Anatomy of thermals: thermal bubble

Whole simulation pictures 200 second action of bubble separation from the ground. There is apparent expansion of the bubble with a height, and constant acceleration. Pictures represent state of the bubble in time 200 seconds. Pictures are viewports of dimensions 120 x 150 m, and bootom side lies in altitude 100m above ground.

temperatures (Celsius)
velocity vectors
airmass ascending (m/s)
movie (MPEG-1), 350 kB
movie (Divx5.03), 70 kB
movie (MPEG-1), 700 kB
movie (Divx5.03), 360 kB
movie (MPEG-1), 590 kB
movie (Divx5.03), 370 kB


turbulence strength (1.0=laminar flow)

Simulation model was created in the form of axisymmetric cylinder with radius of 100m and height of 800m, with upper cap opened. Parameters of air enclosed in cylinder was set according to ISA (International Standard Atmosphere), ie. the temperature gradient was -0.7 C / 100m. This means, that air in cylinder was thermally instable. Beginning of thermal bubble was modeled so that there was created on the ground hot air "pillow" with radius of 50m. Temperature difference between hot air and surroundings was +5 C.

Pictures comes from computer simulation of airflow. In this case was used CFD software FLUENT. Computation took 3 days on graphics station SGI Octane with RISC processor of 195 MHz. Amount of used RAM during computation was 600 MB.

Peter Gasparovic, february 2003