Fluid dynamics basics: pipe and duct flow
A quick guide to laminar flow, turbulent flow, Reynolds number, hydraulic diameter, and pressure loss.
This guide is prepared for quick engineering review. Critical design decisions still require the relevant standard, supplier data, and engineering approval.
Problem / Objective
In pipe and duct flow, velocity, diameter, viscosity, and roughness define the result together. Looking only at flow rate hides pressure loss and pump load.
Assumptions
- Flow is treated as incompressible.
- Fluid properties are selected at operating temperature.
- Pipe or duct geometry is known.
Step by step method
- Calculate velocity from flow, diameter, and area.
- Use Reynolds number to identify flow regime.
- Select roughness and friction factor.
- Calculate loss with Darcy-Weisbach or a suitable method.
- Add elbow, valve, and entrance losses.
Common mistakes
- Forgetting hydraulic diameter in duct calculations.
- Skipping viscosity changes with temperature.
- Calculating only straight pipe loss and missing local losses.
Related calculators
Related glossary terms
Quick FAQ
Question: What does Reynolds number do?
It estimates whether flow behaves as laminar, transitional, or turbulent.
Question: What is hydraulic diameter?
It represents non-circular ducts with an equivalent flow diameter.
Question: Why does pressure loss increase?
Pressure loss rises with velocity, length, roughness, and local resistances.
Related engineering links
Calculators, terms, and companion guides that share the same engineering concepts.
Calculators
Basic Heat Flow
Estimate heat flow through a single layer with manual inputs.
Pipe Pressure Loss
Compute Re, friction factor, and pressure loss via Darcy-Weisbach.
Guides
Pump type selection guide
Compare positive displacement and dynamic pumps by flow, pressure, viscosity, and service conditions.
Viscosity classification guide
Practical notes for choosing viscosity without mixing ISO VG, SAE, and lubricant grade systems.
CNC machining material and cutting speed guide
Quick decisions for cutting speed, feed, and tool life when machining steel, aluminum, stainless steel, and titanium.
Glossary
Laminar Flow
Laminar Flow is a core engineering term. Definition, usage notes, and a practical example.
Turbulent Flow
Turbulent Flow is a core engineering term. Definition, usage notes, and a practical example.
Head Loss
Head Loss is a core engineering term. Definition, usage notes, and a practical example.
Reynolds Number
Reynolds Number is a core engineering term. Definition, usage notes, and a practical example.