Newtonian viscosity and Poiseuille flow
Viscosity relates shear stress to velocity gradient.
Key knowledge
For simple shear in a Newtonian fluid, τ_xy=η∂v_x/∂y. A horizontal rigid circular tube in steady fully developed incompressible laminar flow with no slip has volume flow Q=πR⁴(p_in−p_out)/(8ηL). Tube radius strongly affects flow, while pressure drop supplies viscous dissipation.
Values and equations
τ_xy=η∂v_x/∂y; Q=πR⁴(p_in−p_out)/(8ηL)
- τ_xy
- x-directed viscous shear stress on a y-normal plane Pa
- η
- Dynamic viscosity Pa s
- v_x
- Fluid x velocity m s⁻¹
- y,R,L
- Transverse coordinate, tube radius and length m
- Q
- Volume flow rate, positive from inlet to outlet m³ s⁻¹
- p_in,p_out
- Inlet and outlet pressures Pa
Conditions and scope
- η>0 constant; simple Cartesian shear for the constitutive equation. The tube has uniform R>0 and L>0, fixed walls, negligible entrance effects and no imposed body-force pressure contribution.
- The tube law does not apply unchanged to turbulent flow, slip, non-Newtonian fluids or pulsatile/deforming vessels; laminar is not a universal fixed-Reynolds-number guarantee.
Sources · 1
References
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Evidence and review
Read §14.7 parallel-plate shear law14.17 and Laminar Flow Confined to Tubes, Poiseuille resistance/flow14.22–14.23. The gradient form is the stated simple-shear extension; do not adopt universal Reynolds thresholds or imply that fluids have no viscous shear stress.
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Entry details
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- foundations-refresh.1
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