VIZUALIZATION OF VORTICES BY CONDENSATION
The reason that vortices can be visualized by condensation is due to the fact that the vortex core is a region of low pressure. This lowering of pressure can be understood by considering the fluid mechanist’s trusty tool, the Bernoulli equation.
When a strong vortex is formed, say at a wing tip, the swirl component of the velocity causes the velocities to become considerably larger than the surrounding flow. In the latter regions, the flow speeds are still on the order of (or even approximately equal to) those of the freestream. According to the steady state Bernoulli equation, the pressures should become very small within the vortex. In many cases of interest, the flow density is approximately constant and the temperatures must also drop well below those of the surrounding flow. If the ambient air is sufficiently humid, the low pressures and temperatures will cause the water vapor to condense, forming a “cloud” in the low pressure vortex.
The cores of the vortices are frequently seen to be “empty”. This is the dark region in the center of the vortex seen in the above photo. The most likely reason is that centrifugal effects tend to throw the droplets away from the center.
In many cases, no condensation will be seen if the aircraft is in steady, level flight. It is only when the pilot cranks the plane into a high-g maneuver that the lift increases to the point where condensation can occur. The condensation therefore gives us a “time-accurate” monitor of the lift variation.
In aeronautical applications, such lift-generated vortices are often accompanied by condensation over the wings discussed at the highlighted text.