Rough Open Channel Flow Research Articles

TURBULENT STRUCTURE IN OPEN-CHANNEL FLOW OVER FLAT BED WITH BED-LOAD TRANSPORT

In open channel fows with bed-load transport, mean flow properties, turbulence intensities and Reynolds stress profiles are affected by the bed-load motions. In this study, LDA measurements over flat movable sand bed were conducted with high accuracy. The sediment transport was also measured simultaneously. It was found that the mean velocity gradient near the bed decreases with an increase of the bedload transport rate. Both the horizontal and vertical turbulence intensities, also increase in the inner layer as compared with those of rough fixed-bed open channel flows. Reynolds stress profiles near the bed deviate systematically from the triangular distribution which is theoretically valid in a two dimensional uniform open channel flow.

Turbulent Velocity Profiles for Smooth and Rough Open Channel Flow

Velocity measurements, using a laser‐doppler anemometer, are carried out in a fully developed, rectangular, subcritical open channel flow on smooth and rough beds. The average roughness heights of the “rough” surfaces, used in the experiments, are 1 mm, 4 mm, 8 mm, and 12 mm. A model is presented for determining the reference level of the velocity distribution on “rough” surfaces. The shear velocities are determined from velocity profiles measured close to the bed. The calculated shear velocities show an increasing tendency as the wall roughness increases. In the fully turbulent inner region on a “smooth” wall, the coefficients in the logarithmic law‐of‐the‐wall distribution take values of 2.44 and 5.5; the former corresponds to 0.41 for the von Kármán constant. The velocity‐defect expression of Coles has a profile parameter of 0.1, rather than 0.55. For “rough” channels, the representation of the overall data in terms of law‐of‐the‐wall distribution seems reasonable; however, the velocity‐defect distribution is not satisfactory.

Transverse Mixing in Open Channel Flow

An experimental study motivated by dimensional analysis was carried out to investigate the variation in the transverse mixing coefficient in straight, rectangular, smooth, and rough open channel flows. The findings have led to a number of suggestions relating to the future laboratory simulations of prototype cooling-water or wastewater dispersion in a river.

Some Turbulence Measurements in Open-Channel Flow

Measurements of the characteristics of turbulence in hydrodynamically smooth and hydrodynamically rough open-channel flow using hot-film anemometry technics showed: (a) that the relative turbulent intensity of the vertical velocity component was about 60% of the longitudinal, (b) that the spectral energy distribution was not significantly affected by the type of flow, (c) that most of the turbulent energy is contained in frequencies less than 5 hertz, (d) the macroscale of the turbulence as determined from the autocorrelation function was on the same order as the depth and (e) the ratio of the microscale to depth ranged from 0.1 to 0.2. The measurements were used to verify experimentally the longitudinal direction momentum terms in the Navier-Stokes equation and to determine qualitatively the magnitude of the production, diffusion, and dissipation terms in the energy equation.