مهندسی عمران شریف (Feb 2019)
AN ESTIMATION OF RELATIVE SUBMERGENCE EFFECTS ON TURBULENCE MACRO-SCALE STRUCTURES IN A PLAIN CHANNEL WITH ROUGH BED
Abstract
Previous studies clearly show the importance of surface roughness for natural and artificial boundary-layer flows, such as open-channel flow. The relative submergence (the ratio of water depth to the bed roughness length characteristics) is an essential factor in open-channel rough bed flow. The turbulence macroscales are one of the most important characteristics of turbulent flow, which can be affected by relative submergence. In fact, turbulence macroscales in wall turbulence transport momentum and provide a means of producing turbulent kinetic energy. In the case of rough bed flows, where roughness elements protrusions disrupt homogeneity of near-wall flow characteristic, turbulence macroscales show a complex behavior not only in near wall region, but also in region far from the bed. These issues should be studied considering the importance of relative submergence.The present research is an experimental study that is focused on the role of relative submergence on the structures of turbulent macroscales. To this end, stream-wise and normal-wise components of velocities are measured with the aid of particle image velocimetry (PIV) method in a rectangular open-channel. During laboratory measurements, three distinctly different hydraulic scenarios, where the ratio of flow depth to roughness height (i.e., relative submergence) changes from 7.5 to 10.8 , are covered. Various methods and concepts common in turbulence studies, such as vorticity, two-point correlation, Galilean decomposition, are implemented to determine the role of relative submergence in turbulent macro scales.These measurements show that the overall shape of instantaneous vortices and turbulent structures does not change with relative submergence. However, the length of turbulent macroscale increases with the relative submergence. Furthermore, it is found that the ratio between the length of turbulent macroscale obtained from stream-wise velocity and those obtained from normal-wise velocity increase with the relative submergence. This observation represents that the stream-wise extension of stream-wise velocity is higher in comparison to normal-wise velocity.
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