Hydrodynamic noise from flowing liquids The major root cause of high hydrodynamic noise (noise generated by liquid flow in pipes, valves, and pumps) is:

Introduction / Context:
Hydrodynamic noise arises when liquid flow creates pressure fluctuations that couple into structures and air, producing audible or ultrasonic noise. This phenomenon is common in throttling valves, high-head pumps, and restrictive fittings. Recognising the dominant mechanisms allows engineers to mitigate noise through design and operating changes before resorting to heavy acoustic treatments.

Given Data / Assumptions:

• Incompressible liquid flow with possible local pressure drops.
• Presence of valves, orifices, elbows, or impellers that may induce strong pressure gradients.
• Standard industrial piping systems (metals, typical diameters).

Concept / Approach:
Cavitation occurs when local static pressure falls below the liquid’s vapour pressure, forming vapour bubbles that subsequently collapse in regions of higher pressure. This collapse is highly energetic and generates intense, broadband pressure pulses. These pulses translate to structural vibration and radiated noise, and they can cause erosion and pitting. While boundary layer turbulence and general flow fluctuations contribute to noise, cavitation dominates when present because its bubble collapses emit far stronger impulses over a wide frequency range.

Step-by-Step Solution:

Verification / Alternative check:
Field evidence: reducing valve pressure drop, increasing NPSH available, or using anti-cavitation trims typically yields pronounced noise reductions, confirming cavitation as the primary driver when present.

Why Other Options Are Wrong:

• Pipe wall vibrations due to external impacts: Not the root fluid-mechanics cause; often a consequence of flow excitation.
• Boundary layer separation only: Can generate noise, but typically less severe than cavitation-induced pulses.
• Random flow fluctuations only: Background noise contributor; rarely dominant versus cavitation.
• Poor acoustic insulation: A transmission issue, not the source mechanism.

Common Pitfalls:
Attempting to fix cavitation noise solely with lagging; the correct approach is hydraulic: reduce pressure drop per stage, raise suction pressure, or use multi-hole/anti-cavitation valve trims.

Final Answer:
Cavitation