WilliamHill中文官网航空航天工程系学术报告

报告人： Björn Hof

Institute of Science and Technology Austria

时间：8月16日（周六）10:00-11:30, 14:30-16:00

地点：WilliamHill中文官网入口新奥工学大楼3048

Abstract 1: The addition of small amounts of long chain polymers can dramatically reduce the drag of turbulent flows. Initially the amount of drag reduction increases with polymer concentration but  eventually saturates at what is also known as the maximum drag reduction (MDR) asymptote, a level intermediate between laminar and turbulent flow. Classically believed to be a marginal state of inertial turbulence, more recently it has been suggested that MDR instead corresponds to a distinct dynamical state, dubbed elasto inertial turbulence (EIT). I will discuss recent evidence that supports this new interpretation of the limit of polymer drag reduction. In particular I will present experimental visualizations of the 3D velocity fields of MDR flows and compare the flow structure to low Reynolds number EIT. 

Abstract 2: Low Reynolds number turbulence has first been observed in viscoelastic pipe flows a century ago (Ostwald 1925, Auerbach 1925), yet the origin of this chaotic flow state remains debated. While several instabilities and transition paths have been suggested in recent theoretical and computational studies, the relation to the actual experimental phenomenon is not always clear. In this talk I will discuss experiments in viscoelastic pipe flow at low and vanishing inertia. Covering five decades in elasticity number we show that close to the onset of turbulence the flow structure always corresponds to a centre mode. Being close to two dimensional the flow structure is typically of ‘arrow-head’ type, featuring bullet shaped high speed streaks and characteristic stagnation points at the centre line. With increasing Reynolds number however, the centre mode abruptly gives way to a wall mode. The flow structure is now three dimensional and the power spectra change slope from -2 to -3 (or smaller).  Extending our study to strongly curved (helical) pipes we demonstrate that at high curvature the wall mode sets in as the primary instability and we argue that it is driven by streamline curvature (hoop stress mode). Hence the presented experiments suggest that viscoelastic turbulence at low inertia is driven by two distinct instabilities. 

Bio: Professor Björn Hof is a Full Professor at Institute of Science and Technology Austria, leading the group of Turbulence and Complex Dynamics. He has made important contributions to the field of transition to turbulence, including determining the critical point of transition to turbulence in pipe flow, establishing the relationship between transition to turbulence and directed percolation, and clarifying the instability mechanism of polymer flow. He has published approx. 100 papers in leading journals including Nature, Science, Nature Reviews Physics, Nature Physics, Nature Communication, PNAS, PRL, ARFM and JFM. He is a fellow of American Physical Society.

联系人：宋保方 baofang.song@pku.edu.cn