ANS:A - Low pressure

Cavitation is a phenomenon that occurs in fluid flow systems when the pressure of a liquid drops below its vapor pressure, resulting in the formation of vapor bubbles within the fluid. These vapor bubbles are essentially empty spaces filled with vapor and can form and collapse rapidly as the fluid flows through the system. The process of bubble formation and collapse can lead to significant damage to the components of the fluid system. Here's a more detailed explanation of cavitation:

1. Low Pressure: Cavitation occurs when the pressure of the liquid drops below its vapor pressure. Vapor pressure is the pressure at which a liquid's vapor phase is in equilibrium with its liquid phase at a given temperature. When the pressure in the fluid drops below the vapor pressure, the liquid starts to vaporize, forming bubbles.
2. Formation of Vapor Bubbles: As the pressure drops, small vapor bubbles form within the liquid. These bubbles can form at points of high fluid velocity or low pressure, such as near sharp edges, sudden expansions, or in regions of turbulent flow.
3. Bubble Collapse: The vapor bubbles move with the fluid flow and may eventually encounter regions of higher pressure. When this happens, the bubbles collapse or implode due to the increase in pressure. The collapse of the bubbles generates intense localized forces, resulting in shock waves and high-speed liquid jets. These forces can erode and damage nearby surfaces, including metal components, propellers, impellers, and pump blades.
4. Effects of Cavitation: Cavitation can cause several adverse effects in fluid systems, including:
   • Erosion: The repeated formation and collapse of vapor bubbles can erode solid surfaces, leading to pitting, surface roughness, and material loss.
   • Noise: Cavitation can produce a distinctive noise, often described as a rattling or grinding sound, as the bubbles collapse.
   • Reduced Performance: Cavitation can reduce the efficiency and performance of fluid systems, such as pumps, turbines, and propellers, by disrupting the flow and causing mechanical damage.

To prevent cavitation, engineers design fluid systems to maintain adequate pressure levels, minimize flow turbulence, and avoid sharp edges or sudden changes in flow geometry. Additionally, materials with high resistance to cavitation erosion, such as hardened alloys and coatings, are often used in critical components exposed to cavitation.