Transition of Transient Channel Flow with High Reynolds Number Ratios

Mathur, Akshat; Seddighi, Mehdi; He, Suqin

doi:10.3390/e20050375

Cited by 5 publications

(8 citation statements)

References 37 publications

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“…However, at φ = 1.25π, R z11 has once again become positive for 2z + < 400 throughout the domain, indicating a complete breakdown of the streaks by this point. This behaviour in R 11z bears a strong similarity to that observed in the step-up accelerating flows of Mathur et al (2018b), in which the Reynolds number was increased by a large factor of 6.5. When this factor was Case L10A10 (figure 9c) starts off in a similar manner to case L26A05.…”

Section: Streamwise Velocity Streakssupporting

confidence: 79%

“…The streamwise correlation R 11x quantifies the length of the streaks, whilst the spanwise correlation R 11z quantifies the spanwise spacing between the streaks, which is assumed to be equal to twice the spanwise distance at which R 11z reaches its minimum value, (R z11 ) min . The magnitude of (R z11 ) min indicates the strength of the streaks relative to the surrounding flow in the corresponding x-z plane (Mathur et al 2018b;Falcone & He 2022),…”

Section: Streamwise Velocity Streaksmentioning

confidence: 99%

“…After observing similar trends in ramp-up channel flow, Jung & Kim (2017) suggested that bypass transition may only occur if the additional impulse from the increased flow rate greatly exceeds the additional impulse from the increased shear stress. On the other end of the scale, Mathur, Seddighi & He (2018 b ) observed that increasing the ratio between the final and initial Reynolds numbers amplified the length of the streaks during the pretransition stage, and increased the strength of the turbulent spots at the onset of transition.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Turbulent–turbulent transient concept in pulsating flows

Taylor,

Seddighi

2024

J. Fluid Mech.

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The turbulence behaviour of current-dominated pulsating flows has been investigated. Direct numerical simulations have been carried out for Stokes lengths over a range of $l_s^+=5\unicode{x2013}26$ , and amplitudes spanning 90 % of the current-dominated regime, about a mean flow of $\overline {Re}=6275$ . The results show that the turbulence response in intermediate and low-frequency pulsations is governed by a multistage turbulent–turbulent transition process, which bears a strong similarity to the multistage response of non-periodic acceleration. During the early acceleration period, the flow enters a pretransition stage, in which a new laminar perturbation boundary layer forms at the wall, and the streamwise velocity streaks are stretched. If the low-speed streaks destabilise prior to the deceleration period, then the flow enters a transition stage in which the perturbation boundary layer undergoes a bypass-like transition process. A unique feature of pulsating flows is the ongoing mechanism of turbulence decay, which initiates during the deceleration period and constitutes the main transient turbulence mechanism for much of the cycle. For high-frequency pulsations, the perturbation boundary layer fails to reach the pretransition stage prior to the deceleration period. Instead, the flow alternates between two inertial stages which are characterised by two layers of amplified viscous force; one growing at the wall, and one detached and moving towards the core.

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Section: Streamwise Velocity Streakssupporting

confidence: 79%

Section: Streamwise Velocity Streaksmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Turbulent–turbulent transient concept in pulsating flows

Taylor,

Seddighi

2024

J. Fluid Mech.

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“…The Rayleigh problem studied here is non-periodic, but the laminar flow allows a similarity solution. Experimental results for transition to turbulence in non-periodic unsteady flows by Mathur et al (2018) have shown that if the flow is persistently accelerated the critical Reynolds number associated with transition increases. In addition, for periodic unsteadiness flows or flows where the unsteadiness is changing, for example by acceleration, the flow history is important: the way in which instantaneous flow behaves is based on past history.…”

Section: Discussionmentioning

confidence: 99%

Free-stream coherent structures in the unsteady Rayleigh boundary layer

Johnstone

Hall

2020

IMA Journal of Applied Mathematics

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Results are presented for nonlinear equilibrium solutions of the Navier–Stokes equations in the boundary layer set up by a flat plate started impulsively from rest. The solutions take the form of a wave–roll–streak interaction, which takes place in a layer located at the edge of the boundary layer. This extends previous results for similar nonlinear equilibrium solutions in steady 2D boundary layers. The results are derived asymptotically and then compared to numerical results obtained by marching the reduced boundary-region disturbance equations forward in time. It is concluded that the previously found canonical free-stream coherent structures in steady boundary layers can be embedded in unbounded, unsteady shear flows.

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“…Mathur et al [1] address the importance of structures in the transient behaviour of a channel flow at high Reynolds number Re. Large-eddy simulations of turbulent channel flow subjected to a step-like acceleration reveal the transition of transient channel flow comprised of a three-stage response similar to that of the bypass transition of boundary layer flows; the effect of the structures (the elongated streaks) becomes more important in the transition for large Re.…”

mentioning

confidence: 99%

Intermittency and Self-Organisation in Turbulence and Statistical Mechanics

Kim

2019

Entropy

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There is overwhelming evidence, from laboratory experiments, observations, and computational studies, that coherent structures can cause intermittent transport, dramatically enhancing transport. A proper description of this intermittent phenomenon, however, is extremely difficult, requiring a new non-perturbative theory, such as statistical description. Furthermore, multi-scale interactions are responsible for inevitably complex dynamics in strongly non-equilibrium systems, a proper understanding of which remains one of the main challenges in classical physics. However, as a remarkable consequence of multi-scale interaction, a quasi-equilibrium state (so-called self-organisation) can be maintained.This Special Issue presents different theories of statistical mechanics to understand this challenging multiscale problem in turbulence. The 14 contributions to this Special Issue focus on the various aspects of intermittency, coherent structures, self-organisation, bifurcation and nonlocality. Given the ubiquity of turbulence, the contributions cover a broad range of systems covering laboratory fluids (channel flow, the Von Kármán flow), plasmas (magnetic fusion), laser cavity, wind turbine, air flow around a high-speed train, solar wind and industrial application. The following is a short summary of each contribution.Mathur et al.[1] address the importance of structures in the transient behaviour of a channel flow at high Reynolds number Re. Large-eddy simulations of turbulent channel flow subjected to a step-like acceleration reveal the transition of transient channel flow comprised of a three-stage response similar to that of the bypass transition of boundary layer flows; the effect of the structures (the elongated streaks) becomes more important in the transition for large Re. Their analysis employing conditionally-averaged turbulent statistics elucidates the interplay between structures and active/inactive regions of turbulence depending on Re.Chliamovitch and Thorimbert [2] present a new method of dealing with non-locality of turbulence flows through the formulation of the bilocal kinetic equation for pairs of particles. Based on a maximum-entropy-based generalisation of Boltzmann's assumption of molecular chaos, they utilise the two-particle kinetic equations and derive the balance equations from the bilocal invariants to close their kinetic equations. The end product of their calculation is non-viscous hydrodynamics, providing a new dynamical equation for the product of fluid velocities at different points in space.Jacquet et al. [3] address the formation of coherent structures and their self-organisation in a reduced model of turbulence. They present the transient behaviour of self-organised shear flows by solving the Fokker-Planck equation for time-dependent Probability Density Functions (PDFs) and model the formation of self-organisation shear flows by the emergence of a bimodal PDF with the two peaks for non-zero mean values of a shear flow. They show that the information length-The total number of statisti...

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Transition of Transient Channel Flow with High Reynolds Number Ratios

Cited by 5 publications

References 37 publications

Turbulent–turbulent transient concept in pulsating flows

Turbulent–turbulent transient concept in pulsating flows

Free-stream coherent structures in the unsteady Rayleigh boundary layer

Intermittency and Self-Organisation in Turbulence and Statistical Mechanics

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