Compute pipe friction pressure drop by dP = f(L/D)(rho v^2/2) from friction factor, pipe length and diameter, fluid density and velocity.
由摩擦系数、管长、管径、密度与流速，按 ΔP = f(L/D)(ρv²/2) 计算管道沿程压降。
Constants and standards used on this page with their authoritative sources, for verification and citation.
| Quantity | Value | Unit | Source |
|---|---|---|---|
| Standard gravity g₀ | 9.80665 (9.81 used in practice) | m/s² | ISO 80000-3:2019 |
| Laminar friction factor | f = 64/Re (Re < 2300) | dimensionless | Poiseuille laminar solution, critical Re = 2300 |
| Colebrook-White implicit equation | 1/√f = −2 log₁₀(ε/(3.7D) + 2.51/(Re√f)) | dimensionless | Colebrook C.F. & White C.M. (1939) |
| Equivalent sand-grain roughness, commercial steel | 0.045 | mm | Nikuradse (1933) equivalent sand-grain roughness; adopted in the Moody (1944) chart |
| Kinematic viscosity of water at 20 °C | 1.004 × 10⁻⁶ | m²/s | IAPWS-95 water properties (1.0×10⁻⁶ used in practice) |
| Density of water at 20 °C | 998.2 (1000 used in practice) | kg/m³ | IAPWS-95 |
| Recommended explicit approximation | Swamee–Jain (1976) | — | Swamee P.K. & Jain A.K. (1976), J. Hydr. Div. ASCE |
Note: Colebrook is implicit; the Swamee-Jain explicit approximation or the Moody chart is normally used. ε is an equivalent sand-grain roughness varying widely with pipe material and fouling, and it drives the result directly.
| Item | Formula / Note | Source / Standard |
|---|---|---|
| Darcy-Weisbach equation | hf = f × (L/D) × v²/(2g) with friction factor f | Darcy 1857 / Weisbach 1845 |
| Moody chart | f from Re and ε/D by chart or Colebrook equation | Moody 1944 / Colebrook 1939 |
| Laminar case | For Re < 2300, f = 64/Re (Hagen-Poiseuille) | Hagen-Poiseuille law |
| Typical roughness | PVC ε≈0.0015 mm, new steel 0.045 mm, cast iron 0.26 mm | Hydraulics handbook tables |