Events Calendar

Aerospace & Mechanical Engineering Seminar

Speaker: James J. Riley, PACCAR Professor of Engineering, University of Washington

Talk Title: The Effects of Stable Density Stratification Initially Homogeneous, Isotropic Turbulence

Abstract: Stable density stratification occurs in various situations in the atmosphere and in the oceans. For example, in the atmosphere stable density stratification is found near the tropopause and above, and often in nocturnal boundary layers, while in the oceans it usually is observed below the mixed layer. And, through its effects on turbulent mixing, stable stratification has relevance to a number of important issues such as the overall ocean thermal energy balance and the transfer rates of heat and chemicals to/from the atmosphere.

In this seminar the results are presented of a study of the effects of stable density stratification on the simplest of turbulent flows, initially homogeneous, isotropic turbulence, using direct numerical simulations. Simulations were carried out at an initially moderate Froude number, but for a range of initial Reynolds numbers such that, for the high Reynolds number cases, the flows had buoyancy Reynolds numbers in the hundreds, similar to typical oceanic values. A number of aspects of the flows have been addressed, including their energetics, the behavior of various velocity and length scales describing the flows, their mixing characteristics, and their spectral behavior. In particular, how the behavior of the flows depend on the local Froude and buoyancy Reynolds numbers is emphasized. It is found, for example, that as the flows decay, stratification modifies them such that, compared to non-stratified cases, the energy decay rates decreased, the growth rate of the horizontal scales increased, while the growth rates of the vertical scales became negative. These results are consistent with the analysis of Davidson (J. Fluid Mech., 2010), based upon the behavior of the effects of density stratification on the large-scale motions. On the other hand if the buoyancy Reynolds number becomes too low, then the flows, especially the vertical velocity, begin to decay much more rapidly. It is also found, for example, that the behavior of the spectra of the velocity gradient tensor is consistent with the heuristic arguments of Lilly (J. Atmos. Sci., 1983) and the scaling arguments of Billant & Chomaz (Phys. Fluids, 2001). Finally, previous results of the USC group (e.g., Spedding J. Fluid Mech., 1997) are interpreted in terms of the Froude and buoyancy Reynolds numbers.

Biography: James J. Riley is the PACCAR Professor of Engineering at the University of Washington. He received his PhD from the Johns Hopkins University in 1972, having worked under the guidance of Stanley Corrsin. After a year as a post-doctoral fellow at the National Center for Atmospheric Research, he spent ten years in industry at Flow Research Company in Kent, Washington, ultimately as the Director of the Fluid Mechanics Division. He joined the University of Washington in 1983, where he is now a Professor in the Department of Mechanical Engineering, and an Adjunct Professor in both the Departments of Applied Mathematics and of Aeronautics and Astronautics. While on sabbatical at the Joseph Fourier University in Grenoble, France, Riley occupied the Visiting Chair in Industrial Mathematics. More recently he was a Senior Fellow at the Isaac Newton Institute for the Mathematical Sciences at Cambridge University. Riley's research interests have included particle dispersion in turbulent flows, waves and turbulence in stably-stratified and in rotating fluids, boundary layer and shear layer transition and turbulence, fluid/compliant surface interactions, and chemically reacting turbulent flows. He is an associate editor of the Journal of Fluid Mechanics and of the Journal of Turbulence, and until recently was a member of the editorial boards of the Annual Review of Fluid Mechanics and of the Applied Mechanics Reviews. Riley is a member of the National Academy of Engineering, and of the Washington State Academy of Sciences.

Host: Department of Aerospace and Mechanical Engineering

Location: Seaver Science Library (SSL) - 150

Audiences: Everyone Is Invited

Contact: Ashleen Knutsen