On Coherent Structures of Turbulent Open-channel Flow Above a Rough Bed

Abstract

Present study examines turbulent structures of a rough bed open-channel flow in the context of deterministic approach. Instantaneous velocity field is measured in different hydraulic conditions using two dimensional Particle Image Velocimetry (PIV) in vertical plane and Stereoscopic PIV in horizontal plane. Different techniques and quantities such as swirl strength, two-point and cross-correlations of the swirl strength and fluctuating velocity are estimated to control formation of hairpin vortex. Our observations show in the area far above roughness elements, there is fairly a good agreement with the observations of previous studies for smooth-bed boundary flow. This consistency demonstrates that there is a similarity between outer region of the smooth and rough-bed layer flow. Moreover, some of the observations in the present study such as contourmap of instantaneous vorticity and swirling strength or observed inclination angle in contourmap of two-point correlations of the fluctuating velocity and swirl contourmap can be considered as signatures of the hairpin vortex. Given these observations, due to the lack of direct observation of the hairpin vortex and similarity of the observed features with signatures of other types of coherent structures, it is still too hard to assurly opine that in this condition, hairpin vortex is present.