Quantifying Non-Stationarity with Information Theory

We introduce an index based on information theory to quantify the stationarity of a stochastic process. The index compares on the one hand the information contained in the increment at the time scale τ of the process at time t with, on the other hand, the extra information in the variable at time t that is not present at time t−τ. By varying the scale τ, the index can explore a full range of scales. We thus obtain a multi-scale quantity that is not restricted to the first two moments of the density distribution, nor to the covariance, but that probes the complete dependences in the process. This index indeed provides a measure of the regularity of the process at a given scale. Not only is this index able to indicate whether a realization of the process is stationary, but its evolution across scales also indicates how rough and non-stationary it is. We show how the index behaves for various synthetic processes proposed to model fluid turbulence, as well as on experimental fluid turbulence measurements.

Fluid turbulence simulations of divertor heat load for ITER hybrid scenario using BOUT++

The BOUT++ six-field turbulence code is used to simulate the ITER 11.5MA hybrid scenario and a brief comparison is made among ITER baseline, hybrid and steady-state operation (SSO) scenarios. Peeling-ballooning instabilities with different toroidal mode numbers dominate in different scenarios and consequently yield different types of ELMs. The energy loss fractions (ΔWped/Wped) caused by unmitigated ELMs in the baseline and hybrid scenarios are large (~2%) while the one in the SSO scenario is dramatically smaller (~1%), which are consistent with the features of type-I ELMs and grassy ELMs respectively. The intra ELM divertor heat flux width in the three scenarios given by the simulations is larger than the estimations for inter ELM phase based on Goldston’s heuristic drift model. The toroidal gap edge melting limit of tungsten monoblocks of divertor targets imposes constraints on ELM energy loss, giving that the ELM energy loss fraction should be smaller than 0.4%, 1.0%, and 1.2% for ITER baseline, hybrid and SSO scenarios, correspondingly. The simulation shows that only the SSO scenario with grassy ELMs may satisfy the constraint.

Olfactory Sensing and Navigation in Turbulent Environments

Fluid turbulence is a double-edged sword for the navigation of macroscopic animals, such as birds, insects, and rodents. On one hand, turbulence enables pheromone communication among mates and the possibility of locating food by their odors from long distances. Molecular diffusion would indeed be unable to spread odors over relevant distances in natural conditions. On the other hand, turbulent flows are hard to predict, and learning effective maneuvers to navigate them is challenging, as we discuss in this review. We first provide a summary of the olfactory organs that sense airborne or surface-bound odors, as well as the computational tasks that animals face when extracting information useful for navigation from an olfactory signal. A compendium of the dynamics of turbulent transport emphasizes those aspects that directly impact animals’ behavior. The state of the art on navigational strategies is discussed, followed by a concluding section dedicated to future challenges in the field. Expected final online publication date for the Annual Review of Condensed Matter Physics, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Russell Donnelly and His Leaks

Russell James Donnelly (b. 1930) was an exceptionally creative physicist with many other interests: art, music, history, and scientific societies and their scholarly journals. He reinvigorated the maturing field of low temperature physics by linking it strongly to fluid turbulence by bold and optimistic leadership at the intersection of the two fields. Immediately after achieving his Ph.D. at Yale University with C.T. Lane and L. Onsager, Russ joined the University of Chicago in 1956, where he became a professor at the first possible opportunity. After some ten years at U. Chicago, where he worked for a time with S. Chandrasekhar, he moved to the University of Oregon and led a vigorous life until his death in 2015. Russ was an excellent organizer of scientific meetings and an enthusiastic expositor of his science. He had a profound sense of service to the community, both civic and scientific, and showed exceptional scientific openness and generosity to his colleagues. He was greatly admired by his community. Expected final online publication date for the Annual Review of Condensed Matter Physics, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Liutex Core Line for Vortex Structure in Turbulence

Liutex is a vortex identification method that provides a vector interpretation of local fluid rotation. Liutex produces a vector quantity which can be used to determine the absolute and relative strength of a vortex, the local rotation axis of a vortex, the vortex core center, the size of the vortex core, and the vortex boundary. Vortex identification and visualization is essential in computational fluid turbulence analysis and fluid mechanics in general. Until Liutex, there has not been a way to identify the core of a vortex structure or even the center of rotation of a vortex structure. Since Liutex, tools have been created to assist in the identification and analysis of vortical structures. The Liutex Core Line has been developed to better understand turbulent fluid structures. A Liutex core is defined as a concentration of Liutex vectors and defined to be unique and the Liutex core line is the center of rotation of that Liutex core. Currently, iso-surfaces are the most popular way to visualize the structure of turbulent flow but there is no reason to believe that it is the best way to represent a vortex’s structure. Previous methods that use iso-surface are strongly threshold dependent and since the Liutex core line is unique, it is independent of threshold and can show the real vortex structure. In this paper we show the benefits and promises of the Liutex Core Line as a better way of representing vortex structures.

New Two-Fluid Turbulence Model Based Numerical Simulation of Flow in a Flat Suddenly Expanding Channel

The purpose of the research was to numerically study the structure of the flow in a flat channel in the zone of its sudden step-like expansion. The results of the study are given in the paper. The calculations are carried out with the use of a new two-fluid turbulence model and are based on the numerical solution of a system of nonstationary equations. The profiles of axial velocity and turbulent stress in various sections of the channel before and after the step were obtained, as well as the dependence of the friction coefficient for the lower wall of the channel on the distance after the step. For the difference approximation of the initial equations, the control volume approach was applied; the relationship between the velocities and pressure was found using the SIMPLEC procedure. Meanwhile, the viscosity terms were approximated by the central difference, and for the convective terms the QUICK second-order accuracy scheme was used. To confirm the correctness of the numerical results, we compared them with the experimental data taken from the NASA database for the Reynolds number Re = 36,000. The results obtained using the SA and SST models are also given in the paper. Despite the coarse grid used for numerical calculations, the results based on the new two-fluid turbulence model are not less accurate than the results determined by the RANS models for predicting separated flows in the flat channel in the zone of its sudden backward-facing step expansion