Which of the following is least likely to occur in a round pipe?

1. Laminar flow and turbulent flow in the transitional flow regime
2. Laminar flow in the turbulent flow regime
3. Turbulence in the laminar flow regime
4. Turbulence in the turbulent flow regime

As related to flow measurement, Reynolds number provides an indication of how fluid flows in a pipe. In the laminar flow regime, fluid flows almost straight down the pipe. Eddies form and flow downstream in the turbulent flow regime. Flow in the transitional flow regime can exhibit properties found in the laminar flow regime (straight flow), turbulent flow regime (eddies), or both. Turbulence is a distortion of a fully developed velocity profile.

Therefore, the properties of laminar flow and turbulent flow can be present in the transitional flow regime (Answer A). Turbulence can be found in any flow regime (Answer C and Answer D). However, laminar flow will not likely be found in the turbulent flow regime, so Answer B is the best answer.

Additional complicating factors

Reynolds number is used to provide an indication of which flow regime is present in the pipe. However, it is not the only factor. Using special techniques, operating at Reynolds numbers considered to be in the turbulent flow regime can exhibit the properties of laminar flow. For example, using smooth pipe with few fittings, using long radius bends, making slow flow rate changes and using pumps that are gentle on the fluid (or better yet, gravity) can forestall the onset of eddy formation and allow the fluid to exhibit the properties of laminar flow at Reynolds numbers generally considered to be in the turbulent flow regime.