The Reynolds number increases with ______ flow speed and ______ tube diameter.

The Reynolds number increases with ______ flow speed and ______ tube diameter.

• A. Decreasing; Increasing
• B. Increasing; Enlarging
• C. Increasing; Decreasing
• D. Decreasing; Decreasing

Answer: (B)

Reynolds number is a balance between inertial and viscous forces, and it grows as flow speed and the characteristic length scale increase. In pipe flow, the characteristic length is the diameter, so Re is proportional to the product of velocity and diameter (Re ≈ ρ v D / μ). Therefore, increasing flow speed raises inertial effects, and enlarging the tube diameter enlarges the length scale over which momentum is transported, both raising Re.

Reynolds number is a balance between inertial and viscous forces, and it grows as flow speed and the characteristic length scale increase. In pipe flow, the characteristic length is the diameter, so Re is proportional to the product of velocity and diameter (Re ≈ ρ v D / μ). Therefore, increasing flow speed raises inertial effects, and enlarging the tube diameter enlarges the length scale over which momentum is transported, both raising Re.