)

The elasticity of polymer solutions is found to generate a new state of turbulence, elasto-inertial turbulence (EIT), characterized by an interplay between elastic and flow instabilities. Experiments and ... [more ▼]

The elasticity of polymer solutions is found to generate a new state of turbulence, elasto-inertial turbulence (EIT), characterized by an interplay between elastic and flow instabilities. Experiments and direct numerical simulations (DNS) in pipe and channel flows demonstrate the emergence of EIT at Reynolds numbers much lower than the critical Reynolds number for transition to turbulence in Newtonian flows. EIT causes the friction factor to deviate from the laminar solution and subsequently transition to the maximum drag reduction asymptote around Re=1800. EIT is a self-sustained mechanism that arises from the interactions between fluctuations of extensional viscosity, velocity and pressure. The polymer solution elasticity controls the growth of flow instability, resulting in transitional-like flows at high Reynolds numbers. The existence of EIT is not limited to pipe, channel or boundary layer flows, and evidence of EIT will be discussed in other flows, including natural convection using DNS. [less ▲]

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See detailMechanics and characteristics of transition to turbulence in elasto-inertial turbulence
Terrapon, Vincent ULiege; Dubief, Yves; Soria, Julio

Conference (2012, November 20)

Numerical experiments of transition in elasto-inertial turbulent channel flows are used to highlight the mechanisms of transition and characterize the MDR regime. Specifically, the pressure kernel from ... [more ▼]

Numerical experiments of transition in elasto-inertial turbulent channel flows are used to highlight the mechanisms of transition and characterize the MDR regime. Specifically, the pressure kernel from the generalized pressure Poisson equation is used to demonstrate the role of elastic instabilities in inducing and sustaining a turbulent-like flow. Additionally, dynamic mode decomposition is applied to statistically steady viscoelastic flows at different Reynolds number to identify the relative contributions of elastic and inertial instabilities. It is shown that elastic instabilities can be triggered through long-range interactions from disturbances in the free-stream, similarly to by-pass transition, and are then sufficient to self-sustain. When the Reynolds number is increased, the relative contribution of inertial instabilities becomes more important, and the flow demonstrates features that are characteristic to Newtonian turbulent flows (e.g., streaks, quasi-streamwise vortices), although at lower intensity. [less ▲]

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See detailAnalysis of transitional polymeric flows and elastic instabilities
Dubief, Yves; Terrapon, Vincent ULiege; Soria, Julio

in Center for Turbulence Research, Proceedings of the Summer Program (2012)

The dynamics of turbulence generated and controlled by polymer additives is investigated from the perspective of the coupling between polymer dynamics and flow structures. Direct numerical simulations of ... [more ▼]

The dynamics of turbulence generated and controlled by polymer additives is investigated from the perspective of the coupling between polymer dynamics and flow structures. Direct numerical simulations of channel flow with Reynolds numbers ranging from 1,000 to 10,000 (based on the bulk and the channel height) are used to study the formation and dynamics of elastic instabilities and their effects on the flow. The resulting mechanism of interactions between polymer dynamics and the flow helps resolve a long-standing controversy in the understanding of polymer drag reduction and explains the phenomenon of early turbulence, or onset of turbulence at lower Reynolds numbers than for Newtonian flows, previously observed in polymeric flows. [less ▲]

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See detailTowards a better understanding of polymer drag reduction in turbulent wall-bounded flows
Terrapon, Vincent ULiege; Dubief, Yves; White, Chris et al

Scientific conference (2011, November 14)

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See detailPolymer Maximum Drag Reduction: A Unique Transitional State
Dubief, Yves; White, Christopher M.; Shaqfeh, Eric S.G. et al

E-print/Working paper (2011)

The upper bound of polymer drag reduction is identified as a unique transitional state between laminar and turbulent flow corresponding to the onset of the nonlinear breakdown of flow instabilities.

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See detailPolymer maximum drag reduction: A unique transitional state
Dubief, Yves; White, Christopher M.; Shaqfeh, Eric S. G. et al

in Annual Research Briefs (2010)

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See detailNew answers on the interaction between polymers and vortices in turbulent flows
Dubief, Yves; Terrapon, Vincent ULiege; White, Christopher M et al

in Flow, Turbulence and Combustion (2005), 74(4), 311-329

Numerical data of polymer drag reduced flows is interpreted in terms of modification of near-wall coherent structures. The originality of the method is based oil numerical experiments in which boundary ... [more ▼]

Numerical data of polymer drag reduced flows is interpreted in terms of modification of near-wall coherent structures. The originality of the method is based oil numerical experiments in which boundary conditions or the governing equations are modified in a controlled manner to isolate certain features of the interaction between polymers and turbulence. As it result, polymers are shown to reduce drag by damping near-wall vortices and sustain turbulence by injecting energy onto the streamwise velocity component in the very near-wall region. [less ▲]

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