Turbulent entrainment in sediment-laden flows interacting with an obstacle

Temporal entrainment characteristics and mixing processes of sediment-laden turbidity currents interacting with a rectangular obstacle are investigated through lock-exchange experiments. Building on the Morton-Taylor-Turner hypothesis, dependency of temporal entrainment on non-dimensional parameters is examined. Currents of varying density are analyzed during the slumping phase over smooth and rough substrates. Quantitative and qualitative observations of the currents are captured through high resolution, high framerate binary thresholding techniques. Additionally, siphoning techniques are used to compare the density structure of the currents before and after the obstacle. Upon interaction with the obstacle, currents are found to experience four stages of entrainment: (i) lateral entrainment stage; (ii) jet stage; (iii) collapsing stage; (iv) re-establishment stage. The entrainment parameter was within the range of other studies for both obstacle and no-obstacle cases. Reynolds, Froude, and Richardson numbers are also comparable to previous studies; however, there was no clear relationship with the entrainment parameter. This suggests that entrainment dependency on non-dimensional parameters is not quantifiable where the analysis area length to lock-box length ratio is ≈1. The presence of the obstacle was shown to increase entrainment by approximately 99% immediately downstream of the obstacle, associated with a subsequent entrainment decrease by 14% at the downstream end of the analysis area. For rectangular obstacles 1/6th the initial current height, as used for this study, an obstacle’s role is limited in decreasing net velocity and entrainment and not fit-for-purpose as a barrier to reduce current velocity. Finally, we discuss optimization strategies, weighing up observed minimal net velocity/entrainment decrease with the detrimental effects of jet expansion.