Project Title:

Feasibility of Generating an "Artificial" Burst in a Turbulent Boundary Layer

02.02-8500A
Feasibility of Generating an "Artificial" Burst in a Turbulent Boundary Layer
Flow Industries Inc.
21414 68th Avenue South
Kent
WA
98032
Gad-el-hak
Mohamed
NAS1-18292
Amount:294,000
LaRC
NAS1-17930

Abstract:

   It is generally agreed that the bursting phenomenon is the most significantdynamic event in a turbulent boundary layer.  About 80% of the momentum transport
occurs during these bursts.  Previous attempts to understand the physics and structure
of these events were frustrated by the fact that bursts occur randomly in space and
time and that successive bursts are not necessarily identical.  During Phase I of
this study, "artificial" bursts were generated in laminar and turbulent boundary
layers.  The burst-
like events were produced by withdrawing near-wall fluid from two minute holes separated
in the spanwise direction or by pitching a miniature delta wing that was flush-mounted
to the wall.  Either of these actions generated stream-wise vorticity and a low-speed
streak that resembled a naturally occuring one.  The resulting sequence of events
occurred at a given location and at controlled times, allowing detailed examination
and comparison with natural, random bursts by means of flow visualization and fast-response
probe measurement techniques.  During Phase II of the investigation, the topological
properties of the artificial bursts will be deduced from phase-locked hot-film measurements
and from pattern recognition algorithms.

As part of this investigation, a new technique to reduce skin friction drag in a
turbulent boundary layer is proposed.  The technique combines the beneficial effects
of suction and a longitudinally ribbed surface.  The suction will be applied selectively
in space and time to minimize pumping energy requirements and to alleviate the need
for a porous wall.  It is anticipated that net drag reduction using the proposed
method will far exceed the reduction attained using suction alone or longitudinal
grooves alone.