Drop Dynamics

As raindrops fall, they oscillate due to natural instabilities, turbulence in the air and perhaps also due to vortex shedding from the falling drop. An understanding of these oscillations is important to the study of rain.

The natural instabilities that drops undergo are revealed in the following animations which simulate the three modes of the fundamental harmonic of a drop. These three modes are referred to as axisymmetric, transverse and horizontal (animations by Brian K. Jones).

Axisymmetric Mode

Transverse Mode

Horizontal Mode

The following images show some sample images obtained from a high speed video camera, of falling drops. These images reveal oscillatory behavior. These are not the true drop oscillations that would be seen in falling rain drops because these drops have just been released. However, the images illustrate general oscillatory chacteristics.

(Images by Brian K. Jones)

In the above images, the top of the frame is located about 3cm beneath the hypodermic needle from which these drops were released, except for the far right hand side image where the top of the frame is much further from the needle. This last image shows how the initial oscillations caused by snap off of the drop from the needle dies away. Once the drop falls much further it reaches terminal velocity and other oscillations occur.

To study the oscillations that occur at terminal velocity, we have constructed a vertical drop levitation tunnel. By adjusting the air flow to a value equal to the terminal velocity of the drop, we are able to `float' drops, allowing them to be studied in a fixed coordinate system. Below are images of the vertical levitation tunnel constructed by Brian K. Jones taken shortly after the first drop was levitated (12/20/2002).

This tunnel follows the design developed by A. K. Kamra and can be found in "A vertical wind tunnel for water drop studies," Kamra, A.K. Sathe, A. B. and Ahir, D. V., Mausam , 37:219-222, 1986.