The equation shown is used to determine the Fanning friction factor for laminar flow.

The equation shown is used to determine the Fanning friction factor for laminar flow.

• A. True
• B. False
• C. Cannot be determined
• D. Depends on Reynolds number

Answer: (B)

Laminar pipe flow has simple, inverse relationships between friction factors and Reynolds number, but there are two related definitions to keep straight. The Fanning friction factor for laminar flow is f = 16 / Re. The Darcy–Weisbach friction factor is f = 64 / Re, and these two are related by f_Darcy = 4 × f_Fanning. If the equation shown is 64/Re, that computes the Darcy factor, not the Fanning factor. Therefore it would not determine the Fanning friction factor, making the statement false. The correct form to determine the Fanning factor in laminar flow would be 16 / Re.

Laminar pipe flow has simple, inverse relationships between friction factors and Reynolds number, but there are two related definitions to keep straight. The Fanning friction factor for laminar flow is f = 16 / Re. The Darcy–Weisbach friction factor is f = 64 / Re, and these two are related by f_Darcy = 4 × f_Fanning.

If the equation shown is 64/Re, that computes the Darcy factor, not the Fanning factor. Therefore it would not determine the Fanning friction factor, making the statement false. The correct form to determine the Fanning factor in laminar flow would be 16 / Re.