Waves with periods shorter than the inertial period exist in the atmosphere (as inertia- gravity waves) and in the oceans (as Poincare and internal gravity waves). Such waves owe their origin to various mechanisms, but of particular interest are those arising either from local secondary instabilities or spontaneous emission due to loss of balance. Previous researchers have studied these phenomena in the laboratory, both in the mechanically-forced and the thermally-forced rotating annulus. Their generation mechanisms, especially in the latter system, have not yet been fully understood. This project aims to change that. Firstly, we present a laboratory investigation using the two layer mechanically-forced annulus. We perform a new series of experiments in which we combine an existing polarised light altimetry technique with particle imaging velocimetry. This necessi- tated a substantial rebuild of the apparatus. The new vessel enables us to measure the flow in one of the layers directly, and thus investigate the validity of a torque bal- ance calculation used by previous experimenters that was hitherto unverified. We use these results to discuss the possibility that the inertia-gravity waves seen in the two layer annulus might have been generated by a shear instability; either that of Holm- boe, or an Ekman layer instability. Our investigation suggests that whilst Holmboe's instability is unlikely to be the cause, a localised Ekman layer instability is a possible generation mechanism for the short waves seen in earlier experiments. Secondly, we perform a numerical investigation using a fully nonlinear, finite-difference, 3D Boussinesq N avier-Stokes model of the rotating thermal annulus. The model pre- dicts the generation of short waves from 'wavemaker' regions determined by strong shear and downwelling near the inner cylinder. These then propagate into the geo- strophic interior of the fluid as inertia-gravity waves, where they have been detected in previous laboratory experiments. We then show how these wavemakers are con- sistent with being due to a localised thermal boundary layer instability, which has a number of similarities to the Ekman layer instability of the two-layer annulus. Such a mechanism also has many similarities with those responsible for launching small- and meso-scale inertia-gravity waves in the atmosphere from fronts and local convection.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:558384 |
| Date | January 2011 |
| Creators | Jacoby, Thomas Norman Llyn |
| Contributors | Read, Peter ; Williams, Paul |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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