Kolmogorovian turbulence in transitional pipe flows
Par Gustavo Gioia, Okinawa Institute of Science and Technology Graduate University, Okinawa, Japan
Jeudi 23 Février, 14h, Salle des séminaires (215), 2ème étage, Bâtiment A4N
As everyone knows who has gradually opened a water faucet, pipe flows are initially laminar but become turbulent at high flow velocities. At intermediate velocities there is a transitional regime in which plugs of laminar flow alternate along the pipe axis with “flashes” of fluctuating, non-laminar flow. While it is known that flashes can fission, decay, and crowd out the intervening laminar plugs, the nature of “flash flow” (the flow inside flashes) remains poorly understood. In this talk, we show experimentally that flash flow satisfies the Blasius law of fluid friction, which is diagnostic of turbulence; thus, flash flow is likely to be but turbulent flow. To verify this possibility, we show that the statistics of flash flow are in keeping with Kolmogorov’s theory of the statistical structure of turbulence (so that, for example, at high wavenumbers, the turbulent-energy spectra of flash flow collapse onto the turbulent-energy spectra of turbulent flow, consistent with small-scale universality), with the implication that transitional pipe flows are segregated mixtures of a laminar phase and a turbulent phase. Our findings lend support to the widely-held, but hitherto unsubstantianted, notion that transitional pipe flows signal a non-equilibrium phase transition to turbulence. This research was carried out with Rory Cerbus, Chien-Chia Liu, and Pinaki Chakraborty, Okinawa Institute of Science and Technology Graduate University.