Experimental study on velocity profile and turbulence characteristics in open channel contractions

Channel contractions are widely used in water diversion projects due to topographical and geological limitations. Short transitions would lead to water surface fluctuations and higher turbulence. To investigate the velocity and turbulence characteristics along transitions of different length, laboratory experiments were carried out using a two-dimensional electromagnetic current meter ACM2-RS. Results show that the longitudinal mean velocity increases along the centerline of the contraction while the longitudinal turbulence intensity decreases, which can be explained by vortex stretching mechanism and linear distortion theory. Influenced by secondary currents, the maximum velocity occurs below the surface. The stronger secondary circulation is, the lower the maximum velocity is located. Contractions of different length affect the velocity profiles and turbulence intensities downstream differently. Shorter contractions lead to higher magnitude of turbulence intensity downstream, while its vertical distribution remains the same in the region away from the bed, which decreases first and then increases with the vertical position going up.