Title

A New View on Origin, Role and Manipulation of Large Scales in Turbulent Boundary Layers

Report Number

NASA-CR-165861

Creator

Corke, T. C.

Nagib, H. M.

Guezennec, Y.

Corporate Author

Illinois Institute of Technology

Corporate Report Number

IIT Fluid & Heat Transfer Report R81-3

Date

1981

Date Issued

1981-12

Date Modified

Scanned and posted 2024-10-03

Extent

353

NTRL Accession Number

N8315588

Identifier

N8315588

Abstract

The potential of passive "manipulators" for altering the large-scale turbulent structures in boundary layers was investigated. Utilizing smoke-wire visualization and multi-sensor probes, the experiment verified that the outer scales, defined by the intermittent excursions of potential fluid into the boundary layer could be suppressed by simple arrangements of parallel-plates. A parametric investigation of different types of
parallel-plate devices verified that optimum designs existed for the different configurations of stacked and tandem plate arrangements. A final optimized design consisting of two single plates residing at a height of 0.8δ in the boundary layer and spaced 8δ apart in the streamwise direction was chosen for the final set of measurements to document the characteristics of the
"manipulated" layers.
As a result of suppressing the outer scales in the turbulent layers, a decrease in the streamwise growth of the boundary layer thickness was achieved and was coupled with a 30 percent decrease in the local wall-friction coefficient. After accounting for the drag on the manipulator plates, the net drag reduction reached a value of 20 percent within 55 boundary layer thicknesses downstream of the device. No evidence for the reoccurrence of the outer scales was present at this streamwise distance thereby suggesting that further reductions in the net drag are attainable. The manipulated layers also exhibited a 17 percent thickening of the wall sublayer. In turn, the frequency of the wall "burst" process was reduced by a comparable amount.
Utilizing new techniques, many of them introduced in this investigation, the objective detection and description of the two-dimensional velocity field near the wall was achieved. These included match filtering, two-dimensional velocity reconstruction as well as digital image processing and pattern recognition. The results of these established that the frequency of occurrence of the wall events is simultaneously dependent on the two parameters, Re_δ₂ and Re_x The correlation of the inner layer Strouhal frequency for these events with the two parameters resulted in a simple relation which correlates wall-layer event (or "burst") results of this investigation as well as those in the literature, covering a range of Reynolds numbers from 600 to 38000. As a result of being able to independently control the inner and outer boundary layer characteristics with these manipulators, a new view of these layers emerged. It now appears that the large outer scales in laboratory turbulent boundary layers are remnants of the transition process, i.e., slowly decaying turbulent spots that are embedded in the boundary layer. The manipulators in ffect remove this link with the "birth" of the turbulence and therefore hasten the Reynolds number "aging" of the turbulence.

Distribution Classification

1

Distribution Conflict

No

DTIC Record Exists

No

Illinois Tech Related

Yes

Photo Quality

Complete

Report Availability

Full text available

Type

report