@article{10654, abstract = {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.}, author = {Klotz, Lukasz and Lemoult, Grégoire M and Avila, Kerstin and Hof, Björn}, issn = {1079-7114}, journal = {Physical Review Letters}, number = {1}, publisher = {American Physical Society}, title = {{Phase transition to turbulence in spatially extended shear flows}}, doi = {10.1103/PhysRevLett.128.014502}, volume = {128}, year = {2022}, } @article{9207, abstract = {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.}, author = {Klotz, Lukasz and Pavlenko, A. M. and Wesfreid, J. E.}, issn = {1469-7645}, journal = {Journal of Fluid Mechanics}, publisher = {Cambridge University Press}, title = {{Experimental measurements in plane Couette-Poiseuille flow: Dynamics of the large- and small-scale flow}}, doi = {10.1017/jfm.2020.1089}, volume = {912}, year = {2021}, } @article{9297, abstract = {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.}, author = {Liu, T. and Semin, B. and Klotz, Lukasz and Godoy-Diana, R. and Wesfreid, J. E. and Mullin, T.}, issn = {1469-7645}, journal = {Journal of Fluid Mechanics}, publisher = {Cambridge University Press}, title = {{Decay of streaks and rolls in plane Couette-Poiseuille flow}}, doi = {10.1017/jfm.2021.89}, volume = {915}, year = {2021}, } @article{5943, abstract = {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 R ∈ (0.14, 0.75) and crossflow Reynolds numbers ReD ∈ (260, 640). From spectral analysis we characterize the Strouhal number and amplitude of the hairpin instability as a function of R and ReD. 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.}, author = {Klotz, Lukasz and Gumowski, Konrad and Wesfreid, José Eduardo}, journal = {Journal of Fluid Mechanics}, pages = {386--406}, publisher = {Cambridge University Press}, title = {{Experiments on a jet in a crossflow in the low-velocity-ratio regime}}, doi = {10.1017/jfm.2018.974}, volume = {863}, year = {2019}, } @article{513, abstract = {We present an experimental setup 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 obtain 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) direction.}, author = {Klotz, Lukasz and Lemoult, Grégoire M and Frontczak, Idalia and Tuckerman, Laurette and Wesfreid, José}, journal = {Physical Review Fluids}, number = {4}, publisher = {American Physical Society}, title = {{Couette-Poiseuille flow experiment with zero mean advection velocity: Subcritical transition to turbulence}}, doi = {10.1103/PhysRevFluids.2.043904}, volume = {2}, year = {2017}, }