The boundary condition characterized by a fixed gradient across the boundary, allowing cross-boundary flow, is called Conductive.

• A. No-flow
• B. Constant-Pressure
• C. Leaky
• D. Conductive

Answer: (D)

Prescribing how steeply the pressure (or head) changes across a boundary and letting flow cross that boundary defines a conductive boundary condition. In porous-media flow, the flux across the boundary follows Darcy's law, q = -K dP/dn, so fixing the gradient dP/dn fixes the amount of flow that can cross the boundary. If the boundary were no-flow, the gradient would be zero and nothing would cross. If the boundary fixed the pressure, you’d specify a boundary value rather than the gradient, so the flow would depend on interior conditions instead of a prescribed cross-boundary gradient. A leaky boundary implies some leakage but not necessarily a fixed gradient across the boundary. Therefore, this boundary condition is conductive.