5 research outputs found
Experiments on a jet in a crossflow in the low-velocity-ratio regime
The hairpin instability of a jet in a crossflow (JICF) for a low
jet-to-crossflow velocity ratio is investigated experimentally for a velocity
ratio range of and crossflow Reynolds numbers
. From spectral analysis we characterize the Strouhal number
and amplitude of the hairpin instability as a function of and . We
demonstrate that the dynamics of the hairpins is well described by the Landau
model, and, hence, that the instability occurs through Hopf bifurcation,
similarly to other hydrodynamical oscillators such as wake behind different
bluff bodies. Using the Landau model, we determine the precise threshold values
of hairpin shedding. We also study the spatial dependence of this
hydrodynamical instability, which shows a global behaviour.Comment: 20 pages, 21 figure
Couette-Poiseuille flow experiment with zero mean advection velocity: Subcritical transition to turbulence
We present a new experimental set-up that creates a shear flow with zero mean
advection velocity achieved by counterbalancing the nonzero streamwise pressure
gradient by moving boundaries, which generates plane Couette-Poiseuille flow.
We carry out the first experimental results in the transitional regime for this
flow. Using flow visualization we characterize the subcritical transition to
turbulence in Couette-Poiseuille flow and show the existence of turbulent spots
generated by a permanent perturbation. Due to the zero mean advection velocity
of the base profile, these turbulent structures are nearly stationary. We
distinguish two regions of the turbulent spot: the active, turbulent core,
which is characterized by waviness of the streaks similar to traveling waves,
and the surrounding region, which includes in addition the weak undisturbed
streaks and oblique waves at the laminar-turbulent interface. We also study the
dependence of the size of these two regions on Reynolds number. Finally, we
show that the traveling waves move in the downstream (Poiseuille).Comment: 17 pages, 15 figure
Experimental measurements in plane Couette-Poiseuille flow: Dynamics of the large- and small-scale flow
In this paper we experimentally study the transitional range of Reynolds numbers in
plane Couette–Poiseuille flow, focusing our attention on the localized turbulent structures
triggered by a strong impulsive jet and the large-scale flow generated around these
structures. We present a detailed investigation of the large-scale flow and show how
its amplitude depends on Reynolds number and amplitude perturbation. In addition,
we characterize the initial dynamics of the localized turbulent spot, which includes the
coupling between the small and large scales, as well as the dependence of the advection
speed on the large-scale flow generated around the spot. Finally, we provide the first
experimental measurements of the large-scale flow around an oblique turbulent band
Decay of streaks and rolls in plane Couette-Poiseuille flow
We report the results of an experimental investigation into the decay of
turbulence in plane Couette-Poiseuille flow using 'quench' experiments where
the flow laminarises after a sudden reduction in Reynolds number .
Specifically, we study the velocity field in the streamwise-spanwise plane. We
show that the spanwise velocity containing rolls, decays faster than the
streamwise velocity, which displays elongated regions of higher or lower
velocity called streaks. At final Reynolds numbers above 425, the decay of
streaks displays two stages: first a slow decay when rolls are present and
secondly a more rapid decay of streaks alone. The difference in behaviour
results from the regeneration of streaks by rolls, called the lift-up effect.
We define the turbulent fraction as the portion of the flow containing
turbulence and this is estimated by thresholding the spanwise velocity
component. It decreases linearly with time in the whole range of final .
The corresponding decay slope increases linearly with final . The
extrapolated value at which this decay slope vanishes is , close to at which turbulence is self-sustained.
The decay of the energy computed from the spanwise velocity component is found
to be exponential. The corresponding decay rate increases linearly with ,
with an extrapolated vanishing value at . This value
is also close to the value at which the turbulence is self-sustained, showing
that valuable information on the transition can be obtained over a wide range
of .Comment: 22 pages, 15 figure
Phase transition to turbulence in spatially extended shear flows
Directed percolation (DP) has recently emerged as a possible solution to the century old puzzle surrounding the transition to turbulence. Multiple model studies reported DP exponents, however, experimental evidence is limited since the largest possible observation times are orders of magnitude shorter than the flows’ characteristic timescales. An exception is cylindrical Couette flow where the limit is not temporal, but rather the realizable system size. We present experiments in a Couette setup of unprecedented azimuthal and axial aspect ratios. Approaching the critical point to within less than 0.1% we determine five critical exponents, all of which are in excellent agreement with the 2+1D DP universality class. The complex dynamics encountered at
the onset of turbulence can hence be fully rationalized within the framework of statistical mechanics