8,479 research outputs found
Transition to turbulence in duct flow
The transition of the flow in a duct of square cross-section is studied. Like
in the similar case of the pipe flow, the motion is linearly stable for all
Reynolds numbers; this flow is thus a good candidate to investigate the
'bypass' path to turbulence. Initially the so-called 'linear optimal
perturbation problem' is formulated and solved, yielding optimal disturbances
in the form of longitudinal vortices. Such optimals, however, fail to elicit a
significant response from the system in the nonlinear regime. Thus,
streamwise-inhomogeneous, sub-optimal disturbances are focussed upon; nonlinear
quadratic interactions are immediately evoked by such initial perturbations and
an unstable streamwise-homogeneous large amplitude mode rapidly emerges. The
subsequent evolution of the flow, at a value of the Reynolds number at the edge
between fully developed turbulence and relaminarization, shows the alternance
of patterns with two pairs of large scale vortices near opposing parallel
walls. Such edge states bear a resemblance to optimal disturbance
Intermittency in the transition to turbulence
It is commonly known that the intermittent transition from laminar to turbulent flow in pipes occurs because, at intermediate values of a prescribed pressure drop, a purely laminar flow offers too little resistance, but a fully turbulent one offers too much. We propose a phenomenological model of the flow, which is able to explain this in a quantitative way through a hysteretic transition between laminar and turbulent states, characterized by a disturbance amplitude variable that satisfies a natural type of evolution equation. The form of this equation is motivated by physical observations and derived by an averaging procedure, and we show that it naturally predicts disturbances having the characteristics of slugs and puffs. The model predicts oscillations similar to those which occur in intermittency in pipe flow, but it also predicts that stationary biphasic states can occur in sufficiently short pipes
Transition to turbulence in pipe flow
Since the seminal studies by Osborne Reynolds in the nineteenth century, pipe flow has served as a primary prototype for investigating the transition to turbulence in wall-bounded flows. Despite the apparent simplicity of this flow, various facets of this problem have occupied researchers for more than a century. Here we review insights from three distinct perspectives: ( a) stability and susceptibility of laminar flow, ( b) phase transition and spatiotemporal dynamics, and ( c) dynamical systems analysis of the Navier—Stokes equations. We show how these perspectives have led to a profound understanding of the onset of turbulence in pipe flow. Outstanding open points, applications to flows of complex fluids, and similarities with other wall-bounded flows are discussed. Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 55 is January 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates
Transition to turbulence in particle laden flows
Suspended particles can alter the properties of fluids and in particular also
affect the transition from laminar to turbulent flow. In the present
experimental study, we investigate the impact of neutrally buoyant, spherical
inertial particles on transition to turbulence in a pipe flow. At low particle
concentrations, like in single phase Newtonian fluids, turbulence only sets in
when triggered by sufficiently large perturbations and, as characteristic for
this transition localized turbulent regions (puffs) co-exist with laminar flow.
In agreement with earlier studies this transition point initially moves to
lower Reynolds number (Re) as the particle concentration increases. At higher
concentrations however the nature of the transition qualitatively changes:
Laminar flow gives way to a globally fluctuating state following a continuous,
non-hysteretic transition. A further increase in Re results in a secondary
instability where localized puff-like structures arise on top of the uniformly
fluctuating background flow. At even higher concentration only the uniformly
fluctuating flow is found and signatures of Newtonian type turbulence are no
longer observed
Transition to turbulence in particulate pipe flow
We investigate experimentally the influence of suspended particles on the
transition to turbulence. The particles are monodisperse and neutrally-buoyant
with the liquid. The role of the particles on the transition depends both upon
the pipe to particle diameter ratios and the concentration. For large
pipe-to-particle diameter ratios the transition is delayed while it is lowered
for small ratios. A scaling is proposed to collapse the departure from the
critical Reynolds number for pure fluid as a function of concentration into a
single master curve.Comment: 4 pages, 4 figure
Transition to turbulence in pulsating pipe flow
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, ), others observe a decreasing threshold for identical
parameters and only observe an increasing threshold at low . 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 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 ) from the decay dominated (quasi
steady) threshold to the steady pipe flow level
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