Characterizing the formation and regeneration of hairpin vortices in a laminar boundary layer
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Abstract
- A free surface water channel is used to study hairpin vortex formation created by fluid injection through a narrow slot into a laminar boundary layer. Particle imagevelocimetry flow-field measurements of injections into quiescent cross-flow conditions confirm that elongated ring vortices are produced with a nondimensionalized circulation strength that is approximately linear with formation time. Unlike circular ring vortices, a limiting strength is not observed at a nondimensional formation time of 4 due to the proximity of the counter-rotating vortex pair. Identical injections are made into a laminar boundary layer at different free-stream velocities and streamwise slot positions (485 ≤ Reδ∗ ≤ 584) with average injection velocity ratios between 0.08 and 0.16. Visualizations indicate that the shear layer between the low x-momentum injected fluid and the boundary layer creates a Kelvin-Helmholtz instability that forms the hairpin vortex head which then monotonically decreases in circulation strength with downstream distance. A similar process can form, or regenerate, a secondary hairpin vortex upstream of the primary vortex with a circulation strength of the head that is comparable to the strength of the primary head at the time of regeneration. However, the legs of the primary vortex continue to strengthen up to regeneration. The peak circulation in the legs is not directly correlated to the strength of the original elongated ring vortex. However, when the circulation is scaled with the injection momentum ratio it is linearly related to scaled injection time. It is proposed that the injection momentum ratio and nondimensionalized injection time based on the wall normal penetration time can be used to identify threshold conditions which produce a secondary vortex. It is suggested that this criterion may be used to identify the minimum strength of flow structures that would be capable of regeneration and thus transition initiation.
| Title | Characterizing the formation and regeneration of hairpin vortices in a laminar boundary layer |
|---|---|
| Creator | Maharjan, R. |
| Sabatino, Daniel | |
| Publisher | Physics of Fluids |
| Academic Department | Mechanical Engineering |
| Division | Engineering |
| Organization | Lafayette College |
| Date Issued | 2015 |
| Date Available | 2016-01-04T19:37:00Z |
| Type | Article |
| Language | English |
| Keyword | boundary layer turbulence |
| laminar boundary layers | |
| flow visualization | |
| rotating flows | |
| viscosity | |
| Bibliographic Citation | Sabatino, D. and Maharjan, R. (2015) "Characterizing the formation and regeneration of hairpin vortices in a laminar boundary layer." Physics of Fluids 27: 124104. |
| Standard Identifier | DOI 10.1063/1.4936138 |
| Handle 10385/2063 | |
| Permalink | http://hdl.handle.net/10385/2063 |
| Rights Statement |
In Copyright
|
| Rights Holders | AIP Publishing LLC |
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| File | Sabatino-PhysicsofFluids-vol27-2015.pdf | Uploaded 2019-10-20 | Public | Download |
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