Experimental and numerical study of submerged, round buoyant jets impinging on a horizontal surface.

Abstract

There exists experimental evidence that the dilution of a submerged point source discharge achieved in the radial spreading flow after impingement on a horizontal surface is considerably greater than that predicted for only the submerged jet portion of the flow field. Although previous studies presented data and developed a conceptual model that qualitatively described the experimental results, they do not provide sufficient results to completely understand this phenomenon and disagree on how to model it. In this research, velocity and concentration profiles at various radial distances within the radially spreading flow field were gathered simultaneously with an Acoustic Doppler Velocimeter and an electronic conductivity probe. The experimental setup consisted of a negatively buoyant jet impinging on a submerged plate in a large tank. The purpose was to both identify the deficiencies in the previously proposed model formulation and to perform specific types of experiments that would independently validate the conceptual basis behind the model formulation. Measurements gathered in this study confirmed the existence of a critical flow downstream of the near field mixing zone. By adding a specific downstream control, namely a weir, in the far field zone of the flow, the critical flow control was removed. This resulted in a reduction in the Froude number and the dilution as well, emphasizing the significance of downstream controls on mixing. Experiments involving the submergence of the buoyant jet demonstrated the role of the two layer dynamics on the overall mixing. In the plume case, the upper layer does not contribute to the Froude number or the momentum balance and the experimental results show the dilution to be independent of total depth. Increasing the source momentum increases the influence of the upper layer on the flow dynamics and experiments for this case show that increasing the total depth increases dilution. Numerical model was proposed that is capable of predicting average dilution and average layer thickness at the end of the near field mixing zone and the radial extent of this zone.