86 research outputs found
On the nature of laminar-turbulence intermittency in shear flows
In pipe, channel and boundary layer flows turbulence first occurs
intermittently in space and time: at moderate Reynolds numbers domains of
disordered turbulent motion are separated by quiescent laminar regions. Based
on direct numerical simulations of pipe flow we here argue that the spatial
intermittency has its origin in a nearest neighbor interaction between
turbulent regions. We further show that in this regime turbulent flows are
intrinsically intermittent with a well defined equilibrium turbulent fraction
but without ever assuming a steady pattern. This transition scenario is
analogous to that found in simple models such as coupled map lattices. The
scaling observed implies that laminar intermissions of the turbulent flow will
persist to arbitrarily large Reynolds numbers
Heteroclinic path to spatially localized chaos in pipe flow
In shear flows at transitional Reynolds numbers, localized patches of
turbulence, known as puffs, coexist with the laminar flow. Recently, Avila et
al., Phys. Rev. Let. 110, 224502 (2013) discovered two spatially localized
relative periodic solutions for pipe flow, which appeared in a saddle-node
bifurcation at low speeds. Combining slicing methods for continuous symmetry
reduction with Poincar\'e sections for the first time in a shear flow setting,
we compute and visualize the unstable manifold of the lower-branch solution and
show that it contains a heteroclinic connection to the upper branch solution.
Surprisingly this connection even persists far above the bifurcation point and
appears to mediate puff generation, providing a dynamical understanding of this
phenomenon.Comment: 10 pages, 5 figure
Relaminarization of pipe flow by means of 3d-printed shaped honeycombs
Based on a novel control scheme, where a steady modification of the
streamwise velocity profile leads to complete relaminarization of initially
fully turbulent pipe flow, we investigate the applicability and usefulness of
custom-shaped honeycombs for such control. The custom-shaped honeycombs are
used as stationary flow management devices which generate specific
modifications of the streamwise velocity profile. Stereoscopic particle image
velocimetry and pressure drop measurements are used to investigate and capture
the development of the relaminarizing flow downstream these devices. We compare
the performance of straight (constant length across the radius of the pipe)
honeycombs with custom-shaped ones (variable length across the radius). An
attempt is made to find the optimal shape for maximal relaminarization at
minimal pressure loss. The maximum attainable Reynolds number for total
relaminarization is found to be of the order of 10.000. Consequently the
respective reduction in skin friction downstream of the device is almost by a
factor of 5. The break-even point, where the additional pressure drop caused by
the device is balanced by the savings due to relaminarization and a net gain is
obtained, corresponds to a downstream stretch of distances as low as approx.\
100 pipe diameters of laminar flow
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
Geometry of transient chaos in streamwise-localized pipe flow turbulence
In pipes and channels, the onset of turbulence is initially dominated by
localized transients, which lead to sustained turbulence through their
collective dynamics. In the present work, we study the localized turbulence in
pipe flow numerically and elucidate a state space structure that gives rise to
transient chaos. Starting from the basin boundary separating laminar and
turbulent flow, we identify transverse homoclinic orbits, the presence of which
necessitates a homoclinic tangle and chaos. A direct consequence of the
homoclinic tangle is the fractal nature of the laminar-turbulent boundary,
which was conjectured in various earlier studies. By mapping the transverse
intersections between the stable and unstable manifold of a periodic orbit, we
identify the 'gateways' that promote an escape from turbulence.Comment: Accepted for publication in Physical Review Fluids as a Rapid
Communicatio
Speed and structure of turbulent fronts in pipe flow
Using extensive direct numerical simulations, the dynamics of
laminar-turbulent fronts in pipe flow is investigated for Reynolds numbers
between and . We here investigate the physical distinction
between the fronts of weak and strong slugs both by analysing the turbulent
kinetic energy budget and by comparing the downstream front motion to the
advection speed of bulk turbulent structures. Our study shows that weak
downstream fronts travel slower than turbulent structures in the bulk and
correspond to decaying turbulence at the front. At the
downstream front speed becomes faster than the advection speed, marking the
onset of strong fronts. In contrast to weak fronts, turbulent eddies are
generated at strong fronts by feeding on the downstream laminar flow. Our study
also suggests that temporal fluctuations of production and dissipation at the
downstream laminar-turbulent front drive the dynamical switches between the two
types of front observed up to .Comment: 14 pages, accepted for publication in Journal of Fluid Mechanic
Experimental investigation of laminar turbulent intermittency in pipe flow
In shear flows turbulence first occurs in the form of localized structures
(puffs/spots) surrounded by laminar fluid. We here investigate such spatially
intermittent flows in a pipe experiment showing that turbulent puffs have a
well defined interaction distance, which sets the minimum spacing of puffs as
well as the maximum observable turbulent fraction. Two methodologies are
employed here. Starting from a laminar flow puffs can be created by locally
injecting a jet of fluid through the pipe wall. When the perturbation is
applied periodically at low frequencies, as expected, a regular sequence of
puffs is observed where the puff spacing is given by the ratio of the mean flow
speed to the perturbation frequency. On the other hand, at large frequencies
puffs are found to interact and annihilate each other. Varying the perturbation
frequency an interaction distance can be determined. In the second set of
experiments, the Reynolds number is reduced suddenly from fully developed
turbulence to the intermittent regime.The resulting flow reorganizes itself to
a sequence of constant size puffs which, unlike in Couette and Taylor Couette
flow are randomly spaced. The minimum distance between the turbulent patches is
identical to the puff interaction length. The puff interaction length is found
to be in excellent agreement with the wavelength of regular stripe and spiral
patterns in plane Couette and Taylor-Couette flow. We propose that the same
interaction mechanism is present in these flows
Second-order phase transition in counter-rotating taylor-couette flow experiment
In many basic shear flows, such as pipe, Couette, and channel flow, turbulence does not
arise from an instability of the laminar state, and both dynamical states co-exist. With decreasing flow speed (i.e., decreasing Reynolds number) the fraction of fluid in laminar motion increases while turbulence recedes and eventually the entire flow relaminarizes. The first step towards understanding the nature of this transition is to determine if the phase change is of either first or second order. In the former case, the turbulent fraction would drop discontinuously to zero as the Reynolds number decreases while in the latter the process would be continuous. For Couette flow, the flow between two parallel plates, earlier studies suggest a discontinuous scenario. In the present study we realize a Couette flow between two concentric cylinders which allows studies to be carried out in large aspect ratios and for extensive observation times. The presented measurements show that the transition in this circular Couette geometry is continuous suggesting that former studies were limited by finite size effects. A further characterization of this transition, in particular its relation to the directed percolation universality class, requires even larger system sizes than presently available
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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