Making Air Visible: Communicating Ideas about the Atmosphere

John R Taylor and David J Low
School of Physics, University College UNSW, ADFA

Abstract

Four examples of flow visualisation in the undergraduate laboratory are discussed in terms of their motivation and implementation.

Gravity Currents: a gravity current is a horizontal flow, generated by a lateral gradient in density, often seen in nature as a "front". A simple gravity current can be generated by the removal of a central barrier separating warm water from cold water in a long tank. Dye is used to make the gravity currents visible. The importance of dynamical features such as the "nose" can be visualised, and energy transfer relationships in fluid flow (characterised by the Froude number) along with the influence of viscous forces (via the Reynolds number) can be explored.

Coriolis Force: a density front is set up in a water tank mounted on a turntable by confining a pool of fresh, less-dense water (dyed) above a uniform layer of denser, salty water. Upon release, the horizontal pressure gradient produced by the density gradient accelerates the lighter fluid outwards. The non-rotating case of uniform expansion can be compared with the rotating case, where Coriolis inhibits lateral expansion and geostrophic equilibrium is attained for a while. Breakdown of the unstable equilibrium leads to the development of vortices, seen as rotating volumes of fresh water. Measurements of the frontal expansion speed can be compared with a simple geostrophic model, and concepts such as the inertial period can be introduced by reference to the time for adjustment to geostrophic equilibrium. The development of instabilities on the front can be compared to dynamical models of atmospheric circulation.

Lee Waves: a parcel of air displaced from its equilibrium level in a density-stratified experiences a restoring force due to buoyancy, and will oscillate. If parcels are being swept downstream by the mean flow, periodic displacements of the density surfaces will be seen. Here, the flow over obstacles under density-stratified conditions is explored: the stratification is set up by a twin-reservoir (fresh and salt water) gravity feed, while flow over an obstacle is simulated by moving an obstacle through the fluid at a controlled speed. Students are first introduced to the buoyancy frequency by both measurements of the density profile, and by direct experimentation (the "bounce test"). They can then explore shear waves and see the dispersion of wave modes by observing the movement of a vertical dye line, before moving on to observing lee waves via the perturbations in dye streamers linked to density-stratified surfaces. Different flow regimes (potential, non-hydrostatic, hydrostatic) can be explored by varying the speed of the "flow".

Boundary Layer Convection: a water tank, heated from below, is sampled by a vertical array of thermistors which can be moved through the tank. Pools of dye on the floor of the tank and/or shadowgraph techniques are used to visualise thermal plumes. These visualisations can be compared to the statistics of the convective process (the distribution of temperature fluctuations in the horizontal and vertical), and quantities such as kinematic heat flux can be calculated. Scaling and similarity arguments, such as comparing convective temperature and velocity scales, and their variation with height in a convective layer, can be explored in this context. Finally, by creating a temperature (density) inversion within the tank, the growth of convection in the mixed layer, entrainment zone and capping inversion can be observed and sampled.

© 1997 - 2010 UniServe Science

Page Maintained By: PhySciCH@mail.usyd.edu.au
Last Update: Thursday, 02-Nov-2000 12:13:29 EST
URL: http://science.uniserve.edu.au/disc/phys/aippeg/ozcupe5/low2.html