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Reynolds Number — FRCA Primary MCQ

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ModeratePhysics & Clinical MeasurementReynolds NumberFRCA Primary

For steady flow of a Newtonian fluid through a smooth, straight circular tube, which statement best defines the Reynolds number and identifies the approximate onset of transition from laminar to turbulent flow?

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Correct answer: D — It is the ratio of inertial to viscous forces; transition begins at approximately 2000.

Explanation lettering: C = shown as A · E = shown as B · A = shown as C · B = shown as D · D = shown as E

B is correct. Reynolds number is the dimensionless ratio of inertial to viscous forces. For flow in a circular tube, Re = ρvD/μ, so increasing density, velocity or diameter promotes turbulence, whereas increasing viscosity favours laminar flow. In the conventional FRCA model, flow is predominantly laminar below about Re 2000, with transition beginning around this value; flow is generally regarded as fully turbulent above roughly 4000. These limits are approximate and depend on tube geometry, surface roughness and disturbances. A reverses the force ratio. C and E describe other force relationships and give unsupported thresholds. D approximates a Weber-number concept because it compares inertia with surface tension; 4000 is conventionally associated with established turbulence, not its onset.

Reference: Ben-Dov G, Cohen J. Critical Reynolds number for a natural transition to turbulence in pipe flows. Physical Review Letters. 2007;98:064503. https://pubmed.ncbi.nlm.nih.gov/17358948/