Visualizing shear stress in Görtler vortex flow

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-wu/wy 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 wu/wz 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.

Visualization of pre-set vortices in boundary layer flow over wavy surface in rectangular channel

“…Open square indicates present works using triangle leading edge to pre-set the spanwise wavelength of vortices, VF-type 40°t riangle (left) and FO-type 40°triangle (right) vortices were lifted up above the separation bubble which is formed in the valley of corrugation. As a result, the stem of the vortices that supports the mushroom hat turns out to be weaker as it moves downstream so that the vortex structures are vulnerable to the free-stream flow (Tandiono et al 2009). Finally at the second peak, more disturbed structures were found.…”

Streamwise counter-rotating vortices generated by triangular leading edge pattern in flat plate boundary layer

“…It is also interesting that there are similarities between this type of streamwise vortices and those produced by some other passive devices like vortex generators (Velte et al 2007;Aider et al 2008;Nadesan et al 2014) or by centrifugal forces like Görtler vortices (Winoto and Low 1991;Winoto et al 2005;Tandiono et al 2009;Budiman et al 2014). Therefore, the purpose of this work is to study, by means of flow visualization, the development of streamwise counter-rotating vortices induced by triangular pattern on the leading edge of a flat plate, as shown in Fig 1, under zero-pressure gradient condition.…”

“…In all studies, VGs may have shown increased aerodynamic lift but at the expense of increased drag, which is unfavorable. Another method used to produce streamwise vortices in the boundary layer is by attaching a serrated strips to the airfoil lower surface and near the leading edge (Soderman 1972). The results revealed that the leading edge serrations delay separated flow and increase maximum lift, if they are placed at a proper position to the leading edge.…”