Exploring the effects of an obstruction on the evolution of the Rayleigh-Taylor Instability
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Authors
Brown, Christopher
Advisors
Dalziel, Stuart
Date
2018-07-20Awarding Institution
University of Cambridge
Author Affiliation
Applied Mathematics and Theoretical Physics
Qualification
Doctor of Philosophy (PhD)
Language
English
Type
Thesis
Metadata
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Brown, C. (2018). Exploring the effects of an obstruction on the evolution of the Rayleigh-Taylor Instability (Doctoral thesis). https://doi.org/10.17863/CAM.25160
Abstract
This thesis discusses the effect of an obstruction on the evolution of the Rayleigh-Taylor
instability in a confined geometry at low Atwood numbers. Laboratory experiments are the
principal method of investigation, though these data are supplemented with implicit large
eddy simulations (ILES). The laboratory data are captured using an innovative laser scanning
system which is able to simultaneously record density and velocity data in 3D. A new approach
for calculating density data from laser induced fluorescence measurements is developed and
demonstrated. The technique is used to improve the accuracy of the density measurement from
laser induced fluorescence, by correcting for the damage to dye caused by the laser.
The introduction of an obstacle at the height of the initial interface results in dramatic
changes to the dynamics of mixing, even when this obstacle is only a few percent of the domain
width. Two obstructed scenarios are considered. In both of these an obstruction is placed on the
interface between an upper heavy layer and lower light layer. In the first case, a single horizontal
opening connects the upper and lower layers. A bidirectional flow exchanges fluid through the
opening, establishing a circulation cell in each layer. These cells exist quasi-steadily for long
periods, constantly recirculating and mixing the fluid in each layer. This acts to increase the
time required for mixing compared with the classical unobstructed case, but results in a more
uniformly mixed final stratification.
The second case has two horizontal openings, one either side of the obstruction. This results
in markedly different dynamics. The flow through each of the openings switches back and
forth between being bidirectional (as with the single opening case) and unidirectional, with
unidirectional exchange reversing direction with a constant period. These results are consistent
with the ILES data.
For both of these cases a wide range of analytical techniques are used to connect the new
obstructed dynamics with previously conducted research, such as calculating the molecular
mixing fraction, energetics and mixing efficiency. A multistage mixing process is identified,
unique for cases with an obstruction. For the single opening case a hierarchy of models are
developed that accurately capture the density change of each layer for both the experimental
and numerical data. The effect of changing the aspect ratio of the domain is investigated using
ILES, from which different dynamical regimes are observed, discussed and analysed.
Keywords
fluids, maths, physics
Identifiers
This record's DOI: https://doi.org/10.17863/CAM.25160
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