Author(s): Heqing Huang

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Keywords: Gravity flows; Open channel flows; Densimetric Froude number; Gradient Richardson number; Secondary flow

Abstract: Research concerning open channel flows originated much earlier and is much mature than that for subaqueous gravity flows (SGF). Many important concepts and theory of open channel flows are often transplanted to SGF directly until several years ago. Recent theoretical, experimental and numerical advances on the research of gravity flows reveal that we may need to have more doubts about our traditional thinking by realizing the fundamental differences between the two. First, the critical densimetric Froude number for identifying sub- or super-critical flows may be larger or smaller than unit, or even non-existent in SGF on the reason that a supercritical SGF may dissipate its energy through the entrainment of ambient fluid or sedimentation. Secondly, a value of gradient Richardson number around 0. 25 is generally deemed a critical value for stratification stability. Over the years it is found that at even gradient Richardson higher above unit, high horizontal eddy viscosity and eddy diffusivity are also observed even though vertical turbulence is inhibited by stratification. Thirdly, not long ago there is a heating argument about whether the secondary flow of SGF at sinuous channel bends is river-like, directed at the bed from the outer bank to the inner bank, or river-reversed between a American and an English research group based on their respective limited experimental observations. Then with more experiments and simulations, it is further realized that it may be river-like or river-reversed depending on bed slope or flow regimes. By simulating SGF in both straight and sinuous channels of various sinuosity, we find that small river-like circulation near the bottom corner of outer bank may co-exists with predominant river-reversed secondary flow. The secondary flow pattern of SGF is not only related to channel bed or flow regime but also to channel shape and sinuosity. These findings should have profound implications in our future study of SGF. We may not definitely expect sharp change in turbidites caused by hydraulic jumps when turbidity currents experience a slope change, we may not shut down turbulence in numerical models when the gradient Richardson number is above 0. 25, and we need to realize that similar morphology may be formed due to different mechanisms between open channels and submarine channels.

DOI:

Year: 2013