Fluid mass continuity
Density change balances mass flux through a region.
Key knowledge
Classical mass conservation without sources gives ∂ρ/∂t + ∇·(ρv) = 0. Constant positive density reduces this to ∇·v = 0; steady constant-density tube flow gives A₁v₁ = A₂v₂.
Values and equations
∂ρ/∂t + ∇·(ρv) = 0; for constant ρ > 0: ∇·v = 0; steady tube: A₁v₁ = A₂v₂
- ρ
- Fluid mass density kg m⁻³
- v
- Fluid velocity field m s⁻¹
- t
- Time s
- A₁, A₂
- Tube cross-sectional areas m²
- v₁, v₂
- Cross-section mean speeds m s⁻¹
- ∇
- Spatial derivative operator m⁻¹
Conditions and scope
- Continuum density and velocity fields are smooth enough for the displayed derivatives; no mass production or removal term.
- The tube relation uses cross-section mean speeds and excludes leakage or side inflow.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
MIT Marine Hydrodynamics sections 1.3.1 and 1.5: material-volume conservation and general differential balance; OpenStax and MIT 29.4.14: specialization.
2026-10-10 · General smooth classical mass balance and stated specialization; source terms and discontinuous shock/jump formulations excluded.
Entry details
Permanent link to this entry ↗- Record identity
- physics-continuity-equation
- Revision
- university-reference.1
- Structured reference record
- University reference · University core expansion