Loss of fluid energy through inertia:

• A. Is greatest during steady flow through a rigid tube of constant caliber
• B. Is best described as Poiseuille's law
• C. Is greatest in a tortuous vessel with multiple obstructions
• D. Accounts for less energy loss than viscous losses

Answer: (C)

Inertia-related energy loss comes from changes in the fluid’s velocity and direction. The most dramatic inertial losses occur when flow must be accelerated and decelerated as it navigates constrictions, bends, or obstructions, because momentum must be redirected and sped up or slowed down. A tortuous vessel with multiple obstructions creates many such momentum changes, so the inertial portion of the pressure drop is greatest there.

In contrast, a straight, rigid tube of constant caliber keeps velocity more uniform along its length, so there are fewer momentum changes and inertial losses are small; the pressure drop in this case is dominated by viscous friction, which is described by Poiseuille’s law.

Poiseuille’s law specifically models viscous losses in laminar flow, not inertial losses, so attributing inertia to Poiseuille’s law isn’t accurate. And while viscous losses are common, saying inertial losses are always less isn’t correct when flow must negotiate obstructions, where inertia can be a major contributor.