Role of flow reversals in transition to turbulence and relaminarization of pulsatile flows

To understand the role of fluid dynamics in atherogenesis, especially the effect of the flexibility of arteries, a two-dimensional numerical model for blood flow at the aortic bifurcation with linear viscoelastic walls is developed. The arbitrary Lagrangian-Eulerian method is adopted to deal with the moving boundary problem. The wall expansion induces flow reversals or eddies during the decelerating systole while the wall contraction restricts them during the diastole. A flexible bifurcation experiences the shear stresses about 10 percent lower than those of a rigid one.

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Deep Eastern Boundary Currents: Realistic Simulations and Vorticity Budgets

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0002.1 ◽

2020 ◽

Vol 50 (11) ◽

pp. 3077-3094

Author(s):

Xiaoting Yang ◽

Eli Tziperman ◽

Kevin Speer

Keyword(s):

General Circulation ◽

Large Scale ◽

Circulation Model ◽

Distribution Functions ◽

Boundary Currents ◽

Eastern Boundary ◽

Tracer Distribution ◽

Eastern Boundary Currents ◽

Flow Reversals

AbstractConcentrated poleward flows near the eastern boundaries between 2- and 4-km depth have been observed repeatedly, particularly in the Southern Hemisphere. These deep eastern boundary currents (DEBCs) play an important role in setting the large-scale tracer distribution and have nonnegligible contribution to global transports of mass, heat, and tracers, but their dynamics are not well understood. In this paper, we first demonstrate the significant role of DEBCs in the southeastern Atlantic, Indian, and Pacific Oceans, using the Southern Ocean State Estimate (SOSE) data assimilating product, and using high-resolution regional general circulation model configurations. The vorticity balances of these DEBCs reveal that, over most of the width of such currents, they are in an interior-like vorticity budget, with the meridional advection of planetary vorticity βυ and vortex stretching fwz being the largest two terms, and with contributions of nonlinearity and friction that are of smaller spatial scale. The stretching is shown, using a temperature budget, to be largely forced by resolved or parameterized eddy temperature transport. Strongly decaying signals from the eastern boundary in friction and stretching form the dominant balance in a sublayer close to the eastern boundary. The temporal variability of DEBCs is then examined, to help to interpret observations that tend to be sporadic in both time and space. The probability distribution functions of northward velocity in DEBC regions are broad, implying that flow reversals are common. Although the regions of the simulated DEBCs are generally local minima of eddy kinetic energy, they are still constantly releasing westward-propagating Rossby waves.

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Capital Flow Reversals, Sudden Stops, and International Reserve Adequacy: Further Evidence From the Global Financial Crisis

International Journal of Financial Research ◽

10.5430/ijfr.v10n1p52 ◽

2018 ◽

Vol 10 (1) ◽

pp. 52 ◽

Cited By ~ 1

Author(s):

Levan Efremidze ◽

Ozan Sula ◽

Thomas Willett

Keyword(s):

Global Financial Crisis ◽

Balance Sheet ◽

Capital Flow ◽

Policy Makers ◽

Balance Sheet Effects ◽

Export Competitiveness ◽

The Global Financial Crisis ◽

Flow Reversals ◽

Capital Flow Reversals

Using a dataset of 39 emerging markets, we examined the role of international reserves during currency and capital flow crises. Our analysis revealed that higher levels of reserves are associated with lower intensity crises where intensity is measured by the magnitude of the change in exchange market pressure (EMP) or size of capital flow reversals. We also find evidence for the cushioning effects of reserves during the crises. When used against capital flow reversals, reserves can help mitigate the negative output effects of the crisis. Finally, our findings show that reserve adequacy should be evaluated based on the nature of the potential crisis. Policy makers may prefer to refrain from using reserves if export competitiveness is more important than potential balance sheet effects of currency depreciation.

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Effects of Buoyancy on the Turbulent Wake of a Horizontal Cylinder in Cross-Flow

Volume 1: Symposia, Parts A and B ◽

10.1115/fedsm2012-72012 ◽

2012 ◽

Author(s):

Matthieu Boirlaud ◽

Dominique Couton ◽

Frédéric Plourde

Keyword(s):

Mixed Convection ◽

Shear Layer ◽

Thermal Wave ◽

Thermal Field ◽

Cross Flow ◽

Horizontal Cylinder ◽

Transition To Turbulence ◽

Cylinder Surface ◽

Convection Regime

While heat transfer around bluff-bodies have been extensively studied in natural and forced convection regime, the mixed convection regime has not still yet brought so much attention; however the latter has direct interest either in various engineering applications or for fundamental point of views. Direct Numerical Simulation was applied in this paper to study the buoyancy effects in the wake of a horizontal cylinder in cross-flow for Re∞ = 1000 and Ri = 2.77. In the framework of mixed convection regime, results mainly focus on the role of thermal field and buoyancy effects. The main visible impact in the thermal field introduction is the asymmetry in the cylinder wake. In addition, typical mushroom-like structures driven by thermal field develop along the wake. From an unsteady point of view, a thermal wave develops from the bottom of the cylinder and the latter follows the cylinder surface. As a consequence, the upper shear-layer that occurs in isotherm case is strongly disturbed because of the interaction with the thermal wave and the lower shear-layer is stretched in the flow direction. Comparisons with the isotherm case help us to better understand the role of the thermal field and the effects of buoyancy in the transition to turbulence.

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Activity pulses induce spontaneous flow reversals in viscoelastic environments

10.1101/2021.02.05.429955 ◽

2021 ◽

Author(s):

Emmanuel L. C. VI M. Plan ◽

Julia M. Yeomans ◽

Amin Doostmohammadi

Keyword(s):

Viscoelastic Medium ◽

Cellular Systems ◽

Time Dependent ◽

Cell Functions ◽

Complex Interactions ◽

Active Particles ◽

Flow Generation ◽

Active Flow ◽

Flow Reversals

Complex interactions between cellular systems and their surrounding extracellular matrices are emerging as important mechanical regulators of cell functions such as proliferation, motility, and cell death, and such cellular systems are often characterized by pulsating acto-myosin activities. Here, using an active gel model, we numerically explore the spontaneous flow generation by activity pulses in the presence of a viscoelastic medium. The results show that cross-talk between the activity-induced deformations of the viscoelastic surroundings with the time-dependent response of the active medium to these deformations can lead to the reversal of spontaneously generated active flows. We explain the mechanism behind this phenomenon based on the interaction between the active flow and the viscoelastic medium. We show the importance of relaxation timescales of both the polymers and the active particles and provide a phase-space over which such spontaneous flow reversals can be observed. Our results suggest new experiments investigating the role of controlled pulses of activity in living systems ensnared in complex mircoenvironments.

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Experiments on the role of amplitude and phase modulations during transition to turbulence

Journal of Fluid Mechanics ◽

10.1017/s0022112082002924 ◽

1982 ◽

Vol 123 ◽

pp. 1-29 ◽

Cited By ~ 41

Author(s):

R. W. Miksad ◽

F. L. Jones ◽

E. J. Powers ◽

Y. C. Kim ◽

L. Khadra

Keyword(s):

Dominant Role ◽

Modulation Frequency ◽

Power Spectra ◽

Transition Process ◽

Transition To Turbulence ◽

Nonlinear Interactions ◽

Spectral Broadening ◽

Energy Redistribution ◽

Difference Mode

The transition of a laminar two-dimensional wake is studied experimentally to establish the role of amplitude and phase modulations in the spectral-broadening and energy-redistribution process. Multiple instability modes fo and fi are triggered by acoustic excitation. The spectrum of the fluctuating velocity field formed by the growing and interacting instabilities shows the development of a complicated side- band structure reminiscent of amplitude- and phase-modulated waves. Digital com- plex demodulation techniques are used to obtain quantitative measurements of local instantaneous amplitude and phase modulations. Measurements of the modulation time traces, their modulation indices, the lag between phase and amplitude modula- tions, and the power spectra of the modulations are presented. Our results show that both phase and amplitude modulation play a role in the transition process. The dominant modulation frequency of both amplitude and phase is that of the difference mode fv = f1−f0 produced by the interaction of the two excited instabilities. Phase modulation becomes progressively more important as transition proceeds down- stream, and seems to play the dominant role in the spectral-broadening and energy- redistribution process. Measurements of the bicoherency spectrum indicate that sideband structures, and accompanying modulations, are produced by nonlinear interactions between the low-frequency difference mode and higher-frequency in- stability modes. Some limited measurements indicate that finite-amplitude induced nonlinear dispersion effects ω(k, a2) may provide a physical mechanism by which amplitude modulations generated by nonlinear interactions can induce simultaneous phase modulations.

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The effect of pulsation frequency on transition in pulsatile pipe flow

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.789 ◽

2018 ◽

Vol 857 ◽

pp. 937-951 ◽

Cited By ~ 7

Author(s):

Duo Xu ◽

Marc Avila

Keyword(s):

Pipe Flow ◽

Experimental Studies ◽

Quantitative Agreement ◽

Transition To Turbulence ◽

Pulsation Frequency ◽

Pipe Length ◽

Pulsation Amplitude ◽

Frequency Regime ◽

Pulsatile Flows ◽

The Stability

Pulsatile flows are common in nature and in applications, but their stability and transition to turbulence are still poorly understood. Even in the simple case of pipe flow subject to harmonic pulsation, there is no consensus among experimental studies on whether pulsation delays or enhances transition. We here report direct numerical simulations of pulsatile pipe flow at low pulsation amplitude$A\leqslant 0.4$. We use a spatially localized impulsive disturbance to generate a single turbulent puff and track its dynamics as it travels downstream. The computed relaminarization statistics are in quantitative agreement with the experiments of Xuet al. (J. Fluid Mech., vol. 831, 2017, pp. 418–432) and support the conclusion that increasing the pulsation amplitude and lowering the frequency enhance the stability of the flow. In the high-frequency regime, the behaviour of steady pipe flow is recovered. In addition, we show that, when the pipe length does not permit the observation of a full cycle, a reduction of the transition threshold is observed. We obtain an equation quantifying this effect and compare it favourably with the measurements of Stettler & Hussain (J. Fluid Mech., vol. 170, 1986, pp. 169–197). Our results resolve previous discrepancies, which are due to different pipe lengths, perturbation methods and criteria chosen to quantify transition in experiments.

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The role of nonlinear critical layers in transition to turbulence in boundary layers

10.2514/6.1996-2132 ◽

1996 ◽

Author(s):

M. Goldstein

Keyword(s):

Boundary Layers ◽

Transition To Turbulence ◽

Critical Layers

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Instability modes and transition of pulsatile stenotic flow: pulse-period dependence

Journal of Fluid Mechanics ◽

10.1017/s0022112006003740 ◽

2007 ◽

Vol 573 ◽

pp. 57-88 ◽

Cited By ~ 32

Author(s):

H. M. BLACKBURN ◽

S. J. SHERWIN

Keyword(s):

Harmonic Component ◽

Period Doubling ◽

Vortex Rings ◽

Mean Flow ◽

Instability Modes ◽

Pulsatile Flows ◽

Long Pulse ◽

Low Amplitude ◽

Primary Instability

The instability modes arising within simple non-reversing pulsatile flows in a circular tube with a smooth axisymmetric constriction are examined using global Floquet stability analysis and direct numerical simulation. The sectionally averaged pulsatile flow is represented with one harmonic component superimposed on a time-mean flow. We have previously identified a period-doubling global instability mechanism associated with alternating tilting of the vortex rings that are ejected out of the stenosis throat with each pulse. Here we show that while alternating tilting of vortex rings is the primary instability mode for comparatively larger reduced velocities associated with long pulse periods (or low Womersley numbers), for lower reduced velocities that are associated with shorter pulse periods the primary instability typically manifests as azimuthal waves (Widnall instability modes) of low wavenumber that grow on each vortex ring. Convective shear-layer instabilities are also supported by the types of flow considered. To provide an insight into the comparative role of these types of instability, which have still shorter temporal periods, we also introduce high-frequency low-amplitude perturbations to the base flows of the above global instabilities. For the range of parameters considered, we observe that the dominant features of the primary Floquet instability persist, but that the additional presence of the convective instability can have a destabilizing effect, especially for long pulse periods.

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