The potential for free and mixed convection in sedimentary basins |
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Authors: | J P Raffensperger D Vlassopoulos |
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Institution: | (1) US Geological Survey, 8987 Yellow Brick Road, Baltimore, MD 21237, USA Fax: +1-410-238-4210 e-mail: jpraffen@usgs.gov, US;(2) Department of Environmental Sciences, Clark Hall, University of Virginia, Charlottesville, VA 22903, USA, US |
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Abstract: | Free thermal convection and mixed convection are considered as potential mechanisms for mass and heat transport in sedimentary
basins. Mixed convection occurs when horizontal flows (forced convection) are superimposed on thermally driven flows. In cross
section, mixed convection is characterized by convection cells that migrate laterally in the direction of forced convective
flow. Two-dimensional finite-element simulations of variable-density groundwater flow and heat transport in a horizontal porous
layer were performed to determine critical mean Rayleigh numbers for the onset of free convection, using both isothermal and
semi-conductive boundaries. Additional simulations imposed a varying lateral fluid flux on the free-convection pattern. Results
from these experiments indicate that forced convection becomes dominant, completely eliminating buoyancy-driven circulation,
when the total forced-convection fluid flux exceeds the total flux possible due to free convection. Calculations of the thermal
rock alteration index (RAI=q·∇T) delineate the patterns of potential diagenesis produced by fluid movement through temperature gradients. Free convection
produces a distinct pattern of alternating positive and negative RAIs, whereas mixed convection produces a simpler layering
of positive and negative values and in general less diagenetic alteration.
Received, January 1999/Revised, June 1999/Accepted, July 1999 |
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Keywords: | numerical modeling free convection general hydrogeology sedimentary rocks thermal conditions |
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