Numerical Analysis of Air Injection as a Scour Control Technique in Flow

The present study numerically investigates the air injection method as a scour control technique. Air bubbles are injected into the flow exiting a stilling basin, and their effects on the scour potential of the flow are explored. Because of the existence of both large- and small-scale interface structures, the investigated flow lies in the category of transitional multiphase flows, and a numerical model based on the computational fluid dynamics-population balance method (CFD-PBM) approach is developed for the simulation. Verification of the model is done by simulation of Rayleigh–Taylor instability. To validate the model’s predictions, the skin-friction coefficient on a flat-plate both with and without the presence of air bubbles are compared with the experimental results. The maximum difference in the magnitude of the skin-friction coefficient before air injection between numerical and experimental results is 13%. The numerical model also showed a favourable agreement, with a maximum difference of 9%, between the numerical and the experimental results after air injection to the flow. The flow in a stilling basin is also simulated, and air bubbles are injected at the end of the basin to the flow. The formation process of the hydraulic jump and the air entrainment at the jump toe are in agreement with experimental reports. The exerted shear force on the bed, which is considered as an indicator of the scour potential of the flow, decreases up to 47% due to air injection. The underlying physics of the scour reduction phenomenon caused by air injection is investigated thoroughly by analysing the internal flow field in the presence of air bubbles. Graphical Abstract