The pictures show the visualization of a horseshoe vortex developing in a cascade of turbine blades.
The flow lines at the wall are created by using a layer of oil-titaniumdioxide at the wall of this wind tunnel.
The shear forces in the boundary layer create the characteristic streaks that indicate the direction of the
flow as well as the state (i.e. laminar, transitional, turbulent or separated) of the boundary layer.
In these pictures, a horseshoe vortex forms upstream of the leading edge of the blade, and is subsequently
washed to the suction side of the blade below. Downstream Mach number is about 0.95, Reynolds Number
is about 600,000. Flow is from left to right.