A new computational study by Nahid Mirizadeh and Mohammad Manafpour, published in Results in Engineering, offers the most systematic answer yet. Using high-resolution computational fluid dynamics simulations in the open-source OpenFOAM platform, the researchers modeled a classical hydraulic jump at an upstream Froude number of 7.5, a regime typical of high-velocity spillway flows, while systematically varying the inflow air concentration from zero to fifty percent. Crucially, unlike most previous investigations, which examined only a handful of aeration levels, the study anchored every aerated scenario against a rigorously validated non-aerated baseline, allowing the progressive influence of air to be isolated and quantified for the first time across the full practical spectrum.

The numerical framework itself represents a careful exercise in verification. The team solved the unsteady Reynolds-averaged Navier-Stokes equations for a two-phase air-water mixture, tracking the free surface with the Volume of Fluid method, in which a scalar indicator function defines the fractional volume of water in each computational cell. Three turbulence closures were tested against laboratory measurements from the University of Queensland, where a horizontal rectangular flume 3.2 meters long, half a meter wide, and 0.41 meters high produced a well-defined jump with a Reynolds number near 140,000. The standard k-epsilon model emerged as the clear winner, predicting the roller length with only a 3.7 percent deviation from experiment, while the SST k-omega and Realizable k-epsilon models erred by 20 and 30 percent respectively.

Validation extended well beyond a single metric. The simulated free-surface profile matched laboratory data with a coefficient of determination of 0.99 and a Kling-Gupta Efficiency of 0.94, a score classified as excellent. The predicted secondary flow depth agreed with the theoretical Bélanger equation to within two percent, and velocity fields reproduced the characteristic structure of the jump: a high-velocity jet hugging the bed, a recirculating roller of reverse flow above it, and a shear layer between the two where turbulence is generated most intensely. A grid convergence analysis using the Grid Convergence Index confirmed that results were mesh-independent, with roller length varying by less than 2.5 percent between medium and fine grids.

With the model proven trustworthy, the researchers turned to the central question, and the answer they found is strikingly nonlinear. For aeration levels up to thirty percent, the free-surface profile of the jump barely budges, with normalized water depths changing by less than 1.5 percent. But at forty and fifty percent air concentration, the surface elevation at the beginning of the transition zone rises by roughly six to nine percent compared with the non-aerated case. The mechanism is physical rather than mysterious: the entrained air increases the volume of the air-water mixture and locally reduces its bulk density, forcing the flow to occupy a greater depth. For dam designers, this means that heavily aerated spillways may demand deeper tailwater to keep the jump anchored in its stilling basin.

The roller, the churning recirculation zone that gives the hydraulic jump its characteristic white-water appearance, responds even more dramatically. Low to moderate aeration of up to twenty percent lengthened the roller by 11.6 percent, but at forty and fifty percent aeration the roller stretched by 26.9 and 34.9 percent respectively. The authors attribute this elongation to two-phase flow mechanisms that slow the decay of turbulence: compressible air bubbles temporarily store turbulent energy and release it gradually, while buoyancy-driven secondary currents disrupt the main vortical structures. Because the mean streamwise velocity remains largely unchanged, the turbulence simply lingers longer, and the roller must extend downstream to complete the dissipation of kinetic energy. Since roller length often dictates the required basin length and the placement of baffle blocks, this finding carries direct consequences for structural design.

The most reassuring result concerns cavitation protection, the original motivation for pre-aeration in the first place. In the non-aerated simulation, the void fraction near the channel bed within the jump body remains essentially zero, leaving the boundary vulnerable to cavitation erosion. At forty and fifty percent inflow aeration, however, near-bed air concentrations reach approximately seven and twelve percent, with the higher value comfortably exceeding the eight percent threshold long associated with cavitation mitigation in the classic experimental literature. The study also confirms that peak air entrainment occurs in the initial portion of the shear layer, where the violent vortices at the jump toe drag air bubbles downward and transport them along the high-shear interface, in agreement with decades of experimental observation.

The velocity and turbulence fields reveal further subtleties. High aeration levels increased near-bed streamwise velocity by three to six percent, apparently because bubble penetration and slip effects reduce effective friction at the wall. While moderate, this change implies altered shear stresses that could shift scour patterns within and downstream of the stilling basin. Turbulent kinetic energy, meanwhile, peaked at the jump toe and concentrated in the shear layer, climbing by 18.3 percent at fifty percent aeration to reach approximately 2.76 square meters per second squared. Intriguingly, the normalized turbulence intensity first held steady, then rose sharply at thirty to forty percent aeration, and finally dropped at fifty percent, because the increase in maximum velocity outpaced the growth in turbulent fluctuations. The authors caution that RANS closures may underrepresent large-scale rotational eddies in highly aerated regions, suggesting that large-eddy simulation will be needed to fully resolve these structures.

Perhaps the most counterintuitive finding is what did not change: overall energy dissipation. Across the entire range of inflow air concentrations from zero to fifty percent, computed energy losses varied only between 63 and 64 percent, compared with 64.8 percent for the laboratory reference case. Because most of the entrained air is released downstream of the jump, the tailwater depth and velocity remain nearly identical to the non-aerated case, and the small residual differences stem mainly from variations in initial flow depth. This echoes earlier experimental work reporting less than two percent variation in dissipation between moderate aeration levels, and it means that pre-aeration delivers cavitation protection without sacrificing the jump’s core job of destroying excess kinetic energy.

Taken together, the results sketch a threshold-type picture of aerated hydraulic jumps that engineers can act on directly. Aeration up to twenty percent produces only minor hydraulic modifications, making it a low-cost, low-risk option. At forty percent and above, the benefits of robust cavitation protection arrive alongside measurable costs: deeper flow, rollers up to a third longer, higher near-bed velocities, and elevated turbulence that could increase dynamic loads on basin linings. The authors argue that these trade-offs should be explicitly built into tailwater calculations, basin dimensioning, and scour assessments for chute spillways and stilling basins operating at high Froude numbers. They also acknowledge the study’s limitations, including its reliance on RANS turbulence modeling, a two-dimensional idealization, and a single Froude number, and they call for future work spanning wider flow regimes, prototype-scale field validation, and design aids that translate these quantitative trends into practical engineering guidelines. For the dams that hold back the world’s rivers, even a few well-placed bubbles may prove to be the difference between a structure that endures and one that slowly crumbles.

Subject of Research: Effects of upstream flow aeration on two-phase hydraulic jump dynamics in rectangular channels using CFD simulation

Article Title: Progressive effects of upstream flow aeration on two-phase hydraulic jump dynamics in rectangular channels: a CFD investigation

Article References: Mirizadeh, N., & Manafpour, M. (2026). Progressive effects of upstream flow aeration on two-phase hydraulic jump dynamics in rectangular channels: a CFD investigation. Results in Engineering, 32, Article 113234. https://doi.org/10.1016/j.rineng.2026.113234

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113234

Keywords: hydraulic jump, CFD, OpenFOAM, pre-aeration, cavitation, stilling basin, spillway, two-phase flow, Volume of Fluid, turbulence, roller length, energy dissipation