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Time Resolved Stereo Particle Image Velocimetry Measurements of the Instabilities Downstream of a Backward-Facing Step in a Swept-Wing Boundary LayerTime-resolved particle image velocimetry (TRPIV) measurements are performed down-stream of a swept backward-facing step, with a height of 49% of the boundary-layer thickness. The results agree well qualitatively with previously reported hotwire measurements, though the amplitudes of the fluctuating components measured using TRPIV are higher. Nonetheless, the low-amplitude instabilities in the flow are fairly well resolved using TR- PIV. Proper orthogonal decomposition is used to study the development of the traveling cross flow and Tollmien-Schlichting (TS) instabilities downstream of the step and to study how they interact to form the large velocity spikes that ultimately lead to transition. A secondary mode within the traveling cross flow frequency band develops with a wavelength close to that of the stationary cross flow instability, so that at a certain point in the phase, it causes an increase in the spanwise modulation initially caused by the stationary cross flow mode. This increased modulation leads to an increase in the amplitude of the TS mode, which, itself, is highly modulated through interactions with the stationary cross flow. When the traveling cross flow and TS modes align in time and space, the large velocity spikes occur. Thus, these three instabilities, which are individually of low amplitude when the spikes start to occur (U'rms/Ue <0.03), interact and combine to cause a large flow disturbance that eventually leads to transition.
Document ID
20170005778
Acquisition Source
Langley Research Center
Document Type
Conference Paper
Authors
Eppink, Jenna L. (NASA Langley Research Center Hampton, VA, United States)
Yao, Chung-Sheng (NASA Langley Research Center Hampton, VA, United States)
Date Acquired
June 27, 2017
Publication Date
June 5, 2017
Subject Category
Fluid Mechanics And Thermodynamics
Report/Patent Number
NF1676L-25622
Meeting Information
Meeting: AIAA Aviation 2017 Conference
Location: Denver, CO
Country: United States
Start Date: June 5, 2017
End Date: June 9, 2017
Sponsors: American Inst. of Aeronautics and Astronautics
Funding Number(s)
WBS: WBS 109492.02.07.01.01
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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