Reynolds number is used to determine what aspect of fluid flow?

• A. Whether the flow is laminar or turbulent
• B. Temperature of the fluid
• C. Pressure drop in the pipe
• D. Fluid density

Answer: (D)

Reynolds number tells you how the flow behaves by comparing inertial forces to viscous forces in a fluid. It’s calculated from density, velocity, a characteristic length, and viscosity, but its purpose is to indicate which type of flow regime will dominate. When inertial effects win (high Reynolds number), the flow tends to become unstable and chaotic—turbulent. When viscous effects win (low Reynolds number), the flow stays smooth and orderly—laminar. This is why Reynolds number is used to judge whether the flow is laminar or turbulent. It isn’t a direct measure of temperature, nor does it directly give the pressure drop; those can be influenced by the flow regime but are separate concepts. Density is part of the calculation, but by itself density doesn’t determine the flow pattern; the balance between inertia and viscosity does.