TY - JOUR
AB - 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.
AU - Avila, Kerstin
AU - Hof, Björn
ID - 8999
IS - 1
JF - Entropy
TI - Second-order phase transition in counter-rotating taylor-couette flow experiment
VL - 23
ER -
TY - JOUR
AB - 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.
AU - Klotz, Lukasz
AU - Pavlenko, A. M.
AU - Wesfreid, J. E.
ID - 9207
JF - Journal of Fluid Mechanics
SN - 0022-1120
TI - Experimental measurements in plane Couette-Poiseuille flow: Dynamics of the large- and small-scale flow
VL - 912
ER -
TY - JOUR
AB - 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 Re. 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 Re. The corresponding decay slope increases linearly with final Re. The extrapolated value at which this decay slope vanishes is Reaz≈656±10, close to Reg≈670 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 Re, with an extrapolated vanishing value at ReAz≈688±10. 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 Re.
AU - Liu, T.
AU - Semin, B.
AU - Klotz, Lukasz
AU - Godoy-Diana, R.
AU - Wesfreid, J. E.
AU - Mullin, T.
ID - 9297
JF - Journal of Fluid Mechanics
SN - 0022-1120
TI - Decay of streaks and rolls in plane Couette-Poiseuille flow
VL - 915
ER -
TY - JOUR
AB - High impact epidemics constitute one of the largest threats humanity is facing in the 21st century. In the absence of pharmaceutical interventions, physical distancing together with testing, contact tracing and quarantining are crucial in slowing down epidemic dynamics. Yet, here we show that if testing capacities are limited, containment may fail dramatically because such combined countermeasures drastically change the rules of the epidemic transition: Instead of continuous, the response to countermeasures becomes discontinuous. Rather than following the conventional exponential growth, the outbreak that is initially strongly suppressed eventually accelerates and scales faster than exponential during an explosive growth period. As a consequence, containment measures either suffice to stop the outbreak at low total case numbers or fail catastrophically if marginally too weak, thus implying large uncertainties in reliably estimating overall epidemic dynamics, both during initial phases and during second wave scenarios.
AU - Scarselli, Davide
AU - Budanur, Nazmi B
AU - Timme, Marc
AU - Hof, Björn
ID - 9407
IS - 1
JF - Nature Communications
TI - Discontinuous epidemic transition due to limited testing
VL - 12
ER -
TY - JOUR
AB - Turbulence in the flow of fluid through a pipe can be suppressed by buoyancy forces. As the suppression of turbulence leads to severe heat transfer deterioration, this is an important and undesirable phenomenon in both heating and cooling applications. Vertical flow is often considered, as the axial buoyancy force can help drive the flow. With heating measured by the buoyancy parameter 𝐶, our direct numerical simulations show that shear-driven turbulence may either be completely laminarised or it transitions to a relatively quiescent convection-driven state. Buoyancy forces cause a flattening of the base flow profile, which in isothermal pipe flow has recently been linked to complete suppression of turbulence (Kühnen et al., Nat. Phys., vol. 14, 2018, pp. 386–390), and the flattened laminar base profile has enhanced nonlinear stability (Marensi et al., J. Fluid Mech., vol. 863, 2019, pp. 50–875). In agreement with these findings, the nonlinear lower-branch travelling-wave solution analysed here, which is believed to mediate transition to turbulence in isothermal pipe flow, is shown to be suppressed by buoyancy. A linear instability of the laminar base flow is responsible for the appearance of the relatively quiescent convection driven state for 𝐶≳4 across the range of Reynolds numbers considered. In the suppression of turbulence, however, i.e. in the transition from turbulence, we find clearer association with the analysis of He et al. (J. Fluid Mech., vol. 809, 2016, pp. 31–71) than with the above dynamical systems approach, which describes better the transition to turbulence. The laminarisation criterion He et al. propose, based on an apparent Reynolds number of the flow as measured by its driving pressure gradient, is found to capture the critical 𝐶=𝐶𝑐𝑟(𝑅𝑒) above which the flow will be laminarised or switch to the convection-driven type. Our analysis suggests that it is the weakened rolls, rather than the streaks, which appear to be critical for laminarisation.
AU - Marensi, Elena
AU - He, Shuisheng
AU - Willis, Ashley P.
ID - 9467
JF - Journal of Fluid Mechanics
SN - 00221120
TI - Suppression of turbulence and travelling waves in a vertical heated pipe
VL - 919
ER -