MAE Seminar: Perry Johnson, Postdoctoral Fellow, Center for Turbulence Research, Stanford University, Small-scale Motions in Turbulence: Velocity Gradients and Particle Collisions

Date/Time

02/14/2019
4:00 pm-5:00 pm
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Location

303 MAE-A
939 Sweetwater Drive
Gainesville, FL 32611-6250

Details

Small-scale Motions in Turbulence:
Velocity Gradients and Particle Collisions

Perry Johnson, Postdoctoral Fellow
Center for Turbulence Research, Stanford University

Abstract
The dynamics of small particles in turbulent flows are important for a wide range of industrial and environmental applications. In this talk, I will address modeling challenges for two important small-scale phenomena in turbulence.

First, I will explore the unique behavior of the velocity gradients in turbulent flows. The velocity gradient tensor carries information about the local rotation and energy dissipation rate, which are vital for a fundamental understanding of turbulent dynamics. In particle-laden flows, velocity gradients determine the torque and deformation stresses experienced by a particle along its trajectory. I will demonstrate how a stochastic model can be constructed for a faithful representation of velocity gradient dynamics in the Lagrangian frame. With a carefully contructed closure, this approach reproduces signature topological and non-Gaussian features of small-scale turbulence in a computationally efficient manner and has been successfully coupled with large-eddy simulations for modeling droplet deformation.

Secondly, the nature turbulent motions in a thin layer near a boundary creates a net drift of small solid particles toward the wall. This phenomenon, known as turbophoresis, creates a higher concentration of particles in the near-wall region, but inter-particle collisions can be very effective at mitigating this effect. Despite the nonlinear dependence of collision rate on particle number density, I will show how collisional effects can be accurately represented with Lagrangian tracking of a reduced number of computational particles. This is done by artificially enhancing the collisional diameter of the particles, but the details differ for low and high Stokes number regimes.

Biography
Perry Johnson received his PhD in Mechanical Engineering from Johns Hopkins University (JHU) in 2017. At JHU, he was a National Science Foundation (NSF) Graduate Research Fellow and received the Corrsin-Kovasznay outstanding paper award. He then joined the Center for Turbulence Research at Stanford University as a postdoctoral fellow. His research interests center on modeling and computing various features of turbulence and multiphase flow physics.

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