Reynolds number depends on which variables?

• A. Viscosity, Temperature
• B. Average flow speed, tube diameter, and density
• C. Pressure and Flow rate
• D. Wavelength and frequency

Answer: (B)

Reynolds number quantifies the balance between inertial and viscous forces in a fluid flow. In the standard form, Re = (rho × v × D) / mu, where rho is density, v is average flow speed, D is a characteristic length (like pipe diameter), and mu is dynamic viscosity. This shows that increasing flow speed, increasing the characteristic length, or having a higher density raises Re (inertial effects dominate), while increasing viscosity lowers it (viscous effects dominate).

The option that lists average flow speed, tube diameter, and density aligns with the variables that directly set Re in the common equation. Temperature affects viscosity, and pressure or flow rate aren’t the direct determinants of Re in the same fundamental way, while wavelength and frequency aren’t relevant to this flow regime.