Velocity Distribution and 3D Turbulence Characteristic Analysis for Flow over Water-Worked Rough Bed

Abstract

Abstract:To reproduce the natural flow topography in a laboratory environment, it is crucial to recapture its bed condition in order to ensure the accurate representation. Water-worked bed represents a state-of-the-art experimentally formed bed to imitate the natural-formed channel in most rivers or natural streams. Recently, this technique has been intensively studied through experimental and computational approaches; however, its actual influence towards the near-bed flow as compared to experimentally prepared rough bed in well-packed bedform order are still yet to be investigated deeply. This experimental study systematically investigated and compared the differences in velocity distribution and three-dimensional (3D) turbulence characteristics, including turbulence intensities and Reynolds stresses, between uniform smooth bed, laboratory-prepared rough bed and water-worked bed open channel flows. The flow comparisons were concentrated at near-bed region where clear flow behaviour change can be observed. Through these comparisons, the study inspected the characteristics of water-worked bedform thoroughly, in order to inform future experimental research that tries to reproduce natural stream behaviours.

Dominant features in three-dimensional turbulence structure: comparison of non-uniform accelerating and decelerating flows

“…1 has dimensions of 12 m 9 0.50 m 9 0.45 m, and it is a recently refurbished flume located at the Hydraulic Laboratory, University of Bradford [26]. The flume is operated by a circulating system, where the outlet discharge is directed into a filtering tank then to a water pump system to be re-circulated back into the flume.…”

Water-Worked Gravel Bed: State-of-the-Art Review

“…Recently, Padhi et al [32] examined the effects of roughness on the τ uw and τ f profiles in a WGB and an SGB. Akin to Pu et al [30], Padhi et al [32] found that the roughness height in the WGB was also higher than that in the SGB. However, the results of Padhi et al [32] do not correspond to those of Pu et al [30].…”

“…Besides, after Nezu et al [29], the bed topography can be considered as one of the most influencing factors in estimating the turbulence parameters. Therefore, to quantify the impact of the bed topography on the flow velocity, Pu et al [30] carried out experiments over three different beds (a smooth bed, a WGB and an SGB), using an Acoustic Doppler Velocimeter (ADV), and compared the results. They used the following equations of log-wake laws for velocity profiles:…”

“…where u * is the frictional velocity, z is the vertical distance, ν is the kinematic viscosity, B r is the constant of integration, κ is the von Kármán coefficient, Π is the Coles' wake parameter, ∆z is the virtual bed level (≈0.25 k s , according to Dey et al [31]), z 0 is the zero velocity level, k s is the average roughness height, and h is the flow depth. Pu et al [30] used the velocity data of each bed to obtain the fitted curves for the log-wake laws (Figure 7). Interestingly, the values of B r are lower in both the smooth and the rough (WGB and SGB) beds than the traditional values: B r = 5.5 and 8.5 for the smooth and rough beds, respectively.…”

“…Pu et al [30] compared the τ uw profiles in a smooth bed with those in the WGB and SGB. They showed that the τ uw profile in the smooth bed converges with the gravity line at a shorter vertical distance than those in the WGB and SGB.…”