In the context of gas flow in airways, helium’s lower density compared with nitrogen results in what effect on the onset of turbulence?

In the context of gas flow in airways, helium’s lower density compared with nitrogen results in what effect on the onset of turbulence?

• A. Higher critical velocity for turbulence
• B. Lower critical velocity for turbulence
• C. No change in turbulence onset
• D. Increased viscosity with flow

Answer: (A)

The key idea is the Reynolds number, which governs when flow becomes turbulent. Reynolds number is Re = ρ v D / μ, so it depends on density, velocity, a characteristic dimension, and viscosity. Helium has a much lower density than nitrogen, while its viscosity is not dramatically different. With a lower density, the inertial forces driving turbulence are reduced, so for the same airway geometry and flow conditions the Reynolds number is smaller. Since turbulence sets in when Re reaches a critical value, a lower Re means you must push the flow to a higher velocity to reach that threshold. Therefore the onset of turbulence occurs at a higher velocity when helium is used. The idea that viscosity would cause an immediate increase with flow isn’t the driving factor here; the density change is the dominant effect.

The key idea is the Reynolds number, which governs when flow becomes turbulent. Reynolds number is Re = ρ v D / μ, so it depends on density, velocity, a characteristic dimension, and viscosity. Helium has a much lower density than nitrogen, while its viscosity is not dramatically different. With a lower density, the inertial forces driving turbulence are reduced, so for the same airway geometry and flow conditions the Reynolds number is smaller. Since turbulence sets in when Re reaches a critical value, a lower Re means you must push the flow to a higher velocity to reach that threshold. Therefore the onset of turbulence occurs at a higher velocity when helium is used. The idea that viscosity would cause an immediate increase with flow isn’t the driving factor here; the density change is the dominant effect.