Air Entrainment Model
Air entrainment occurs when gas bubbles are drawn into a liquid at a free surface through turbulent mixing, impingement, or mechanical agitation. Once entrained, these bubbles are transported by the flow, rise due to buoyancy, and may escape at a free surface or remain trapped within the liquid. The consequences range from beneficial, aerated spillways resist cavitation damage and oxygenated water supports aquatic life; to detrimental, such as gas porosity defects in metal castings or unwanted gas ingestion in fuel systems.
The FLOW-3D product family’s Air Entrainment Model simulates the entrainment, transport, and escape of undissolved gas bubbles at free surfaces across a wide range of applications and industries. The same underlying model applies whether the fluid is water, molten metal, or liquid fuel; and whether the goal is to maximize aeration or minimize it.
Modeling Capabilities
The Air Entrainment model works in conjunction with the Variable Density and the Drift Flux Models to fully represent the physics of aerated flow. Together, these three models approximate the air entrained at the free surface, the motion of entrained air bubbles through the fluid, and the effect of variable air concentration on the local fluid density. The combined capability includes:
- Local air entrainment rate at the free surface, driven by turbulent kinetic energy exceeding surface stabilizing forces
- Transport of entrained air bubbles through the flow field
- Bubble rise due to buoyancy forces
- Bubble escape when air reaches a free surface
- Bulking — the increase in effective fluid volume due to entrained air — affecting free surface elevation
- Variable density flow fields in aerated regions
- Dynamic average bubble sizes based on breakup and coalesce models
- Full postprocessing output including local and integrated air concentrations, average bubble size, and entrained and escaping air fluxes
Dissolved Oxygen Model
The Air Entrainment Model is coupled with a dissolved oxygen model that tracks the transfer of oxygen from entrained air bubbles into the surrounding liquid. Three options are available for the definition of the mass transfer coefficient:
- User-defined constant — for cases where the transfer coefficient is known from experiment or empirical data
- Clean bubble assumption (Higbie’s penetration theory) — appropriate for bubbles rising in clean water without surfactant effects
- Dirty bubble assumption — accounts for surfactant effects that reduce oxygen dissolution at the bubble interface
The Dissolved Oxygen Model is particularly valuable for water quality assessments downstream of aeration structures, stilling basins, and diffuser systems.
Example: Plunging Jet
This FLOW-3D HYDRO simulation of a plunging jet shows both the air entrainment and dissolved oxygen models active simultaneously. The top panel shows dissolved oxygen concentration as it is absorbed and transported through the fluid following air entrainment at the impingement point. The bottom panel shows the corresponding entrained air volume fraction, revealing where air is entrained and how it is carried into the water column.
FLOW-3D HYDRO simulation of a plunging jet showing dissolved oxygen concentration (top) and entrained air volume fraction (bottom).
Water and Environmental Applications
Spillway Aeration and Cavitation Protection
At high flow velocities, irregularities on a spillway surface can cause flow separation and local pressure drops that lead to cavitation damage. Entraining air into the flow is an effective mitigation strategy: the presence of air bubbles damps the pressure waves produced by collapsing cavitation voids, significantly reducing surface erosion. FLOW-3D simulates the turbulent boundary layer growing from the spillway surface, the inception point where air first reaches the free surface, and the subsequent aeration and bulking of the flow downstream.
FLOW-3D HYDRO simulation of air entrainment on a self-aerating stepped spillway, showing the 3D free surface (left) and entrained air volume fraction in cross-section (right).
Fish Passage and Water Quality
Dissolved oxygen concentration is a key parameter in fish passage design. FLOW-3D determines the concentrations of air entrained at spillways and weirs and tracks how that oxygen is transported and released downstream. This allows engineers to assess whether dissolved gas levels will remain within safe limits for target fish species, and to identify where supersaturation may become a concern.
Municipal Water and Wastewater Systems
Aerated flows are common in municipal infrastructure — drop shafts, sewer overflows, and stormwater tunnels all involve conditions where air is entrained and transported. The model’s buoyancy and bulking outputs accurately represent the variable-density flow conditions in these structures, which is essential for sizing shafts, predicting hydraulic grade lines, and avoiding surcharging.
Diffuser and Aeration Systems
Mass sources in FLOW-3D can introduce air directly into the water column, enabling simulation of bubble diffusers used in wastewater treatment, reservoir destratification, and aquaculture oxygenation systems. The turbulent diffusion of entrained air and dissolved oxygen terms present in the FLOW-3D product family turbulence models allow for accurate prediction of the very low air concentrations typical of fine-bubble diffuser systems.
Metal Casting Applications
In metal casting, entrained air is a primary source of casting defects. Foundry engineers refer to this phenomenon as air entrapment — air or gas bubbles drawn into the melt during filling that become trapped as the metal solidifies, producing porosity, surface blemishes, and compromised mechanical properties. The FLOW-3D air entrainment model estimates the amount and location of air entrapment across all casting process types.
Gravity and Permanent Mold Casting
In gravity-poured processes, turbulent filling conditions — particularly at the gate, runner transitions, and thin sections — entrain air into the melt. FLOW-3D CAST tracks the transport of entrained air through the filling cavity and models bubble rise due to buoyancy, including the escape of air at a free surface. This allows engineers to predict where gas porosity is most likely to form and adjust gating geometry or filling speed accordingly.
High Pressure Die Casting
In high pressure die casting (HPDC), the high velocities of molten metal through the runner and gate system create conditions where air entrapment is difficult to avoid entirely. The model estimates entrained and entrapped air concentrations throughout the fill, identifying regions of the cavity where air is most likely to be trapped at the end of fill. This insight informs vent placement, overflow design, and vacuum system requirements.
FLOW-3D CAST simulation of entrained and entrapped air during mold filling of an HPDC structural component, showing the geometry (left) and air distribution at end of fill (right).
Aerospace and Industrial Applications
Aerospace Fuel Systems
During aircraft maneuvers and spacecraft operations, fuel sloshing can entrain air into the liquid propellant. Air ingestion into fuel delivery systems can disrupt flow to engines or affect the accuracy of fuel quantity measurements. FLOW-3D simulates fuel sloshing dynamics and the entrainment of air at the liquid surface, helping engineers assess the risk of gas ingestion under defined maneuver profiles and design propellant management devices accordingly.
Air Persistence in Industrial Mixing
In high-shear mixing operations such as food production, cosmetics manufacturing, and chemical processing, rotating impellers and agitators entrain air at the free surface of the product. Unlike water, many mixed products are viscous enough that entrained bubbles rise slowly or not at all, causing air to persist within the product long after it is introduced. Trapped air increases the product volume, reduces bulk density, and can degrade texture, appearance, and shelf life. FLOW-3D simulates the mixing dynamics along with the entrainment and transport of the air through high-viscosity fluids allowing process engineers to evaluate mixer geometry and operating conditions for their effect on air incorporation and retention.