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The preferential organisation of coherent vortices in a turbulent boundary layer in relation to local large-scale streamwise velocity features was investigated. Coherent vortices were identified in the wake region using the Triple Decomposition Method (originally proposed by Kolář) from 2D particle image velocimetry (PIV) data of a canonical turbulent boundary layer. Two different approaches, based on conditional averaging and quantitative statistical analysis, were used to analyze the data. The large-scale streamwise velocity field was first conditionally averaged on the height of the detected coherent vortices and a change in the sign of the average large scale streamwise fluctuating velocity was seen depending on the height of the vortex core. A correlation coefficient was then defined to quantify this relationship between the height of coherent vortices and local large-scale streamwise fluctuating velocity. Both of these results indicated a strong negative correlation in the wake region of the boundary layer between vortex height and large-scale velocity. The relationship between vortex height and full large-scale velocity isocontours was also studied and a conceptual model based on the findings of the study was proposed. The results served to relate the hairpin vortex model of Adrian et al. to the scale interaction results reported by Mathis et al., and Chung and McKeon.
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Control of Artificially-Generated Hairpin Vortices in a Laminar Boundary Layer
Experiments were conducted to investigate the use of coherent vortical structures for boundary layer re-energization in a laminar boundary layer. The structures were generated artificially in the form of a hairpin train using a synthetic jet actuator to ensure repeatability for assessing the control. The structures act as a proxy for naturally occurring large-scale motions found in turbulent boundary layers. The hairpin train was controlled by both a static pin and a jet-assisted surface-mounted actuator (JASMA), and the results were captured using particle image velocimetry (PIV). Planar PIV was used to run a parameter study on several actuator strengths, angles, and heights along the domain mid-plane. Stereoscopic PIV measurements conducted downstream were aggregated into a volume of data for further analysis of the control of one particular hairpin train generation case. It was found that both the passive control pin and the active control JASMA contribute turbulent kinetic energy and downwash downstream to increase mixing and momentum injection into the boundary layer.
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- Award ID(s):
- 2129493
- PAR ID:
- 10519830
- Publisher / Repository:
- AIAA
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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