University of Minnesota
Aerospace Engineering and Mechanics
Spring 1997 Seminar Series


Investigating Similarity in Turbulent Plane Wakes



Dr. Robert Moser
University of Illinois - Urbana/Champaign
Department of Theoretical and Applied Mechanics

Abstract

It has been recognized for some time that certain turbulent free shear flows, in particular plane wakes, may have growth rates and turbulence statistics that differ from experiment to experiment, depending on the details of how the flow is created. These differences have been observed to persist into the "far wake" where the flow appears to be self-similar, though there continues to be debate as to whether the differences are permanent or a long lived "transient." While we cannot currently settle this debate, there are many things that can be learned about nonuniqueness of similarity states and its causes, whether permanent or not. In this talk, we will explore the implications of such nonuniqueness using a careful, though elementary, similarity analysis of the mean and Reynolds stress equations of a plane wake, and some possible causes through the study of direct numerical simulation results. Among the noteworthy results of this investigation are that, at least to the level of the Reynolds stress equations, a family of similarity states parameterized by the growth rate are allowed, and that different states can indeed be accessed by manipulating the initial conditions of the wake. Furthermore, it is possible to scale out much of the magnitude variation in various statistical quantities using factors of the growth rate, but significant variations in profile shape remain. Finally, it is noted that simulated wakes with differing growth rates have distinctly different large-scale structural features.

The observed nonuniqueness poses problems for turbulence prediction (modeling) since common models may not even be sensitive to the controlling differences in the initial conditions. It also presents an opportunity for flow manipulation, since the effects of initial conditions appear to persist well into the far wake.


Friday, April 4, 1997
209 Akerman Hall
2:30 - 3:30 p.m.


Refreshments served after each seminar in 227 AKERMAN HALL .
Disability accommodations provided upon request.
Contact Leslie Petrus : Secretarial Assistant, (612) 625-8000.