# University of Minnesota

Aerospace Engineering and Mechanics

**Spring 1997 Seminar Series**

*Investigating Similarity in Turbulent Plane Wakes*

__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.