We consider the mixing dynamics of an air–liquid system driven by the rotation of a pitched blade turbine (PBT) inside an open, cylindrical tank. To examine the flow and interfacial dynamics, we use a highly parallelised implementation of a hybrid front-tracking/level-set method that employs a domain-decomposition parallelisation strategy. Our numerical technique is designed to capture faithfully complex interfacial deformation, and changes of topology, including interface rupture and dispersed phase coalescence. As shown via transient, a three-dimensional (3-D) LES (large eddy simulation) using a Smagorinsky–Lilly turbulence model, the impeller induces the formation of primary vortices that arise in many idealised rotating flows as well as several secondary vortical structures resembling Kelvin–Helmholtz, vortex breakdown, blade tip vortices and end-wall corner vortices. As the rotation rate increases, a transition to ‘aeration’ is observed when the interface reaches the rotating blades leading to the entrainment of air bubbles into the viscous fluid and the creation of a bubbly, rotating, free surface flow. The mechanisms underlying the aeration transition are probed as are the routes leading to it, which are shown to exhibit a strong dependence on flow history.

High levels of larval mortality are a significant barrierto the artificial mass production of Pacific bluefin tuna (Thunnusorientalis). Mortality may occur when larvae sink and comeinto contact with the bottom of the rearing tank during the first10 days after hatching. We evaluated the effect of flow controlby aeration on the survival of T. orientalis larvae.These larvae were held in 500-L tanks in which the aeration ratewas varied during the night. Larval survival increased with airsupply. We documented the cross-sectional flow pattern and gravitationalsinking velocities of larvae to assess the correlation between survivaland circulation patterns in the tank. The sinking velocity of T. orientalis larvaeat night increased with larval body density, which varied with swimbladdervolume. Larvae with uninflated swimbladders sank significantly fasterthan larvae with inflated swimbladders. Both water circulation speedand survival increased at higher aeration rates. Our results suggestthat aeration rates >900 ml min–1 may increase larval survival bycounteracting sinking.

Segmentation of the aeration system and its anatomy was studied in rhizomes of common reed (Phragmites australis). Segmentation is achieved by nodal diaphragms which allow the passage of pressurised gas-flow but which also form effective barriers against flooding of the internal space in case of local injury. The pressures required to force water through the diaphragms were measured and compared with the anatomical structure of the diaphragm. The fine hydrophilic stellate parenchyma of the diaphragms was shown to act as a matrix supporting menisci of air–water interfaces and therefore preventing water movement until the ‘limit’ pressure (measured values ranged from 18 to >40 kPa) was overcome. Although the surfaces of the large stellate parenchyma and sclerenchyma strands covering the diaphragm are hydrophobic these components do little to prevent the ingress of water.