## Weekly Seminar: Spring 2018

Speaker:** Prof. Chris White** (University of New Hamshire)

Title: *"Properties of the Mean Momentum Balance in Polymer Drag-Reduced Channel Flow" *

Hosted By: Dennice Gayme (ME)

Date: Friday, May 4, 2018

Time: 3:00 p.m.

Location: Gilman Hall # 132

### Abstract

In the first part of the talk, Professor White will briefly outline his ongoing research projects. In particular, recent work on the development of a simple dynamical model of the turbulent boundary layer will be presented. The formulation of the model is based on recent findings that reveal that at large Reynolds numbers the inertially-dominated region of the turbulent boundary layer is composed of large scale zones of nearly uniform momentum segregated by narrow fissures of concentrated vorticity. It will be shown that a simple model that exploits these essential elements of the turbulent boundary layer structure can reproduce statistical profiles of the streamwise velocity that agree remarkably well with those acquired from direct numerical simulation at high Reynolds number.

The main part of the talk will discuss research related to the phenomenon of polymer drag reduction in wall-bounded turbulent flows. Here a mean momentum equation based analysis of polymer drag reduced channel flow is performed to evaluate the redistribution of mean momentum and the mechanisms underlying the redistribution processes. Similar to channel flow of Newtonian fluids, polymer drag reduced channel flow is shown to exhibit a four layer structure in the mean balance of forces that also connects, via the mean momentum equation, to an underlying scaling layer hierarchy. The self-similar properties of the flow related to the layer hierarchy appear to persist, but in an altered form (different from the Newtonian fluid flow), and dependent on the level of drag reduction. With increasing drag reduction, polymer stress usurps the role of the inertial mechanism, and because of this the wall-normal position where inertially dominated mean dynamics occurs moves outward, and viscous effects become increasingly important farther from the wall. For the high drag reduction flows of the present study, viscous effects become non-negligible across the entire hierarchy and an inertially dominated logarithmic scaling region ceases to exist. It follows that the state of maximum drag reduction is attained only after the inertial sublayer is eradicated. According to the present mean equation theory, this coincides with the loss of a region of logarithmic dependence in the mean profile.

**Bio**

Dr. White received his Ph.D. in Mechanical Engineering from Yale University in 2001. From 2001-2004 he was Postdoctoral Research Fellow at Stanford University. Following his post-doctoral work, he joined Sandia National Laboratories as a Senior Member of the Technical Staff in the Combustion Research Facility. His principal duties at Sandia included lead investigator in the Advanced Hydrogen Fueled Engine Laboratory. In 2006, he joined the Mechanical Engineering Faculty at the University of New Hampshire.

Dr. White's research is broadly motivated by applications related to the production, storage, distribution, conversion and end-use applications of energy. His research to date is of both fundamental and applied nature in the areas of combustion, piston engines, biomass, ocean energy, and turbulent drag reduction. He and co-author Godfrey Mungal's 2008 paper "Mechanics and prediction of turbulent drag reduction with polymer additives" is designed as a Highly Cited Paper (top 1% in the field of Physics) by the Thompson Reuters Essential Science Indicators. In 2009, Dr. White received an NSF CAREER award to study the flow properties and rheology of liquified biomass suspensions. He currently has funding from NSF, ONR, and NAVIR.