The flap-gate spillway is popular in northern countries because it can dischargefloating objects like ice and wood without additional construction. However, froma hydraulic point of view, the discharge of water over the crest of the flood gate canexperience cavitation phenomena by creating a water jet that forms an air cavityinside it. Cavitation in water spillways must be eliminated to prevent structuraldamage and ensures afety. Cavitation also reduces operational efficiency andincreases maintenance and repair costs. This thesis will study this hydraulicphenomenon in 2D and 3D for various discharge cases and evaluate a commonsolution of airing the cavity below the jet at atmospheric pressure. Scalabilityeffects on the model will also be examined.

This method requires learning CFD basics and using Ansys FLUENT. Thisprogram will design, mesh, apply boundary conditions and simulate thegeometries. For each case, we will compare the phase fraction, pressure, andspeed distributions over the simulated space to understand flow behavior. Theresults show that the laws of similarity fail under sub-atmospheric pressure andthat smaller models have more turbulence. Flow asymmetries make cavity sealingmore difficult in 3D models, especially at low discharges. Larger dischargesreduce asymmetry, but the cavity negative pressure is less extreme than in 2Dmodels. At low discharges, ventilation is ineffective. Airing larger dischargesimproves flow symmetry and prevents cavitation by keeping the jet open tothe atmosphere. Ventilation increases cavity pressure and reduces pressurefluctuations in high discharge situations, but it does not eliminate cavitation.

By demonstrating that aeration significantly improves flow behavior, our findingscan influence the design and construction of safer, more durable, and moreefficient new hydraulic structures, thereby contributing to the advancement ofhydraulic engineering.