Final answer:
The critical Reynolds number indicates the transition from laminar to turbulent flow. For a value below about 2000, the flow is laminar, and for a value above about 3000, it becomes turbulent. The correct answer to the student's question is that Critical Re is when the flow becomes turbulent.
Step-by-step explanation:
The critical Reynolds number (Re) is a dimensionless quantity used in fluid mechanics to predict the flow regime of a fluid flowing over a surface or through a pipe. The Reynolds number is calculated using the density (ρ) of the fluid, its velocity (v), its viscosity (n), and a characteristic length such as the tube radius (r) for flow in a pipe. In the context of flow over a flat plate, the critical Reynolds number is typically used to determine when the flow transitions from laminar to turbulent.
For a Reynolds number below approximately 2000, the flow is considered laminar, which means it flows in parallel layers with minimal mixing between them. When the Reynolds number is above approximately 3000, the flow becomes turbulent, exhibiting chaotic changes in pressure and velocity. Between Reynolds numbers of 2000 and 3000, the flow is unstable, and small perturbations, such as surface roughness or obstructions, can transition the flow from laminar to turbulent or vice versa. This range is often referred to as the transitional flow regime.
The correct answer to the student's question is c. Critical Re is when the flow becomes turbulent.