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Views of the two conductive zones in the structured column: (a) low permeable annulus and high permeable core with an attractant source along the central axis (b) radial cross-section. The attractant is released from the inner source to form an attractant gradient in the radial direction transverse to flow. (c) axial cross-section. Because of the contrasting conductivities in the two layers, the flow velocities in the two layers are also different.
