Iranian Water Researches Journal Laboratory study of rough bed open channel turbulent flow

Study of open-channel turbulent flow over rough bed is beneficial to provide a better understanding of many riverine physical processes. Indeed, the knowledge of rough bed turbulent flow has a significant effect on sustainable environmental management of the mountainous rivers. The structure of rough bed turbulent boundary layer in inner region is affected by bed geometry in addition to flow characteristics (e.g. viscosity). Presence of rough elements in the channel bottom makes differences between boundary layer flow over the smooth and rough beds. Presence of roughness enhances the secondary currents. The secondary currents’ enhancement can play an important role in sediments’ transversal and longitudinal transport. The secondary currents’ formation in rectangular open-channels is attributed to the side wall effects. If the channel width is small, i.e., the side walls are not too far from each other, the secondary currents are formed in whole cross-section. Accordingly, based on the channel aspect ratio (which is defined as the ratio of channel width to the water depth; i.e. B⁄H), Nezu and Nakagawa [14] classified the rectangular open-channels: First class is called narrow channels which has an aspect ratio smaller than 5. In narrow channels, the secondary currents are present in whole cross-section. The second class is wide channels where B⁄H is larger than 2.5~5. In this category, no secondary currents are formed in the channel central part (i.e. y⁄(H>2.5) where y is transversal location) and so flow characteristics do not noticeably undulate in the lateral direction. Despite many studies about narrow channels, only a few number of studies deal with secondary current presence of absence and also their characteristics in wide channels. This paper deals with turbulent flow hydraulic characteristics in straight open channel under the roughness elements’ effects, with high aspect ratio (i.e. wide channel). More precisely, in this paper, characteristics of the flow due to formation of secondary currents in the rough bed condition are studied. For this purpose a series of laboratory measurements using ADV probe were conducted which will be explained in the following section. The laboratory experiments were conducted in a straight rectangular flume, 11 m long and 1 m wide, at the Hydraulic Laboratory of Tarbiat Modares University. The channel side walls and bed were made from glass and smooth Perspex, respectively. Free surface profiles were measured with an ultrasonic distance transducer to control the flow uniformity condition. Small surface waves at the flume entrance were eliminated using a 1.5 m long and 0.95 m wide polystyrene plate held parallel to the upper water surface just at the end of the water intake. The flow rate was controlled with a gate valve and was measured using a magnetic flow meter. During experiments, four different hydraulic scenarios were performed in subcritical flow condition. A summary of the experimental conditions is reported in Table 1. Also, the rough bed characteristics during the measurements are reported in Table 2. Flow field was measured by a vectorino type 10 MHz down looking micro- Acoustic Doppler Velocimeter (ADV) capable of measuring instantaneous three velocity components. The velocity measurements were done at a sampling rate of 100 Hz over a 45 mm3 sampling volume. Each velocity point was sampled for at least 300 s in order to assure that the number of independent samples is enough for statistical analysis. In normal- and span-wise directions, distance between two consecutive measurement points were 5 cm and 5 mm, respectively. Measurements were performed in just half of the channel, after verifying symmetry of the velocity distribution with respect to the channel centerline. In addition to the flow field, the local bed shear stresses were measured using a homemade Preston tube named three-tube device in the present study. The manufactured probes were connected to four capacitive types, the Keller 41X pressure transducers using silicon tubes and so the collected data could be stored in the computer. Each point measurement with three tube pressure instrument was continued for around 300 s similar to the measurement of the velocity field, while three tube pressure instrument data were measured with frequency of 50 Hz. It has been observed that all the flow characteristics are spatially changed in transversal directions. Mean streamwise velocity and turbulent intensity profiles are diverted from available theoretical equations as secondary currents’ result. These diversions are clearer near side walls region. Also, shear stress in transversal direction is changed significantly. The undulation amplitude in transversal direction is 35% of shear stress mean value in span-wise direction.