Characterization of double-averaged velocity profile in an open-channel with intermediate relative roughness

Flow with intermediate relative roughness (the ratio of roughness height to water depth higher than 1/80 and lower than 1/40) is common in most of mountainous streams. Despite this fact and numerous studies on flow with intermediate relative roughness, it is still unclear that how profile of the streamwise velocity varies along water depth. In this study, instantaneous velocity of flow in an open-channel with rough bed has been measured using Particle Image Velocimetry (PIV). In order to analyze profile of streamwise velocity, double averaging method (spatial averaging of time-averaged values in a thin slab parallel to the channel bed) is used. It was observed that near the rough bed, vectors of instantaneous velocity showed strong spatial variations that makes extraction of unique behavior for velocity profile impossible without double averaging. Results also showed that values of double averaged velocity is not sensitive to priority in averaging (i.e. time and then spatial averaging or spatial and then time averaging); thus double averaging regardless of priority can be used. To investigate double averaged velocity profile, three approaches including logarithmic profile with variable parameter, linear profile and mixing layer profile were employed. Results showed that all three profiles could be fitted properly to our experimental data. However, logarithmic profile with variable Von-Karman constant and integration constant which is supported by strong scientific background is the most suitable profile and is therefore recommended.