Is there an asymptotic effect of initial and upstream conditions on turbulence?
William K. George
Chalmers University of Technology
Abstract:
More than two decades ago the first strong experimental results appeared suggesting that turbulent flows might not be asymptotically independent of
their initial (or upstream) conditions. And shortly thereafter the first theoretical explanations were offered as to why we came to believe something about turbulence that might not be true. It was recognized immediately that if turbulence was indeed asymptotically dependent on its initial conditions, it meant that there could be no universal single point model for turbulence, certainly consistent with experience, but not easy to accept for the turbulence community. Even now the ideas of asymptotic independence still dominate most texts and teaching of turbulence. This lecture will review the substantial additional experimental, numerical and theoretical evidence for the asymptotic effect of initial and upstream conditions that has accumulated over the past 20 years. Emphasis will be placed on the canonical turbulent flows (especially wakes, jets, and homogeneous decaying turbulence), which have been the traditional building blocks for our understanding. Some of the implications for the future of turbulence modeling and research, especially LES and turbulence control, are also considered.
Abstract:
I will describe, hopefully with some humor, my nearly 40 year quest to understand turbulent jets. I’ll start with the early attempts of my students and colleagues in the 1970’s at CFD, which could never seem to both conserve momentum and agree with the jet data. All of which led to the obvious conclusion from the many experiments of the previous three decades that Newton was clearly wrong, and that ~ f = 1/2 m~a (i.e., force really did equal only one-half mass times acceleration). Then I’ll review the ensuing 15 year battle with the turbulence community (most of whom failed to see the humor and absurdity in this) to overcome their rationalizations and convince them that it really was
not unreasonable to ask that proper measurements should conserve momentum, and to identify the sources of the problem. And finally I’ll trace the attempts of me and my collaborators over the past two decades to design experiments using the rapidly evolving optical and digital technology for the purpose of actually understanding what the flow in a jet was doing, the latest incarnations of which are the experiments currently underway at DTU. My personal jet saga will probably be recognized by senior investigators as representative of scientific investigations their own fields, but more importantly might serve as a model for the young investigator of the importance of perseverance. Or perhaps for all, only another example of the difficulties and futility of taking on the entire world.