What type of flow condition occurs if viscosity is significant while roughness is negligible?

When viscosity is significant while roughness is negligible, the flow condition is classified as laminar flow. In laminar flow, fluid moves in parallel layers with minimal disruption between them, and the effects of viscosity dominate over inertial forces. This means that the fluid particles slide smoothly past one another, and the velocity of the flow is typically uniform across any cross-section of the pipe.

In laminar flow, the Reynolds number, which is a dimensionless number used to predict flow patterns in different fluid flow situations, is low (typically less than 2000). At these low velocities, the viscous forces are strong enough to keep the flow orderly and stable, leading to a predominantly laminar regime. In this scenario, since roughness is negligible, the flow is further streamlined, reinforcing the characteristic smooth and orderly behavior expected in laminar conditions.

Understanding this distinction helps clarify why in situations where viscosity is significant, laminar flow is expected, especially in applications where fluids move slowly or in highly viscous fluids.