@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}, } @article{651, abstract = {Superhydrophobic surfaces reduce the frictional drag between water and solid materials, but this effect is often temporary. The realization of sustained drag reduction has applications for water vehicles and pipeline flows. }, author = {Hof, Björn}, issn = {00280836}, journal = {Nature}, number = {7636}, pages = {161 -- 162}, publisher = {Nature Publishing Group}, title = {{Fluid dynamics: Water flows out of touch}}, doi = {10.1038/541161a}, volume = {541}, year = {2017}, } @article{662, abstract = {We report a direct-numerical-simulation study of the Taylor-Couette flow in the quasi-Keplerian regime at shear Reynolds numbers up to (105). Quasi-Keplerian rotating flow has been investigated for decades as a simplified model system to study the origin of turbulence in accretion disks that is not fully understood. The flow in this study is axially periodic and thus the experimental end-wall effects on the stability of the flow are avoided. Using optimal linear perturbations as initial conditions, our simulations find no sustained turbulence: the strong initial perturbations distort the velocity profile and trigger turbulence that eventually decays.}, author = {Shi, Liang and Hof, Björn and Rampp, Markus and Avila, Marc}, issn = {10706631}, journal = {Physics of Fluids}, number = {4}, publisher = {American Institute of Physics}, title = {{Hydrodynamic turbulence in quasi Keplerian rotating flows}}, doi = {10.1063/1.4981525}, volume = {29}, year = {2017}, } @article{1160, abstract = {We investigate fundamental nonlinear dynamics of ferrofluidic Taylor-Couette flow - flow confined be-tween two concentric independently rotating cylinders - consider small aspect ratio by solving the ferro-hydrodynamical equations, carrying out systematic bifurcation analysis. Without magnetic field, we find steady flow patterns, previously observed with a simple fluid, such as those containing normal one- or two vortex cells, as well as anomalous one-cell and twin-cell flow states. However, when a symmetry-breaking transverse magnetic field is present, all flow states exhibit stimulated, finite two-fold mode. Various bifurcations between steady and unsteady states can occur, corresponding to the transitions between the two-cell and one-cell states. While unsteady, axially oscillating flow states can arise, we also detect the emergence of new unsteady flow states. In particular, we uncover two new states: one contains only the azimuthally oscillating solution in the configuration of the twin-cell flow state, and an-other a rotating flow state. Topologically, these flow states are a limit cycle and a quasiperiodic solution on a two-torus, respectively. Emergence of new flow states in addition to observed ones with classical fluid, indicates that richer but potentially more controllable dynamics in ferrofluidic flows, as such flow states depend on the external magnetic field.}, author = {Altmeyer, Sebastian and Do, Younghae and Lai, Ying}, issn = {20452322}, journal = {Scientific Reports}, publisher = {Nature Publishing Group}, title = {{Dynamics of ferrofluidic flow in the Taylor-Couette system with a small aspect ratio}}, doi = {10.1038/srep40012}, volume = {7}, year = {2017}, } @article{1087, abstract = {Using extensive direct numerical simulations, the dynamics of laminar-turbulent fronts in pipe flow is investigated for Reynolds numbers between and 5500. 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.}, author = {Song, Baofang and Barkley, Dwight and Hof, Björn and Avila, Marc}, issn = {00221120}, journal = {Journal of Fluid Mechanics}, pages = {1045 -- 1059}, publisher = {Cambridge University Press}, title = {{Speed and structure of turbulent fronts in pipe flow}}, doi = {10.1017/jfm.2017.14}, volume = {813}, year = {2017}, }