Seminar - Yadan Mao - Unsteady natural convection in near-shore waters

Monday 7 November 2009, 3.00 pm - 3.50 pm
Civil Engineering Lecture Theatre 3

Abstract:
In the near-shore waters, an increasing bathymetry in the offshore direction is a geometric factor for differential heating or cooling across shore. Exposed to daylight radiation, the volumetric heating rate in the shallow region is greater than in the deep region, generating a warm surface layer flowing offshore. At night, a circulation in the opposite direction is induced by differential cooling owing to heat loss from the water to the atmosphere. Field experiments demonstrate that this natural convection in calm near-shore waters plays a significant role in cross-shore exchanges with significant biological and environmental implications. Our research aims at providing detailed quantification of this thermal flow for various thermal forcing.

A survey of the literature indicates that existing analyses did not capture distinctive flow regions and variations of thermal flow properties with offshore distance. Based on a wedge model, an improved scaling analysis is proposed in the present study to reveal more detailed features of the flow than the previous scaling analysis, especially the dependency of flow properties on offshore-distance. Four different thermal forcing (radiative heating, isoflux cooling, constant and ramped isothermal heating) are investigated and the scaling and analytical results are verified by numerical simulations.

Through the improved scaling analysis, critical functions of offshore distance have been derived to identify the distinctness and the stability of thermal boundary layers. The possible flow scenarios are predicted depending on the bottom slope, and for radiation case also the penetration depth of radiation. Flow regimes for each scenario are classified based on the Rayleigh number. For each flow regime, distinctive sub-regions are identified. Different sets of scaling relations incorporating the offshore-distance dependency have been derived for distinctive regions. Furthermore, a spectral analysis has been conducted on the numerical results to reveal detailed characteristics of instability of radiation-induced natural convection.

Finally, some comments are made with respect to the implications of these results for cross shore transport in lakes and reservoirs.

Short bio of Ms Yadan Mao
Yadan Mao obtained her Bachelor and Master degrees in Geophysics from China University of Geosciences. After obtaining her Master degree, she was awarded a PhD scholarship by China Scholarship Council and James Cook University and has been doing her PhD study at James Cook University under the supervision of A/Prof. Chengwang Lei and Prof. John Patterson. She has submitted her thesis early December, 2009. Invited by her supervisors, she will give a presentation about her thesis project here today. The research of her PhD study focused on unsteady natural convection, induced by solar radiation, surface cooling or heating, in a triangular enclosure, which can be applied to near shore waters of lakes and reservoirs. Her research up to now has covered several geophysical fields: the propagation of seismic waves in porous media, the measurement of sea surface current with High Frequency Ocean Surface Radar, the response of sea surface current to wind, and natural convection in the littoral region.