The Experts below are selected from a list of 270 Experts worldwide ranked by ideXlab platform
Björn Hof - One of the best experts on this subject based on the ideXlab platform.
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Transition to turbulence in pulsating Pipe Flow
Journal of Fluid Mechanics, 2017Co-Authors: Sascha Warnecke, Baofang Song, Björn HofAbstract:Fluid Flows in nature and applications are frequently subject to periodic velocity modulations. Surprisingly, even for the generic case of Flow through a straight Pipe, there is little consensus regarding the influence of pulsation on the transition threshold to turbulence: while most studies predict a monotonically increasing threshold with pulsation frequency (i.e. Womersley number, $\unicode[STIX]{x1D6FC}$ ), others observe a decreasing threshold for identical parameters and only observe an increasing threshold at low $\unicode[STIX]{x1D6FC}$ . In the present study we apply recent advances in the understanding of transition in steady shear Flows to pulsating Pipe Flow. For moderate pulsation amplitudes we find that the first instability encountered is subcritical (i.e. requiring finite amplitude disturbances) and gives rise to localized patches of turbulence (‘puffs’) analogous to steady Pipe Flow. By monitoring the impact of pulsation on the lifetime of turbulence we map the onset of turbulence in parameter space. Transition in pulsatile Flow can be separated into three regimes. At small Womersley numbers the dynamics is dominated by the decay turbulence suffers during the slower part of the cycle and hence transition is delayed significantly. As shown in this regime thresholds closely agree with estimates based on a quasi-steady Flow assumption only taking puff decay rates into account. The transition point predicted in the zero $\unicode[STIX]{x1D6FC}$ limit equals to the critical point for steady Pipe Flow offset by the oscillation Reynolds number (i.e. the dimensionless oscillation amplitude). In the high frequency limit on the other hand, puff lifetimes are identical to those in steady Pipe Flow and hence the transition threshold appears to be unaffected by Flow pulsation. In the intermediate frequency regime the transition threshold sharply drops (with increasing $\unicode[STIX]{x1D6FC}$ ) from the decay dominated (quasi-steady) threshold to the steady Pipe Flow level.
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transition to turbulence in pulsating Pipe Flow
arXiv: Fluid Dynamics, 2017Co-Authors: Sascha Warnecke, Baofang Song, Björn HofAbstract:Fluid Flows in nature and applications are frequently subject to periodic velocity modulations. Surprisingly, even for the generic case of Flow through a straight Pipe, there is little consensus regarding the influence of pulsation on the transition threshold to turbulence: while most studies predict a monotonically increasing threshold with pulsation frequency (i.e. Womersley number, $\alpha$), others observe a decreasing threshold for identical parameters and only observe an increasing threshold at low $\alpha$. In the present study we apply recent advances in the understanding of transition in steady shear Flows to pulsating Pipe Flow. For moderate pulsation amplitudes we find that the first instability encountered is subcritical (i.e. requiring finite amplitude disturbances) and gives rise to localized patches of turbulence ("puffs") analogous to steady Pipe Flow. By monitoring the impact of pulsation on the lifetime of turbulence we map the onset of turbulence in parameter space. Transition in pulsatile Flow can be separated into three regimes. At small Womersley numbers the dynamics are dominated by the decay turbulence suffers during the slower part of the cycle and hence transition is delayed significantly. As shown in this regime thresholds closely agree with estimates based on a quasi steady Flow assumption only taking puff decay rates into account. The transition point predicted in the zero $\alpha$ limit equals to the critical point for steady Pipe Flow offset by the oscillation Reynolds number. In the high frequency limit puff lifetimes are identical to those in steady Pipe Flow and hence the transition threshold appears to be unaffected by Flow pulsation. In the intermediate frequency regime the transition threshold sharply drops (with increasing $\alpha$) from the decay dominated (quasi steady) threshold to the steady Pipe Flow level.
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Experimental Observation of Nonlinear Traveling Waves in Turbulent Pipe Flow
Science (New York N.Y.), 2004Co-Authors: Björn Hof, Bruno Eckhardt, Frans T. M. Nieuwstadt, Jerry Westerweel, Casimir W. H. Van Doorne, Holger Faisst, Håkan Wedin, Richard R Kerswell, Fabian WaleffeAbstract:Transition to turbulence in Pipe Flow is one of the most fundamental and longest-standing problems in fluid dynamics. Stability theory suggests that the Flow remains laminar for all Flow rates, but in practice Pipe Flow becomes turbulent even at moderate speeds. This transition drastically affects the transport efficiency of mass, momentum, and heat. On the basis of the recent discovery of unstable traveling waves in computational studies of the Navier-Stokes equations and ideas from dynamical systems theory, a model for the transition process has been suggested. We report experimental observation of these traveling waves in Pipe Flow, confirming the proposed transition scenario and suggesting that the dynamics associated with these unstable states may indeed capture the nature of fluid turbulence.
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Transition to Turbulence in Pipe Flow
Fluid Mechanics and its Applications, 1Co-Authors: Björn HofAbstract:Transitional Pipe Flow is investigated in two different experimental set-ups. In the first the stability threshold and the initial growth of localized perturbations are studied.
V. Shankar - One of the best experts on this subject based on the ideXlab platform.
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Linear instability of viscoelastic Pipe Flow
Journal of Fluid Mechanics, 2020Co-Authors: Indresh Chaudhary, Piyush Garg, Ganesh Subramanian, V. ShankarAbstract:A modal stability analysis shows that pressure-driven Pipe Flow of an Oldroyd-B fluid is linearly unstable to axisymmetric perturbations, in stark contrast to its Newtonian counterpart which is linearly stable at all Reynolds numbers. The dimensionless groups that govern stability are the Reynolds number, the elasticity number, and the ratio of solvent to solution viscosity. The unstable mode has a phase speed close to the base-state maximum over the entire unstable region in the relevant parameter space, implying that the unstable mode belongs to a class of viscoelastic center modes. Unlike the Newtonian transition which is dominated by nonlinear processes, the linear instability discussed here could be very relevant to the onset of turbulence in viscoelastic Pipe Flows. The prediction of an instability is, in fact, consistent with several experimental studies on Pipe Flow of polymer solutions, ranging from previous reports of early turbulence to the more recent discovery of elasto-inertial turbulence. The instability identified in this study comprehensively dispels the prevailing notion of Pipe Flow of viscoelastic fluids being linearly stable in the Reynolds-Weissenberg plane, marking a possible paradigm shift in our understanding of transition in rectilinear viscoelastic shearing Flows.
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Viscoelastic Pipe Flow is Linearly Unstable.
Physical review letters, 2018Co-Authors: Piyush Garg, Indresh Chaudhary, Mohammad Khalid, V. Shankar, Ganesh SubramanianAbstract:Newtonian Pipe Flow is known to be linearly stable at all Reynolds numbers. We report, for the first time, a linear instability of pressure-driven Pipe Flow of a viscoelastic fluid, obeying the Oldroyd-B constitutive equation commonly used to model dilute polymer solutions. The instability is shown to exist at Reynolds numbers significantly lower than those at which transition to turbulence is typically observed for Newtonian Pipe Flow. Our results qualitatively explain experimental observations of transition to turbulence in Pipe Flow of dilute polymer solutions at Flow rates where Newtonian turbulence is absent. The instability discussed here should form the first stage in a hitherto unexplored dynamical pathway to turbulence in polymer solutions. An analogous instability exists for plane Poiseuille Flow.
Ganesh Subramanian - One of the best experts on this subject based on the ideXlab platform.
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Linear instability of viscoelastic Pipe Flow
Journal of Fluid Mechanics, 2020Co-Authors: Indresh Chaudhary, Piyush Garg, Ganesh Subramanian, V. ShankarAbstract:A modal stability analysis shows that pressure-driven Pipe Flow of an Oldroyd-B fluid is linearly unstable to axisymmetric perturbations, in stark contrast to its Newtonian counterpart which is linearly stable at all Reynolds numbers. The dimensionless groups that govern stability are the Reynolds number, the elasticity number, and the ratio of solvent to solution viscosity. The unstable mode has a phase speed close to the base-state maximum over the entire unstable region in the relevant parameter space, implying that the unstable mode belongs to a class of viscoelastic center modes. Unlike the Newtonian transition which is dominated by nonlinear processes, the linear instability discussed here could be very relevant to the onset of turbulence in viscoelastic Pipe Flows. The prediction of an instability is, in fact, consistent with several experimental studies on Pipe Flow of polymer solutions, ranging from previous reports of early turbulence to the more recent discovery of elasto-inertial turbulence. The instability identified in this study comprehensively dispels the prevailing notion of Pipe Flow of viscoelastic fluids being linearly stable in the Reynolds-Weissenberg plane, marking a possible paradigm shift in our understanding of transition in rectilinear viscoelastic shearing Flows.
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Viscoelastic Pipe Flow is Linearly Unstable.
Physical review letters, 2018Co-Authors: Piyush Garg, Indresh Chaudhary, Mohammad Khalid, V. Shankar, Ganesh SubramanianAbstract:Newtonian Pipe Flow is known to be linearly stable at all Reynolds numbers. We report, for the first time, a linear instability of pressure-driven Pipe Flow of a viscoelastic fluid, obeying the Oldroyd-B constitutive equation commonly used to model dilute polymer solutions. The instability is shown to exist at Reynolds numbers significantly lower than those at which transition to turbulence is typically observed for Newtonian Pipe Flow. Our results qualitatively explain experimental observations of transition to turbulence in Pipe Flow of dilute polymer solutions at Flow rates where Newtonian turbulence is absent. The instability discussed here should form the first stage in a hitherto unexplored dynamical pathway to turbulence in polymer solutions. An analogous instability exists for plane Poiseuille Flow.
Joseph Klewicki - One of the best experts on this subject based on the ideXlab platform.
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emergence of the four layer dynamical regime in turbulent Pipe Flow
Physics of Fluids, 2012Co-Authors: Joseph Klewicki, Chenq Chin, H M Blackburn, Andrew Ooi, Ivan MarusicAbstract:Direct numerical simulations of fully developed turbulent Pipe Flow that span the Reynolds number range 90 ≲ δ+ ≲ 1000 are used to investigate the evolution of the mean momentum field in and beyond the transitional regime. It is estimated that the four layer regime for Pipe Flow is nominally established for δ+ ⩾ 180, which is also close to the value found for channel Flow. Primary attention is paid to the magnitude ordering and scaling behaviors of the terms in the mean momentum equation. Once the ordering underlying the existence of four distinct balance layers is attained, this ordering is sustained for all subsequent increases in Reynolds number. Comparisons indicate that Pipe Flow develops toward the four layer regime in a manner similar to that for channel Flow, but distinct from that for the boundary layer. Small but discernible differences are observed in the mean momentum field development in Pipes and channels. These are tentatively attributed to variations in the manner by which the outer region...
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Mixing mechanisms in turbulent Pipe Flow
Physics of Fluids, 1997Co-Authors: James E. Guilkey, Alan R. Kerstein, Patrick Mcmurtry, Joseph KlewickiAbstract:An experimental investigation of passive scalar mixing in turbulent Pipe Flow is carried out using a new non-intrusive scalar initialization technique. The measurements support a recently predicted similarity scaling of concentration spectra in Flows that are unbounded in one direction. Reflecting this scaling, the scalar variance exhibits a power-law rather than exponential decay, indicating that the traditional plug-Flow reactor picture of turbulent Pipe-Flow mixing omits key physical mechanisms.
Indresh Chaudhary - One of the best experts on this subject based on the ideXlab platform.
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Linear instability of viscoelastic Pipe Flow
Journal of Fluid Mechanics, 2020Co-Authors: Indresh Chaudhary, Piyush Garg, Ganesh Subramanian, V. ShankarAbstract:A modal stability analysis shows that pressure-driven Pipe Flow of an Oldroyd-B fluid is linearly unstable to axisymmetric perturbations, in stark contrast to its Newtonian counterpart which is linearly stable at all Reynolds numbers. The dimensionless groups that govern stability are the Reynolds number, the elasticity number, and the ratio of solvent to solution viscosity. The unstable mode has a phase speed close to the base-state maximum over the entire unstable region in the relevant parameter space, implying that the unstable mode belongs to a class of viscoelastic center modes. Unlike the Newtonian transition which is dominated by nonlinear processes, the linear instability discussed here could be very relevant to the onset of turbulence in viscoelastic Pipe Flows. The prediction of an instability is, in fact, consistent with several experimental studies on Pipe Flow of polymer solutions, ranging from previous reports of early turbulence to the more recent discovery of elasto-inertial turbulence. The instability identified in this study comprehensively dispels the prevailing notion of Pipe Flow of viscoelastic fluids being linearly stable in the Reynolds-Weissenberg plane, marking a possible paradigm shift in our understanding of transition in rectilinear viscoelastic shearing Flows.
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Viscoelastic Pipe Flow is Linearly Unstable.
Physical review letters, 2018Co-Authors: Piyush Garg, Indresh Chaudhary, Mohammad Khalid, V. Shankar, Ganesh SubramanianAbstract:Newtonian Pipe Flow is known to be linearly stable at all Reynolds numbers. We report, for the first time, a linear instability of pressure-driven Pipe Flow of a viscoelastic fluid, obeying the Oldroyd-B constitutive equation commonly used to model dilute polymer solutions. The instability is shown to exist at Reynolds numbers significantly lower than those at which transition to turbulence is typically observed for Newtonian Pipe Flow. Our results qualitatively explain experimental observations of transition to turbulence in Pipe Flow of dilute polymer solutions at Flow rates where Newtonian turbulence is absent. The instability discussed here should form the first stage in a hitherto unexplored dynamical pathway to turbulence in polymer solutions. An analogous instability exists for plane Poiseuille Flow.