Measurements of turbulence structure in a compound channel

Abstract

Experimental research was undertaken to investigate the changes in spatial turbulence intensity, water turbulent kinetic energy, the time and spatial macro-scale, scales of turbulent eddies (macro- and microeddies) in a compound channel. Three tests for two various roughness values were realized. The surface of the main channel bed was smooth and made of concrete, whereas the floodplains and sloping banks were covered by cement mortar composed of terrazzo. Instantaneous velocities were measured with the use of a three-component Acoustic Doppler Velocimeter (ADV). The distributions of relative turbulence intensity (u′/U, v′/U, w′/U) in the main channel and on the floodplains were presented. It was found that the longitudinal (u′/U) and transverse (v′/U) turbulence values decreased from the bottom upwards to the floodplain elevation (z/h = 0.56) in the main channel, but remained constant above the floodplain level. Vertical relative turbulence intensity (w′/U) increased going up from the bottom until z/h = 0.15, decreased until about z/h = 0.7, and then increased again upwards to the water surface. The distributions of relative turbulence intensities were described with regression equations. The distributions of turbulent kinetic energy at different water depths were described by regression equations. Vertical distributions of turbulent kinetic energy on the floodplains and over the banks of the main channel were divided into three zones. Over the bottom of the main channel, four zones were determined, containing the middle zone of the flow field divided into two zones of different trends. Measurements of instantaneous velocities are used to investigations of the longitudinal sizes of the smallest eddies (Kolmogorov’s microscale). Presented analyses were based on the already published results.