Which statement correctly indicates the cavitation risk when sigma is lower?

• A. Lower σ indicates higher cavitation risk
• B. Higher σ indicates higher cavitation risk
• C. Sigma has no relation to cavitation
• D. Cavitation risk is independent of σ

Answer: (A)

The idea is that cavitation risk depends on how close the local pressure in the flow gets to the vapor pressure, relative to the flow’s dynamic energy. The cavitation number, sigma, is defined as (P_in − P_vap) divided by (0.5 ρ V^2). It measures the pressure margin above the vapor pressure, scaled by the flow’s dynamic head. When sigma is lower, either the pressure margin is smaller (P_in is closer to P_vap) or the dynamic term is larger (higher velocity), or both. In any case, the flow is nearer to the conditions where vapor bubbles can form, so cavitation occurs more readily. Hence, lower sigma means higher cavitation risk. The other statements contradict this relationship: increasing sigma reduces risk, sigma having no relation, or cavitation risk being independent of sigma would all ignore the defined margin and the way it controls bubble formation.