Which statement about Reynolds number is true?

• A. It predicts heat transfer coefficient in all fluids
• B. It represents the ratio of inertial forces to viscous forces, indicating laminar vs turbulent flow
• C. It is used to calculate the viscosity directly
• D. It is defined as the ratio of kinetic energy to potential energy

Answer: (B)

Reynolds number is a dimensionless measure that compares inertial forces to viscous forces in a fluid flow. It’s defined as Re = rho * V * L / mu (or V * L / nu), where rho is density, V is a characteristic velocity, L is a characteristic length, mu is dynamic viscosity, and nu is kinematic viscosity. This ratio tells you how dominant inertia is versus viscosity: at low Reynolds numbers, viscous forces smooth out disturbances and the flow tends to be laminar; at high Reynolds numbers, inertia can overcome viscous damping and the flow becomes turbulent. This is why the statement about Reynolds number being the ratio of inertial to viscous forces and indicating laminar vs turbulent flow is the true one.

It doesn’t by itself give heat-transfer coefficients. Heat transfer is described by other dimensionless groups like the Nusselt number, which often depends on Reynolds number along with Prandtl number. Similarly, Reynolds number isn’t a direct method for calculating viscosity; while viscosity appears in its denominator, Re is used to classify flow given viscosity is known, rather than to determine viscosity from scratch. And it’s not the ratio of kinetic energy to potential energy—the Reynolds number has nothing to do with that energy ratio.