Randomness in flow turbulence around a bridge pier in a sand mined channel

Abstract

Mining activities have the potential to cause excess scour and affect the streambed stability around bridge piers present in the vicinity. Randomness characteristics of turbulence around the pier in a mining infested channel can be helpful in better understanding of this phenomenon. In this study, the effect of a mining pit on the randomness in turbulent flow around the pier has been explored on the basis of Kolmogorov complexity (KC) measure and its derivative, i.e. KC spectrum. Conventionally, turbulent stresses are supposed to govern the sediment transport characteristics, hence the Reynolds shear stresses (RSS) and bursting events are also analyzed. Results of two cases have been comparatively studied; for the first case, only an oblong pier is embedded in the channel while in the second case, a rectangular mining pit is excavated upstream of the pier. Quadrant analysis of the Reynolds shear stress revealed that strong ejection events in the upstream side and inward interaction events in the downstream side of the pier cause excess momentum transport in presence of a mining pit. Streamwise and vertical turbulence intensities also increase by 20% and 33% respectively in the down-flow zone in front of the pier. KC value of turbulence around the pier, especially in the down-flow zone and separation zone is found to be 10% and 12% higher due to pit excavation. Comparing the KC spectra for both cases suggests that dredging of a pit in the channel increases the degree of randomness of higher velocity scales around the bridge pier. These alterations in turbulent stresses and randomness characteristics can be helpful in explaining the streambed scour and undermining of bridge piers due to mining activities.