Final answer:
When velocity is between 180-300 and the viscosity ratio (VR) is 2.5-3.4, the flow is classified as transitional. Rescanning is required when there is a significant change in either velocity or viscosity that affects the Reynolds number.
Step-by-step explanation:
When a velocity is between 180-300 and viscosity ratio (VR) is 2.5-3.4, the flow is classified as transitional. It is between laminar and turbulent flow, since the Reynolds number (NR) is likely between 2000 and 3000, where flow can be either or both laminar and turbulent. To determine the exact nature of the flow or a change in the flow regime, rescanning should be done if there is a significant change in either velocity or viscosity ratio that might push the NR value outside of the transitional range.
Rescanning is often employed when the flow properties alter, such as with changes in velocity or fluid properties that would affect the Reynolds number. For instance, if the average velocity decreases below 180 or increases above 300, or if the viscosity ratio falls outside of the range of 2.5 to 3.4, then the flow regime may change from transitional to either laminar or turbulent. Observing these changes would necessitate rescanning to understand the new flow characteristics.