Affiliations: Department of Mechanical Engineering, National
University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore,
E-mail: [email protected]
Abstract: Shear stress distributions were obtained from velocity measurements
in a concave surface boundary layer flow in the presence of Görtler vortices by
means of a single hot-wire probe for several streamwise (x) locations. A set of
vertical wires of 0.20 mm diameter were positioned at a distance of 10 mm
upstream from the leading edge of a concave surface of radius of curvature R =
1.0 m to pre-set the wavelength of the vortices so to obtain the most amplified
wavelength Görtler vortices. Consequently, the wavelength of the vortices was
set equal to the wire spacing and preserved downstream. In addition to the high
shear regions near the wall, one positive peak at the head of the mushroom-like
structures and two relatively weak negative peaks at the vicinity of the
low-speed streaks are found in the iso-∂u/∂y contours. They are believed to be
related to the formation of the inflectional point in the velocity profile
across boundary layer. The occurrence of the inflection points in the spanwise
distributions of streamwise velocity component u is associated with the
appearance of the second peak of the ∂u/∂z shear near the boundary layer edge.
The nonlinear effect of Görtler instability is to increase the wall shear
stress, and further enhancement beyond the turbulent values is due to the
presence of secondary instability.