Fluids have flow rate through area {flux, fluid}|. Flux is energy, mass, momentum, or charge change D divided by cross-sectional area A times time t: D / (A * t).
pipe
In pipes, masses entering and leaving cross-sectional areas are equal. Otherwise, fluid builds, or vacuum happens. Mass m flowing through pipe equals fluid density d times fluid velocity v times cross-sectional area A: m = d*v*A. For liquid, fluid density is constant, and fluid velocity going in vi times cross-sectional area at entrance Ai equals fluid velocity going out vo times cross-sectional area at exit Ao: vi * Ai = vo * Ao. For gas, fluid density varies, and fluid density at entrance di times fluid velocity going in vi times cross-sectional area at entrance Ai equals fluid density at exit do times fluid velocity going out vo times cross-sectional area at exit Ao: di * vi * Ai = do * vo * Ao.
Flux equals constant times gradient {Fick's first law of diffusion} {Fick first law of diffusion}: dm / (A * dt) = dC / ds, where m is mass, A is cross-sectional area, t is time, C is concentration difference, and s is distance.
Pressure, temperature, concentration, or force change over time relates to quantity change over distance {Fick's second law of diffusion} {Fick second law of diffusion}: dP / dt = dm / ds, where P is pressure, t is time, m is mass, and s is distance.
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Date Modified: 2022.0225