Hot-wire measurements in the boundary layer developing on a concave surface of 2.0 m radius of curvature in the presence of forced wavelength Görtler vortices have been conducted for a free-stream velocity of 3.0 m / s. The wavelengths of vortices were preset by vertical perturbation wires of 0.2 mm diameter located 10 mm upstream of the concave surface leading edge. The velocity contours in the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices that are similar to those found in the transitional flow field. It shows the occurrence of the second instability mode that is indicated by the formation of small horseshoe eddies generated between the two neighboring vortices traveling in the streamwise direction to form mushroom-like structures as a consequence of the nonlinear growth of the Görtler vortices. The breakdown of these structures before the boundary-layer flow becomes turbulent is also shown to qualitatively predict the start of the transition in the flow. The Görtler number where the start of the transition was predicted is found to be within the range of transitional Görtler numbers previously reported for naturally developed Görtler vortices. The average of the spanwise wavelength after being normalized by / u is comparable with the generally quoted value of 100 for turbulent boundary layers.

From an experimental study on the onset of secondary instability in forced wavelength Görtler vortices, it is found that the breakdown process of the Görtler vortices is due to the development of varicose and sinuous mode instabilities. The development of the varicose mode is characterized by the formation of horseshoe vortices that evolve downstream to form mushroom-like structures. This phenomenon is then followed by meandering of the vortices as an indication of the onset of sinuous mode instability, prior to turbulence. The spectrum analysis applied to the fluctuating velocity component shows the occurrence of peak frequency of about 150Hz, which is attributed to the fundamental secondary instability mode with its wavelength comparable to the spanwise wavelength of the primary Görtler vortices, at the location where the mushroom-like structures are clearly depicted in the mean velocity contours on the y-z plane. This confirms that the secondary instability is of the varicose type at the onset that is followed by the sinuous type downstream, prior to the breakdown of the vortices that will lead the boundary layer flow to turbulence. It is also found that the growth rate of the secondary instability observed is about 6.5 times higher than that of the Görtler instability, which leads to a breakdown of the streamwise structures and hence to turbulence.

Smoke-wire flow visualization is used to study the development of pre-set counter-rotating streamwise vortices in boundary layer flow over a wavy surface in a rectangular channel. The formation of the vortices is indicated by the vortical structures on the cross-sectional plane normal to the wavy surface. To obtain uniform spanwise vortex wavelength which will result in uniform vortex size, two types of spanwise disturbances were used: a series of perturbation wires placed prior and normal to the leading edge of the wavy surface, and a jagged pattern in the form of uniform triangles cut at the leading edge. These perturbation wires and jagged pattern induce low-velocity streaks that result in the formation of counterrotating streamwise vortices that evolve downstream to form the mushroom-like structures on the crosssectional plane of the flow. The evolution of the most amplified disturbances can be attributed to the formation of these mushroom-like structures. It is also shown that the size of the mushroom-like structures depends on the channel entrance geometry, Reynolds number, and the channel gap.

The use of leading edge with different geometrical patterns will affect the development of boundary layer flow on a flat plate and its heat transfer properties. In this work, the effects of three patterns namely saw-tooth, semicircular and slots with same wavelength and amplitude were examined. The experiments were carried out for Reynolds number based on wavelength of patterns ranging from 1540 to 3850. For all cases, after each valley, an oval shape region was formed containing a counter-rotating vortex pair. It is also shown that for the flat plate with slots, another vortex was visualized between each valley.

The development of Görtler vortices with pre-set wavelength of 15 mm has been visualized in the boundary-layer on a concave surface of 2.0 m radius of curvature at a free-stream velocity of 3.0 m/s. The wavelength of vortices was pre-set by vertical wires of 0.2 mm diameter located 10 mm upstream of the concave surface leading edge. The velocity contours in the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different regions can be identified based on different growth rate of the vortices. The occurrence of a secondary instability mode is indicated by the formation of a small horseshoe eddies generated between the two neighboring vortices traveling streamwise, to form mushroom-like structures as a consequence of the non-linear growth of the Görtler vortices.