densitetsdriven

Densitetsdriven, or density-driven, describes physical processes in which variations in fluid density generate buoyant forces that move fluid and solutes. In hydrogeology and geoscience, it refers to flows where density differences, caused by temperature changes, salinity, or chemical composition, are the primary driver rather than pressure gradients alone.

In porous media, the flow is often described by Darcy's law extended to variable density: q = - (k/μ) (∇p - ρ g). When ρ varies with temperature or chemical concentration, the buoyancy term dominates in regions with modest hydraulic gradients. The Boussinesq approximation is commonly used, treating density variations as small and only important in the buoyancy term.

Contexts include groundwater systems with thermal or chemical gradients, such as thermally induced convection in aquifers, saltwater intrusion in coastal aquifers due to density contrast between freshwater and saline water, and density-driven mixing in geothermal reservoirs. In carbon capture and storage, injected CO2 can alter brine density and trigger convective mixing, accelerating plume migration.

In oceanography and climatology, density-driven convection contributes to thermohaline circulation and double-diffusion phenomena, where salinity and temperature gradients create complex flow patterns. Numerical modelling of densitetsdriven processes requires solving coupled mass, momentum, energy, and solute transport equations, often at high Rayleigh-Darcy numbers that indicate unstable, convective regimes.

Understanding densitetsdriven processes is essential for accurate predictions of contaminant transport, resource management, and storage safety, as neglecting density effects can misrepresent plume evolution and mixing rates.