Experimental Investigation Into The Routes To Bypass Transition And The Shear-Sheltering Phenomenon

Journal Article · Thu Nov 01 04:00:00 EDT 2007 · Journal of Fluid Mechanics

OSTI ID:923514

McEligot, D M; Hernon, D; Walsh, E J

The objective of this investigation is to give experimental support to recent direct numerical simulation (DNS) results which demonstrated that in bypass transition the flow first breaks down to turbulence on the low-speed streaks (or so-called negative jets) that are lifted up towards the boundary-layer edge region. In order to do this, wall-normal profiles of the streamwise fluctuation velocity are presented in terms of maximum positive and negative values over a range of turbulence intensities (1.3–6%) and Reynolds numbers for zero pressure gradient flow upstream of, and including, transition onset. For all turbulence intensities considered, it was found that the peak negative fluctuation velocity increased in magnitude above the peak positive fluctuations and their positions relative to the wall shifted as transition onset approached; the peak negative value moved towards the boundary-layer edge and the peak positive value moved toward the wall. An experimental measure of the penetration depth (PD) of free-stream disturbances into the boundary layer has been gained through the use of the skewness function. The penetration depth (measured from the boundary-layer edge) scales as PD (? Rextw)-0.3), where ? is the frequency of the largest eddies in the free stream, Rex is the Reynolds number of the flow based on the streamwise distance from the leading edge and tw is the wall shear stress. The parameter dependence demonstrated by this scaling compares favourably with recent solutions to the Orr–Sommerfeld equation on the penetration depth of disturbances into the boundary layer. The findings illustrate the importance of negative fluctuation velocities in the transition process, giving experimental support to suggestions from recent DNS predictions that the breakdown to turbulence is initiated on the low-speed regions of the flow in the upper portion of the boundary layer. The representation of pre-transitional disturbances in time-averaged form is shown to be inadequate in elucidating which disturbances grow to cause the breakdown to turbulence.

• Research Organization: Idaho National Laboratory (INL)
• Sponsoring Organization: DOE - NE
• DOE Contract Number: AC07-99ID13727
• OSTI ID: 923514
• Report Number(s): INL/JOU-06-11275
• Journal Information: Journal of Fluid Mechanics, Journal Name: Journal of Fluid Mechanics Vol. 591; ISSN JFLSA7; ISSN 0022-1120
• Country of Publication: United States
• Language: English

Related Subjects

42 ENGINEERING
ASYMMETRY
BOUNDARY LAYERS
BREAKDOWN
DISTRIBUTION
DISTURBANCES
FLUCTUATIONS
PENETRATION DEPTH
PRESSURE GRADIENTS
REYNOLDS NUMBER
SHEAR
SIMULATION
STATISTICS
TURBULENCE
VELOCITY
shear-sheltering phenomenon