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Direct Numerical Simulation of Mixed Convection in Turbulent Channel Flow: On the Reynolds number dependency of momentum and heat transfer under unstable stratification Sid, Samir ; ; Terrapon, Vincent 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 ▲] Detailed reference viewed: 85 (21 ULg)On the role of pressure in elasto-inertial turbulence Terrapon, Vincent ; ; 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 ▲] Detailed reference viewed: 91 (18 ULg)Elasto-inertial turbulence in polymeric flows Terrapon, Vincent ; ; 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 ▲] Detailed reference viewed: 21 (7 ULg)On the mechanism of elasto-inertial turbulence ; Terrapon, Vincent ; 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 ▲] Detailed reference viewed: 161 (28 ULg)Dynamics of Elasto-Inertial Turbulence in Flows with Polymer Additives Terrapon, Vincent ; ; 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 ▲] Detailed reference viewed: 64 (13 ULg)Dynamics of Elasto-Inertial Turbulence in Flows with Polymer Additives Terrapon, Vincent ; ; 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 ▲] Detailed reference viewed: 53 (17 ULg)Studying the Topology and Dynamics of Elasto-inertial Channel Flow Turbulence Using the Invariants of the Velocity Gradient Tensor and Dynamic Mode Decomposition ; Terrapon, Vincent ; 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 ▲] Detailed reference viewed: 59 (6 ULg)A New State of Turbulence: Elasto-Inertial Turbulence ; ; 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 ▲] Detailed reference viewed: 93 (9 ULg)Mechanics and characteristics of transition to turbulence in elasto-inertial turbulence Terrapon, Vincent ; ; 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 ▲] Detailed reference viewed: 45 (13 ULg)Analysis of transitional polymeric flows and elastic instabilities ; Terrapon, Vincent ; 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 ▲] Detailed reference viewed: 91 (14 ULg)Towards a better understanding of polymer drag reduction in turbulent wall-bounded flows Terrapon, Vincent ; ; et al Scientific conference (2011, November 14) Detailed reference viewed: 45 (6 ULg)Polymer Maximum Drag Reduction: A Unique Transitional State ; ; 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. Detailed reference viewed: 20 (6 ULg)Polymer maximum drag reduction: A unique transitional state ; ; et al in Annual Research Briefs (2010) Detailed reference viewed: 60 (5 ULg)New answers on the interaction between polymers and vortices in turbulent flows ; Terrapon, Vincent ; 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 ▲] Detailed reference viewed: 47 (6 ULg)On the coherent drag-reducing and turbulence-enhancing behaviour of polymers in wall flows ; ; Terrapon, Vincent et al in Journal of Fluid Mechanics (2004), 514 Numerical simulations of turbulent polymer solutions using the FENE-P model are used to characterize the action of polymers on turbulence in drag-reduced flows. The energetics of turbulence is ... [more ▼] Numerical simulations of turbulent polymer solutions using the FENE-P model are used to characterize the action of polymers on turbulence in drag-reduced flows. The energetics of turbulence is investigated by correlating the work done by polymers on the flow with turbulent structures. Polymers are found to store and to release energy to the flow in a well-organized manner. The storage of energy occurs around near-wall vortices as has been anticipated for a long time. Quite unexpectedly, coherent release of energy is observed in the very near-wall region. Large fluctuations of polymer work are shown to re-energize decaying streamwise velocity fluctuations in highspeed streaks just above the viscous sublayer. These distinct behaviours are used to propose an autonomous regeneration cycle of polymer wall turbulence, in the spirit of Jimenez & Pinelli (1999). [less ▲] Detailed reference viewed: 71 (6 ULg)Simulated polymer stretch in a turbulent flow using Brownian dynamics Terrapon, Vincent ; ; et al in Journal of Fluid Mechanics (2004), 504 We examine the phenomenon of polymer drag reduction in a turbulent flow through Brownian dynamics simulations. The dynamics of a large number of single polymer chains along their trajectories is ... [more ▼] We examine the phenomenon of polymer drag reduction in a turbulent flow through Brownian dynamics simulations. The dynamics of a large number of single polymer chains along their trajectories is investigated in a Newtonian turbulent channel flow. In particular, the FENE, FENE-P and multimode FENE models with realistic parameters are used to investigate the mechanisms of polymer stretching. A topological methodology is applied to characterize the ability of the flow to stretch the polymers. It is found using conditional statistics that at moderate Weissenberg number Wi the polymers, that are stretched to a large fraction of their maximum extensibility, have experienced a strong blaxial extensional flow. When Wi is increased other flow types can stretch the polymers but the few highly extended molecules again have, on average, experienced a biaxial extensional flow. Moreover, highly extended polymers are found in the near-wall regions around the quasi-streamwise vortices, essentially in regions of strong biaxial extensional flow. [less ▲] Detailed reference viewed: 49 (6 ULg)Brownian dynamics simulation in a turbulent channel flow Terrapon, Vincent ; ; et al in Proceedings of the 4th ASME/JSME Joint Fluids Engineering Conference; No FEDSM2003-45680 (2003, July) The dynamics of different bead-spring models is investigated in a turbulent channel flow. In particular, the FENE, the FENE-P and the FENE multichain models are compared. In the case of the FENE-P model ... [more ▼] The dynamics of different bead-spring models is investigated in a turbulent channel flow. In particular, the FENE, the FENE-P and the FENE multichain models are compared. In the case of the FENE-P model, both the Brownian Dynamics and the constitutive equations are used. It is shown that the different models produce qualitatively similar results for the mean extension and the mean stresses. This qualitative behaviour is also reproduced for different extensibility parameters. It is also found that the action of polymers is confined in the near wall region where the polymers are mainly oriented in the streamwise direction. [less ▲] Detailed reference viewed: 23 (1 ULg)Numerical Simulation of High Drag Reduction Regime in Polymer Solutions ; ; Terrapon, Vincent et al in Proceedings of the 4th ASME/JSME Joint Fluids Engineering Conference; No FEDSM2003-45652 (2003, July) The simulation of drag reduced channel flows has to rely on consitituve models such as FENE-P. Their implementation is not straightforward to achieve convergence and stability of the solution. This paper ... [more ▼] The simulation of drag reduced channel flows has to rely on consitituve models such as FENE-P. Their implementation is not straightforward to achieve convergence and stability of the solution. This paper discusses the problem of advection embedded in the FENE-P equation and the issue of the domain size. Finally we present results of simulations for High Drag Reduction regime, and show the subsequent modification of the vortical structures. [less ▲] Detailed reference viewed: 31 (4 ULg) |
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