Experimental investigation of turbulent bursting events in weakly mobile channel bed

Prem Lal Patel, Dhvani Y. Patwa, Praful V. Timbadiya

Thursday 2 july 2015

12:00 - 12:15h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 11A. Sediment - Transport

Present study deals with measurement of turbulent flow structures near coarse sediment bed in a mobile boundary channel. Measured velocity data, using 3D-ADV, were filtered, as suggested by Goring and Nikora (2002), before performing quadrant analysis, and quantifying the dominance of bursting events. The quadrant threshold method, defining a hole-size parameter, H, as advocated by Lu and Willmarth (1973), is used to eliminate the fractional contribution of weak bursting events in total Reynolds shear stress (RSS). From fractional contribution of bursting events, it has been found that contribution of sweep event is maximum in RSS near the channel bed. The situation is reversed in outer flow region wherein the contribution of ejection event is maximum in RSS. As far as fractional contributions of bursting events with reference to time is concerned, the sweep and ejection events are in dominance with 29.92% and 28.42% of the total time respectively. The shear velocity using the different approaches, viz. using mean bed shear approach, logarithmic-law of wall approach, extrapolating Reynolds shear stress distribution approach (Nezu and Nakagawa, 1993) and VPA Excel-spreadsheet approach (Le Roux and Brodalka, 2004), is computed from the measured data, and found to vary from 4.74 to 3.97 cm/s. The quantification of turbulent bursting events and shear velocity would be helpful in modelling the sediment transport and morphological processes in alluvial channels. References Goring, D. and Nikora, V. (2002). “Despiking acoustic Doppler velocimeter data.” Journal of Hydraulic Engineering, ASCE, 128(1), pp. 117-126. Le Roux, J. P., & Brodalka, M. (2004). “An Excel™-VBA programme for the analysis of current velocity profiles.” Computers & geosciences, 30(8), pp.867-879. Lu, S. S., and Willmarth, W. W. (1973). “Measurements of the structure of the Reynolds stress in a turbulent boundary layer.” Journal of Fluid Mechanics, 60(03), pp.481-511. Nezu, I., and Nakagawa, H. (1993). “Turbulence in open-channel flows.” Balkema, Rotterdam, The Netherlands.