Author(s): Nadeeka S. Miguntanna, Muttucumaru Sivakumar, Shu-Qing Yang, Keith Enever, Quan Gong
Keywords: Slender cylinder; Open channel; Wake; Turbulence;
Abstract: The complex wake created by an emergent circular cylinder in a shallow open channel flow is studied using Laser Doppler Velocimetry (LDV) Technique. While previous wake studies in shallow flow condition are focused on the small aspect ratio cylinder, here the wake features of shallow flows past a bed mounted cylinder with large aspect ratio (hv/D) of 13.7; is investigated where hv is the wetted length of the cylinder and D is the cylinder diameter. The complexity of the wake is investigated by conducting velocity measurements on horizontal planes at near-bed, mid-depth, and near the free surface. Measurements were taken in both upstream and downstream of the cylinder. The wake region is clearly demarcated from series of measurements in a very fine grid. Velocity fields are studied at a cylinder Reynolds number (ReD) of 3300 in water and at a subcritical Froude number of 0.58. It is found that stagnant re-circulation region behind the cylinder in all horizontal planes, where the size of the mean re-circulation region is found to be different. It is found that wake closure length (Lwc) (distance to the wake closure point from the centre) tends to decrease as the distance from channel bed increases; i.e near the channel bed Lwc is highest and lowest Lwc is obtained near free surface. On the other hand, length of the re-circulation region (Lc) tend to increase from near bed to free surface confirming the bed suppression effect. Further, in the near-bed plane, higher turbulence intensities were observed. In addition, after comparing the similar studies in the literature it is also noted that wake characteristics of slender cylinders are significantly different to wake generated by cylinders with small aspect ratio.